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• Published: 28 January 2021

Development and characterization of GR2E Golden rice introgression lines

• B. P. Mallikarjuna Swamy 1 ,
• Severino Marundan Jr. 1 ,
• Mercy Samia 1 ,
• Reynante L. Ordonio 2 ,
• Democrito B. Rebong 2 ,
• Ronalyn Miranda 2 ,
• Anielyn Alibuyog 2 ,
• Anna Theresa Rebong 2 ,
• Ma. Angela Tabil 2 ,
• Roel R. Suralta 2 ,
• Antonio A. Alfonso 2 ,
• Partha Sarathi Biswas 3 ,
• Md. Abdul Kader 3 ,
• Russell F. Reinke 1 ,
• Raul Boncodin 1 &
• Donald J. MacKenzie 4  

Scientific Reports volume  11 , Article number:  2496 ( 2021 ) Cite this article

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• Biotechnology
• Plant sciences

Golden Rice with β-carotene in the grain helps to address the problem of vitamin A deficiency. Prior to commercialize Golden Rice, several performance and regulatory checkpoints must be achieved. We report results of marker assisted backcross breeding of the GR2E trait into three popular rice varieties followed by a series of confined field tests of event GR2E introgression lines to assess their agronomic performance and carotenoid expression. Results from confined tests in the Philippines and Bangladesh have shown that GR2E introgression lines matched the performance of the recurrent parents for agronomic and yield performance, and the key components of grain quality. Moreover, no differences were observed in terms of pest and disease reaction. The best performing lines identified in each genetic background had significant amounts of carotenoids in the milled grains. These lines can supply 30–50% of the estimated average requirements of vitamin A.

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Introduction

Rice ( Oryza sativa ) is the major source of energy and nutrition for more than half the world’s population 1 . However, rice supplies minimal micronutrients in its milled form and completely lacks β-carotene which is the precursor for vitamin A. Thus, resource-poor people primarily dependent on rice with little access to diverse diets suffer from micronutrient deficiencies, also termed hidden hunger 2 , 3 . Even though efforts are being made to address micronutrient deficiencies by supplementation, fortification, and dietary diversification, the problem still persists globally. Biofortification of major staple crops has been recognized as one of the sustainable means to tackle micronutrient deficiencies especially in the vulnerable target groups in rural areas 4 .

Vitamin A is essential for various functions in the human body such as development and functioning of the visual system, differentiation and maintenance of cells, epithelial membrane integrity, and production of red blood cells, immune system, reproduction, and iron metabolism 5 , 6 . An estimated 190 million children and 19 million pregnant women have vitamin A deficiency (VAD), and almost a million children go blind every year 7 . In the Philippines, VAD ranges between 19.6 to 27.9% in infants and preschool children 8 , while in Bangladesh, over half of the preschool age (56.3%) and school age children (53.3%) at the national level were found to exhibit at least a mild grade of VAD 9 .

Several crops such as maize, cassava, and sweet potato have been successfully biofortified with elevated levels of provitamin A 10 , 11 . However, there is no naturally-occurring variation for provitamin A in grains in rice germplasm, so this has been achieved by using genetic engineering approaches. The genetic modification was made by the addition of two genes, phytoene synthase ( Zmpsy1 ) from Zea mays and carotene desaturase ( crtI ) gene from the common soil bacterium, Pantoea ananatis (syn. Erwinia uredovora ) into a temperate japonica rice variety, Kaybonnet, from the USA. This completed the carotenoid pathway in the grain and resulted in the accumulation of β-carotene in the endosperm 12 , 13 . However, the transfer of this golden rice trait from Kaybonnet into additional locally-adapted and widely-grown rice varieties is required for the successful release and adoption of golden rice in Asia.

Among the six second-generation Golden Rice (GR2) events received by the International Rice Research Institute (IRRI), event GR2E was found to contain a single intact copy of the inserted DNA integrated at a single site within the rice genome, giving rise to agronomically desirable progeny with suitable grain carotenoid content. This event has been transferred into Asian rice varieties through marker-assisted backcrossing (MABC). MABC has been successfully used to transfer high value genes/QTLs for disease resistance, submergence and drought tolerance traits into popular rice varieties without altering their desirable traits 14 , 15 , 16 .

Development of stable golden rice breeding lines with nutritionally relevant levels of provitamin A and without trait-associated yield, grain quality, or disease resistance penalties relative to the recipient parental varieties is essential for the successful adoption of golden rice. Introgression of the GR2E locus from GR2E Kaybonnet into PSBRc82, IR64, and BRRI dhan29 (BR29) was performed at IRRI through MABC along with selections for desirable agronomic and grain quality traits. The phenotypic evaluation was conducted under screen house conditions. Selection of homozygous plants and lines were carried out under field conditions at IRRI. Agronomic evaluations of selected lines were carried out under field conditions in a series of confined field tests (CTs) at IRRI, PhilRice and BRRI.

The main objectives of the present work were to: develop agronomically desirable lines of provitamin A enriched GR2E golden rice in the genetic backgrounds of popular rice varieties from Asia; to understand the effects of genetic background and environment on carotenoid expression, and to identify stable and productive lines of GR2E golden rice for varietal evaluation.

Introgression of event GR2E into multiple genetic backgrounds

A series of five backcrosses of event GR2E Kaybonnet into three widely-grown rice varieties, IR64, PSBRc82, and BR29, resulted in the identification of introgression lines that were agronomically similar to their respective recipient parents. The stability and inheritance of the GR2E locus was confirmed using event-specific PCR in every generation, where it was found to segregate without distortion in a typical 1:1 Mendelian ratio in all the backcross generations (BC 1 to BC 5 ) and genetic backgrounds. All seeds containing the GR2E event showed the typical golden yellow color, indicating the expression of the provitamin A trait in the endosperm. Hemizygous (It is a condition in a diploid organism, where only one copy of the locus is present) plants phenotypically similar to their respective recipient parents were identified, backcrossed and advanced up to BC 5 F 1 , and with each successive backcross there was a progressive increase in similarity of the progenies to their respective recurrent (recipient) parents (Fig.  1 ). A total of 400, 190, and 94 BC 5 F 1 plants of IR64, PSBRc82, and BR29, respectively, were phenotyped and genotyped by event-specific PCR. Yellow BC 5 F 2 seeds were selected and analyzed for total carotenoid content, which ranged from 3.6–6.2 ppm in IR64, 3.1–6.4 ppm in PSBRc82, and 3.2–8.0 ppm in BR29. The BC 5 F 2 plants were closer to respective recipient parents for key agronomic traits with average days to flowering (DTF), plant height (PH) and number of panicles (NP) of the selected BC 5 progenies were 71.5 days, 108 cm and 15 for IR64, 82.5 days, 122.3 cm and 15.4 for PSBRc82 and 83 days, 117 cm and 17 for BR29 respectively. The final set of BC 5 F 3 selected lines had background recovery of more than 98%. Agro-morphological traits, panicle characteristics, and grain parameters were similar to the recipient parents and no unintended, unexpected, effects due to the presence of the GR2E event were observed throughout the backcross breeding program. Based on the overall agronomic performance, carotenoid levels, and genetic background recovery, 40 BC 5 F 1 plants in the IR64 background, and 20 BC 5 F 1 plants in each of the PSBRc82 and BR29 backgrounds were selected. The BC 5 F 2 seeds produced by each of these plants were further evaluated under field conditions in confined tests and plants homozygous for the GR2E locus were selected.

( a – c ) GR2E introgression lines.

Selection of homozygous and agronomically acceptable GR2E lines

The first confined field test of GR2E breeding lines was carried out during the 2015WS at IRRI to make individual homozygous plant selections. From among 8000 BC 5 F 2 plants tested, a total of 602, 439, and 471 plants homozygous for the GR2E locus were identified in IR64, PSBRc82, and BR29, respectively (Fig S1 ). Efforts were focused on the lines homozygous for GR2E; however, hemizygous and null plants were also phenotyped to determine the impact of the presence of the GR2E locus on agronomic traits. The pair-wise t-tests were conducted between families derived from single BC 5 F 1 plants within each of the three genetic backgrounds. Significant differences between families for total carotenoids were noted in a number of the possible pair-wise comparisons (data not shown). The mean comparisons between homozygous, hemizygous and null GR2E plants within each of the three populations did not show any abnormal deviations for key agronomic traits (Fig S2 ). The mean PH of lines carrying GR2E were marginally shorter than the respective recipient parent. For the remaining traits there were no clear differences between plants carrying GR2E and the respective parent variety. A total of 70 BC 5 F 3 ILs similar to their respective parents and having higher levels of carotenoids were selected for IR64 and PSBRc82 genetic backgrounds.

Evaluation of GR2E introgression lines in multi-location replicated confined tests

Agronomic performance of GR2E Introgression Lines (ILs) and their respective control varieties were assessed in a series of CTs at IRRI (2015WS, 2016DS and 2016WS), PhilRice (2015WS and 2016DS) and BRRI in Bangladesh (2016 Boro). A total of 70 ILs similar to their respective parents in agronomic performance and having the greatest levels of carotenoids were selected from each of IR64 and PSBRc82 backgrounds. A total of 14 agronomic, yield and yield-related traits and carotenoid content were measured from the different confined tests. Among the 70 ILs tested during the 2015WS at IRRI, PSBRc82 GR2E ILs showed small but statistically significant differences from non-transgenic PSBRc82 for eight traits including days to flowering (DTF), plant height (PH), Flag leaf length (FL), flag leaf width (FW), filled spikelets (FS), total number of spikelets per plant (TSP), grain length (GL) and hundred seed weight (HSW) (Table 1 ). However, in successive CTs conducted using 32 GR2E PSBRc82 ILs at IRRI and PhilRice, only FL, GL and HSW (2016DS), and GL, HSW and plot yield (PY) (2016WS; IRRI) showed significant differences. On the other hand, no significant differences were observed during the 2016DS and only GL and HSW showed significant differences at PhilRice in 2016WS (Table 2 ). Similarly GR2E IR64 ILs showed small but significant differences to the recipient parent for FL, TSP, GL, GW and HSW in 2015DS and for FW, FS, spikelet fertility (SF) and PY in 2016DS, while only GL showed significant difference in 2016WS. For the CT conducted with GR2E BR29 ILs in Bangladesh in the 2016 Boro season there were no significant differences from BR29 for all the traits measured (Table 3 ). Significant variations in total carotenoids among different families were observed in all backgrounds. The highest concentration of total carotenoids was observed in the BR29 background, followed by the PSBRc82 background, while the IR64 background had the lowest concentration of total caroteneoids (Tables 1 , 2 , 3 ). The grain samples of GR2E ILs along with recipient parents are shown in Fig.  2 . Grain quality traits amylose content (AC), gel consistency (GC) and alkali spreading value (ASV) were measured for PSBRc82, IR64 and BR29 (Tables 1 , 2 , 3 ). There were no significant differences for AC between GR2E PSBRc82 ILs and PSBRc82 in all the trials. There were no significant differences in ASV and AC between GR2E IR64 ILs and the IR64 parent, while for BR29 there were no differences between the transgenic and the control except for AC. The background recovery of final set of selected BC 5 F 3 ILs showed more than 98% recipient genome in all the three genetic backgrounds (Fig S3 – S5 ). There was no significant difference in AC except in BR29, similarly for GC some minor significant differences were observed in PSBRc82 and IR64 in some seasons.

Grain samples of GR2E golden rice and respective recipient parents.

Correlation between yield, yield related traits and carotenoid content

The correlation among yield and yield related traits; and with total carotenoid content is presented in the Figs S6 – S8 . Over all there was no specific trend in correlations among different yield and yield related traits. Except in one environment carotenoid content was negatively but non-significantly associated with PY in all the three genetic backgrounds. The correlation analysis of carotenoid content between different seasons showed highly significant correlation in all the three genetic backgrounds.

Effect of genetic background and environment on expression of carotenoids

The combined analysis of variance for carotenoid content at two months after harvest showed that there were significant genotypic, seasonal and location effects on the expression of carotenoid content. However, there were no significant genotype and environmental interactions (G × E) for carotenoid content except CT2 PR vs CT4 (Table 4 ). However, among the three genetic backgrounds, expression of carotenoids was higher in GR2E BR29 ILs followed by PSBRc82 and lowest in GR2EIR64 ILs (Fig.  3 , Fig S9 ). There were very highly positive significant correlations for carotenoid content estimated in different locations both within and between seasons (Figs S10 – S12 ). In general carotenoids expression was bit higher in WS than in DS, but also among most of the CTs no significant G × E interaction was observed (Table 4 ).

Carotenoid levels in different genetic backgrounds.

Identification of superior GR2E NILs for multi-location evaluation

We selected five GR2E introgression lines each for PSBRc82 and IR64, for BR29 eight lines were selected from the CTs. These lines will be further evaluated in multi-location field testing in the Philippines and Bangladesh respectively. The list of selected lines and their corresponding agronomic performance is provided in Table 5 . The ILs were similar to the respective recipient parents in all the agronomic, yield and yield traits measured, and the total carotenoids ranged from 3.8 to 5.5 ppm in the DS and 4.1 to 6.1 in the WS. Among the eight selected GR2E BR29 ILs no significant variation was observed in any trait except yield, with an advantage of 12.8% over BR29.

Most of the dietary vitamin A is of plant origin in the form of provitamin A that is converted to vitamin A in the body 17 . VAD is persistent in most of the rice eating countries in Asia, Africa and Latin America 18 , 19 . Therefore, enriching rice with provitamin A through biofortification is a viable and complementary intervention to tackle the VAD. The provitamin A trait was introduced into the rice variety Kaybonnet through genetic engineering 13 , which has a temperate japonica genetic background and is not well adapted to the tropical conditions in most rice growing Asian countries. We developed GR2E event introgressed golden rice ILs in the genetic backgrounds of IR64, PSBRc82 and BR29.

Introgression of the GR2E produced agronomically superior plants

Golden rice GR2E is genetically stable and molecularly clean event useful for breeding ( https://www.dropbox.com/sh/qpiz0cftefcaceq/AAByIpj_HED3zgqH7ufW7A-ta?dl=0 ; https://www.foodstandards.gov.au/code/applications/Documents/A1138%20Application_Redacted.pdf ). The breeding process to develop GR2E introgression lines did not show any abnormal plant phenotypes both in homozygous and hemizygous conditions indicating the genetic stability of the GR2E gene and trait expression. Both the phenotypic and genotypic based segregation analysis showed typical Mendelian segregation ratio in different segregating generations. GR2E advance backcross progenies were phenotypically very similar to their respective recipient parents. Transgenic events with single copy, clean integration and showing normal Mendelian segregation are considered ideal for research and breeding purposes, as they do not alter the host plant genome 20 , 21 , 22 .

Agronomic performance at field level and G × E studies showed that the GR2E gene did not alter any of the traits of the recipient parents in all its zygosity conditions. Overall plant performance was better during DS and among the genetic backgrounds the GR2EPSBRc82 lines performed better than the GR2EIR64 lines. Morphological traits such as panicle type, panicle exertion, grain shape, flag leaf length and width were similar for the GR2E ILs. Many lines performed equally similar to the respective recurrent parents, allowing the selection of advanced lines in all backgrounds for further testing in multi-location trials. The results showed that back cross process recovered almost all the desirable agronomic, yield and grain quality traits of the respective parents with significant expression of vitamin A. Despite many typhoons, heavy rains and high winds during the trials. There were no severe lodging incidences observed. Insects and diseases incidences were monitored during the two growing seasons at two different plant growth stages: maximum tillering stage (vegetative stage) and 50% flowering. Generally, crop stand was good with manageable level of insect pests and diseases during the growing seasons. Insects observed (both pest and beneficial insects) were found to be present in both test materials. We did not notice any difference between GR2E introgression lines and their respective recipient parents for the pest or diseases pressure on the crop across the confined field tests.

Woodfield and White 23 , and Badenhorst et al . 24 opined that development of transgenic product is not limited only to transformation, but also includes breeding through further backcrossing of transgenes with recipient parents and selection for desired traits of interest, in order to expedite commercial product development. For commercial deployment of any new variety with one or more introduced new trait(s) of a staple crop, in parallel to yield and other key agronomic traits, the newly developed variety should have essentially similar or better performance against biotic and abiotic stresses and grain quality traits compared to recipient variety; the introduced trait(s) should not alter these traits of the recipient variety 25 , 26 .

Grain quality and proximate composition of GR is similar to recipient rice varieties

Furthermore, different cooking and eating quality traits like, AC and ASV did not show any significant difference between the ILs and their respective recipient parents in any CTs. The golden rice breeding lines with significant amount of provitamin A accumulated in the grains helps to tackle VAD in high risk countries such as Bangladesh and the Philippines. However, it is a requirement to assess the composition of genetically modified crops to see if any significant changes in grain quality, nutrients and anti-nutrients contents in comparison to traditional counterpart and to assess the safety of the intended or unintended changes 27 , 28 . The compositional analysis of golden rice showed that all the compounds measured are within the biologically acceptable range and does not pose any risk to human health 29 . Earlier reports on transgenic products for insect and herbicide tolerance have also shown that little biologically meaningful changes in grain quality, nutrient and anti-nutrient composition 30 . There was a clear environmental effect, even though total carotenoids varied with environments, the genotypes with high carotenoids were always the best in all the locations. Such variations in trait expression due to environmental and agronomic factors and genetic basis have been well explained 31 , 32 .

Genetic background and environment influences carotenoid expression

Stable trait expression and minimal G × E for any trait of importance, especially for grain micronutrients and vitamins is essential for varietal release as well as for their successful adoption 4 , 33 , 34 . Total carotenoids were well correlated across the sites and generations; and expressed stably across the environments but there is a genetic background effect. Carotenoids expression varied even within segregating lines of different generations in each of the genetic backgrounds. So targeted breeding and careful selection of progenies with carotenoids test in each generation is necessary for advancing the lines. Mapping background QTLs and genes and using them in MAB can provide opportunity for precise development of GR lines with highest expression. The carotenoid levels were found to vary across the genetic backgrounds, locations and seasons but there were no significant G × E interactions. The highest expression of carotenoids was observed in BR29 background and the lowest in IR64 background. Several earlier attempts to develop golden rice events and introgression lines had to face the genetic background effects. Transgenic events developed in the indica backgrounds of IR64 and BR29 reported lower expression of GR genes in IR64 and higher expression in BR29 transformants, even ILs developed in IR64 showed lesser expression 35 . Moreover, ILs did not show any significant difference in yield when expressing the genes in the carotenoid pathway 36 . In our study also lowest expression was noticed in IR64. Simultaneously efforts are being made to develop next generation golden rice events with elevated levels of carotenoids with longer stability 37 , 38 , 39 . However, a genetic background effect is still a major bottle neck for introgression of carotenoid trait. Background effect on the expression of introduced traits was reported in rice for submergence tolerance, yield and related traits, disease resistance and drought tolerance 15 , 16 , 40 , 41 .

The variation in carotenoid concentration in grains might be due to variations in sunlight exposure and intensity across the locations and seasons 42 . Differential accumulation of β-carotene due to variation in exposure period and intensity of sunlight was also observed in algae, carrots, pumpkin and maize 43 , 44 , 45 , 46 . Moreover, like other carotenoids containing crops the carotenoid concentration in the grains of golden rice degrades over time after harvest. The degradation rate is very high at first few weeks after harvest and it becomes very slow after 6–8 weeks (data not shown). The carotenoids degradation rate is highly influenced by the storage temperature, moisture and exposure to light of the storage environment 22 , 47 . So, development of golden rice varieties with stable carotenoids expression is essential to achieve the impact 37 . However, there might be genotypic effect on the retention ability for carotenoids in rice grain. Understanding background effect and standardization of post-harvest handling is needed to achieve desired level of carotenoids in the introgression lines of multiple backgrounds.

Superior introgression lines were identified for multi-location trials

The five back crosses of GR2E gene into three genetic backgrounds resulted in identification of ILs similar to respective recipient parents. Adoption by the farmers and preference by the consumers for a specific crop variety particularly rice introduced with a new trait largely depends on its yield, grain quality and eating quality parameters. The introduced trait should be stable over locations and seasons to expedite the adoption level. Considering the present levels of carotenoids and per capita consumption in these target countries, the resulting ILs would be able to supply 30–50% of the EAR for vitamin A for the high risk population group if GR2E rice is consumed regularly.

Materials and methods

Development of gr2e near isogenic lines.

Kaybonnet is a high yielding japonica rice variety with blast resistance and excellent milling quality commercially cultivated in the USA. The genetic modification was made by the addition of two genes, phytoene synthase (Zmpsy1) from Zea mays and carotene desaturase (crtI) gene from the common soil bacterium, Pantoea ananatis (syn. Erwinia uredovora ). The GR2E Kaybonnet was crossed with the popular high yielding and adopted rice varieties such as IR64, PSBRc82, and BR29. IR64 is popular in most of the Asian countries, PSBRc82 in the Philippines, and BR29 in Bangladesh. In each generation, segregating materials were genotyped using GR2E event specific molecular marker. Plants containing the GR2E event and phenotypically similar to respective recipients were selected and backcrossed in each backcross generation to advance the materials to BC 5 F 2 . Background selections were performed using 100 randomly selected SSR markers in BC 1 and BC 2 , while selected plants from BC 3 , BC 4 and BC 5 were genotyped using the 6 K SNPs set at Genotyping Service Laboratory, IRRI. Only yellow-colored BC 5 F 2 seeds were separated and analyzed for total carotenoid content. A total of 40 BC 5 F 2 families for IR64 and 20 families each for PSBRc82 and BR29 were selected for evaluation in the confined test at IRRI. We have provided details of MAB scheme and evaluation of introgression lines in the Fig.  4 .

Development and evaluation of GR2E introgression lines.

Experimental materials used in the confined tests

A total of 8000 individual plants comprised of 4000 BC 5 F 2 plants from GR2E IR64, 2000 plants each from GR2E PSBRc82 and GR2E BR29 were included in a CT in the dry season of 2015 (2015DS). Plants were genotyped using GR2E specific markers and homozygous plants were selected. Selected BC 5 F 3 homozygous plants from each genetic background along with the respective recipient and donor parents were evaluated in a series of CTs at IRRI and PhilRice in the Philippines and at BRRI in Bangladesh. The list of GR2E materials evaluated and the details of the CTs is provided in the Supplementary Table S1 . Three CTs were conducted for GR2E IR64 and GR2E PSBRc82 at IRRI, while the selected lines of GR2E PSBRc82 were evaluated for two seasons at PhilRice. Further, BC 5 F 3 seeds of GR2E BR29 were sent to Bangladesh, multiplied in the screen house, and further evaluated in a CT at BRRI, Gazipur, for one season in 2016.

Crop management and observations

Seeds of the selected plants of GR2E introgression lines, recipient and donor parents were seeded in trays. Seedlings were transplanted at 21 days after sowing with a standard spacing of 20 × 20 cm. Details of the experimental design and layout are provided in Tables S1 and S2 . Standard agronomic practices were followed to raise a good crop, including the application of need-based plant protection measures to protect the crop from diseases and insect pests. Data were gathered on key agronomic, yield and yield-related traits; and total carotenoid content was measured two months after harvest. Grain quality data were generated from the selected lines of CT2 and from all lines included in CT3 and CT4. Insect pest infestations and disease incidences were recorded at maximum tillering and at 50% flowering. Agronomic traits were measured on five random plants from each entry. Days to 50% flowering was recorded on a whole plot basis. At maturity, five selected plants were harvested from individual plots and the remaining inner plants were harvested in bulk. Final plot yield was adjusted to a uniform grain moisture content of 14%.

DNA was extracted using fresh leaf samples and following a modified cetyl trimethylammonium bromide (CTAB) protocol 48 . Nanopore was used to check the quality and quantity of the DNA extracted. The DNA samples were diluted with distilled water into an equal concentration of 25 ng/µl. Amplification of event specific markers using polymerase chain reaction (PCR) was carried out with a 10 µl reaction mixture that contained 1.5 µl of DNA template, 1.0 µl of 10 × PCR buffer with MgCl 2 , 0.5 µl each of forward and reverse primers, 0.2 µl of 1 mM dNTP and 0.1 µl of Taq DNA polymerase and 5.7 µl distilled water. The amplification reaction was carried out in a 96-well PCR plate in a thermocycler using the following temperature profile: denaturation, 95 °C for 5 min; 35 cycles of denaturation at 95 °C for 45 s, annealing at 55 °C for 45 s and extension at 72 °C for 45 s; and final extension at 72 °C for 8 min and long-term storage at 10 °C. Amplification products were separated by gel electrophoresis on 1.2% agarose (0.5 × TBE; 160 V for 45 min) and visualized using SYBR Safe DNA stain and imaging using an AlphaImager HP (Protein Simple, San Jose, CA) gel documentation system. The GR2E specific primer sequences as follows.

ZD-E1-P1 5′-GCTTAAACCGGGTGAATCAGCGTTT-3′

ZD-E1-P2 5′-CGAGAGGAAGGGAAGAGAGGCCACCAA-3′

ZD-E1-P3 5′-CTCCCTCACTGGATTCCTGCTACCCATAGTAT-3′

Grain quality analysis

Grain quality analysis was carried out at the Analytical Service Laboratory (ASL) of IRRI. We measured/analyzed grain length and width, amylose content, alkali spreading value and gel consistency, using standard protocols 49 . Similar analyses were performed at BRRI on grain samples of GR2E BR29.

Amylose content

Amylose content (AC) was determined on milled rice extracts using a segmented flow analyzer. Rice samples were ground to a fine powder using a cyclone mill. Sodium Hydroxide and Ethanol were added to a test portion of the sample and heated in a boiling bath for 10 min. Acetic acid and Iodine solution was mixed with the aliquot of the test solution to form a blue starch iodine complex and its absorbance was measured at 620 nm using a colorimeter 49 . The result of the analysis was reported as apparent amylose to take into account the contribution of amylopectin present in the rice, which also forms a blue color starch iodine complex.

Gelatinization temperature

Rice starch gelatinization temperature (GT) was estimated by determining the alkali spreading value (ASV) of milled rice grains in potassium hydroxide solution. Six kernels of whole milled rice were incubated with 10 ml of 1.7% KOH for 23 h at ambient temperature (25 °C). The appearance and disintegration of the endosperm was visually rated depending on the intensity of spreading and swelling. ASV of 1–2 was classified as high GT, 3 for intermediate to high GT, 4–5 for intermediate GT and 6–7 for low GT.

Gel consistency

Samples of milled rice were ground to a fine powder, placed in a culture tube and suspended in a mixture of ethanol and 0.2 N KOH containing thymol blue and incubated in a boiling water bath for 15 min, followed by cooling to room temperature (15 min) and placing in an ice bath (20 min). Gel consistency of the rice paste (4.4% w/v) was determined by measuring the length of the cold gel in the culture tube after placing horizontally for 1 h. Rice was differentiated into three consistency types—soft (61 to 100 mm), medium (41 to 60 mm) and hard (27 to 40 mm).

Carotenoid concentrations

Total carotenoid concentration was estimated following the protocol developed by Gemmecker et al . 50 . Dehulled and polished rice seeds were ground to a fine powder using a modified paint shaker and accurately weighed amounts (ca. 500 mg) were dispensed into 15-ml Falcon tubes, mixed by sonication with 2 ml distilled water and incubated for 10 min at 60 °C. Cooled samples were centrifuged (3000 g , 5 min) and the supernatant fractions were transferred to new 15-ml tubes. Acetone (2 ml) and 100 μl of the lipophilic metallo organic dye, VIS682A (20 μg/ml; QCR Solutions Corp.), as an internal standard were added to each sample followed by mixing with short pulses of sonication and centrifugation (3000 g , 5 min). Supernatants were transferred to 15-ml tubes and the pellets were re-extracted twice more with 2-ml volumes of acetone and the resulting supernatant fractions were combined. Two ml petroleum ether (PE): di-ethyl ether (DE) (2:1 v/v) was added to each combined supernatant fraction (ca. 8 ml) and volumes were adjusted to 14 ml with distilled water. After vortexing, phase separation was achieved by centrifugation (3000 g , 5 min). The organic phase was recovered by pipetting out and transferred into a 2 ml graduated Eppendorf tube and the remaining aqueous phase was re-extracted with another 2 ml PE:DE (2:1 v/v), followed by centrifugation (3000 g , 5 min). The combined organic phases were dried using a vacuum-concentrator (Eppendorf concentrator 5301) and re-dissolved in 1 ml acetone. Maximum absorbance of sample extract at 450 nm and maximum absorbance of internal standard at 680 nm was determined using DU730 Beckman Coulter UV/VIS spectrophotometer. Concentrations of total carotenoids were determined from A450 nm assuming an average E450  nm = 142, 180 l mol −1  cm −1 in acetone using the Beer-Lambert law corrected for sample dilution and normalized to the internal standard.

Statistical analysis

All statistical analyses were performed as a linear mixed model using R 51 and PB Tools v1.0 52 .

Mixed model for single site analysis:

where µi denotes the mean of the ith entry (fixed effect), bj denotes the effect of the jth block, and eij denotes the residual error.

Mixed model for multiple site analysis:

where µi denotes the mean of the ith entry (fixed effect), lk denotes the effect of the kth site, bj(k) denotes the effect of the jth block within the kth site, (µl)ik denotes the interaction between the entries and sites (random effect), and eijk denotes the residual error.

Mean comparison and correlation analysis

The differences in least square (LS)-mean values between GR2E rice and the control rice were tested at first step followed by significant difference (p < 0.05) was identified in the multi-year combined-sites analysis 53 . Correlation among different traits from all the replicated trials was carried out using R Program 51 .

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Challenges and opportunities in productivity and sustainability of rice cultivation system: a critical review in Indian perspective

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Rice–wheat cropping system, intensively followed in Indo-Gangetic plains (IGP), played a prominent role in fulfilling the food grains demand of the increasing population of South Asia. In northern Indian plains, some practices such as intensive rice cultivation with traditional method for long-term have been associated with severe deterioration of natural resources, declining factor productivity, multiple nutrients deficiencies, depleting groundwater, labour scarcity and higher cost of cultivation, putting the agricultural sustainability in question. Varietal development, soil and water management, and adoption of resource conservation technologies in rice cultivation are the key interventions areas to address these challenges. The cultivation of lesser water requiring crops, replacing rice in light-textured soil and rainfed condition, should be encouraged through policy interventions. Direct seeding of short duration, high-yielding and stress tolerant rice varieties with water conservation technologies can be a successful approach to improve the input use efficiency in rice cultivation under medium–heavy-textured soils. Moreover, integrated approach of suitable cultivars for conservation agriculture, mechanized transplanting on zero-tilled/unpuddled field and need-based application of water, fertilizer and chemicals might be a successful approach for sustainable rice production system in the current scenario. In this review study, various challenges in productivity and sustainability of rice cultivation system and possible alternatives and solutions to overcome such challenges are discussed in details.

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Introduction

Rice ( Oryza sativa L.)–wheat ( Triticum aestivum L.) is the largest cropping system practised in South Asian countries (Nawaz et al. 2019 ). About 85% of this cropping system falls in Indo-Gangetic plains (IGP), covering nearly 13.5 million hectares (mha) area (Saharawat et al. 2012 ). India alone covers approximately 76% of IGP, spreading in the states of Punjab, Haryana, Uttar Pradesh, Bihar and West Bengal. Being staple food crops in the country, rice and wheat played a key role in minimizing the gap between food grains demand and production. In recent years, country witnessed surplus food grains production through an integrated approach of high-yielding varieties, disease and pest management, nutrient management, irrigation water management and better mechanization. Rice and wheat production was reported as 34.6 million tonnes (mt) and 11 mt, respectively, during 1960–1961, which is expected to rise to 122.3 and 109.5 mt, respectively, during 2020–2021 (PBAS 2019; PIB 2021). In the last one decade, slow growth in crop productivity has been registered, which may further decline in near future due to some ongoing resource guzzling practices. The trend of rice yield in South Asia is presented in Fig.  1 . From 1998 onwards, Bangladesh witnessed a noble growth in rice yield, surpassing India and Pakistan, and continues to uphold the growing trend. It was due to intensive use of modern technologies such as cultivation of high-yielding varieties, adoption of improved irrigation technologies and balanced fertilizer application (Ahmed 2004 ; Shew et al. 2019 ). In past few years, rice productivity in India looks stagnant, even it may decline in future due to over-exploitation of natural resources (Ladha et al. 2009 ), low seed replacement rate (UPSDR 2019), poor management of irrigation water, fertilizer and crop residue (Ladha et al. 2009 ), same cropping pattern over the years (Nambiar and Abrol 1989 ) and lack of awareness about consequences of faulty cultivation practices among farmers (Dis et al. 2015 ; UPSDR 2019). The problem is not limited to India but also extends to other countries of IGP, where intensive tillage practices and confined agro-biodiversity degraded natural resources to a great extent. Researchers questioned the sustainability of rice–wheat cropping system under present challenges of stagnant yield (Ladha et al. 2003a ), soil degradation (Bhandari et al. 2002 ; Tripathi and Das 2017 ), declining water table (Humphreys et al. 2010 ) and environmental pollution (Bijay-Singh et al. 2008 ). The trend of the area covered under rice cultivation in South Asia is shown in Fig.  2 . In IGP, most of the rice cultivated area falls under Indian Territory, but this area was bounded within 40–45 mha during 1988–2018. The stagnant and limited spatial coverage of rice area is due to unavailability of irrigation facility, high water requirement of the crop, declining water table, labour-intensive cultivation, poor feed quality of by-product (straw), degradation of soil structure and irregular nature of rainfall. In fact, rice cultivation using the conventional method is believed as water-, energy- and capital-exhaustive practice (Bhatt et al. 2016 ).

Source : FAOSTAT )

Trend of rice yield in South Asia (

Trend of rice area in South Asia (

India, a home to 17.7% of the world population, is the prime consumer of water requiring 3000 billion cubic meters annually (Vyas et al. 2019 ). India is the largest consumer of groundwater accounting for about 230 km 3 of groundwater use every year (TWB 2012). India receives nearly 4000 billion cubic meters of precipitation every year. However, only 48% of this water is stored in the surface and groundwater bodies due to losses in various hydrological processes such as runoff, water discharge through rivers to oceans, evaporation and evapotranspiration (Verma and Phansalkar 2007 ; Dhawan 2017 ). A major portion (88–90%) of groundwater extracted is used for irrigation purpose in agricultural fields (Siebert et al. 2010 ; GoI 2014). Rice crop requires huge amount of water than other cereal crops, and it consumes about 3000–5000 L of water to produce 1 kg of rice (Bouman 2009 ; Geethalakshmi et al. 2011 ). Tuong and Bouman ( 2003 ) reported that around 75% of global rice is produced by raising the seedlings in a nursery followed by transplanting operation in puddled field. In addition to excessive water, capital and energy demand, this practice of rice cultivation is associated with soil degradation (Bhatt et al. 2016 ), loss of ecosystem (Nawaz et al. 2019 ) and environmental pollution (Jimmy et al. 2017 ).

In the current scenario, when degradation of soil structure, declining soil health, residue handling issues and harmful emissions from rice cultivated fields are taking place, the sustainability of rice production system is questionable. In India, rice is cultivated on 44 mha area, accounting 20% of total rice production worldwide (Oo et al. 2018 ). It is estimated that India needs to produce 130 mt rice by 2030 to meet the demand of the growing population (Gujja and Thiyagarajan 2009 ). To achieve the projected demand, use of high-yielding varieties, expansion of rice cultivation area and wet tillage would be required, but latter two practices would further increase the irrigation water demand and greenhouse gas emissions (Oo et al. 2018 ). Considering all these aspects, an attempt has been made to critically review the challenges and opportunities in productivity and sustainability of rice cultivation system in Indian perspective. Also, attempts were made to highlight the possible alternatives and solutions to overcome the present challenges in rice cultivation system. The key challenges and intervening areas in rice cultivation system are discussed in details under the following sections:

Underground water table depletion

India is the top user of groundwater around the world (Mukherjee et al. 2015 ), and it has about 25% share in global groundwater consumption. In fact, the groundwater consumption of India is higher than collective groundwater use of China and USA (Margat and van der Gun 2013 ). The unsystematic use of groundwater for irrigation caused widespread over-exploitation of groundwater resources (Rodell et al. 2009 ), which is not sustainable in long-term. In India, out of 160 mha cultivable land, only 68 mha cultivated area is covered with irrigation facilities, while about two-third area is still rain-dependent (Dhawan 2017 ). About 61.6% of irrigation water is extracted from groundwater through wells, dug wells, shallow tube wells and deep tube wells (Suhag 2016 ). The rate of groundwater level fall in India is probably the fastest globally (Aeschbach-Hertig and Gleeson 2012 ). During the last three decades, underground water levels in northern region of India have dropped from 8 to 16 m below ground level, and in rest of India, it has declined from 1 to 8 m below ground level (Sekhri 2013 ). Another estimate reports that north-western India lost 109 giga cubic meter of groundwater between 2002 and 2008 (Rodell et al. 2009 ). The rapid extraction and slow groundwater recharge caused groundwater table to fall at a rate of about 1 m per year (m y −1 ) in Punjab and Haryana, which may fall more rapidly in the coming years (Humphreys et al. 2010 ; Singh et al. 2014 ). In many cities of north-western India, the groundwater table is declining at a rate of 1.6 m y −1 (Singh et al. 2015 ). The huge volumetric loss of groundwater and its faster declining rate might be the cause for India becoming a home for 25% of worldwide population living under water-scarce conditions (Mekonnen and Hoekstra 2016 ; Anonymous 2019a ). The continuous decline of groundwater table has created water-stressed condition, affecting the per-capita water availability. In 1951, per-capita water availability was 5177 cubic meter per year (m 3 y −1 ), which reduced to 1598 m 3 y −1 in 2011 as presented in Table 1 . It has made India a water-stressed country according to international norms (Dhawan 2017 ; GoI 2018). Further, projected per-capita water availability is expected to fall to 1174 m 3 y −1 by 2051 (GoI 2018). Water stress to scarce condition would put enormous pressure on the sustainability of water-guzzling crops like rice. Traditionally grown rice requires around 200–240 cm of the water column from nursery preparation to harvesting stage (Humphreys et al. 2008 ; Chauhan et al. 2012 ). However, the actual amount of water applied by the farmers is much higher especially in light-textured soils (Timsina and Connor 2001 ). Over the years, flood irrigation has become a common practice, even water ponding is considered as necessary part of rice cultivation. Easily accessible and sufficient availability of irrigation water in north-western India turned out rice–wheat cropping system, a classical example of high productive system in non-ideal soils for rice cultivation, which are porous, coarse and highly permeable in nature (Chauhan et al. 2012 ). However, intensive cultivation of rice–wheat cropping system in these regions has forced the farmers to extract the groundwater with submersible pumps, which resulted in over-exploitation of groundwater. Singh and Kasana ( 2017 ) reported that area under the safe limit of groundwater (3.1–10 m) in Haryana state reduced from 44 to 34%, while the area under critical and over-exploited category of groundwater increased from 56 to 64% and 4 to 23%, respectively, during 2004–2012. The decline in groundwater of many districts of Haryana was in the tune of 0.7–1.1 m y −1 . It was concluded that variations in groundwater levels could be due to rice–wheat cropping systems, irregular distribution of rainfall, over urbanization, variation in hydrogeological setup and different aquifer conditions. The irregularity in annual rainfall of India is presented in Fig.  3 . The deviation of annual rainfall from mean value could be very high during the drought years. Moreover, rainfall pattern makes this problem more complicated as during the monsoon season, events of excessive rainfall and the large interval between two consecutive rainfall events take place. In the absence of rainfall events at a certain interval, rice cultivation requires a huge amount of irrigation water, causing rapid extraction of groundwater, which is associated not only with water table depletion but also with carbon dioxide (CO 2 ) emissions, where engines and tractors are used as the prime mover for pumping unit. Undoubtedly, excessive rice cultivation in non-ideal soils, traditional rice cultivation practices and major dependency of irrigation on groundwater would put enormous pressure on natural resources. Furthermore, the excessive use of chemicals and fertilizers in rice cultivation under coarse-textured soils also poses other threats of soil and groundwater contamination with harmful chemicals.

Source: Somasundar ( 2014 ), Jaganmohan ( 2020 )

Annual rainfall and deviation from mean rainfall of India during 1988–2018

Groundwater pollution

Groundwater pollution is a serious concern, which affects grain quality and health of human and animals. The excess and untimely use of N-fertilizer is associated with nitrate leaching, which pollutes the groundwater (Bhatt et al. 2016 ). In a study, researchers found higher nitrate content in groundwater of the regions where intensive rice–wheat cropping system was practised (Bajwa 1993 ). The problem of groundwater pollution is more serious in rice cultivating regions with coarse-textured soils, where frequent and heavy irrigation is applied. Bouman et al. ( 2002 ) found higher N leaching losses under wet season rainfed rice than irrigated rice. Pathak et al. ( 2009 ) observed higher cumulative leaching losses of nitrogen (46–69 kg N ha −1 ) in rice field than the wheat field (16–22 kg N ha −1 ). Rainfall plays an important role in N losses, which can be as high as 18% of applied nitrogen in high rainfall years (Pathak et al. 2009 ). Wang et al. ( 2015a ) reported that intensive rice cultivation practice in subtropical China led to moderate ammonium-N (NH 4 -N) pollution of shallow groundwater. It was concluded that flooded land and excessive N-fertilizer rate could lead to worse NH 4 –N and nitrate–N (NO 3 –N) pollution, respectively. Coarse-textured soils leach N more rapidly than heavy-textured soils, and N leaching under such soils is highly dependent on N-fertilizer application (Benbi 1990 ). Though it is very difficult to stop the nitrogen leaching completely, better management practices by adopting the proper irrigation and fertilizer scheduling can minimize the leaching losses and improve N-use efficiency (Singh et al. 1995 ). The cultivation of high water requiring crop like rice in arsenic-contaminated soils like in middle IGP of northern India carries the threat of groundwater contamination with arsenic (Srivastava et al. 2015 ). In many locations, arsenic content of groundwater under rice cultivation exceeded the acceptable limit (10 µg L −1 ), raising the contamination level up to 312 µg L −1 (Srivastava et al. 2015 ). The application of such polluted groundwater for irrigation purpose can lead to other problems of soil and grain toxicity.

Soil and grain toxicity

It is extremely important to relook the practice of intensive rice cultivation under toxic soils and toxic irrigation water as it could lead to grain toxicity, affecting the human health. The practice of growing rice in arsenic-contaminated soils like in middle IGP escalates the possibility of soil and grains contamination with arsenic beyond the safe limit (Srivastava et al. 2015 ). It was reported that arsenic content in soil under rice cultivation exceeded the allowable limits of 20 mg kg −1 , raising the contamination level up to 35 mg kg −1 . Moreover, arsenic toxicity in the grains was found in the range of 0.179–0.932 mg kg −1 , leaving 8 of 17 varieties unsafe for human consumption. Dhillon and Dhillon ( 1991 ) found selenium toxicity in the soil and plants when selenium contaminated irrigation water was used for irrigation in rice–wheat cropping system under silty loam soils for a longer period. The intensive cultivation of frequent irrigation requiring crops like low land rice turned out one of the major factors responsible for the deposition of seleniferous material in the soil, leaving more than 100 ha area under selenium toxicity (Dhillon and Dhillon 1991 ). Sara et al. ( 2017 ) observed that arsenic and selenium content of soil increased with duration of rice monoculture system. The increase in arsenic and selenium concentration in soil caused toxicity in rice grain. The anaerobic condition in rice cultivation affects nutrient uptake by the plants and production of toxic substances (De Datta 1981 ). Tran ( 1998 ) also reported that long-term soil puddling and rice monoculture system increases the risk of soil toxicities. Shah et al. ( 2021 ) highlighted the toxic residues of pesticides and metalloids in rice grain under flooded rice cultivation system. Needless to say that intensive rice cultivation with puddling and flooding method projects the health risk associated with soil and grain toxicity in long-term. Sara et al. ( 2017 ) recommended to control these elements with prior importance by employing the different actions including crop rotations, soil amendments, etc.

Degradation of soil structure

Rice cultivation using conventional method requires intensive wet tillage primarily to reduce the percolation losses and to suppress the weed growth. The repeated puddling operation creates an impervious layer at 15–20 cm depth, which restricts water infiltration and root growth (Aggarwal et al. 1995 ; Kukal and Aggarwal 2003 ). The negative effects of subsurface compaction on the establishment, seed emergence, root growth and yield of succeeding crop are of major concern (Kukal and Aggarwal 2003 ). The puddling operation deteriorates the soil structure by damaging the soil aggregates, breaking the capillary pores and dispersing the fine clay particles (Aggarwal et al. 1995 ). Bakti et al. ( 2010 ) recommended that in fine-textured soil like clay having low percolation rate, puddling, which is capital intensive and detrimental to soil structure, should be minimized. It would be beneficial for soil health and its functionality to replace the puddled transplanted rice (PTR) with lesser intensive cultivation practices such as zero-till-based mechanized transplanting, direct-seeded rice (DSR) and strip tillage-based transplanting. The adoption of such rice cultivation practices under conservation agriculture (CA) either on a flat or permanent bed and diversified cropping systems with wetting and drying irrigation method could be effective to improve the soil structure (Singh et al. 2005a ; Bakti et al. 2010 ; Chauhan et al. 2012 ).

Soil health deterioration

The intensive tillage, puddling operation and excessively cultivation of rice–wheat cropping system deteriorated health, structure and nutrient balance of the soils in north-western India. Killebrew and Wolff ( 2010 ) reported that long-term intensive rice cultivation system led to soil salinization, nutrient deficiencies, soil toxicities and reduced capacity of the soil to supply the nitrogen to the plant roots. Such changes can lead to reduced yield and abandonment of paddy fields in long-term. In other studies, Boparai et al. (1992) and Mohanty and Painuli ( 2004 ) observed that long-term water submergence and mineral fertilization practices in conventional rice cultivation resulted in degraded soil quality in terms of disintegration of stable aggregates and reduced soil organic matter. The concerns have been expressed on the sustainability of high yield of crops due to intensive rice cultivation system and multiple harvests of crops in a year (Livsey et al. 2019 ). The sustainability of rice production under rice–wheat cropping system in Punjab has been reported at risk due to soil degradation and declining water table (Dhaliwal et al. 2020 ) along with inadequate crop residue recycling and lack of organic fertilization. These changes in soil–water environment led to micro-nutrients deficiencies and yield stagnation (Dobermann and Fairhurst 2002 ; Yadvinder-Singh and Bijay-Singh 2003). However, such negative impacts can be lowered by adopting rice in combination with leguminous crops and rice–oilseed crop rotation (Chen et al. 2012 ; Meetei et al. 2020 ). Moreover, shifting the rice monoculture to rice–fish farming showed positive effects on soil health in terms of labile pool of C fractions, microbial populations, nutrients and soil fertility in addition to environmental sustainability (Bihari et al. 2015 ). The problem of declining soil health becomes worse with the burning of rice residue, which results in 20–100% loss of precious nutrients retained in the residue (Singh et al. 2008 ). In response to nutrient losses with residue burning, farmers have to apply more fertilizers to obtain a similar crop yield, which raises the cost of cultivation. It needs urgent attention to improve the soil health in which residue retention on the soil surface and seeding with zero-till practice can play significant roles (Malik and Yadav 2008 ; Sidhu et al. 2008 ). Extending the resource conservation technologies (RCTs) for rice cultivation under conventional and CA along with soil water potential-based irrigation scheduling could be effective to improve the soil health and environmental quality (Dwivedi et al. 2003 ; Gupta and Sayre 2007 ; Jat et al. 2010 ).

Declining crop response

The decline in crop response to applied fertilizers is a serious concern, causing the farmers to apply fertilizers above the recommended dose in an injudicious way. Although crop response to P and K fertilizers can be realized only after 5–10 years, it is necessary to apply these fertilizers along with N as the application of N-fertilizer alone in long-term can cause yield decline in rice–wheat cropping system (Bhatt et al. 2016 ). The low fertilizer use efficiency due to fertilizer losses as surface runoff, leaching, volatilization and unfavourable soil moisture is one of the major reasons for declining crop response to applied fertilizers. Moreover, long-term practice of same cropping sequence like rice–wheat in IGP over the years, injudicious and unbalanced application of fertilizers, inappropriate timing of fertilizer application and low soil organic matter are other factors responsible for declining crop response to applied fertilizers (Chauhan et al. 2012 ; Bhatt et al. 2016 ). In rice–wheat cropping system, the net negative balance of NPK is 2.22 mt per annum for IGP (Tandon 2007 ). The current trend of decline in crop response to applied fertilizers would create more difficulties for any further improvement in crop productivity. Therefore, soil and water management, integration of green or brown manuring, growing of dual-purpose pulses and addition of organic manure along with inorganic fertilizers are required to reverse the trend and improve the crop response in long run.

Decreasing water productivity

In the scenario of depleting groundwater table, decreased water productivity is of major concern, which has been reported from different agro-climatic zones of the country (Humphreys et al. 2010 ; Bhatt 2015 ). Decreased water productivity along with deteriorating water table can hamper the objective of sufficient grains production in future. It requires urgent attention to increase the water productivity of crops especially C3 crops like rice, which are less water efficient. This can be achieved by grabbing the opportunities at biological, environment and management levels (Sharma et al. 2015 ). Rice (lowland) is a less water productive crop (0.2–1.2 kg m −3 ) as compared to wheat (0.8–1.6 kg m −3 ) and maize (1.6–3.9 kg m −3 ) (Sharma et al. 2015 ). While the Punjab and Haryana states of India report the highest land productivity (4 tonnes per hectare) for rice, the water productivity is relatively low at 0.22–0.60 kg m −3 , even though these states have almost 100% irrigation coverage. It signifies the inappropriate use of irrigation water. Puddling and flooding operations in lowland rice production system consume a major portion of irrigation amount, causing lesser water productivity. The PTR requires 15–25 cm water column for saturation and flooding of soil (Tuong 1999 ). However, puddling method also reduces deep drainage losses by lowering the infiltration rate, which is generally high in the absence of puddling in coarse-textured soils (Sharma et al. 2004 ). The reduction in infiltration rate depends on soil texture, tillage intensity and puddling operations, water table and depth of floodwater (Gajri et al. 1999 ; Kukal and Aggarwal 2002 ). Bouman and Tuong ( 2001 ) reported that rice performs well in terms of yield when continuous flooding or saturated soil condition is maintained. Rice yield reduces when soil moisture drops below to saturation level. Technologies such as alternate wetting and drying (AWD), a system of rice intensification (SRI), bed planting, DSR and soil mulching have been adopted to reduce the water inputs and improving the water productivity (Tuong et al. 2005 ). Tabbal et al. ( 2002 ) reported that rice cultivation in saturated soil culture required 30–60% lesser water, which increased the water productivity by 30–115% over conventional practice. However, a yield penalty of 4–9% was levied on rice cultivation in saturated soil culture as compared to conventional practice. Water-saving in AWD method is attributed to a reduction in seepage and drainage losses (Tuong et al. 1994 ). This practice of irrigation is usually applied to DSR in which water required for raising the nursery and transplanting the rice is eliminated. However, the duration of DSR is longer than PTR, which would require higher water for evapotranspiration process than conventionally cultivated rice (Cabangon et al. 2002 ; Humphreys et al. 2010 ). Researchers asserted that net water savings depends on water saved from longer irrigation interval and additional water required in pursuance to deep drainage losses in DSR as compared to PTR. A few researchers reported that lesser irrigation amount was required in DSR than PTR with or without yield penalty (Jat et al. 2009 ; Yadav et al. 2010 ). The yield of DSR reduced rapidly when the soil was permitted to dry beyond soil moisture tension of 20 kPa (Yadav et al. 2010 ). These findings suggest that it is essential to reduce the unproductive water outflows to improve the water productivity of rice, which may be accomplished by soil water potential-based frequently irrigated DSR. Water-saving techniques such as micro-irrigation systems (sprinkler and drip irrigation) proved as cutting edge technology for improving the water use efficiency and conserving the water due to elimination of conveyance losses, evaporation from the water surface, runoff losses, etc. (Meena et al. 2015 ). Technologies such as CA should be promoted and practised on a large scale to improve the water productivity of crops. Agronomical practices such as rice cultivation on a raised bed with furrow irrigation, DSR with cultivars of high stress tolerance index, unpuddled transplanted rice and DSR with straw mulching would be effective approaches to increase the water productivity without much effect on the rice yield (Mahajan et al. 2011 ; Kar et al. 2018 ). Needless to say that India also need to review the present scenario of producing the higher water requiring crops such as rice and sugarcane in water-stressed areas (Dhawan 2017 ).

Declining factor productivity

The declining trend of total factor productivity in agriculture is a severe threat to sustainable farming and food security. In recent years, a significant portion of the cultivable land faced stagnation or negative growth in total factor productivity (Kumar and Mittal 2006 ). In low land of Asia, excessive tillage led to degradation of land resource base, which reduced the productivity growth of primary cereals like rice and wheat (Pingali and Heisey 2001 ). In north-western India, the rice–wheat cropping system has been associated with environmental degradation along with stagnant or declining crop productivity, thereby posing a threat to sufficient grain production (Aggarwal et al. 2000 ). A few researchers stated that declining factor productivity and degrading soil and water resources have threatened the sustainability of rice–wheat cropping system (Hobbs and Morris 1996 ; Ladha et al. 2003a ). A more yield decline has been witnessed in rice as compared to wheat under rice–wheat cropping system (Ladha et al. 2003b ). However, generally, it is argued that wheat yield suffers more after PTR due to soil structure degradation (Humphreys et al. 1994 ; Bhushan and Sharma 1999 ). Ladha et al. ( 2003b ) suggested to adopt the suitable agronomic and soil management practices for sustaining and improving the crop productivity.

Diverse weed flora

Weeds are the major problem in rice cultivation. Effective weed management plays an important role in the overall profitability of any cropping system. The destruction of weeds with puddling is the main reason for ongoing traditional practice in rice cultivation. However, intensive rice cultivation over the years confined the eco-biodiversity and weed spectrum, and therefore, specific weeds develop more resistance against herbicides and compete with crop plants for water, nutrient and energy. Crop diversification can effectively change the weed spectrum and reduce weed infestation and resistance (Chhokar and Malik 2002 ). Unlike in traditional practice, DSR restricts the weed seed distribution and weed killing and leaves 60–90% weed seeds in the top layer of the soil (Swanton et al. 2000 ; Chauhan et al. 2006 ). The diverse weed flora consisting of grasses, broadleaved and sedges infest rice crop depending on the rice culture and management practices adopted as well as soil and climate conditions. The major weeds found in the rice fields in South Asia are mentioned in Table 2 . Echinochloa crus-galli and Echinochloa colona are the major weeds found in different rice ecologies (aerobic as well as anaerobic rice) in Asian countries. There are many weeds such as Dactyloctenium aegyptium, Digitaria sanguinalis, Digera arvensis , Trianthema portulacastrum and Cyperus rotundus, which do not infest puddle transplanted rice but found in abundance in DSR and cause huge yield reductions (Chhokar et al. 2014 ) . Overall, DSR has diverse weed flora due to alternate wetting and dry conditions. Further, the losses caused by weeds in rice depend upon weed densities, nature of weed flora, duration of weed competition as well as crop establishment methods (Diarra et al. 1985 ; Fischer and Ramirej 1993 ; Eleftherohorinos et al. 2002 ; Chhokar et al. 2014 ). Crop establishment methods such as direct seeding (under dry or wet conditions) or transplanting (under puddled or unpuddled conditions) have strong influence on weed diversity and intensity. Numerous studies have reported higher yield losses in direct seeding compared to transplanting in rice cultivation. (Walia et al. 2008 ; Chauhan 2012 ; Chhokar et al. 2014 ). Based on the large number of farm trials (Gharade et al. 2018), weeds in India caused a loss of about 15–66% in DSR and 6–30% in PTR. Similarly, other workers also reported that weeds cause worldwide, 30–100 per cent rice grain yield reductions in DSR (Oerke and Dehne 2004 ; Rao et al. 2007 ; Kumar and Ladha 2011 ; Chhokar et al. 2014 ). The higher yield reductions in DSR compared to PTR are due to infestation of diverse weed flora in abundance and their emergence before or along with the crop as well as in several flushes, whereas in PTR crop has an advantage of about one-month-old seedlings over weeds (Chhokar et al. 2014 ; Rao et al. 2007 ). Moreover, standing water during the initial stages reduces weeds germination and also improves the herbicides effects. Hill and Hawkins ( 1996 ) reported that same relative E. crus-galli density caused a 20% yield reduction in PTR compared to 70% in DSR. Besides yield losses, weed infestation also reduces rice quality (Menzes et al. 1997 ). Worldwide, rice is grown under different ecologies ranging from an upland to lowland situations, but maximum area is occupied with PTR, where fields are flooded during the most of the crop duration. The depth of the water influences the type and density of the weed flora (Kent and Johnson 2001 ; Kumar and Ladha 2011 ). However, the scarce and costly labour for transplanting is forcing to shift towards the DSR. The labour problem has been aggravated recently due to Covid-19 pandemic in northern India (Haryana and Punjab) and as a result, many farmers shifted from PTR to DSR. However, for long-term success of DSR, two pre-requisites are selection of suitable varieties and efficient weed management (Chhokar et al. 2014 ).

In DSR, single pre- or post-application of herbicide fails to control the diverse weed flora and combination of herbicides either in tank mixture or in sequence is required to have effective control of broad-spectrum weeds. The application of pre-emergence pendimethalin or oxadiargyl followed by either bispyribac or penoxsulam in combination with ethoxysulfuron or pyrazosulfuron controls the diverse weed flora in DSR. Fenoxaprop + safener (Rice Star) effectively controls the problematic weeds, Dactyloctenium aegyptium and Digitaria sanguinalis. Also, the ready mixture of triafamone + ethoxysulfuron as well as penoxsulam + cyhalofop can be utilized for diverse weed flora control. The sole dependency on herbicide is not desirable due to the risk of evolution and spread of herbicide resistant weeds. Weedy rice or red rice ( O. sativa f. spontanea ) has turned out as a major challenge in rice cultivation where PTR has been replaced with DSR (Kumar and Ladha 2011 ). In fact, weedy rice problem in Malaysia has left some farmers to switch back to transplanting method of rice cultivation to control it. Therefore, for effective weed management in long-term, herbicides in mixtures and rotations should be supported with multiple non-chemical weed control strategies such as stale seed bed, competitive cultivars, crop rotation, use of weed free seed and mechanical weeding to remove the weeds before seed setting. In addition, the development and large-scale adoption of herbicide-tolerant rice in future will simplify and provide cost-effective diverse weed flora control in DSR.

Labour scarcity

The labour scarcity and higher labour cost are the emerging challenges in rice production system (Lauren et al. 2008 ). The labour shortage causes the delay in rice transplantation, which may reduce the yield by 30–70% upon delay of 1–2 months (Rao and Pradhan 1973 ). The problem of a labour shortage during the rice transplantation and wheat-sowing season arises due to engagement of labour in assured working scheme like MGNREGA by Government of India. Rice transplantation is very laborious, tedious and time-consuming operation, which requires 300–350 man-h ha −1 (Bhatt et al. 2016 ). It has also been observed that manual random transplanting of rice results in lesser seedlings per unit area compared to the recommended level of 30–40 plants per square meter. Mechanical transplanting of rice is being adopted, which requires only 40 man-h ha −1 to tackle the issues of labour scarcity, higher labour cost and delay in rice transplantation (Mohanty et al. 2010 ). After harvesting the rice with combine harvesters, the problems of critical window period between rice harvest and wheat sowing, labour scarcity and higher labour cost involved in manual residue handling encourage the farmers to adopt the practice of residue burning to avoid any delay in wheat sowing. The farmers of Punjab and Haryana regions are more concerned about timely seeding of wheat as its yield is reduced by 26.8 kg day −1  ha −1 , when sowing is done after 30th November (Tripathi et al. 2005 ). The research focus on machinery development, subsidiary on residue handling machines and ban on crop residue burning by Government of India have prompted the farmers to adopt alternate practices for residue management. However, it would require more research focus on machinery development for multi-cropping systems, awareness of farmers about consequences of residue burning, set-up of industries engaged in manufacturing of residue-based products at block level and schemes like incentives for supplying the raw materials, i.e. crop residues to such industries.

Residue management challenges

In India, more than 686 mt of crop residue is generated every year, of which 234 mt is surplus (Hiloidhari et al. 2014 ). Around 368 mt crop residue is generated from cereal crops in which rice and wheat contribute approximately 154 and 131 mt, respectively (Hiloidhari et al. 2014 ). Along with the crop production, residue generated from the agriculture sector is increasing every year as given in Table 3 . Among the various crop residues, management of rice residue and sugarcane trash has been very challenging due to its poor feed quality owing to higher silica content, narrow window period between rice harvest and wheat sowing, higher cost of residue handling machines, labour-intensive operation of residue removal and lack of storage and energy generation systems. These challenges force the farmers of north-western India to adopt the injudicious practice of residue burning as an economical option for timely sowing of wheat into combine harvested rice fields. Such unfair practices degrade the environment by contaminating the air with carbon monoxide (CO), carbon dioxide (CO 2 ), methane (CH 4 ) and particulate matter. In fact, air quality index of National Capital Region of India falls sever to emergency level during the rice-harvest and wheat-sowing season (APRC 2018). Crop residue burning is also associated with other problems such as loss of nutrients retained in the residue, global warming and soil health deterioration. Hence, the farmers have been suggested to use the rice residue for manure, energy production, biogas production, ethanol generation, gasification, biochar and mushroom cultivation according to easily accessible option to them (Fig.  4 ). A few researchers reported that incorporation of residue in the soil is an effective in-situ residue management option, which improves the soil health in long-term (Kumar and Goh 2000 ; Sidhu and Beri 2005 ; Bijay-Singh et al. 2008 ). However, higher energy requirement and temporary immobilization of nitrogen are the key challenges in this method, which increases the cost of cultivation (Singh et al. 2005b , 2020 ). The surface retention of rice residue by direct seeding the wheat or other crops with resource conserving machines such as zero-till drill, strip-till drill, mulcher, punch planter, Happy Seeder and Rotary Disc Drill emerged as more promising option for residue management (Sidhu et al. 2007 , 2015 ; Sharma et al. 2008 ). Researchers reported multiple benefits of reduced soil erosion, improved soil organic carbon, reduced water losses through evaporation and less emergence of weeds in direct seeding of wheat under residue covered field (Ding et al. 2002 ; Humphreys et al. 2010 ; Sidhu et al. 2015 ). Busari et al. ( 2015 ) concluded that conservation tillage either zero tillage or reduced tillage along with anchored crop residue can build up a better soil environment along with lessened impact on the environment, leading to climate resilience crop production system. The non-conventional seeding practice, i.e. direct drilling, allows in-situ management of crop residue and timely seeding of crops. It also provides the yield advantage to crops, while saving the time, water (10–15%) and diesel (70–80%) along with reduced impact on the environment (Erenstein and Laxmi 2008 ; Erenstein 2009 ; Mishra and Singh 2012 ). Despite multiple benefits, the adoption of these technologies is not very impressive at farmers’ field. Therefore, more efforts on the development of suitable seeding machines for multi-cropping systems under conventional and CA and their popularization are required for effective in-situ residue management on large scale at farmers’ field. Custom hiring service needs to be promoted at block and village level to overcome the issue of costly residue handling and seedling machines for farmers belonging to small- and medium-land holdings. Moreover, utilization of crop residue for industrial and energy applications requires infrastructure development, establishment of residue collection centres at block level, build-up of strong supply chains, policy interventions, large-scale trainings and incentives to farmers to drive the sustainable residue management mission.

Different in-field and off-field options for residue management

Environmental pollution

The agriculture sector has been a major source of methane (CH 4 ) and nitrous oxide (N 2 O) emissions, primarily driven from flood-based rice cultivation (Kritee et al. 2018 ), use of synthetic fertilizers (Zschornack et al. 2018 ) and residue burning practices (Jain et al. 2014 ). Such emissions can raise the global warming potential to 10 times in rice season than winter (Zschornack et al. 2018 ). It is estimated that agriculture is the largest sector, contributing about 44% of anthropogenic methane emissions (Janssens-Maenhout et al. 2019 ). The graph plotted using the data taken from FAO shows a consistent decrease in the contribution of the agriculture sector to CH 4 emission during 1990–2017 (Fig.  5 a). However, interestingly amount of CH 4 emission emitted from agriculture sector consistently increased for the same period (Fig.  5 b). Needless to say that other sectors emitted CH 4 emissions in a faster way than agriculture. But changes in agricultural practices such as increased cultivable area especially under rice cultivation, an overdose application of fertilizers and residue burning have elevated CH 4 emissions significantly. Similarly, the amount of N 2 O emission emitted from agriculture sector consistently increased during 1990–2017 (Fig.  5 c and 5d). Apart from CH 4 and N 2 O emissions, the traditional practice of rice cultivation significantly contributes to other greenhouse gas emissions, too. Puddling operation in mechanized rice cultivation consumes much amount of fuel and thereby raises CO 2 level in the environment. Also, more water requiring crops are responsible for higher CO 2 emission as compared to other crops in the areas where stationary diesel engines or tractors are used for pumping out the water. The burning of 1 L of diesel supplies 2.67 kg of CO 2 to the environment. The problems of environmental pollution from rice cultivation are not limited to its growth period but also after harvesting of rice. Economic constraints, unavailability of suitable residue handling machines and poor feed quality of rice residue encourage the farmers to adopt the unfair practice of residue burning for quick in-situ management of residue and timely seeding of wheat. It creates a huge burden on the environment during the rice-harvesting and wheat-sowing season. Kumar et al. ( 2019 ) estimated the loss due to residue burning by taking nutrient losses, yield loss, soil biodiversity, irrigation, health and other factors into consideration. It was observed that residue burning in north-western India caused losses to the tune of Rs. 8953 per hectare. As far as CH 4 and N 2 O emissions are concerned, better water management practices can lower these emissions from the rice fields. CH 4 emission reduces significantly with intermittent irrigation approach, while N 2 O emission rises under such conditions, thereby creating a trade-off between CH 4 and N 2 O emissions (Yue et al. 2005 ). However, CH 4 emission plays a dominant role in greenhouse gas emissions. The excessive use of fertilizer, chemicals and non-renewable energy in PTR raises other emissions of CO 2 , oxides of nitrogen (NO x ), oxides of sulphur (SO x ) and heavy metal (Jimmy et al. 2017 ). It is important to optimize N-fertilizer doses to improve its uptake efficiency and to reduce the losses and emission load on the environment (Ju et al. 2009 ; Qiao et al. 2012 ). A shift in cultivation method from PTR + residue retention to non-puddled transplanting using strip tillage + residue retention can mitigate 15–30% greenhouse gas emissions (CO 2 equivalent emission) along with the benefit of carbon storage in the soil (Alam et al. 2016 , 2019 ). The adoption of cultivation practices such as DSR on flat or permanent beds, zero-till mechanized transplanting and strip tillage + transplanting can alleviate harmful impacts of puddling method on the environment. However, it requires more research efforts to address weed control, soil-borne pathogens and grain quality challenges of rice cultivated under non-puddled practices (Kumar et al. 2011). A shift from intensive cereal–cereal production system to leguminous-cereal cultivation or replacing rice–wheat with maize–wheat cropping system periodically under zero-till or CA practice could be beneficial for sustainable food grain production. The integrated approach of adopting low duration and lesser water requiring varieties, water management, residue management and RCTs in rice cultivation can mitigate the environmental pollution.

Figure depicting ( a ) share of agriculture sector in CH 4 emission, ( b ) amount of CH 4 emission from agriculture sector, ( c ) share of agriculture sector in N 2 O emission, ( d ) amount of N 2 O emission from agriculture sector (

• Global warming

Global warming is an emerging serious threat to agriculture sector. Greenhouse gases like CH 4 , CO 2 and N 2 O trap the short wave radiation, causing a net increase in the global temperature. The comparative assessment of different crops should be made not only based on yield potential but also their emission intensity, i.e. net return to the environment. For instance, the production of 1 kg rice returns 0.71 kg CO 2 equivalent (CO 2 -eq) emissions to the environment as compared to 0.27 kg CO 2 -eq emissions per kg production of other cereals ( Source: FAOSTAT ). In addition to this, huge amount of residue generated from rice and sugarcane crops creates management challenges and farmers burn the residue for timely sowing of wheat especially in IGP. The total carbon present in rice residue converts to CO 2 (70%), CO (7%), CH 4 (0.66%) and particulate matter, while 2.09% nitrogen to N 2 O gas upon burning (NPMCR 2014 ). The burning of crop residue is not only associated with air pollution but also with loss of precious nutrients retained in the crop residue. During the crop residue burning, almost 100% carbon, more than 90% nitrogen, 20–25% phosphorus and potassium and about 60% sulphur are lost in the form of various gases and particulate matter (Singh et al. 2008 ). The gases emitted from crop residue burning can cause radiation imbalance, leading to harmful effects such as more aerosols in the region, acid rain and ozone layer depletion. Hence, like in other crops, farmers should adopt residue management and RCTs in rice cultivation as well for a sustainable farming. Ma et al. ( 2019 ) found that global warming potential (GWP) and greenhouse gas intensity (GHGI) reduced by 12.6–59.9% and 10.5–65.8%, respectively, by returning the wheat crop waste to the soil in the form of straw, straw-derived biochar and straw with straw-decomposing microbial inoculants over no straw return practice. Sapkota et al. ( 2017 ) and Chen et al. ( 2021 ) highlighted the use of no-tillage with residue retention practice to combat the global warming potential in rice–wheat and rice–rice cropping systems. The return of crop residue to the soil should be in the form of mulching as residue incorporation into soil can raise CH 4 emissions by 3.2–3.9 times of straw-induced SOC sequestration rate, thereby worsening the GWP rather than mitigating climate change (Xia et al. 2014 ). In a different study, Pittelkow et al. ( 2014 ) found that potential yield of rice along with minimal yield-scaled GWP is achievable by using the optimal doses of N-fertilizer. Nemecek et al. ( 2012 ) highlighted the lowest GWP for sugar crops (< 0.05 kg CO 2 -eq kg −1 ) followed by root crops (< 0.15 kg CO 2 -eq kg −1 ) and vegetable and fruits (< 0.35 kg CO 2 -eq kg −1 ). Cereals (except rice) and pulses were found to have medium GWP (< 0.6 kg CO 2 -eq kg −1 ), while oil crops (cotton, peanuts) and rice exhibited the highest GWP (1.2–2.4 kg CO 2 -eq kg −1 ). Needless to say that it would be beneficial to the environment and agro-ecosystem to replace the higher GWP posing cereal crop with vegetable, sugar or root crops in cereal–cereal cropping system. The better water management techniques replacing the continuous flooding in rice cultivation might be effective to reduce the GWP further from rice-based cropping systems (Jiang et al. 2019a ).

Abiotic stress challenges in rice

Rice can be grown in most diverse ecologies; however, its growth and productivity are severely affected by abiotic factors such as heat stress, cold stress, salinity, flood and drought (Biswal et al. 2019 ). The severity and intensity of these abiotic stresses are increasing due to climate change (Pereira 2016 ). With the continuous increase in greenhouse gases and extensive human interference in the environment, adverse effects of climate change are likely to increase. The prediction models have shown severe rice yield losses under intensive climate warming scenarios (Zhao et al. 2016 ). Increased concentration of CO 2 and fluctuations in temperature and precipitation would impact the rice growth and productivity severely due to significant effects of these factors in photosynthesis and other important metabolic processes (Liu et al. 2017 ; Wang et al. 2020 ). A recent study suggested that elevated levels of CO 2 also affected protein, iron, zinc and vitamins content of rice cultivars grown in Asia, thereby posing a serious challenge to human health (Zhu et al. 2018 ). Temperature is one of the most critical abiotic factors which influences the rice production, productivity and grain quality directly. Heat stress affects rice growth and metabolism and has severe impact on all the growth phases, especially seedling and reproductive stage (Sailaja et al. 2015 ; Bhogireddy et al. 2021 ). In a recent study, Zhao et al. ( 2017a ) estimated the global yield loss of rice by 3.2% for every 1 °C increase in global mean temperature by compiling the extensive published results from different analytical methods. On the contrary, positive effects of temperature and increased CO 2 on rice growth were predicted in Madagascar (Gerardeaux et al. 2012 ) suggesting that climate change may bring better scenario for rice cultivation in this region.

Little efforts have been made towards mapping the quantitative trait locus (QTL) for heat stress tolerance (Shanmugavadivel et al. 2017 ; Kilasi et al. 2018 ). Moreover, further characterization of these QTLs to understand the mechanisms and causal genes has not been very impressive. Few genes like ERECTA (ER), a homolog of Arabidopsis receptor like kinase and α2 subunit of the 26S proteasome have been identified as potential regulators imparting heat stress tolerance in rice (Li et al. 2015 ; Shen et al. 2015 ). The O . glaberrima allele of TT1 was shown to be more efficient in degradation of cytotoxic denatured proteins during the heat stress. Another gene OsDPB3-2 ( LOC_Os03g63530 ) imparts heat stress tolerance in rice through positive regulation of dehydration-responsive element binding protein 2A (DREB2A). Notably, the overexpression of DPB did not show any phenotypic aberrations suggesting that it can be used as candidate gene for improving thermotolerance in rice (Sato et al. 2016 ).

Similarly, stress due to cold temperature at seedling and booting stages can cause severe loss to rice grain production (Xiao et al. 2018 ). In rice, a pathway mediated by CBF/DREB1 play a crucial role in cold tolerance (Chinnusamy et al. 2007 ; Ritonga and Chen 2020 ). Other transcription factors such as OsMYB4 , MYBS3 , OsbHLH002 and OsMAPK3 positively regulate the cold stress tolerance response in rice (Su et al. 2010 ). Fujino et al. ( 2008 ) identified that qLTG3–1 (Os03g0103300) encoding protein of unknown function is important for germination at low temperature. Cultivars harbouring tolerant allele of qLTG3–1 or overexpressing rice lines showed low-temperature germinability phenotype, suggesting variations in promoter region of tolerant and susceptible alleles. In a crucial study, a gene responsible for cold tolerance of japonica rice was cloned and characterized through QTL analysis. COLD1 (Chilling Tolerance; LOC_Os04 g51180) was found to be a key player associated with chilling tolerance, which acts through activation of Ca ++ channel by interacting with G protein and regulating G protein signalling at plasma membrane (Ma et al. 2015 ). Interestingly, a single nucleotide polymorphism (SNP) at the 15th nucleotide of the 4th exon of COLD1A was attributed to difference in low-temperature-tolerant japonica and susceptible indica cultivars. The susceptible genotypes had T/C instead of A present in tolerant genotypes, which resulted in Met187/Thr187 (susceptible) to Lys187 (tolerant) substitution. The tolerant allele was suggested to be derived from O. rufipogan wild rice (Ma et al. 2015 ). An SNP in coding sequence of LOC_Os10g34840 was identified through genome-wide association study of 1033 rice accessions, which contribute low-temperature tolerance at seedling stage. This SNP at 18,598,921 (G in tolerant while A in susceptible) caused Gly (tolerant) to Ser (susceptible) substitution (Xiao et al. 2018 ). Another such gene Os09g0410300 was shown to contribute cold tolerance at seedling stage, and the phenotype was attributed to nucleotide variations present in its promoter resulting in tolerant and susceptible alleles of a gene (Zhao et al. 2017b ). In addition to genes for cold tolerant at seedling stage, few genes imparting tolerance at vegetative and booting/reproductive stages have also been characterized. Ctb1 (cold tolerance at booting stage) encoding a F box protein and CTB4a encoding a conserved leucine rich repeat receptor like kinase have been cloned and demonstrated their role in conferring cold tolerance at booting stage (Zhang et al. 2017 ). The tolerant allele of CTB4a contained 5 SNPs (at positions 2536, 2511, 1930, 780 and 2063) in its promoter, which helps in better expression of gene in tolerant genotypes (Zhang et al. 2017 ). In another study, a gene contributing cold tolerance at vegetative growth stage was mapped and characterized (Lu et al. 2014 ). The Low-Temperature Growth 1 ( LTG1 ) encoding a casein kinase I regulates cold tolerance through auxin dependent pathway. The tolerant allele of LTG1 has a SNP, i.e. T at 1070 in place of A in susceptible allele, causing amino acid substitution Iso357 (in tolerant) to Lys357 (in susceptible) (Lu et al. 2014 ). A few genetic engineering approaches for developing the abiotic stress tolerance in rice are presented in Table 4 .

Genetic resources and molecular approaches of rice improvement

Rice is one of the most widely adapted crops due to the vast genetic diversity and its wild relatives (Singh et al. 2018). There are 22 wild and 2 cultivated species ( Oryza sativa  and  Oryza glaberrima) under the genus  Oryza (Vaughan 1989 ). The  O. sativa  covers most of the area under rice cultivation and has been classified into five major groups: indica , aromatic japonica , tropical japonica , temperate japonica and aus (Garris et al. 2005). These genomic resources conserved by national and international organizations have been used in crop improvement programs and also for basic research. A total of 132,000 accessions of rice were maintained by International Rice Genebank Collection Information System (IRGCIS) of International Rice Research Institute (IRRI) as on December 2019. A large number of indigenous, exotic and wild rice accessions are also maintained by National gene bank of India of National Bureau of Plant Genetic Resources (NBPGR), New Delhi. Among the crops, rice is the first to have complete genome sequence, which helped in developing genetic resources for gene discovery, molecular markers and crop improvement (IRGSP 2005). Recent efforts of sequencing of 3,000 rice accessions from 89 countries have helped in identification of superior alleles and haplotypes for rice breeding programs (T3RGP 2014). Genomic information of 3,010 diverse Asian cultivated rice including 3000 rice accessions of 3 K rice genome project was used to identify 29 million SNPs, 2.4 million small indels, 10,000 novel full-length protein-coding genes and more than 90 thousand structural variations, which will serve as an extremely important genetic resource for breeding and biotechnology research (Wang et al. 2018 ). Several databases and genomic resources of rice are available in public domain for gene/allele discovery, molecular marker designing and basic studies (Kamboj et al. 2020 ). These resources have facilitated the QTL discovery and gene cloning for marker-assisted breeding programs and transgenic research. Novel resources such as gene activation mutants, EMS mutants and T-DNA-tagged rice mutant populations are powerful genetic resources for functional genomics and crop improvement (Yi and An 2013 ; Mohapatra et al. 2014 ; Reddy et al. 2020 ). Recently, a genomic resource based on CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats–associated nuclease 9) genome editing has been developed wherein more than 34,000 genes of rice have been targeted (Lu et al. 2017 ). Many high-throughput sequencing-based genomic resources for abiotic stress-related traits are discussed by Bansal et al. ( 2014 ). Transcriptomic and micro-RNA-based genomic resources for abiotic stress traits are also available in rice (Bansal et al. 2014 ; Mangrauthia et al. 2016 , 2017 ). Such resources have been utilized in various molecular approaches such as marker-assisted breeding, genome-wide association studies, cis-and transgenic and genome editing for crop improvement (Varshney et al. 2020 ). Marker-assisted selection and introgression have been used for developing biotic and abiotic stress-tolerant rice genotypes (Das et al. 2017 ). Three major bacterial blight resistance genes ( Xa21, xa13 and xa5 ) were introduced through marker-assisted breeding to produce a bacterial blight resistant rice cultivar, Improved Samba Mahsuri (Sundaram et al. 2008 ). Transgenic rice lines for various traits have been developed using a number of genes and genetic elements (Fraiture et al. 2016 ). Recently, genome editing is projected as the potential breeding technique due to its precision and efficiency (Aglawe et al. 2018 ). Several traits and genes of rice are being targeted and improved using the CRISPR/Cas technology of genome editing (Zafar et al. 2020 ).

Grain quality challenges in rice

Rice grain quality is a permutation of several traits such as appearance, cooking, nutritional and milling qualities (Yu et al. 2008 ). Several factors such as cultivars, production and harvesting conditions, post-harvest management, milling and marketing techniques determine the rice grain quality. Rice endosperm is composed of 80–90% starch with 6–28% amylose content and 5–7% proteins, which serve as energy and protein source of the global population especially in developing countries. The grain appearances vis-à-vis cooking, eating and milling quality are largely determined by the combination of several starch properties such as gelatinization temperature, amylose content and gel consistency (Bao et al. 2008 ). Various approaches including genetic and molecular utilized to improve the starch properties of rice have been extensively reviewed by various researchers (Fujita 2014 ; Birla et al. 2017 ). The off-putting nutritional value of rice proteins is mainly due to the deficiency in certain amino acids such as lysine and tryptophan (Ufaz and Galili 2008 ). Compared to maize, efforts towards increasing the content of deficient amino acids such as lysine and tryptophan have not been extensively attempted in rice due to limited genetic variability, and side-effects of nutrient enrichment on germination and abnormal plant growth. Also, due to the absence of expression of some of the enzymes of the carotenoid pathway, rice is not able to synthesize and accumulate sufficient quality of carotenoids. Therefore, efforts have been put forth to genetically alter the rice plants to produce golden rice that produces b-carotene in the endosperm giving rise to a characteristic yellow colour (Ye et al. 2000 ). Similarly, micro-nutrients such as Fe and Zn, vitamins such as folate and thiamine, antinutritional factor such as phytate and other bioactive compounds have been recently reviewed by Birla et al. ( 2017 ) and Custodio et al. ( 2019 ).

Owing to sufficient production, studies during the past have focussed towards quality traits including nutritional quality. It is usually agreed that rice quality depends on both genetic and environmental factors (Cheng et al. 2003 ). Increase in the night temperature is linked to poor grain quality such as decreased head rice ratio, increased chalkiness and reduced grain width (Shi et al. 2016 ; Li et al. 2018 ). Being complex polygenic traits, chalkiness and amylose content, protein content, grain length, grain width and aspect ratio of rice are highly influenced by environmental conditions such as light, temperature and humidity, and certain cultural practices particularly during the grain-filling stage (Siebenmorgen et al. 2013 ; Li et al. 2018 ). Similarly, fertilizer application, plant density and irrigation management especially during the grain-filling period significantly affect the rice grain quality (Huang et al. 2016 ; Wei et al. 2018 ). However, little is known about the role of optimized cultivation managements on rice grain quality (Zhang et al. 2019a ). Besides, deep flood irrigation has been shown to reduce the chalky grains due to the increased supply of carbohydrates to the panicles (Chiba et al. 2017 ). In the recent, several reports have suggested the significant harmful effect of global warming on crop quality (Morita et al. 2016 ; Ishigooka et al. 2017 ). Taken together, systematic work on rice cultivation in varying environmental conditions in combination with genetic studies has widened our current understanding of rice grain quality. Even though, there are significantly more challenges coupled with opportunities to work on enhancing the quality of rice grain, the various approaches to improve rice grain quality are explicitly shown in Fig.  6 .

Key intervention areas to ensure consumer pro high rice grain quality

Way forward with conservation agriculture and resource conservation technologies

Conservation agriculture (CA) is an alternate farming practice, which emphasizes on minimum soil disturbance, soil cover with crop residue (≥ 30%) and crop rotation (Hobbs et al. 2008 ). It has the potential to address the sustainability issues in rice production system. Many farmers partially adopted CA mainly in the form of zero-till-based direct seeding and direct rice transplantation on untilled or unpuddled field. The minimum soil disturbance component of CA or zero-till-based seeding provides multiple benefits of reducing the negative impact of tillage and heavy machinery on soil structure, while saving time, labour and fuel along with lesser harmful air pollutants (Sharma et al. 2003 ; Malik and Yadav 2008 ). Soil cover component of CA acts as an effective moisture conserving technique by reducing the evaporation rate. Moreover, it also provides physical protection to the soil from rainfall, runoff and wind-induced erosion, while improving the structure, organic carbon and physico-chemical properties of soil (Kassam et al. 2009 ; Rockström et al. 2009 ). The crop rotation in CA promotes the biodiversity and helps in soil nutrient balance and weed spectrum (Kumar et al. 2020a ). The threat of pest and disease incidence is also reduced with regular crop rotation (Farooq et al. 2011 ). The effects of CA practice on soil organic carbon, yield and other parameters under different cropping systems are presented in Table 5 . CA practice in rice-based cropping systems can provide a beneficial effect on soil properties like soil organic carbon, bulk density, soil compaction, microbial biomass, infiltration rate, soil enzymatic activities, macro- and water-stable aggregates, water productivity, etc., with a similar or higher yield than CT practice. Laik et al. ( 2014 ) reported 46–54% and 10–24% higher yield of wheat and rice, respectively, in wheat–cowpea–rice cropping system under CA over conventional practice. The water productivity and benefit–cost ratio were also higher under this cultivation practice. Gathala et al. ( 2015 ) concluded that it is uncertain to have yield advantage in rice-based cropping systems under CA establishment methods; however, in terms of cultivation cost, labour cost and net profit, CA-based cultivation methods are advantageous over CT practice. Haque et al. ( 2016 ) observed lesser cultivation cost and higher profit for minimum tillage unpuddled transplanted rice under CA as compared to conventionally grown rice. In a different study, Mohammad et al. ( 2018 ) reported higher crop and water productivity of DSR under CA over CT practice. Chaki et al. ( 2021 ) found that system production, water productivity and nitrogen use efficiency of wheat–mungbean–rice cropping system increased by 5.4, 40 and 5%, respectively, under CA over conventional practice in fine-textured soils. However, grain and water productivity of rice depleted under CA over conventional practice in coarse-textured soils. From the cited studies, it is evident that CA offers savings in time, labour, water and input cost, while improving the soil characteristics and diminishing GWP simultaneously. In the scenario of declining factor productivity coupled with climate change, it is extremely imperative to bring the rice crop under CA for long-term sustainability of crop production system. The use of RCTs such as leaf colour chart and normalized difference vegetation index (NDVI) sensors-based fertilizer application and electrostatic and variable rate spraying for chemical applications need to be integrated with CA for a sustainable rice cultivation system. Further research efforts are required on developing suitable rice cultivars and variety selection for CA and development of cost-effective RCTs such as zero-till rice transplanter and seeder integrated with pre-emergence herbicide applicator. The future studies on weed, nutrients and pest dynamics and quality aspects of rice under CA are desirable for effective weed and pest control and to have comparable rice yield as with PTR. In line with this, policy interventions, large-scale training and field-level demonstrations would also be required to accelerate the adoption of CA among farmers.

Conclusions

The continuous rice cultivation with traditional method imposed serious threats to natural resources and agricultural sustainability. In the scenario of declining factor productivity, crop response and water table and rising air pollution, researchers and policymakers need to intervene through a systematic and integrated approach to produce more rice with less water in a sustainable way. The cultivation of some alternative and lesser water requiring crops should be encouraged by various measures like incentives and minimum support price for the regions of light-textured soils and rainfed condition. Resource use efficiency needs to be enhanced through multi-dimensional approach on varietal development, soil and water management, adoption of resource conserving machines and need-based application of fertilizers and chemicals for sustainable rice cultivation in medium-to-heavy soils. The integrated resource conserving approach like delayed direct seeding of short duration, high-yielding and stress tolerant rice varieties with a zero-till seeder or transplanting such varieties with zero-till transplanter under CA with drip irrigation system should be encouraged for rice cultivation. However, more research studies and analysis are required to explore the yield aspect and profitability with promising results to convince the farmers for shifting from PTR to a new rice cultivation system. Policy reforms are needed to stop the subsidy on methods and systems that contribute to low water productivity on a system basis. Reforms on water security to users, the decentralization and privatization of water management functions to suitable levels, water pricing, markets in tradable property rights and introducing water conserving technologies for irrigation purposes should be in vogue.

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Kumar, N., Chhokar, R.S., Meena, R.P. et al. Challenges and opportunities in productivity and sustainability of rice cultivation system: a critical review in Indian perspective. CEREAL RESEARCH COMMUNICATIONS 50 , 573–601 (2022). https://doi.org/10.1007/s42976-021-00214-5

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Nutritional and functional properties of coloured rice varieties of South India: a review

• Rathna Priya T. S. 1 ,
• Ann Raeboline Lincy Eliazer Nelson 1 ,
• Kavitha Ravichandran 1 &
• Usha Antony 1  

Journal of Ethnic Foods volume  6 , Article number:  11 ( 2019 ) Cite this article

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Rice is a major cereal food crop and staple food in most of the developing countries. India stands second in the production of rice next to China. Though almost 40,000 varieties of rice are said to exist, at present, only a few varieties are cultivated extensively, milled and polished. Even if white rice is consumed by most people around the world, some specialty rice cultivars are also grown. These include the coloured and aromatic rice varieties. The nutritional profile of the specialty rice is high when compared to the white rice varieties. The coloured rice, which usually gets its colour due to the deposition of anthocyanin pigments in the bran layer of the grain, is rich in phytochemicals and antioxidants. Rice bran, a by-product of the rice milling industry is under-utilised, is rich in dietary fibre which finds application in the development of functional foods and various other value-added products. Thus, more focus on specialty rice and its by-products will not only save it from becoming extinct but also lead a step forward towards nutrition security of the country as they are abundant in vitamins, minerals and polyphenols.

Introduction

Rice is a major cereal crop consumed as a staple food by over half of the world’s population. Consumption of rice is very high in developing countries and nations in Asia. Almost 95% of the rice production is done in Asian countries and about half of the world’s population consumes it. The cultivation of rice ranks third in the production of agricultural commodity next to sugarcane and maize. It is the predominant dietary energy source of 17 countries in Asia and the Pacific, 9 countries in North and South America and 8 countries in Africa. India is one of the major centres for rice production. The area for rice cultivation in India comprises about 43,388,000 hectares of land [ 1 ] and rice contributes to 780 and 689 kcal/capita/day of the food supply in Asia and India, respectively. Furthermore, India is one of the largest countries in terms of energy consumption from agriculture and rice comprises a major part of it [ 2 ].

Rice is rich in genetic diversity, with thousands of varieties grown throughout the world and India is home to 6000 varieties, at present. Originally, India had more than 110,000 varieties of rice until 1970, which were lost during the Green Revolution with its emphasis on monoculture and hybrid crops [ 3 ]. Paddy comes in many different colours, including brown, red, purple and even black. The colourful varieties of rice are considered valuable for their health benefits. The unpolished rice with its bran has high nutrient content than milled or polished white rice. However, rice consumers prefer to consume polished white rice, despite the fact that brown rice contains valuable nutrient content [ 4 ]. A detailed analysis on the nutrient content of rice suggests that the nutrition value varies depending upon several factors such as the strain or variety (i.e. white , brown , red and black /purple), nutrient quality of the soil in which rice is cultivated, the degree of milling and the method of preparation before consumption.

Origin and spread of rice

Oryza sativa , the dominant rice species, is a member of the Poaceae family. Historically, rice was cultivated widely in the river valleys of South and Southeast Asia 10,000 years ago [ 5 ] and it is believed to have originated probably in India. Domestication of rice in India is mainly attributed to the Indus valley civilization c. 3000–1500 BC [ 6 ]; however, the evidence of rice cultivation in India has been pushed to 4000 years ahead with the discovery of rice grains and early pottery found in the site of Lahuradewa, Uttar Pradesh, situated in the middle Ganges plains dating to c. 6409 BC [ 7 , 8 ].

Rice is highly adaptable to its environment of growth and this is evident from the fact that it is grown in north-eastern parts of China at latitude 53°N, on the equator in central Sumatra, and at 35°S in New South Wales, Australia. In India, it is grown below sea level in Kerala; most rice-growing areas are present at or near sea level and also, at elevations above 2000 m in Kashmir. Today, rice is cultivated in all parts of the world except Antarctica [ 9 ].

Importance of rice in India

India ranks second in the production of rice in the world next to China, accounting for 22.5% of overall world rice production. Rice is India’s pre-eminent crop and is the staple food of the people of the eastern and southern parts of the country. Apart from being nutritionally rich, rice has greater significance in India and holds great spiritual and ritual importance. As per Indian tradition, rice is revered as a potent symbol of auspiciousness, prosperity and fertility because of its life-sustaining qualities. Several rituals involving rice are performed during different occasions and festivals. In Tamil Nadu, kolam , a kind of geometric pattern, is drawn using rice flour at the threshold of the houses by women before sunrise. Rice also plays a vital role in wedding ceremonies in India. Dhanpan is a ritual wherein the family of bridegroom sends paddy, betel and/or turmeric to the house of the bride [ 10 ]. Rice mixed with turmeric is thrown on the couples during the wedding ceremony as a symbol of prosperity, eternity, continuity and fertility. The father of the bride organises a feast called Bhat (means, boiled rice) for the family and relations of the bridegroom [ 10 ]. The brides throw five handfuls of rice before leaving their parents’ home after the wedding to wish prosperity and wealth and remain with the family members. The bride enters her new home by pushing a glass or a jar full of rice while, rice is the first food offered to the bridegroom by the bride after marriage. In Tamil Nadu, the groom is offered a special variety of rice named Maappillai Samba to improve fertility [ 11 ]. Rice also plays a vital role during the baby shower function, named godh bharai in North India, valaikaapu in Tamil Nadu and seemandham in Kerala; on the event of birth; at the time of giving first solid food to the baby that is 6 or 7 months old; and during puberty in Kerala and Tamil Nadu. Flattened rice made from a variety called Thavala Kannan is given as offering in Kerala.

Rice also plays a prominent role in cultural celebrations of India, such as the festivals are based on sowing of seeds in the paddy field, transplanting the saplings in the fields, removal of weeds from the fields, harvesting of paddy, thrashing of paddy and storage of paddy [ 10 ]. The harvest festivals include Thai Pongal celebrated in the Tamizhian calendar month of Thai (falls in the month of January) in Tamil Nadu; Onam celebrated in the Malayalam month of Chingam (falls in the month of August or September) and Sankranti in Andhra Pradesh and Telangana, Makar Sankranti in Karnataka, Na-Khuwa Bhooj in rural Assam, Nabanna in West Bengal, and Nua khia or Navanna in Odisha; and Bihu in Assam celebrates the harvest of paddy. Thus, rice has not only shaped the history, culture, diet and economy of people but also the growth stage of the rice crop marks the passage of time and season. In India, rice is considered the root of civilization [ 12 ].

Production and market demand for rice varieties

Rice is a fundamental food in many cultural cuisines around the world. According to Ricepedia, more than 90% of production and consumption of rice in the world occur in Asia and the current share in global rice consumption is around 87%. In African countries, per capita consumption continues to increase than production [ 13 ]. The volume of international rice trade has increased almost sixfold, from 7.5 million tonnes annually in the 1960s to an average of 44.2 million tonnes during 2015–2016.

Based on the global market scenario with respect to rice, the production has increased slightly with years. The use of rice as food remains predominant compared to feed and other uses. The supply and utilisation of rice have also increased slightly (Table  1 ).

Similarly, rice is a major cereal crop and is consumed as a staple food by the majority of the population in India. India is one of the major centres for the production of rice. Both the Himalayan red rice and the Assam red rice find their place in international trade. The production of rice, wheat and maize has grown steadily over this period and that of rice is the highest followed by wheat (Table  2 ). In contrast, the production of other grains such as sorghum, pearl millet, finger millet, little millet and coarse cereals have either remained steady or have declined.

Rice is consumed by the rich and poor as well as rural and urban households. The per capita net availability of food grains increased after the Green Revolution, and rice is a part of the balanced diet along with vegetables, pulses, eggs, meat and fruits. The per capita net availability of rice increased to 69.3 kg/year in 2017 from 58.0 kg/year in 1951 [ 15 , 16 ]. Although rice is widely consumed, with years, the expenditure on cereals decreased from 26.3% in 1987–1988 to 12.0% in 2011–2012 and from 15% in 1987–1988 to 7.3% in 2011–2012 in rural and urban households, respectively. This overall dip in the expenditure may be due to the fact that more money is spent on non-food items in both rural and urban households [ 16 ].

Rice varieties

Among the 40,000 varieties of rice cultivated worldwide, only two major species are cultivated widely— Oryza sativa or the Asian rice and Oryza glaberrima or the African rice. The cultivation of Oryza sativa is practised worldwide; however, the cultivation of the Oryza glaberrima is confined to Africa [ 17 ].

Oryza sativa has two major subspecies: the Indica , long-grain rice and the Japonica , round-grain rice. Japonica rice is mainly cultivated and consumed in Australia, China, Taiwan, Korea, the European Union, Japan, Russia, Turkey and the USA. Indica rice varieties are grown widely in Asia [ 17 ]. These varieties also comprise of the fragrant ones which are priced as premium. The principal fragrant varieties are Hom Mali from Thailand and the various types of Basmati exclusively grown on the Himalayan foothills of India (in the states of Haryana and Punjab) and Pakistan (in the state of Punjab) [ 18 ].

The Indian rice varieties cultivated widely are Basmati , Joha , Jyothi , Navara , Ponni , Pusa , Sona Masuri , Jaya , Kalajiri (aromatic), Boli , Palakkad Matta , etc. The coloured variety includes Himalayan red rice; Matta rice, Kattamodon , Kairali , Jyothy , Bhadra , Asha , Rakthashali of Kerala; Red Kavuni , Kaivara Samba , Mappillai Samba , Kuruvi Kar , Poongar of Tamil Nadu, etc.

The shelf life of rice

In general, it is recommended to store rice in the form of paddy rather than as milled rice, since the husk provides protection against insects and helps prevent quality deterioration. Rice can be stored for long periods only if the following three conditions are met and maintained: (1) the moisture levels of grains, 14% or less and that of seeds must be 12% or less; (2) grains must be well protected from insects, birds and rodents; and (3) grains must be protected from rains or imbibing moisture from the atmosphere. In addition to its nutritive and medicinal properties, red rice and black rice possess several other special features and the most common one is their resistance to insects and pests during storage than brown rice. From the cultivation point of view, red rice possesses resistance to drought, flood, submergence, alkalinity, salinity, and resistance to pests and diseases [ 19 ].

Structure of rice grain

The paddy (also, rough rice or rice grain) consists of the hull, an outer protective covering, and the fruit or rice caryopsis (brown or dehusked rice) [ 20 ]. Rice primarily consists of carbohydrates, proteins and small quantities of fat, ash, fibre and moisture. Vitamins and minerals are largely confined to the bran and germ [ 21 ].

The polished white rice, usually consumed, is the highly refined version of raw rice. The processing and milling of raw rice take away significant parts of the grain, namely the bran and the germ. Both bran and germ are rich in dietary fibre as well as nutrients that are beneficial for human health. Further, if white rice undergoes additional polishing, its aleurone layer getsremoved leading to loss of more nutrients, as this layer is rich in vitamin B, proteins, minerals and essential fats.

In this aspect, the coloured rice finds an advantage as a healthier alternative to white rice. Coloured rice varieties and brown rice varieties have the same harvesting process apart from possessing similar nutritional profiles. These varieties are usually either dehulled or partially hulled with the bran and germ intact. Brown rice is found worldwide, while red rice is confined to the Himalayas, Southern Tibet, and Bhutan, as well as parts of North East and South India. After the removal of husk, brown rice still consists of few outer layers—the pericarp, seed-coat and nucellus; the germ or embryo; and the endosperm. The endosperm consists of the aleurone layer, the sub-aleurone layer and the starchy or inner endosperm (Fig.  1 ). The aleurone layer encloses the embryo. Pigments are confined to the pericarp layer [ 20 ].

Structure of rice grain (Copyright: FAO) [ 22 ]. Paddy consists of the husk, bran, aleurone layer, starchy endosperm and embryo. Brown rice is semi-polished, so it retains embryo while white rice is more polished than brown rice, lacking bran, aleurone and embryo. The removal of bran, aleurone and embryo provides aesthetic appeal to rice and improves shelf life; however, it also removes nutrients and minerals found in the grain

The hull (also, husk) constitutes about 20% of the rough rice weight, but values range from 16 to 28%. The aleurone layer varies from one to five cell layers; it is thicker at the dorsal than at the ventral side and thicker in short-grain than in long-grain rice [ 23 ]. The aleurone and embryo cells are rich in protein and lipid bodies [ 24 ].

The different layers of rice contain different quantities of nutrients. The bran layer is rich in dietary fibre, minerals and vitamin B complex while the aleurone layer contains the least. The endosperm of rice is rich in carbohydrate and also contains a reasonable amount of digestible protein, with favourable amino acid profile than other grains [ 25 ].

Rice processing

Processing of rice mainly involves milling of rice which converts paddy into rice by removing the hull and all or part of the bran layer. Milling of rice is a crucial stage and the objective of milling is to remove the husk and bran so as to produce an edible white rice kernel that is free from impurities.

Rabbani and Ali [ 26 ] report that as a result of processing, some essential nutrients like thiamine and vitamin B are lost. The milling process followed by polishing destroys 67% of the vitamin B 3 , 80% of vitamin B 1 , 90% of vitamin B 6 , 50% of manganese and phosphorus, 60% of the iron, and all of the dietary fibre, as well as the essential fatty acids present in the raw unmilled variety.

The rough rice (also, paddy) on milling produces brown rice, milled rice, germ, bran, broken and husk. Each of these has unique properties and can be used in numerous ways. The extent of value addition in rice and rice products depends upon the utilisation pattern of these components directly or as derivatives. For coloured rice varieties, only the first three steps of milling, namely, pre-cleaning, dehusking and separation, are applied and bran and germ are left intact.

Nutritional information

Raw, long-grain white rice is a good source of carbohydrates, calcium, iron, thiamine, pantothenic acid, folate and vitamin E when compared with maize, wheat and potatoes. It does not contain vitamin C, vitamin A, beta-carotene, lutein and zeaxanthin. It is also notably low in dietary fibre.

• Coloured rice

Brown rice retains its bran layer (containing vitamins, minerals and fibre), as this has not been polished more to produce white rice. The coloured rice varieties are either semi-polished or unpolished (Fig.  2 ). Red-coloured rice varieties are known to be rich in iron and zinc, while black rice varieties are especially high in protein, fat and crude fibre. Red and black rice get their colour from anthocyanin pigments, which are known to have free radical scavenging and antioxidant capacities, as well as other health benefits.

Some traditional South Indian rice varieties. a Red Kavuni . b Kaivara Samba . c Kuruvi Kar . d Poongar . e Kattu Yanam . f Koliyal . g Maappillai Samba. h Black Kavuni . Kavuni possesses anti-microbial activity. Kaivara Samba lowers blood sugar levels. Kuruvi Kar is resistant to drought and consumed by the locals for its health benefits. Poongar is consumed by women after puberty and is believed to avert ailments associated with the reproductive system. Kattu Yanam lowers glucose level in blood and also imparts strength. Koliyal is widely consumed as puttu , a specialty dish. Maapillai Samba has a hypocholesterolemic effect and anti-cancer activity and also improves fertility in men. Black Kavuni is resistant to drought and is popular among locals for its health benefits

Brown rice is highly nutritious. It has low calorie and has a high amount of fibre. Furthermore, it is a good source of magnesium, phosphorus, selenium, thiamine, niacin, vitamin B 6 and an excellent source of manganese. Brown rice and rough rice are rich in vitamins and minerals; this is due to the fact that the vitamins are confined to the bran and husk of the paddy. Rice bran and husk contain a higher amount of calcium, zinc and iron (Table  3 ).

Rice is rich in glutamic and aspartic acids but has a lower amount of lysine. The antinutritional factors that are concentrated mainly in the bran are phytate, trypsin inhibitors, oryzacystatin and haemagglutinin-lectin [ 25 ].

The moisture content plays a significant role in determining the shelf life of foods [ 29 ]. Xheng and Lan [ 30 ] report that moisture influences the milling characteristics and the taste of cooked rice. The differences in genetic makeup and the climatic conditions in which they are cultivated determine the moisture content in rice varieties. As seen from Table  4 , the moisture content of the red rice varieties is variable from 9.3 to 12.94%, the moisture content of brown rice and milled rice is lower than other rice varieties.

Protein is the second major component next to starch; it influences the eating quality and the nutritional quality of rice. In India, the dietary supply of rice per person per day is 207.9 g, this provides about 24.1% of the required dietary protein [ 2 ]. Rice has a well-balanced amino acid profile due to the presence of lysine, in superior content to wheat, corn, millet and sorghum and thus makes the rice protein superior to other cereal grains [ 36 ]. The lysine content of rice protein is between 3.5 and 4.0%, making it the highest among cereal proteins. The endosperm protein comprises of 15% albumin (water soluble), globulin (salt soluble), 5–8% prolamin (alcohol soluble), and the rest glutelin (alkali soluble) [ 27 ].

The coloured rice has high protein content than polished white rice due to the presence of bran. The Srilankan and Chinese varieties have higher protein content than the Indian varieties (Table  4 ). Rice bran proteins are rich in albumin than endosperm proteins. The aleurone protein bodies contain 66% albumin, 7% globulin and 27% prolamin and glutelin [ 37 ].

The fat present in rice is a good source of linoleic acid and other essential fatty acids. The rice does not contain cholesterol [ 36 ]. The lipids or fats in rice are mainly confined to the rice bran (20%, dry basis). It is present as lipid bodies in the aleurone layer and bran. The core of the lipid bodies is rich in lipids and the major fatty acids are linoleic, oleic and palmitic acids [ 38 , 39 ]. Starch lipids present in rice is composed of monoacyl lipids (fatty acids and lysophosphatides) complexed with amylose [ 40 ]. The amount of fat present in various fractions of rice and red rice indicate that red rice varieties from Sri Lanka and India have about 1% fat, while the China red rice has almost doubled this value (Table  4 ).

The presence of fibre in the diet increases the bulk of faeces, which has a laxative effect in the gut. The fibre content is 0.5–1.0% for well-milled rice [ 41 ]. Arabinoxylans, along with β-d-glucan, are the major component of soluble dietary fibre in rice. In addition, rhamnose, xylose, mannose, galactose and glucose are also present in soluble dietary fibre. Insoluble dietary fibre is made up of cellulose, hemicellulose, insoluble β-glucan and arabinoxylans. However, the quantity and amount of non-starch polysaccharide depend upon the rice cultivar, the degree of milling and water solubility [ 42 ]. Among the red rice varieties, Chak-hao amubi (Manipur black rice) has a significantly lower content of crude fibre (Table  4 ).

The variation in ash content of different cultivars of rice may be due to genetic factors or the mineral content of the soil [ 43 ]. The zinc and iron content of red rice is two to three times higher than that of white rice [ 44 ]. The most common minerals found in rice include calcium, magnesium, iron and zinc (Table  3 ).

The proximate composition of rice and its fractions are influenced by the kind of rice and degree of milling, as milling completely or partially removes the bran layer, aleurone layer and embryo. Thus, variation occurs in the nutrition content between the rice fractions of the same rice variety. The variations can be found in the amount of fats, fibre and minerals present in the grain.

Phytochemical composition

The non-nutritive plant chemicals that have a protective or disease-preventing property are known as phytochemicals. The phytochemical compounds are mainly accumulated in the pericarp and bran of the rice kernel. They prevent oxidative damage in foods and also have a wide spectrum of beneficial biological activities.

Phytochemicals present in rice can be divided into the following sub-groups namely carotenoids, phenolics, alkaloids, nitrogen and organo-sulphur containing compounds. Phenolic compounds are further sub-grouped as phenolic acids, flavonoids, coumarins and tannins. Anthocyanins, the major pigment responsible for the colour of red and black rice, are a kind of flavonoids. Maapillai Samba , a kind of red rice from Tamil Nadu, has the highest amount of total polyphenolic compounds and anthocyanin content than the varieties from Sri Lanka, China red rice and Manipur black rice (Table  5 ).

The pigmented cereal grains, such as red and purple/black rice, have phytochemical compounds in higher amounts than non-pigmented varieties. The phytochemicals such as cell wall-bound phenolics and flavonoids are gaining more interest as these compounds can be broken down by digestive enzymes and gut microflora, and as a result, they can be easily absorbed into the body [ 45 ].

The coloured rice bran contains anthocyanins that possess inhibition of reductase enzyme and anti-diabetic activities [ 46 ]. The reductase inhibitors possess anti-androgen effects and are used in the treatment of benign prostatic hyperplasia and to lower urinary tract symptoms. β-sitosterol present in Maappillai Samba (Fig.  2 g) has a hypocholesterolemic effect, improves fertility and also heals colon cancer. Furthermore, stigmasterol found in this variety is a precursor in the production of semi-synthetic progesterone [ 11 ].

Garudan Samba contains 9,12-octadecadienoic acid ( Z , Z ) which has the potential to act as hypocholesterolemic, anti-arthritic, hepatoprotective, 5-alpha-reductase inhibitor, anti-histaminic, anti-coronary and anti-androgenic effects. In addition to these compounds, it also contains several other bioactive compounds [ 47 ].

3-Cyclohexene-1-methanol and α, α,4-trimethyl- present in red Kavuni (Fig.  2 a) possess the anti-microbial activity, and also, 3-hydroxy-4 methoxy benzoic acid is used as a precursor for the synthesis of morphine. In addition to these compounds, fatty acid esters and fatty acids such as dodecanoic acid, ethyl ester (lauric acid ester) and octadecanoic acid are present. Among these bioactive compounds, octadecanoic acid and ethyl esters increase low-density lipoprotein (LDL) cholesterol in the human body [ 48 ].

Health benefits

Depending upon the flavours, culinary needs, availability and its potential health benefits, people choose different varieties of rice. Rice has the ability to provide fast and instant energy. Brown rice and red rice are great sources of fibre, B vitamins, calcium , zinc and iron, manganese, selenium, magnesium and other nutrients. The red and black rice variety gets its rich colour from a group of phytochemicals called anthocyanins, which are also found in deep purple or reddish fruits and vegetables.

Diabetes mellitus

Unlike white polished rice, brown rice releases sugars slowly thus helping to stabilise blood sugar in a sustained manner. This trait makes it a better option for people who are suffering from diabetes mellitus. Further, studies in Asia have shown a relationship between the consumption of white rice and risk of type 2 diabetes. Dietary fibres reduce the absorption of carbohydrates by providing an enclosure to the food, hindering the action of hydrolytic enzymes in the small intestine on food, and increasing the viscosity of food in the intestine [ 49 ]. This plays a vital role in reducing the GI of food thereby preventing the risk of diabetes type 2 [ 50 ]. Proanthocyanidins present in red rice provide protection against type 2 diabetes [ 51 ]. Similarly, anthocyanins present in black rice is said to have a hypoglycemic effect [ 52 ].

Brown rice is rich in manganese and selenium, which play a vital role against free radicals, which acts as a major cancer-causing agent. Due to the presence of these elements and high dietary fibre, brown rice is associated with a lowered risk of cancer. Studies have also correlated the use of whole grains like brown rice with lowered levels of colon cancer. This may be related to its high fibre content, as fibre gets attached to carcinogenic substances and toxins helps to eliminate them from the body, and also keep them away from attaching to the cells in the colon. Proanthocyanins, present in red rice, modulate the inflammatory response and protect against some cancers [ 51 ]. Similarly, anthocyanins which are found abundantly in black rice have anti-carcinogenic properties based on epidemiological and in vivo animal and human-based studies [ 53 ].

Cardiovascular disease

Brown rice may help in lowering the risk of metabolic syndrome, while metabolic syndrome itself is a strong predictor of cardiovascular disease. Red rice contains magnesium that prevents the risk of heart attacks [ 54 ]. Various high-fat diet-induced risk factors for cardiovascular disease were ameliorated by anthocyanin-rich extracts from black rice in rat models [ 55 ].

Cholesterol

Brown rice contains naturally occurring bran oil, which helps in reducing LDL forms of cholesterol. Intake of black rice has found to eliminate reactive oxygen species (ROS) such as lipid peroxide and superoxide anion radicals and lower cholesterol levels due to the presence of compounds such as anthocyanins, polyphenolic compounds, flavonoids, phytic acid, vitamin E and γ-oryzanol [ 56 , 57 ]. Modulation of inflammatory responses by proanthocyanidins in red rice provided protection from cardiovascular disease [ 51 ]. Based on these studies, it is evident that whole grains can lower the chances of arterial plaque buildup, thus reducing the chances of developing heart disease.

Hypertension

Both brown and red rice have high magnesium content than white rice. Magnesium is an important mineral that plays a vital role in the regulation of blood pressure and sodium balance in the body [ 54 ].

Rice varieties such as brown, red and black rice are rich in fibre and have the ability to keep healthy bowel function and metabolic function. Anthocyanins present in red rice have properties that can help in weight management [ 54 ].

Rice protein is hypoallergenic; products from other plant sources such as soy and peanut and animal sources like eggs and milk are a good source of proteins, yet they may cause allergy when consumed. Rice protein provides a solution to this problem because it is hypoallergenic. Furthermore, the anthocyanins present in red rice also have the property to reduce allergy [ 54 ].

Medicinal uses of coloured rice

Among several types of rice, few varieties are used to treat ailments. Every variety of rice is unique in its properties, so the treatment of diseases using rice is not limited to a single variety alone. Many different varieties of rice are employed in treating ailments because of their different properties and characteristics. According to practitioners of Ayurveda, rice creates balance to the humours of the body. Rice enriches elements of the body; strengthens, revitalises and energises the body by removing toxic metabolites; regulates blood pressure; and prevents skin diseases and premature ageing. Rakthasali (a kind of red rice) is efficient in subduing disturbed humours of the body and good for fevers and ulcers; improves eyesight, health, voice and skin health; and increases fertility [ 58 , 59 , 60 , 61 ]. In Ayurveda, Sali , Sashtika and Nivara rice are used to treat bleeding from haemorrhoids (piles); Sali rice is used to treat burns and fractures; Nivara rice is used to treat cervical spondylitis, paralysis, rheumatoid arthritis, neuromuscular disorders, psoriasis, skin lesions, reduce backache, stomach ulcers and snakebite; and Nivara rice is also used in the preparation of weaning food for underweight babies [ 58 , 62 ].

Rice water prepared by soaking rice in water or boiling rice in excess water is used to control diseases. In Ayurvedic preparations, rice varieties such as Mahagandhak ras , Kamdudha ras , Sutsekhar ras , Amritanav ras , Swarnmalti ras , Pradraripu ras , Laghumai ras , Dughdavati , Pradaknasak churna , Pushpnag churna , Sangrahat bhasm and Mukta sukti are used to control ailments such as vaginal and seminal discharges, diarrhoea, constipation and dysentery [ 58 ]. Red rice varieties are known to be used in the treatment of ailments such as diarrhoea, vomiting, fever, haemorrhage, chest pain, wounds and burns [ 63 ]. Matali and Lal Dhan are used for curing blood pressure and fever in Himachal Pradesh. Another red rice variety called Kafalya from the hills of Himachal Pradesh and Uttar Pradesh is used in treating leucorrhoea and complications from abortion [ 64 ]. Kari Kagga and Atikaya from Karnataka are used for coolness and also as a tonic, whereas Neelam Samba of Tamil Nadu is used for lactating mothers [ 65 ]. Kuruvi Kar is resistant to drought and consumed by the locals for its health benefits [ 66 ]. Raktasali is efficient in subduing deranged humours [ 60 , 61 ]. It was also regarded as a good treatment for ailments such as fevers and ulcers. It is also believed that it improves eyesight and voice; acts as diuretic, spermatophytic, cosmetic and tonic; and was also antitoxic [ 59 ].

Traditional food and its importance

Ayurvedic treatises mention red rice as a nutritive food and medicine, so the red rice is eaten as a whole grain. Red rice varieties such as Bhama , Danigora , Karhani , Kalmdani , Ramdi , Muru , Hindmauri and Punaigora of Jharkhand and Chattisgarh are rich in nutrition and provide energy and satiety for a whole day [ 67 , 68 ]. Traditionally, various foods such as pongal , puttu , adai , appam , idli , dosai , idiyappam , adirasam , kozhukattai , modakam , payasam , semiya , uppuma , flaked rice, puffed rice, etc. are prepared and consumed. In Tamil Nadu and Kerala, paddy is parboiled prior to milling. This hydrothermal process facilitates the migration of nutrients such as vitamins and minerals from the bran and the aleurone layer to the endosperm [ 69 ]. Rice takes the place of major cereal consumed in the South Indian diet while it is wheat that holds the position in North Indian diet. Dosai , idli , pongal , appam , semiya , uppuma , kichadi and idiyappam are prepared and consumed for breakfast along with wide varieties of chutney. The specialty dish called puttu made from rice is also prepared and consumed for breakfast. The lunch of South India is a combination of cooked parboiled rice, poriyal , eggs, meat, sambar , dal curry, rasam , pappad , moore (buttermilk) or curd and/or dessert, payasam . The dinner usually consists of idli , dosai , idiappam , cooked rice and curries. Various other dishes are also prepared from rice and include biryani, pulao, fried rice, curd rice, tamarind rice, sambar rice, jeera rice, lemon rice, coconut rice, etc. In Tamil Nadu, appams and idlis are also made using the red rice. Koliyal and Garudan Samba ( Kaadai Kazhuththaan ) of Tamil Nadu are used in the preparation of a specialty dish called puttu [ 47 ]. Flatbread and chapatti are made from red Gunja and glutinous rice is used in making puttu , a South Indian meal [ 70 ]. Several products such as cookies, murruku (a type of South Indian snack), are also made using the various coloured rice varieties.

Rice also plays a major role in festivals celebrated in India. The harvest festivals are celebrated with several delicacies made from freshly harvested paddy. In Tamil Nadu, sarkarai pongal is made from raw rice, green gram, milk and jaggery; in Assam, fried rice balls named ghila pitha are prepared and consumed; in West Bengal, traditional Bengali delicacies are made from freshly harvested rice and jaggery, the most famous one is home-made sweets from rice pitha and karpursal or banapuli , and Basmati rice is also used to make Bengali paish .

Parboiled red rice widely consumed in Kerala includes Thondi , Matta , Paal Thondi , Kuruva , Chitteni and Chettadi . Seeraga Samba is an aromatic rice variety consumed widely in Tamil Nadu and Kerala; it is known as ‘Basmati of South India’ and used in the preparation of biryani. Similarly, Jatu of Kulu valley, Ambemohar of Maharastra, Dubraj of Madhya Pradesh, Joha of Assam, Kamod of Gujarat, Badshah bhog of West Bengal and Odisha, Radhunipagla of West Bengal, Katrini and Kalanamak of Uttar Pradesh and Bihar, Gandha samba of Kerala, Kalajira of Odisha and Chakhao varieties of Manipur are prized for its aroma [ 64 , 67 ].

Today, the spotlight is on the increased production of these traditional varieties, promoting the consumption among the younger generation and production of nutritious and novel value-added products from coloured rice.

Although India is home to traditional red rice varieties and their use has been common among the practitioners of traditional medicine and communities as part of their cultural heritage, their functional effects and health benefits in terms of modern scientific methodology are far and few. Due to the insufficient availability of data, the beneficial properties of these varieties still remain unknown to a majority of the population. So, to leverage their health benefits, extensive research on these native coloured varieties by the stakeholders needs to be promoted so that they are available to consumers as a part of the daily diet or specialty functional foods.

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RP initiated the idea of the article and authored all sections of the article except sections on medicinal uses of coloured rice, traditional food products and value-added products and new products. ARLEN authored sections on medicinal uses of coloured rice, traditional food products and value-added products and new products; co-authored other sections of the article KR co-authored the sections on the importance of rice in India, rice processing, production and demand of rice varieties, origin and spread of rice and value-added products and new products; and provided critical inputs to revise the manuscript. UA co-authored the sections on structure of grain, nutrition, health benefits and traditional food products; and provided critical inputs to revise the manuscript. All the authors read and approved the final manuscript.

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Rathna Priya, T., Eliazer Nelson, A.R.L., Ravichandran, K. et al. Nutritional and functional properties of coloured rice varieties of South India: a review. J. Ethn. Food 6 , 11 (2019). https://doi.org/10.1186/s42779-019-0017-3

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The Relevance of Rice

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• Adam Barclay 1 , 2  

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Research into rice—the world’s most important food crop—is crucial for the development of technologies that will increase productivity for farmers who rely on rice for their livelihood. This is particularly the case throughout the developing countries of Asia and is also true for much of Latin America and, increasingly, Africa. The benefits of such increased productivity will flow through to rice-growing countries’ landless rural and urban poor, all of whom (1) are net consumers of rice and (2) spend a large proportion of their income on rice. Recent steep rises in the price of rice have amplified the need for investment in high-quality research targeted toward both the intensive irrigated rice-based systems (in which 75% of the world’s rice is grown and that must provide the rice for rapidly increasing urban populations) and the rainfed rice-based systems (many of which are characterized by unfavorable environments and extreme poverty).

The price of rice

Rice has always been relevant to global food security and socioeconomic stability. But it was not until one of the steepest price rises in food history that the mainstream media really started to comprehend the grain’s importance. As export prices tripled in a mere few months at the beginning of 2008 (Fig.  1 ), rice became front-page news not only in the Asian countries where it is the staple but also in countries halfway across the world where it is not grown at all and eaten only a little.

Monthly export price (US$/ton FOB) of Thai rice 5%-broken, 1998–2008 (March 1998 to May 2008). By the time of publication, rice prices had settled somewhat, but remained around double those of 1 year previously. Source of raw data: The Pinksheet, World Bank.

The reasons for the rice price increase were numerous, but in many ways, it is research that lies at the heart of the issue. The modern era of farming began in the 1960s when high-yielding rice varieties developed by the International Rice Research Institute (IRRI) in the Philippines were adopted on a large scale throughout Asia. The resultant jump in yields, which heralded the Asian Green Revolution, prompted an era of steadily increasing productivity that kept rice production ahead of the population-growth curve. This, in turn, allowed governments to shore up stocks of rice and prices dropped steadily from the food-crisis peaks of the early 1970s (Fig.  2 ).

Trends in world rice production and price, 1961–2008. Source: Production: USDA, 13 May 2008. Rice Price: www.worldbank.org ; 2008 is May 2008 price. Relate to Thai rice 5%-broken deflated by G-5 MUV Index deflator (adjusted based on 17 April 2008 data update).

Why is the price of rice, Oryza sativa , so important? Domesticated from the wild grass Oryza rufipogon 10,000 to 14,000 years ago, this tropical cereal is the main staple for about half of the world’s population—more than 3 billion people. It provides about 20% of direct human calorie intake worldwide, making it the most important food crop. Rice consumption exceeds 100 kg per capita annually in many Asian countries (compared with the US average of 10 kg, for example) and is the principal food for most of the world’s poorest people, particularly in Asia, which is home to 70% of those who earn less than $1 a day (Fig.  3 ). For such people, the more productive rice farming and lower prices brought about by the Green Revolution had a huge impact on poverty [ 1 ].

Cartogram showing country size as a function of the number of people living on less than one dollar per day. Source: R. Hijmans, IRRI.

In Asia, the poorest of the poor spend up to 50% of their total income on rice alone. For them, any money saved on cheaper rice can be used to buy more nutritious food, to meet medical needs, or to clothe and educate children. Furthermore, as rice rapidly gains popularity in Africa, more and more of that continent’s poorest stand to benefit from advances in rice research and production.

Therefore, anything that lowers the price of rice will benefit hundreds of millions of poor consumers, and anything that increases rice-farming productivity will benefit millions of rice farmers and their families. The Green Revolution in Asia, spurred by IRRI’s development of high-yielding, short-duration, short-stature, fertilizer-responsive rice varieties, did just that and led directly to the Asian economic miracle of the last 40 years [ 6 ].

The cheap food myth

As the benefits stemming from agricultural research and development became evident, governments and funding agencies opened their wallets and invested. IRRI’s budget, for example, climbed rapidly from the early 1970s and stayed healthy for almost 20 years. However, in the early 1990s, complacency set in (Fig.  4 ). Public agricultural research found itself losing out to more fashionable (but worthy) causes such as the environment. The fundamental food problem—producing enough—had been solved. Cheap food was here to stay. Or so people thought.

IRRI total funding (inflation adjusted), 1960–2007. Source: IRRI.

While funding for institutes like IRRI dwindled, the flow of improved farming technologies slowed. The pipeline did not dry out, but important programs were diminished or cut. The trouble with the idea that the food problem had more or less been solved is that agricultural research is never finished. A new rice variety, for example, may be resistant to a particular disease but not forever. There is no ultimate variety whose development will signal the end of the need for research. We will need agricultural research for as long as we need agriculture.

With the flow of improved technologies stemmed, the productivity growth of the previous decades stagnated (Fig.  5 ). Sure enough, in the last few years, we have seen clear signs that the world is consuming more rice than it is producing. A steady reduction in stocks is the clearest indicator (Fig.  6 ).

Trend in average world rice yield (1960 to 2007) and the key technological interventions associated with it. Changes in breeding objectives and release years of selected IRRI rice varieties are indicated in the bottom half. Major changes in management and emerging new management objectives are indicated above the yield trend line [ 4 ].

Rice stocks, 1990–2007. Source of raw data: PSD Online ( www.fas.usda.gov/psdonline/psdhome.aspx ), USDA, 2008.

If there is a positive to be gained from the price spike of early 2008, it is that agriculture—including public agricultural research—is back on the development agenda. Ahead of a United Nations summit held on 3–5 June 2008 to address the current food crisis, Food and Agriculture Organization (FAO) Director General Jacques Diouf talked of “re-launching” agriculture. A key policy document prepared for the Summit calls for support to agricultural research that serves the needs of poor farmers, noting that “high food prices represent an excellent opportunity for increased investments in agriculture by both the public and private sectors to stimulate production and productivity.”

The question, of course, is whether or not the words will turn into the dollars required to revitalize agricultural research and, ultimately, agriculture itself.

Simultaneous research revolutions

Nevertheless, it would be dishonest to paint a picture of utter doom and gloom. Despite the withdrawal of investment in research, the last 15 years have seen some impressive successes. This is true for our understanding of the rice plant and consequent genetic improvements, and it is also true for the less glamorous but equally important agronomic side of the equation: Improvements in crop management incorporating such practices as site-specific nutrient management and conservation agriculture have had demonstrable impact in farmers’ fields. Similarly, water-saving practices such as alternate wetting and drying, which allows farmers to grow rice with up to 25% less water, are becoming increasingly important as water becomes an ever scarcer agricultural resource [ 3 ].

Simultaneous revolutions in molecular biology and genetics, computational power and storage capacity, and communications have the potential to help scientists dramatically accelerate the pace of their research. Using the information revealed by the sequencing of the rice genome, techniques such as marker-assisted selection allow new varieties to be bred in a fraction of the time required as recently as 20 years ago. Advances in biotechnology are allowing the development of nutritionally enhanced strains of rice that have the potential to avert the hidden hunger of malnutrition that afflicts so many of the poor. The internet, along with exponentially increasing computing power, has permitted scientists the world over to share and analyze vast volumes of data and knowledge. Although it is difficult to know whether this has had significant impact in farmers’ fields, it has undoubtedly helped scientists in developed and developing countries improve their research capabilities.

More specifically, several key areas of research are bringing together scientists’ increasing knowledge of the biology of the rice plant with work to help farmers improve productivity.

First, researchers are developing, and must further develop, novel and robust approaches to use the wealth of genetic diversity of rice. IRRI is mobilizing the scientific community to establish a public genetic diversity research platform using a variety of germplasm and specialized genetic stocks.

Second, we must continue to develop methods to understand complex traits. By fully exploiting functional genomics tools, it will be possible to bridge the many existing genotype–phenotype gaps [ 2 ]. If we take progress in human genetics as a guide—in which scientists have been able to see how complex traits can be defined despite limited capacity to do controlled genetics—much more can be done in rice research in terms of discovering novel genetic control. In many ways, it is not technology that limits researchers, but the resources and investment needed to apply various toolboxes to rice.

Third, we must continue to develop the rice plant as both a crop and biological model for plant-science research and, in so doing, build a critical mass of knowledge directed to solving practical problems. We cannot expect every plant-science graduate to become an agricultural scientist, but having rice as a research model will enable us to tap into a vast pool of talented people and channel their energy and knowledge into solving some of the greatest agricultural challenges we face.

There are two key points in this paper. First, our capacity to perform research—to increase our understanding of the rice plant and the environments in which it is grown and to thus develop technologies that can help millions, if not billions, of people—is greater than ever. Second, given the world’s current food situation, the need for such research is equally great. The potential is enormous, but it will take commensurate will—political, economic, educational, and scientific—to approach that potential.

Another crucial element, especially for IRRI and its national partners throughout the rice-growing world, is the need to target research toward areas where it is most needed. To do that, we must have the best possible understanding of not only the rice plant, its physiology, and its agronomy, but also the big picture—knowledge of where and how it is grown and by whom, of how it is processed, transported, and marketed, and of how it is consumed and stored.

Rice, global food security, and poverty alleviation

Broadly speaking, rice is grown in more than a hundred countries, with a total harvested area of about 153 million hectares (1 ha = 2.5 acres), producing more than 600 million tons annually. About 90% of the rice in the world is grown in South Asia (58 million hectares), Southeast Asia (43 million hectares), and East Asia (31.5 million hectares). In Asia and sub-Saharan Africa (8 million hectares), almost all rice is grown on small farms of 0.5−3 ha. Yields range from less than 1 ton/ha under very poor rainfed conditions to 10 tons/ha in intensive temperate irrigated systems. Small, and in many areas shrinking, farm sizes account for the low incomes of rice farm families.

About 50% of the rice area is grown under intensive irrigated systems, which account for 75% of global rice production. These are systems in which the water supply is assured from either surface sources (rivers and dams) or wells and where controlled drainage is possible. Modern high-yielding varieties do very well under these conditions, and farmers typically apply fertilizer to obtain high and reliable yields. These systems were the home of the Green Revolution in rice, and global food security will continue to depend upon their continued ability to sustain high yields.

The other half of global rice area is rainfed, meaning that it depends exclusively on rainfall and, in some cases, unpredictable floods, for water. Rainfed rice can grow on steeply sloping lands, such as in the mountainous areas of Southeast Asia. But the largest areas are in the flat rainfed lowlands that predominate over much of the delta and coastal areas of South and Southeast Asia. These are level fields in which farmers construct bunds or levees to capture rainwater and maintain standing water in the field for as long as possible. Because rainfall can be so variable, rice in rainfed areas typically is prone to stresses such as drought and catastrophic flooding—sometimes in the same year. Farmers thus rarely apply fertilizer even when they grow improved varieties because these varieties are intolerant of the stresses. For resource-poor farmers, the risk of losing their investment is unacceptably high. Rainfed lowland rice predominates in those areas of greatest poverty (Fig.  7 ): South Asia, parts of Southeast Asia, and parts of Africa. Yields are very low (1–2 tons/ha), and farm families remain trapped in poverty. Even though these farmers are very poor, it is important to keep in mind that, for most, without rice, they would have no livelihood at all.

Poverty and rice distribution and irrigation by country, and subdivisions for China and India. The size of the pie diagram is scaled (not linear) to the total rice area in a country. There is a clear relationship between the prevalence of rainfed rice and the level of poverty. Source: R.Hijmans, IRRI.

To keep up with the demand and to rebuild rice stocks, the world needs to produce 8–10 million tons more each year than it did the previous year. In 10 years, for example, global production will need to be 80–100 million tons above today’s 600 million tons. This is a daunting challenge because, as much of Asia develops economically, urban expansion and industrial development are displacing some of the world’s best rice lands. And, in Asia at least, little suitable extra land is available for rice production. Cities and industry also demand water that previously entered irrigation schemes for rice production. Furthermore, in fast-developing countries such as India and China, animal feed is displacing rice as people add protein-rich meat and dairy foods to their diet. At the same time, in many countries where increasing wealth had allowed people to begin to diversify their diets, per-capita rice consumption has started to increase again because higher food prices mean that, once more, people cannot afford the more expensive alternatives. And, unchecked, the burgeoning biofuel industry threatens to displace food crops across the globe.

Therefore, tomorrow’s rice needs will have to be met from less favorable lands and using less water. Put in another way, much of the world’s extra rice, especially in Asia, will need to come through increased productivity—more per unit area—rather than through establishing new rice fields.

Research challenges ahead

We are rapidly approaching a time when more than half the world’s population is urban. It is therefore increasingly important to assure affordable food for the urban poor. Across much of the globe, this means maintaining inexpensive rice supplies, and these must come from the intensive irrigated systems. Boosting the productivity of intensive systems is thus one of the main challenges in rice that must be addressed to prevent global poverty from spiraling out of control. Specifically, in the intensive rice systems, we need research that leads to technologies that can achieve the following:

Exploit all options for raising the yield potential of rice. Increasing the yield potential of inbred rice cultivars has proven to be difficult but must be revisited. Hybrid rice improvement may allow for additional yield increases but will require a better understanding of heterosis in rice.

Close yield gaps, increase yield stability, and improve net returns through improved germplasm with multiple resistance to abiotic and biotic stresses and improved crop management. Irrigated rice fields can produce stable yields with highly efficient resource use [ 8 ], but significant options still exist for developing more stress-resistant varieties through new precision-breeding methods. Likewise, breeding for adaptation to specific rice-based cropping systems and management practices can lead to greater fine-tuning of system performance. Research must also be done on second-generation problems such as partial irrigation due to water shortages, salinity, soilborne diseases, micronutrient depletion, and pest/weed buildup related to emerging crop management and land-use patterns.

Add value by improving grain quality and/or the nutritional composition of rice through germplasm improvement and resource management.

Develop sustainable management technologies for diversifying rice systems. Researchers must exploit opportunities to increase the productivity of rice in seasons when rice is the best adapted crop (the rainy season) and provide management options for additional crops or the replacement of rice by crops such as maize or vegetables in other seasons. However, diversification may jeopardize the intrinsic sustainability of irrigated lowland rice systems and reduce the amount of rice available for domestic consumption.

Manage ecosystem services. We must develop the ability to value, monitor, and optimize a wide range of supporting, provisioning, regulating, and cultural ecosystem services.

Understand and adjust to global climate change. Substantial opportunities exist to adapt rice crops and systems to become less vulnerable to climatic extremes, take advantage of rising atmospheric CO 2 levels and, at the same time, emit fewer harmful greenhouse gases and remain profitable for farmers.

Improve delivery of technologies. We require a better understanding of the cultural, social, and economic factors that influence the adoption and adaptation of robust integrated technological advances for increased and ecologically sustainable rice production.

A second challenge is to significantly increase the productivity of rainfed lowland rice. Reliable and sustainable productivity increases will increase farm income directly. However, the indirect effects may be greater. Knowing that a crop will give a good yield—even if, for example, it is subjected to full submergence for 2 weeks or a moderately severe drought—will encourage farmers to apply inputs and obtain even higher yields. A reasonable assurance of enough rice to eat and a surplus to sell will provide farmers with the means to invest in diversification and obtain off-farm employment. Most important, perhaps, children will be able to attend school without interruptions caused by crop failures. Such interruptions often result in children being permanently withdrawn from school and condemning yet another generation to poverty [ 9 ].

Research has been less successful in producing technologies that will improve the productivity of rainfed systems. However, advances in genomics and molecular biology of rice, enabled by the sequencing of its genome [ 7 ]—the first of the crop species—and improved analytical approaches, have allowed rice scientists—breeders, geneticists, and physiologists—to make dramatic progress in developing rice lines that tolerate complete submergence, drought, and salinity. There is now an unprecedented opportunity to make strong contributions to the well-being of farmers and the landless in rainfed systems. The incorporation of major tolerance of complete submergence into varieties already grown on millions of hectares is a concrete proof that this opportunity can be translated into reality [ 10 ]. The following work also needs to be done:

Identify additional genes conferring tolerance of abiotic stresses. The deployment of cultivars carrying the submergence-tolerance gene shows that such traits can be transferred to widely grown varieties, as its strong effect is independent of genetic background. Genes conferring tolerance of drought and salinity stresses must be identified and evaluated for their expression in different backgrounds.

Transfer stress-tolerance genes into present and future mega-varieties (grown on more than 1 million hectares). The most suitable approach is by marker-assisted backcrossing, which can be completed in less than 3 years.

Develop new mega-varieties through modern precision-breeding methods. The currently grown mega-varieties are becoming susceptible to new pests, and there is also a need for varieties with higher grain quality that can be sold at a higher price to increase income.

Evaluate new breeding lines and improved crop management practices in farmer participatory trials and under different cropping systems. As farmers become more confident in the performance of their varieties under stress conditions, options for diversifying their systems should be explored.

New frontiers: climate change and a new rice engine

Attempting to overcome abiotic stresses for rainfed rice generates a “convenient convergence” in two key ways. First, rainfed rice farmers in Asia and Africa all face the same basic physical constraints to rice productivity. Thus, addressing problems that are important for millions of rice farmers in Asia will also address some of the critical needs of poor rice farmers in Africa. Second, it is increasingly likely that climate change will bring about more severe weather that will translate into droughts, floods, and sea-water intrusion. The development of drought-, heat-, submergence-, and salt-tolerant rice essentially translates into “climate-ready” rice. This will be important for both rainfed systems and intensive irrigated systems.

Taking a longer view, rice research also offers one of the most exciting examples of science in any field: the ambitious plan to reengineer rice photosynthesis to make it similar to that of the more efficient maize, sorghum, and sugar cane. The latter have a photosynthetic mechanism (termed C 4 ) that reduces losses of fixed carbon and therefore increases biological yield potential by some 30–50%. It also improves water- and nitrogen-use efficiencies. Success here would mean the scientific equivalent of moving from horse-drawn vehicles to the motor car, with “supercharged” rice having the clear distinction of being environmentally beneficial. Since C 4 photosynthesis has evolved independently several times within the grasses and since the metabolic components of the C 4 pathway already exist in rice, the research, still in its early stages, is generating much enthusiasm. It may take as long as 10–15 years of dedicated work by a global scientific team, but C 4 rice could boost yields and increase the efficiency of resource use more than any other advance since the first modern varieties of the Green Revolution.

Conclusions

Agricultural research in a development context does more than simply produce knowledge and technologies that can be used to improve productivity in developing countries. We know that countries that have had access to advances in agricultural technology have done better, economically, than those that have limited or no access. And we also know that regions that the Green Revolution failed to reach, such as sub-Saharan Africa, are now a long way behind those countries that experienced the Revolution not only in terms of existing technologies but also in terms of capability in and access to modern agricultural research techniques. Thus, the gap continues to widen between the countries that missed out and the countries that benefited 40 years ago [ 5 ].

Whether or not we realize the potential for science to help solve the previously intractable problems in rainfed systems and also to meet the challenges facing intensive systems depends upon how well acceptable products are developed and how well they can be marketed. The experience and lessons of the rice Green Revolution in Asia and Latin America are that farmers will take up new varieties and enabling technologies on a massive scale, and consumers will eat these varieties as long as the price is attractive and the quality acceptable.

Worryingly, however, when the crisis of the 1970s struck, investment in agricultural research had injected the R&D pipeline with the Green Revolution technologies that were able to help farmers turn things around quickly. The current crisis has hit at a time when investment has dropped off. There are technologies in the pipeline, but there could be many more. The new awareness will need to be translated into genuine support if good ideas are to become the technologies we need to keep the world’s most important grain plentiful and affordable for all.

It has taken a food crisis that is echoing that of the early 1970s, but after almost two decades, there is a growing realization once again that agricultural research in general and rice research in particular are worthy of major international support. If any rice scientists ever doubted that their career was worthwhile, they need not doubt any longer.

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Acknowledgments

Achim Dobermann, IRRI deputy director general for research, and Hei Leung, senior plant pathologist and leader of IRRI’s program on Rice genetic diversity and discovery: meeting the needs of future generations for rice genetic resources.

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Zeigler, R.S., Barclay, A. The Relevance of Rice. Rice 1 , 3–10 (2008). https://doi.org/10.1007/s12284-008-9001-z

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Research on the Application of Molecular Image Processing in Rice Quality Inspection

• PMID: 38581337

With the improvement of living standards, consumers are paying more and more attention to the quality of rice. Traditional rice quality detection relies on human sensory judgment, which is inaccurate and inefficient. With the continuous development of molecular imaging technology, more and more scholars at home and abroad have begun to pay attention to its application in the nondestructive testing of agricultural products. Molecular imaging technology combines the advantages of spectral technology and image technology, which can achieve rapid, nondestructive and accurate detection of rice quality. In this paper, taking rice as the research object, we carried out nondestructive detection research on rice varieties, moisture and starch content using molecular imaging technology. We proposed a rapid detection method based on molecular imaging technology for rice variety identification, moisture content and starch content. Molecular images of the rice samples from four origins were obtained using a molecular imaging system, the regions of interest of the rice were identified and, spectral data, textural features and morphological features of the rice were extracted. Spectral, textural and morphological features were selected by principal component analysis (PCA), and nine feature wavelengths were obtained and an optimal model was established with an accuracy of 91.67%, which demonstrated the feasibility of molecular imaging. By comparing the models, the BCC-LS-SVR model based on the RB function had the highest accuracy with R2 of 0.989, RMSEP of 0.767%, R2 of 0.985, and RMSEC of 0.591%. Moreover, starchy rice was detected using molecular imaging. The PCA-SVR model based on the RBF kernel function had the highest accuracy with R2 of 0.989, RMSEC of 0.445%, R2 of 0.991, and RMSEP of 0.669%. Our models demonstrated high accuracy in identifying rice varieties, as well as quantifying moisture and starch content, showcasing the feasibility of molecular imaging technology in rice quality assessment. This research offers a rapid, nondestructive, and accurate method for rice quality assessment, promising significant benefits for agricultural producers and consumers.

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Rice Husk Research: From Environmental Pollutant to a Promising Source of Organo-Mineral Raw Materials

Baimakhan satbaev.

1 RSE Astana Branch, National Center on Complex Processing of Mineral Raw Materials of the Republic of Kazakhstan, Nur-Sultan 010000, Kazakhstan; ur.liam@anatsa-cnf (B.S.); ur.kb@s_ilagrun (N.S.); ur.liam@veyabtas_nesra (A.S.)

Svetlana Yefremova

2 National Center on Complex Processing of Mineral Raw Materials of the Republic of Kazakhstan RSE, Almaty 050036, Kazakhstan; ur.liam@65nemraj (A.Z.); ur.liam@sa_vokebnalbak (A.K.); ur.liam@ter-vse (S.Y.)

Abdurassul Zharmenov

Askhat kablanbekov, sergey yermishin, nurgali shalabaev, arsen satbaev, vitaliy khen.

3 Scientific and Technical Society (STS), KAHAK, Almaty 050010, Kazakhstan; ur.tsil@nehkiylativ

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Rice husk is a large-tonnage waste left from rice production. It is not subject to humification and therefore becomes a serious environmental pollutant. Due to the presence of two essential elements—carbon and silicon—in its composition, rice husk is a promising organo-mineral raw material. The known methods for processing of rice husk are associated with the formation of even more aggressive waste. The creation of a waste-free technology for processing this plant material requires a detailed study. Rice husk of Kyzylorda oblast was studied using IR, SEM, TA, TPD-MS, EPR, and TEM methods. It was determined that under a temperature up to 500 °C, the ligno-carbohydrate component of rice husk decomposes almost completely. Three main peaks are recorded during the decomposition: hemicellulose at 200 °C, cellulose at 265 °C, and lignin at 350–360 °C. This process is endothermic. However, above of 300 °C the exothermic reactions associated with the formation of new substances and condensation processes in the solid residue begin to prevail. This explains the increase in the concentration of paramagnetic centers (PMCs) in products of rice husk carbonization in the range of up to 450 °C. Further increase in temperature leads to a decrease in the number of PMCs as a result of carbon graphite-like structures formation. The silicon–carbon product of rice husk carbonization (nanocomposite) is formed by interconnected nanoscale particles of carbon and silicon dioxide, the modification of which depends on the temperature of carbonization. The obtained data allow management of the rice husk utilization process while manufacturing products in demand based on ecofriendly technologies.

1. Introduction

Within the last few years, works on plant waste processing have multiplied all over the world, creating a wide range of products to be used in various areas of economic activity. Kazakhstan too is focusing on environmental protection and ecological development [ 1 , 2 ]. The “ Birge—taza Qazaqstan ” campaign is being carried out, aiming to foster environmental values in society and cultivate a caring attitude toward nature. The country’s leadership is keen to develop agribusiness, inevitably contributing to the growth of vegetable agricultural waste. This strongly calls for the implementation of the country’s Green Economy Concept [ 3 ]. Since Kazakhstan is a rice-growing country, it is now experiencing the problem of efficient processing of rice production waste—rice husk, which is globally one of the major environmental pollutants [ 4 ]. It is known that the performance characteristics of finished materials and the scope of their application largely depend on the technological parameters of processing of raw materials. Hence, as it was revealed by an earlier review of the literature [ 5 ], the studies of rice husk are mainly devoted to the research of conditions and operating parameters of processes of its refining. There are positive results of approbation of rice husk and products of its processing in different areas of practical use [ 5 , 6 ]. The two most common of them are the production of activated carbon [ 4 , 7 , 8 , 9 , 10 ] and the extraction of silicon dioxide as a source of silicon for the production of pure metal and its compounds, concrete, cement, and refractory ceramics [ 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ].

However, the large number of methods of processing rice husk, proposed by various researchers, does not eliminate the problem of its utilization. In Kazakhstan, industrial processing of this waste is not implemented, and rice husk continues to accumulate. Due to a number of environmental and economic reasons, other countries are holding back the organization of such industrial enterprises [ 5 ]. Firstly, the majority of the proposed methods for rice husk processing lead to the formation of new waste. These are either the acidic effluents generated during the cleaning of rice husk, toxic gases released during its incineration, or dumps of finely dispersed ash [ 21 ]. Secondly, they do not ensure the profitability of production [ 5 ]. It is obvious that a technology is needed that would ensure the maximum utilization of this agricultural waste. However, the creation of an effective technology requires a comprehensive study of not only the recycling process, but also of the raw materials themselves. As a rule, the research in the field of rice husk processing is traditionally limited to determining its composition.

Consequently, the present work is devoted to the study of Kyzylorda oblast rice husk structure by the methods of IR spectroscopy (IR) and scanning electron microscopy (SEM), determining the qualitative and quantitative composition of its organic and mineral components, the study of the decomposition of rice husk by the method of thermal analysis (TA) while determining the role of the constituent components in this process by the method of temperature-programmed desorption mass spectrometry (TPD-MS), the study of the paramagnetic centers formation in the course of rice husk carbonization and the carbonization temperature influence on it, and the study of the supramolecular structure of carbonized rice husk. The obtained data make it possible to provide insight into rice husk as an organo-mineral raw material containing two important elements—carbon and silicon. The optimum decomposition temperature for rice husk has been identified. It is shown that the destruction of rice husk occurs as a result of the disintegration of energetically weak links and removal of easily moving groups with an increase in concentration of paramagnetic centers. The temperature ranges for endo- and exothermic processes leading to the formation of graphite-like structures on the basis of carbon-containing components have been identified. The dynamics of the transformation of silicon dioxide from one form to another with an increase in the temperature of heat treatment of rice husk have been traced. Such data make it possible to effectively manage the processes of using rice husk to produce a silicon–carbon nanocomposite with a unique structure or as an independent ingredient for the production of in-demand products.

2. Materials and Methods

2.1. materials.

Rice husk from different rice-growing farms of Kyzylorda oblast was used as the object of study. The combined batch of rice husk was washed with cold distilled water at a ratio of 1:5 in order to remove the residual flour and dust. Washed rice husk was separated from the liquid by filtering through a mesh with cell size of 5 mm and left on the mesh until an air-dry condition was reached. Rice husk in an air-dry condition was put into the drying oven, where it was kept at a temperature of 105 °C until it reached a constant weight.

2.2. Methods of Analysis

2.2.1. infrared spectroscopy.

IR spectra were recorded on a Specord M80 spectrophotometer (Carl Zeiss, Jena, Germany) in the form of press tablets with KBr in the range of 4000–400 cm −1 . The IR spectrum presented is the average of three measurements. Mineralogical analysis was performed by comparing the obtained IR spectra with correlation diagrams of group frequencies, as well as with reference IR spectra of monominerals [ 22 , 23 ].

2.2.2. Scanning Electron Microscopy

Scanning electron microscopy and electron probe microanalysis were performed on a Superprobe 783 microanalyzer (JEOL, Tokyo, Japan). Analyses and photography of secondary and backscattered (composition) electrons were performed by using an INCA Energy Dispersive Spectrometer (Oxford Instruments, London, England). To avoid the formation of a charge on the analyzed materials, which are capable of deflecting the electron beam, the samples were precoated with a thin structureless gold film in a fine coat ion sputtering apparatus (JEOL, Tokyo, Japan). To clarify the distribution of individual elements, footage of characteristic X-ray radiation of corresponding elements was taken.

2.2.3. Determining Rice Husk Composition

Rice husk composition determination was performed as per the methods described in [ 24 ]: the cellulose content was identified by the method of Kurschner and Hoffer, using a nitric acid alcoholic solution. For comparison, the cellulose amount was identified by determining hardly hydrolyzable polysaccharides using 80 wt.% sulfuric acid. The hemicellulose content was identified by determining easy hydrolyzable polysaccharides using 2 wt.% hydrochloric acid. The quantitative identification of lignin was performed using 72 wt.% sulfuric acid as per Komarov’s modification. The total quantity of extractive substances was identified by treatment with an alcohol–benzol mixture in a Soxhlet apparatus, as well as with hot water. The content of mineral components in rice husk was identified based on silicate chemical analysis.

2.2.4. Thermal Analysis

Thermal analysis was performed on a Hungarian Paulik F.–Paulik J.–Erdey L. system derivatograph Q-1500D (MOM, Budapest, Hungary). The smooth heating of the sample was performed to a temperature of 1000 °C with a temperature increase rate of 12 °C min −1 in the atmosphere of the exhaust gas.

2.2.5. Temperature-Programmed Desorption Mass Spectrometry

TPD-MS was performed on a MX-7304A monopole mass spectrometer (Electron, Sumy, Ukraine) with electron ionization, which was re-equipped for thermal desorption measurements. A sample weighing 1–20 mg was placed on the bottom of a quartz-molybdenum ampoule and before the start of the experiment was pumped out at room temperature to a pressure of ~5 × 10 −5 Pa. The programmed linear heating of the sample was performed at a speed rate of 0.15 °C s −1 to a temperature of ~750 °C. The volatile products of thermolysis directly entered the ionization chamber of the mass spectrometer through a high-vacuum valve 5.4 mm in diameter, ionized and fragmented under the action of electrons. After the separation by masses in a mass analyzer, the intensity of the ion current of the products of desorption and thermolysis was recorded by a VEU-6 secondary electron multiplier (“Gran” Federal State Unitary Enterprise, Vladikavkaz, Russia). The registration and analysis of mass spectra and curves of dependence of the pressure of volatile destruction products on the temperature of the sample (P-T) were performed by the automated computer-based data recording and processing system. The registration of mass spectra was performed in the range of 1–210 amu. During the TPD-MS experiment, about 240 mass spectra were recorded. During the thermal desorption experiment, the samples were heated rather slowly and the rate of evacuation of volatile thermolysis products was high, allowing us to neglect the diffusion effects; therefore, the intensity of the ion current was proportional to the rate of desorption.

2.2.6. EPR Spectroscopy

EPR spectroscopy of the initial and carbonized rice husk in the range starting from 200 °C to 800 °C with a step of 50 °C was performed on an upgraded EPR IRES-1001-2M homodyne spectrometer (JEOL, Tokyo, Japan), which operates in the 3 cm wavelength range, at room temperature and optimal conditions for registration of spectra: a magnetic field of 120 Oe with a microwave power of 16 mW and modulation amplitude of 1 Oe. These conditions of spectra recording were chosen as optimal because the microwave power value eliminated the effects of EPR saturation and the magnetic modulation amplitude eliminated broadening of the EPR line. EPR spectra were recorded between 3 and 4 components of the reference sample, which were ions Mn 2+ in MgO. G-factor and an EPR line width of tested samples were determined using known reference sample parameters. The concentration of free radical states of tested samples was calculated by comparison of areas of their spectra and the calibrated reference sample (third line of EPR spectrum ions Mn 2+ in MgO).

2.2.7. Transmission Electron Microscopy (TEM)

Transmission electron microscopy studies were performed on different instrumental equipment:

• - On the EM-125K device (Sumy electronic devices plant, Sumy, Ukraine) by the method of direct observation of translucency by using the microdiffraction. The samples were prepared by the method of dry preparation, i.e., by the method of dry application of the agent to a collodion backing film and by the method of one-stage carbon replicas with extraction. During the microdiffraction studies, the photographing of diffraction patterns was performed.
• - On the Transmission Electron Microscope Philips EM 301 (Philips, Amsterdam, Netherlands) at an accelerating voltage of 80 kV in the range of electron microscopic magnifications of 13–80 thousand times. The images were recorded with an Olympus C-3040 digital camera, which was operated via computer using the Image Scope M software (Systems for microscopy and analysis (SMA), Moscow, Russia). The objects were prepared as follows. A small quantity of the sample was ground in an agate mortar. The resulting powder was applied to an object copper grid previously coated with an amorphous carbon backing film. The object grid with the applied sample was fixed into the microscope object holder and inserted into the microscope column [ 25 ].

2.2.8. Rice Husk Carbonization and Extraction of Silicon Dioxide

Rice husk carbonization for further research was performed in a shaft furnace SSHOL-8/11 (Tula-Term, Tula, Russia) in an atmosphere of exhaust steam gases. For this purpose, the reactor was filled with 200 g of rice husk, hermetically sealed with a cap that has a tube for exhaust gas removal and placed into the working area of the furnace. Heating to a specified temperature (in the range of 200 to 1000 °C with a step of 50 °C) was performed at a speed rate of 15 °C min −1 , keeping the sample at this temperature for 30 min.

In order to extract silicon dioxide, rice husk carbonized at 650 °C was heat-treated at 800 °C in the open air in order to burn off the carbon. The process was performed in a rice husk carbonization unit. During the process, the reactor was closed with cap, which, in addition to the gas outlet tube, had the air inlet tube that supplied the air inside of the reactor through the whole layer of carbonized rice husk. The process was conducted for 1 h. The yield of the resulting product of yellow-white color in terms of rice husk was equal to 14.8 wt.%.

The separation of carbon and silicon dioxide in carbonized rice husk was performed with the help of a sodium hydroxide solution with a concentration of 70 g dm −3 at a solid:liquid ratio (S/L) of 1:15. The content of silicon dioxide in the carbon residue was equal to 2–3 wt.%.

3.1. Infrared Spectroscopy Study of Rice Husk

As can be seen from the IR spectrum ( Figure 1 ), rice husk of Kyzylorda region has a complex functional composition. In general, the spectrum is characterized by band widening. This may be due to various reasons: irregular intra- and intermolecular interactions, overlapping absorptions of different types of vibrations, and absorption at frequencies slightly different from each other. As a result, the bands merge, and many of them do not have independent peaks but are recorded as a shoulder on more intense lines. All of this creates certain difficulties in the spectrum interpretation.

IR spectrum of rice husk.

In the IR spectrum of rice husk, a series of bands of different intensity are observed with maxima at 3410, 2975, 1640, 1460, 1425, 1375, 1320, 1160, 1065, 1035, and 895 cm −1 . They can be caused by vibrations of functional groups of cellulose, which is the main component of plant tissues [ 26 ]. Cellulose is a polysaccharide whose molecules (C 6 H 10 O 5 ) n are long chains with a spatially regular structure. These chains consist of β-D-glucose (β-D-glucopyranose) links connected by glucoside bonds 1–4 [ 24 ].

Bands at certain wavenumbers can be caused by vibrations of the functional groups of different compounds that make up rice husk. For example, the band at 1160 cm −1 can also belong to the vibrations of the C–O bonds of oxygen-containing groups of hemicelluloses. Hemicelluloses are found in the cell walls of plants and are composed of polysaccharides containing elementary units of five to six carbon atoms. Most of the hemicelluloses are not homogeneous polysaccharides, but mixed. Mixed polysaccharides are composed of various monosaccharide residues linked by glucoside bonds at various positions. Therefore, a broad band with a maximum at 3410 cm −1 can correspond to stretching vibrations of hydroxyl groups included in hydrogen bonds of hemicelluloses as well [ 27 ].

The bands with maxima at wavenumbers 1595 and 1512 cm −1 are typical for skeletal vibrations of aromatic rings. Their presence indicates the presence of lignin [ 28 ]. Lignin is a natural polymer built from the structural elements of C 6 C 3 oxygen derivatives of phenylpropane produced from carbohydrates [ 29 ].

According to paper [ 28 ], the band at 1640 cm −1 can also be caused by vibrations of carbonyl groups conjugated with condensed nuclei. The peak with a maximum at 1725 cm −1 is due to vibrations of conjugated aldehyde and ketone groups and non-conjugated carbonyl and carboxyl groups [ 30 ]. The opinions of researchers differ on the interpretation of the absorption band at 1275 cm −1 . Some believe [ 31 ] that this band is due to asymmetric stretching vibrations of C-O-Si bonds in the methoxyl groups of lignin. Others attribute it mainly to bending vibrations of methylene CH 2 groups [ 32 ].

The bands at 1085, 795, and 465 cm −1 are characteristic bands of the silica component [ 22 ]. No clear absorption bands corresponding to the presence of C-O-Si or Si-CH 3 bonds in rice husk were recorded. However, the intense band with a maximum at 1085 cm −1 has shoulders at 1070 cm −1 and 1040 cm −1 , as well as diffuse absorption bands at 1275 cm −1 and 1225 cm −1 , typical for the valence vibrations of C-O-Si or Si-CH 3 , respectively [ 33 ].

3.2. SEM Study of Rice Husk

Figure 2 a shows a longitudinal section of rice husk, characterizing its shape. Rice husk has a base formed by heterogeneous fibers. A wave-like “shell”, the inner filling of which is represented by a loose mass of plant tissue, is held at the base. Numerous cracks give visible friability to the specimen. Dispersed particles, possibly introduced as a result of mechanical destruction of rice husk, are observed in the deep cracks. Figure 2 shows the distribution of carbon ( Figure 2 b), silicon ( Figure 2 c), and oxygen ( Figure 2 d) in a longitudinal section ( Figure 2 a) of rice husk. The image was obtained in backscattered electrons (in the characteristic emission of C, Si, and O, respectively). The distribution of elements in the figure is characterized by the cluster of light dots. As shown in Figure 2 c, silicon is predominantly located in the outer surface layer and is also localized in some places of the plant tissue. Local accumulations on the inner layer are also explained by the presence of destroyed surface layer particles. The distribution of oxygen ( Figure 2 d) follows the shape of silicon and carbon. However, the image contrast in the case of carbon and oxygen is lower in comparison to silicon. This may be explained as follows. It is known that the higher the atomic number of the element and the greater the difference between the atomic numbers of the elements being compared, the higher the probability of backscattering and the higher the image contrast. The presence of silicon dioxide in rice husk composition distinguishes it from other plant materials.

SEM micrographs of the rice husk sample: ( a ) longitudinal section of rice husk; ( b ) distribution of carbon; ( c ) distribution of silicon; ( d ) distribution of oxygen.

After rice husk thermal treatment, a redistribution of chemical elements was registered by SEM ( Figure 3 ). As a result of the destruction of the rice husk organic component, fusion of the material occurs ( Figure 3 a). Deep channels are formed in the places of burnout of longitudinal fibers and along the contour of the “shell”. A peculiar carbon matrix is formed ( Figure 3 b). It is evenly filled with a silicon-containing phase ( Figure 3 c). As can be seen from the accumulation of white dots in Figure 3 d, the oxygen distribution corresponds to the silicon distribution form. Thus, the thermal destruction of rice husk leads to the formation of a silicon-carbon composite.

SEM micrographs of the rice husk carbonized at 650 °C sample: ( a ) longitudinal section of rice husk carbonized at 650 °C; ( b ) distribution of carbon; ( c ) distribution of silicon; ( d ) distribution of oxygen.

3.3. Rice Husk Composition

Due to the intricate structure of plant tissue, all of its constituent components are very closely related to each other. This makes it difficult to separate them. Thus, cellulose in plant cell walls is closely related to hemicelluloses, lignin, and extractive substances, while lignin is partially penetrating the cellulose microfibrils. Some hemicelluloses (cellulosans) form associates with cellulose and cannot be removed from plant tissue without noticeable damage to the cellulose itself. Therefore, the techniques described in Section 2.2.3 were used to perform the most accurate quantification of rice husk composition.

When treating rice waste with a mixture of concentrated nitric acid and ethyl alcohol, the lignin is nitrated and partially oxidized, and transferred to the alcohol solution. Hemicelluloses are hydrolyzed for the most part. Alcoholic medium moderates the oxidizing and hydrolyzing effect of nitric acid on cellulose. The cellulose content in the studied sample of rice husk, determined by this method, was 33 wt.%. This figure agrees well enough with the amount of cellulose (30 wt.%) established by the method of determination of hard-to-hydrolyze polysaccharides using 80 wt.% sulfuric acid. Hemicellulose content was determined according to the method of determination of easily hydrolyzable polysaccharides using 2 wt.% hydrochloric acid. Their quantity in the composition of rice husk was 18 wt.%. Since the composition of extractive substances is extremely diverse and quantitative isolation of individual components is rather complicated, the total amount of extractive substances soluble in the alcohol–benzene mixture (i.e., resins) and hot water was determined in the composition of rice husk, which was 2.0 wt.% and 6.1 wt.%, respectively. Unlike polysaccharides (cellulose and hemicelluloses), which are hydrolyzed to simple sugars, lignin is resistant to the action of mineral acids. Therefore, its content was determined using 72 wt.% sulfuric acid at room temperature on rice husk previously deresinated with an alcohol and benzene mixture. The reaction mixture was then diluted with water and boiled. The amount of lignin determined by this way was 26 wt.%.

By the silicate chemical analysis scheme, it was found that rice husk contained mineral components such as silicon dioxide (~14.0 wt.%), calcium oxides (~0.2 wt.%), magnesium (~0.1 wt.%), iron (~0.02 wt.%), aluminum (<0.1 wt.%), potassium (~0.4 wt.%), sodium (~0.03 wt.%), and other elements at impurity levels.

3.4. Investigation of Rice Husk Thermal Degradation Process by Thermal Analysis

Thermal analysis of rice husk in the region of 50–150 °C captures on the DTA curve an endo effect related to the loss of free water ( Figure 4 ). The decomposition of rice husk in the exhaust gas atmosphere ends at 770 °C, as can be seen on the TG curve. The DTG curve shows that the bulk of rice husk decomposed with the highest rate at 238 and 265 °C. This process runs with heat absorption, since two blurred endo effects are fixed on the DTA curve at these temperatures. The exothermic effect recorded on the DTA curve at 300 °C is caused by the formation of new substances and condensation processes in the solid residue. It is alternately replaced by endo (450, 550 °C) and exo (500, 600 °C) effects caused by structuring and burnout of carbonaceous residue. Rice husk mass loss reached 79.5%. The excess mass of the residue (20.5wt.%) over the mineral component in the composition of rice husk (~15wt.%) is explained by the presence of carbon formed during condensation–destruction processes and subsequent graphitization (since TA was performed in the atmosphere of the exhaust gas) and is probably firmly bound to silicon dioxide.

Rice husk derivatogram.

3.5. Study of Rice Husk Pyrolysis by TPD-MS

Analysis of correlation curves between the pressure of volatile degradation products of plant materials and heating temperature (P/T) showed ( Figure 5 ) that rice husk decomposes in the temperature range of 150–500 °C, with the maximum decomposition being fixed at T max ~265 °C. The data obtained are in good agreement with the TA results. Comparison of Figure 5 and Figure 6 indicates that the release of the bulk of rice husk pyrolysis products at T max ~265 °C is caused by cellulose degradation due to the desorption of pyran and furan derivatives ( m / z 98). The formation of pyran derivatives is caused by the dehydration of elementary links in the pyranose form; the formation of carboxylic acids followed by decarboxylation promotes the formation of furan derivatives; at the same time, the appearance of aromatic structures at this temperature indicates the sequential course of intermolecular and intramolecular aldol condensation reactions [ 34 ]. The pyrolysis stage with T max ~350 °C occurs due to the degradation of aromatic compounds of lignin, which proceeds in a wider temperature range with the formation of phenol ( m / z = 94, T max = 350 °C), pyrocatechol ( m / z = 110, T max = 290–330 °C), cresols ( m / z = 107, T max = 365 °C), tropylium ion, C 7 H 7 + ( m / z = 91, T max = 340 °C), 4-vinyl-methylguaiacol ( m / z = 164, T max = 290 °C), benzene ( m / z = 78, T max = 360, 545 °C), naphthalene ( m / z = 128, T max = 520 °C), and 4-vinylphenol ( m / z = 120, T max = 220, 290 °C). The formation of these compounds is caused by thermal transformations of corresponding aromatic links and functional groups of lignin [ 35 ]. The stage with T max ~200 °C corresponds to the destruction of hemicelluloses [ 36 ]. However, due to their close relationship with other plant matter constituents, the above compounds predominate in the pyrolysis products at 150–250 °C ( Figure 6 b). At the same time, the ion with m / z = 60 (HOCHCHOH + ) observed in Figure 6 a is known to be the most intense marker ion in the mass spectra of carbohydrate pyrolysis products and is usually detected already at 150 °C [ 37 , 38 ].

Temperature–pressure (P-T) curves of rice husk.

TPD-MS results: ( a ) mass spectrum of pyrolysis products of rice husk at 347 °C obtained after electron impact ionization; ( b ) TPD-curves of the ions with m / z 164, 128, 120, 110, 107, 98, 94, 91, and 78 under pyrolysis of rice husk.

3.6. Investigation of Rice Husk Structural Changes during Carbonization by EPR Spectroscopy

When studying the structural changes of rice husk during heat treatment up to 800 °C using EPR spectroscopy, it was found that the material already exhibits an EPR signal at room temperature. This signal can be caused by the formation of free radicals under mechanical impact during rice husk grinding. The parameters of the EPR spectra are presented in Table 1 . For illustrative purposes, the dependence of paramagnetic centers (PMCs) content on the processing temperature of rice husk is shown in Figure 7 . The concentration of paramagnetic centers reaches a maximum value (1000 × 10 16 spin g −1 ) at 450 °C. There is a general tendency of reducing the width of the EPR line while increasing temperature of heat treatment, as it follows from Table 1 . However, despite general narrowing of the line, it broadens at certain temperatures (400 and 450 °C). Obviously, at these temperatures two kinds of paramagnetic centers are formed (free radicals and clusters as a result of closure of the former) with similar values of g-factor, the superposition of their signals leads to the broadening.

Dependence of the concentration of paramagnetic centers on the carbonization temperature of rice husk.

Parameters of EPR spectra of original and carbonized rice husk.

Comparing the pattern of PMCs concentration changes as the plant raw material processing temperature increases with the results of TA, IR spectroscopy, and TDS-MS, it can be concluded that the growth of PMCs amount when heating rice husk to 450 °C is caused by an increase in the concentration of free-radical states (FRS) as a result of splitting of energy-weak bonds and removal of easily mobile groups. Carbonization of plant material is inevitably accompanied by the formation of condensed carbon rings that form a graphite-like structure. This leads to a decrease in the value of the PMCs index. Consequently, on the ascending part of the paramagnetic centers concentration dependence on the treatment temperature ( Figure 7 ), the ΔH decrease ( Table 1 ) is explained by intensification of exchange interactions in the FRS spin system as their concentration increases, while on the descending part—by a decrease in the dipole–dipole interaction and appearance of delocalized π electrons in graphite-like structures. However, treatment of rice husk carbonized at 650 °C with a sodium hydroxide solution causes PMCs concentration to increase to 1.1 × 10 19 spin g −1 . This is obviously associated with the removal of silicon dioxide from carbonizate due to the breaking of C-SiO 2 bonds and formation of a large amount of FRS. In the studied temperature range of rice husk heat treatment, gradual decrease of g-factor values also occurs, which approach the free electron g-factor value (g = 2.0023) in graphite structures. Thus, we can conclude that rice husk structural transformations during heat treatment undergo a free radical formation stage followed by the formation of hexagonal meshes of cyclically polymerized carbon.

3.7. Characteristics of Carbonized Rice Husk Supramolecular Structure

The presence of particles of different morphology and sizes was recorded in rice husk carbonized at different temperatures (600, 650, and 1000 °C) using transmission electron microscopy ( Figure 8 ). Mainly, there are lamellar translucent and dense particles ( Figure 8 a); aggregates formed by translucent particles of round or oval shape of 15–20 nm and larger ( Figure 8 b). However, more attention should be given to hybrid structures, which are a combination of two phases ( Figure 8 c,d): a lamellar formation of one phase permeated by another denser dispersed (10–20 nm) phase. Moreover, the hybrid particles can be porous (pore size 15–20 nm, Figure 8 c). Particles whose microdiffraction pattern ( Figure 8 e,f) is represented by rings (0.337; 0.210; 0.122 nm), symmetric reflexes (0.46; 0.406; 0.337; 0.251; 0.245; 0.236 nm) and broad rings with a set of interplanar distances of 0.253; 0.212; 0.152; 0.121 nm were determined in the rice husk carbonizate by microdiffraction studies along with amorphous particles. According to paper [ 39 ], this indicates the presence of the following carbon and silica-containing phases: C (26–1080), H 2 Si 14 O 29 ⋅5.4H 2 O (31–584), SiC/Unnamed mineral, syn. (29–1129).

TEM micrographs and microdiffraction patterns of carbonized rise husk: ( a ) rice husk particles carbonized at 650 °C; ( b ) rice husk particles aggregates carbonized at 1000 °C; ( c ) porous hybrid formation obtained by rice husk carbonization at 600 °C; ( d ) cluster of differently shaped particles of rice husk carbonized at 650 °C; ( e ) microdiffraction patterns from the particles shown in Figure 8 a; ( f ) microdiffraction patterns from the particles shown in Figure 8 d.

To study the structure of silica, carbonized rice husk was heat-treated at 800 °C in open air to burn out the carbon component. Figure 9 shows clusters of dense particles of 20 nm and larger ( Figure 9 a) and translucent rounded particles of 20 to 100 nm in diameter ( Figure 9 c). The microdiffraction pattern from dense particles is represented by reflexes with a set of interplanar distances of 0.432; 0.375; 0.286; 0.261; 0.231; 0.207; 0.154; 0.15 nm ( Figure 9 b), corresponding to a mixture of SiO 2 (12–708), SiO 2 (18–1169), and H 2 Si 2 O 5 (27–606) [ 39 ]. The appearance of a 0.428 nm diffuse ring in the microdiffraction pattern from the translucent particles ( Figure 9 d) indicates the nucleation of the SiO 2 /Tridymite-20H, syn. (14–260).

TEM micrographs and microdiffraction patterns of rice husk silica: ( a ) cluster of dense particles; ( b ) microdiffraction pattern from dense particles; ( c ) cluster of translucent particles; ( d ) microdiffraction pattern from translucent particles.

4. Discussion

Analysis of rice husk and products of its heat treatment showed the following. Rice husk is an organo-mineral material. As a large-tonnage waste of rice production, it is a large-scale environmental pollutant, since due to the presence of silicon dioxide it is not subject to the humification process. At the same time, the combined presence of carbon and silicon dioxide in its composition opens up broad prospects for its use as a valuable raw material. The value of rice husk lies in ensuring the formation of a unique silica–carbon structure of the materials produced from it.

The state of carbon and silicon dioxide in the resulting products depends on the processing conditions of plant raw materials. It is possible to get carbon with an amorphous or graphite-like structure. The situation is similar to silicon dioxide. Starting as amorphous, various crystalline forms of silicon-containing products can be obtained: cristobalite, tridymite, and even silicon carbide as a result of the interaction of silicon dioxide with the resulting carbon. Carbonized rice husk is made up of different-shaped particles. The hybrid structures formed by particles of carbon and silicon dioxide have the greatest interest among them.

Due to the fact that silicon dioxide is connected strongly with organic components in rice husk after the thermal destruction of plant tissue, it does not form an isolated structure but rather has a strong interaction with carbon (up to chemical bond), and as a result it builds a nanocomposite ensemble. Obviously, being formed inside the plant cell in the form of soluble silicic acid (which the presence of was registered by a TEM microdiffraction technique even in carbonized rice husk), silica diffuses through the membranes of plant tissue to its outer surface, causing silicification of the cellulose scaffold, which was confirmed by the results of SEM study of the elements distribution in the rice husk structure. Some of it remains in the inner layer. In both cases, the formation of bonds between silica tetrahedra, carbohydrates, and lignin is possible. According to TA and EPR spectroscopy, the temperature of 650 °C appears to be the necessary and sufficient temperature for the carbonization process of rice husk, although according to TPD-MS data, rice husk decomposes in the range up to 500 °C in three stages corresponding to the decomposition of the main components (hemicellulose, cellulose, lignin). At 650 °C, the destruction of the initial raw materials is completed, as shown by TEM study of the supramolecular structure with the formation of a silicon–carbon nanocomposite in which some of the C-SiO 2 bonds remain intact. This was confirmed by the results of different techniques used. Firstly, as it was found during TA, the residue mass after rice husk decomposition in the exhaust steam gas atmosphere (under analogous conditions carbonization of the feedstock was also performed for the study by other methods) exceeded the mass of the mineral component in rice husk due to the formation of cyclically polymerized carbon and probably SiC, as evidenced by the EPR and TEM results. A subsequent attempt to obtain carbon freed from silicon dioxide by the chemical way was unsuccessful. The residual content of SiO 2 in carbonized rice husk treated with an alkaline solution was 2–3 wt.%. Moreover, after this treatment of the obtained product, according to EPR spectroscopy data the number of free radicals increased significantly as a result of the destruction of the C-SiO 2 bonds present in the silicon–carbon composite. Thus, the results of studies carried out by different methods within the framework of the present work are in good agreement and complement each other.

Carbonized rice husk with a unique structure has a wide range of applications: as an active filler for elastomers [ 40 ], a reducing agent in electrothermal metallurgical processes [ 6 ], a sorbent [ 41 ], and a feed additive [ 6 ]. However, in a number of processes, it seems economically more profitable to use rice husk as raw material with the formation of the necessary compounds directly during the process of obtaining the final product. Examples of such processes are the production of new generation refractory materials by the method of self-propagating high-temperature synthesis (SHS) [ 42 , 43 , 44 , 45 ] and the production of plate material in the process of vapor-explosive hydrolysis without the use of any types of synthetic plastics [ 46 ].

The modern concept for the development of the refractory industry consists of the transition to the production of resource-saving refractories of a new generation, distinguished by increased environmental safety and wear resistance, as well as ensuring an increase in the quality of the final product. The feasibility of creating a new generation of refractories is due to the increasing requirements of consumers, as well as the need to improve the operating conditions of refractories and reduce energy costs in their manufacturing. For the development of refractory production, the Republic of Kazakhstan has sufficient raw materials. There are significant reserves of refractory clays, quartzite, chrome ores, small deposits of magnesite, zircon, talc-magnesite, bauxite, man-made raw materials represented by waste from the mining and metallurgical industry [ 47 , 48 ], as well as mineral raw materials for the production of composite materials and ceramics based on corundum, zircon, andalusite, and barite.

Carborundum (silicon carbide) is often used to increase the strength of refractory materials. As part of the present work, we recorded silicon carbide formation during the carbonization process of rice husk. This process was studied and described in detail in papers [ 21 , 49 ]. Therefore, it seems reasonable to introduce rice husk into the charge to produce refractories. When a specially prepared charge is heated to 950 °C, the process of self-propagating high-temperature synthesis will take place. Under the conditions of this SHS process, the organic component of rice husk will be carbonized to form nanoscale carbon. As a result of carbon interaction with silicon dioxide present in rice husk in its active form, silicon carbide will be formed. On the one hand, this will reduce or completely eliminate the consumption of silicon-containing ingredients to be introduced into the charge. On the other hand, enrichment of reaction products with highly fire-resistant (low porosity and high density) compounds will contribute to the formation of durable and dense refractory, increasing thermal durability of refractory lining to be used in chemically aggressive environments of ferrous and nonferrous metallurgy, energy, chemical industry, and construction materials production.

At present, the issues of using plant fibers from agricultural waste in the manufacturing of building boards are quite pressing. There are a number of problems that need to be solved at the same time [ 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 ]:

• - Release of volatile organic substances, aldehydes, and terpenes, a small amount of which causes adverse effects on health.
• - Unsatisfactory physical and mechanical properties of the resulting materials, providing mainly as use for decoration and furniture production, but limiting their use in the construction industry.
• - Use of a large number of highly toxic and inflammable artificial organic polymers (formaldehyde, epoxy, and other resins such as binders, various types of hardeners, plasticizers, and adhesives) since almost all technologies are based on pressing plant biomass. Strengthening the mechanical properties usually requires increasing the amount of resin binder. Increasing fire resistance and improving other properties such as sound absorption, impact resistance, and thermal conductivity is associated with a more complex composition of the blend, i.e., increasing the number of ingredients and different methods of processing can increase in the cost of the finished material.

In this regard, there is independent interest to try and use rice husk for the production of building boards by the method of steam explosive hydrolysis, which determines the effective decomposition of organic raw materials [ 46 ]. The method is based on implementing hydrothermal degradation of polysaccharides by alternating the stages of pressing with charge heating and decompression to form a silicon–lignin solid residue. Plasticizing properties of lignin will allow eliminating the use of synthetic binders. The presence of silicon dioxide in the board composition will contribute to increasing its strength. This direction of research is attractive because it opens the prospect of developing the production of efficient construction boards on the basis of ecofriendly technology using renewable raw materials.

5. Conclusions

Rice husk of Kyzylorda region has a complex functional composition. The main components are polysaccharides (48–52 wt.%) and lignin (26 wt.%). A distinctive feature of rice husk is its high ash content due to the presence of silicon dioxide (14 wt.%). Silicon dioxide is predominantly evenly located in the outer surface layer of rice husk and in the form of local accumulations in its internal surface layer.

Thermal destruction of rice husk occurs at up to 500 °C in three stages. Hemicelluloses decompose at 200 °C. The maximum decomposition at 265 °C is caused by the destruction of cellulose. In the range of 350–360 °C, the destruction of lignin takes place. The decomposition process of rice husk is endothermic. Above 300 °C, exothermic reactions predominate due to the formation of new substances and condensation processes in the solid residue.

In the process of rice husk carbonization at 450 °C, the concentration of paramagnetic centers increases due to the splitting of energetically weak bonds and the removal of easily mobile groups. A further increase in temperature to 800 °C is accompanied by a decrease in the number of PMCs as a result of the formation of graphite-like structures since the g-factor value approaches free electron g-factor value (g = 2.0023) in graphite structures. The preferred carbonization temperature is 650 °C.

Carbonized rice husk is a silicon–carbon nanocomposite formed by nanosized particles of carbon and silicon dioxide with the presence of silicon carbide. Thanks to its unique structure, the silicon–carbon nanocomposite has a wide range of applications. The studies performed and the results obtained make it possible in the future to test rice husk as an independent charge ingredient in the preparation of refractories by the SHS method and building plates by the vapor-explosive hydrolysis method.

Acknowledgments

The authors would like to thank N.T. Cartel, Director of the Chuiko Institute of Surface Chemistry, National Academy of Sciences of Ukraine, academician of the National Academy of Sciences of Ukraine; T. Kulyk, Head of the Laboratory of the Kinetics and Mechanisms of Chemical Transformations on Solid Surfaces of the Chuiko Institute of Surface Chemistry, National Academy of Sciences of Ukraine, PhD in Chemistry, and Senior Researcher for performing of the TPD-MS; V. Levin, Head of the Mineralogy Laboratory of the K.I. Satpayev Institute of Geological Sciences and Candidate of Geological and Mineralogical Sciences for performing of the scanning electron microscopy; L. Komashko, microscopist of the D.V. Sokolskiy Institute of Fuel, Catalisys and Electrochemistry JSC, V. Matveev and F. Pisarev, microscopists of the Frumkin Institute of Physical chemistry and Electrochemistry Russian academy of sciences for performing of the transmission electron microscopy; Y.A. Ryabikin, Leading Researcher of the Institute of Physics and Technology and Candidate of Physical and Mathematical Sciences for performing of the EPR spectroscopy.

Author Contributions

Conceptualization, S.Y. (Svetlana Yefremova) and B.S.; methodology, S.Y. (Svetlana Yefremova); investigation, S.Y. (Svetlana Yefremova), B.S., A.K., S.Y. (Sergey Yermishin), N.S., A.S., and V.K.; data curation, S.Y. (Svetlana Yefremova); writing—original draft preparation, S.Y. (Svetlana Yefremova); writing—review and editing, A.Z.; supervision, A.Z. and B.S.; project administration, S.Y. (Svetlana Yefremova) and B.S.; funding acquisition, B.S. All authors have read and agreed to the published version of the manuscript.

This research was funded by the Ministry of Education and Science of the Republic of Kazakhstan over the years, including grant numbers 1123/GF3 and AP05132122.

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