presentation on transport of water in plants

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30.5: Transport of Water and Solutes in Plants

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• Page ID 1986

Skills to Develop

• Define water potential and explain how it is influenced by solutes, pressure, gravity, and the matric potential
• Describe how water potential, evapotranspiration, and stomatal regulation influence how water is transported in plants
• Explain how photosynthates are transported in plants

The structure of plant roots, stems, and leaves facilitates the transport of water, nutrients, and photosynthates throughout the plant. The phloem and xylem are the main tissues responsible for this movement. Water potential, evapotranspiration, and stomatal regulation influence how water and nutrients are transported in plants. To understand how these processes work, we must first understand the energetics of water potential.

Water Potential

Plants are phenomenal hydraulic engineers. Using only the basic laws of physics and the simple manipulation of potential energy, plants can move water to the top of a 116-meter-tall tree (Figure \(\PageIndex{1}\)a). Plants can also use hydraulics to generate enough force to split rocks and buckle sidewalks (Figure \(\PageIndex{1}\)b). Plants achieve this because of water potential.

Water potential is a measure of the potential energy in water. Plant physiologists are not interested in the energy in any one particular aqueous system, but are very interested in water movement between two systems. In practical terms, therefore, water potential is the difference in potential energy between a given water sample and pure water (at atmospheric pressure and ambient temperature). Water potential is denoted by the Greek letter ψ ( psi ) and is expressed in units of pressure (pressure is a form of energy) called megapascals (MPa). The potential of pure water (Ψ w pure H2O ) is, by convenience of definition, designated a value of zero (even though pure water contains plenty of potential energy, that energy is ignored). Water potential values for the water in a plant root, stem, or leaf are therefore expressed relative to Ψ w pure H2O .

The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and factors called matrix effects. Water potential can be broken down into its individual components using the following equation:

where Ψ s , Ψ p , Ψ g , and Ψ m refer to the solute, pressure, gravity, and matric potentials, respectively. “System” can refer to the water potential of the soil water (Ψ soil ), root water (Ψ root ), stem water (Ψ stem ), leaf water (Ψ leaf ) or the water in the atmosphere (Ψ atmosphere ): whichever aqueous system is under consideration. As the individual components change, they raise or lower the total water potential of a system. When this happens, water moves to equilibrate, moving from the system or compartment with a higher water potential to the system or compartment with a lower water potential. This brings the difference in water potential between the two systems (ΔΨ) back to zero (ΔΨ = 0). Therefore, for water to move through the plant from the soil to the air (a process called transpiration), Ψ soil must be > Ψ root > Ψ stem > Ψ leaf > Ψ atmosphere .

Water only moves in response to ΔΨ, not in response to the individual components. However, because the individual components influence the total Ψ system , by manipulating the individual components (especially Ψ s ), a plant can control water movement.

Solute Potential

Solute potential (Ψ s ), also called osmotic potential, is negative in a plant cell and zero in distilled water. Typical values for cell cytoplasm are –0.5 to –1.0 MPa. Solutes reduce water potential (resulting in a negative Ψ w ) by consuming some of the potential energy available in the water. Solute molecules can dissolve in water because water molecules can bind to them via hydrogen bonds; a hydrophobic molecule like oil, which cannot bind to water, cannot go into solution. The energy in the hydrogen bonds between solute molecules and water is no longer available to do work in the system because it is tied up in the bond. In other words, the amount of available potential energy is reduced when solutes are added to an aqueous system. Thus, Ψ s decreases with increasing solute concentration. Because Ψ s is one of the four components of Ψ system or Ψ total , a decrease in Ψ s will cause a decrease in Ψ total . The internal water potential of a plant cell is more negative than pure water because of the cytoplasm’s high solute content (Figure \(\PageIndex{2}\)). Because of this difference in water potential water will move from the soil into a plant’s root cells via the process of osmosis. This is why solute potential is sometimes called osmotic potential.

Plant cells can metabolically manipulate Ψ s (and by extension, Ψ total ) by adding or removing solute molecules. Therefore, plants have control over Ψ total via their ability to exert metabolic control over Ψ s .

Art Connection

Positive water potential is placed on the left side of the tube by increasing Ψ p such that the water level rises on the right side. Could you equalize the water level on each side of the tube by adding solute, and if so, how?

Pressure Potential

Pressure potential (Ψ p ), also called turgor potential, may be positive or negative (Figure \(\PageIndex{3}\)). Because pressure is an expression of energy, the higher the pressure, the more potential energy in a system, and vice versa. Therefore, a positive Ψp (compression) increases Ψ total , and a negative Ψ p (tension) decreases Ψ total . Positive pressure inside cells is contained by the cell wall, producing turgor pressure. Pressure potentials are typically around 0.6–0.8 MPa, but can reach as high as 1.5 MPa in a well-watered plant. A Ψ p of 1.5 MPa equates to 210 pounds per square inch (1.5 MPa x 140 lb in -2 MPa -1 = 210 lb/in -2 ). As a comparison, most automobile tires are kept at a pressure of 30–34 psi. An example of the effect of turgor pressure is the wilting of leaves and their restoration after the plant has been watered (Figure \(\PageIndex{3}\)). Water is lost from the leaves via transpiration (approaching Ψ p = 0 MPa at the wilting point) and restored by uptake via the roots.

A plant can manipulate Ψ p via its ability to manipulate Ψ s and by the process of osmosis. If a plant cell increases the cytoplasmic solute concentration, Ψ s will decline, Ψ total will decline, the ΔΨ between the cell and the surrounding tissue will decline, water will move into the cell by osmosis, and Ψ p will increase. Ψ p is also under indirect plant control via the opening and closing of stomata. Stomatal openings allow water to evaporate from the leaf, reducing Ψ p and Ψ total of the leaf and increasing ii between the water in the leaf and the petiole, thereby allowing water to flow from the petiole into the leaf.

Gravity Potential

Gravity potential (Ψ g ) is always negative to zero in a plant with no height. It always removes or consumes potential energy from the system. The force of gravity pulls water downwards to the soil, reducing the total amount of potential energy in the water in the plant (Ψ total ). The taller the plant, the taller the water column, and the more influential Ψ g becomes. On a cellular scale and in short plants, this effect is negligible and easily ignored. However, over the height of a tall tree like a giant coastal redwood, the gravitational pull of –0.1 MPa m -1 is equivalent to an extra 1 MPa of resistance that must be overcome for water to reach the leaves of the tallest trees. Plants are unable to manipulate Ψ g .

Matric Potential

Matric potential (Ψ m ) is always negative to zero. In a dry system, it can be as low as –2 MPa in a dry seed, and it is zero in a water-saturated system. The binding of water to a matrix always removes or consumes potential energy from the system. Ψ m is similar to solute potential because it involves tying up the energy in an aqueous system by forming hydrogen bonds between the water and some other component. However, in solute potential, the other components are soluble, hydrophilic solute molecules, whereas in Ψ m , the other components are insoluble, hydrophilic molecules of the plant cell wall. Every plant cell has a cellulosic cell wall and the cellulose in the cell walls is hydrophilic, producing a matrix for adhesion of water: hence the name matric potential. Ψ m is very large (negative) in dry tissues such as seeds or drought-affected soils. However, it quickly goes to zero as the seed takes up water or the soil hydrates. Ψ m cannot be manipulated by the plant and is typically ignored in well-watered roots, stems, and leaves.

Movement of Water and Minerals in the Xylem

Solutes, pressure, gravity, and matric potential are all important for the transport of water in plants. Water moves from an area of higher total water potential (higher Gibbs free energy) to an area of lower total water potential. Gibbs free energy is the energy associated with a chemical reaction that can be used to do work. This is expressed as ΔΨ.

Transpiration is the loss of water from the plant through evaporation at the leaf surface. It is the main driver of water movement in the xylem. Transpiration is caused by the evaporation of water at the leaf–atmosphere interface; it creates negative pressure (tension) equivalent to –2 MPa at the leaf surface. This value varies greatly depending on the vapor pressure deficit, which can be negligible at high relative humidity (RH) and substantial at low RH. Water from the roots is pulled up by this tension. At night, when stomata shut and transpiration stops, the water is held in the stem and leaf by the adhesion of water to the cell walls of the xylem vessels and tracheids, and the cohesion of water molecules to each other. This is called the cohesion–tension theory of sap ascent.

Inside the leaf at the cellular level, water on the surface of mesophyll cells saturates the cellulose microfibrils of the primary cell wall. The leaf contains many large intercellular air spaces for the exchange of oxygen for carbon dioxide, which is required for photosynthesis. The wet cell wall is exposed to this leaf internal air space, and the water on the surface of the cells evaporates into the air spaces, decreasing the thin film on the surface of the mesophyll cells. This decrease creates a greater tension on the water in the mesophyll cells (Figure \(\PageIndex{4}\)), thereby increasing the pull on the water in the xylem vessels. The xylem vessels and tracheids are structurally adapted to cope with large changes in pressure. Rings in the vessels maintain their tubular shape, much like the rings on a vacuum cleaner hose keep the hose open while it is under pressure. Small perforations between vessel elements reduce the number and size of gas bubbles that can form via a process called cavitation. The formation of gas bubbles in xylem interrupts the continuous stream of water from the base to the top of the plant, causing a break termed an embolism in the flow of xylem sap. The taller the tree, the greater the tension forces needed to pull water, and the more cavitation events. In larger trees, the resulting embolisms can plug xylem vessels, making them non-functional.

Which of the following statements is false?

• Negative water potential draws water into the root hairs. Cohesion and adhesion draw water up the xylem. Transpiration draws water from the leaf.
• Negative water potential draws water into the root hairs. Cohesion and adhesion draw water up the phloem. Transpiration draws water from the leaf.
• Water potential decreases from the roots to the top of the plant.
• Water enters the plants through root hairs and exits through stoma.

Transpiration —the loss of water vapor to the atmosphere through stomata—is a passive process, meaning that metabolic energy in the form of ATP is not required for water movement. The energy driving transpiration is the difference in energy between the water in the soil and the water in the atmosphere. However, transpiration is tightly controlled.

Control of Transpiration

The atmosphere to which the leaf is exposed drives transpiration, but also causes massive water loss from the plant. Up to 90 percent of the water taken up by roots may be lost through transpiration.

Leaves are covered by a waxy cuticle on the outer surface that prevents the loss of water. Regulation of transpiration, therefore, is achieved primarily through the opening and closing of stomata on the leaf surface. Stomata are surrounded by two specialized cells called guard cells, which open and close in response to environmental cues such as light intensity and quality, leaf water status, and carbon dioxide concentrations. Stomata must open to allow air containing carbon dioxide and oxygen to diffuse into the leaf for photosynthesis and respiration. When stomata are open, however, water vapor is lost to the external environment, increasing the rate of transpiration. Therefore, plants must maintain a balance between efficient photosynthesis and water loss.

Plants have evolved over time to adapt to their local environment and reduce transpiration(Figure \(\PageIndex{5}\)). Desert plant (xerophytes) and plants that grow on other plants (epiphytes) have limited access to water. Such plants usually have a much thicker waxy cuticle than those growing in more moderate, well-watered environments (mesophytes). Aquatic plants (hydrophytes) also have their own set of anatomical and morphological leaf adaptations.

Xerophytes and epiphytes often have a thick covering of trichomes or of stomata that are sunken below the leaf’s surface. Trichomes are specialized hair-like epidermal cells that secrete oils and substances. These adaptations impede air flow across the stomatal pore and reduce transpiration. Multiple epidermal layers are also commonly found in these types of plants.

Transportation of Photosynthates in the Phloem

Plants need an energy source to grow. In seeds and bulbs, food is stored in polymers (such as starch) that are converted by metabolic processes into sucrose for newly developing plants. Once green shoots and leaves are growing, plants are able to produce their own food by photosynthesizing. The products of photosynthesis are called photosynthates, which are usually in the form of simple sugars such as sucrose.

Structures that produce photosynthates for the growing plant are referred to as sources . Sugars produced in sources, such as leaves, need to be delivered to growing parts of the plant via the phloem in a process called translocation . The points of sugar delivery, such as roots, young shoots, and developing seeds, are called sinks . Seeds, tubers, and bulbs can be either a source or a sink, depending on the plant’s stage of development and the season.

The products from the source are usually translocated to the nearest sink through the phloem. For example, the highest leaves will send photosynthates upward to the growing shoot tip, whereas lower leaves will direct photosynthates downward to the roots. Intermediate leaves will send products in both directions, unlike the flow in the xylem, which is always unidirectional (soil to leaf to atmosphere). The pattern of photosynthate flow changes as the plant grows and develops. Photosynthates are directed primarily to the roots early on, to shoots and leaves during vegetative growth, and to seeds and fruits during reproductive development. They are also directed to tubers for storage.

Translocation: Transport from Source to Sink

Photosynthates, such as sucrose, are produced in the mesophyll cells of photosynthesizing leaves. From there they are translocated through the phloem to where they are used or stored. Mesophyll cells are connected by cytoplasmic channels called plasmodesmata. Photosynthates move through these channels to reach phloem sieve-tube elements (STEs) in the vascular bundles. From the mesophyll cells, the photosynthates are loaded into the phloem STEs. The sucrose is actively transported against its concentration gradient (a process requiring ATP) into the phloem cells using the electrochemical potential of the proton gradient. This is coupled to the uptake of sucrose with a carrier protein called the sucrose-H + symporter.

Phloem STEs have reduced cytoplasmic contents, and are connected by a sieve plate with pores that allow for pressure-driven bulk flow, or translocation, of phloem sap. Companion cells are associated with STEs. They assist with metabolic activities and produce energy for the STEs (Figure \(\PageIndex{6}\)).

Once in the phloem, the photosynthates are translocated to the closest sink. Phloem sap is an aqueous solution that contains up to 30 percent sugar, minerals, amino acids, and plant growth regulators. The high percentage of sugar decreases Ψ s , which decreases the total water potential and causes water to move by osmosis from the adjacent xylem into the phloem tubes, thereby increasing pressure. This increase in total water potential causes the bulk flow of phloem from source to sink (Figure \(\PageIndex{7}\)). Sucrose concentration in the sink cells is lower than in the phloem STEs because the sink sucrose has been metabolized for growth, or converted to starch for storage or other polymers, such as cellulose, for structural integrity. Unloading at the sink end of the phloem tube occurs by either diffusion or active transport of sucrose molecules from an area of high concentration to one of low concentration. Water diffuses from the phloem by osmosis and is then transpired or recycled via the xylem back into the phloem sap.

Water potential (Ψ) is a measure of the difference in potential energy between a water sample and pure water. The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and matric potential. Water potential and transpiration influence how water is transported through the xylem in plants. These processes are regulated by stomatal opening and closing. Photosynthates (mainly sucrose) move from sources to sinks through the plant’s phloem. Sucrose is actively loaded into the sieve-tube elements of the phloem. The increased solute concentration causes water to move by osmosis from the xylem into the phloem. The positive pressure that is produced pushes water and solutes down the pressure gradient. The sucrose is unloaded into the sink, and the water returns to the xylem vessels.

Art Connections

Figure \(\PageIndex{2}\): Positive water potential is placed on the left side of the tube by increasing Ψ p such that the water level rises on the right side. Could you equalize the water level on each side of the tube by adding solute, and if so, how?

Yes, you can equalize the water level by adding the solute to the left side of the tube such that water moves toward the left until the water levels are equal.

Figure \(\PageIndex{4}\): Which of the following statements is false?

• About Organismal Biology
• Phylogenetic Trees and Geologic Time
• Prokaryotes: Bacteria & Archaea
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• Land Plants
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Water Transport in Plants: Xylem

• Sugar Transport in Plants: Phloem
• Nutrient Acquisition by Animals
• Animal Gas Exchange and Transport
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• The Mammalian Cardiac Cycle
• Ion and Water Regulation and Nitrogenous Wastes in Animals
• The Mammalian Kidney: How Nephrons Perform Osmoregulation
• Plant and Animal Responses to the Environment

Learning Objectives

• Explain water potential and predict movement of water in plants by applying the principles of water potential
• Describe the effects of different environmental or soil conditions on the typical water potential gradient in plants
• Identify and differentiate between the three pathways water and minerals can take from the root hair to the vascular tissue
• Explain the three hypotheses explaining water movement in plant xylem, and recognize which hypothesis explains the heights of plants beyond a few meters
• Define transpiration and identify the source of energy that drives transpiration

Water Potential and Water Transport from Roots to Shoots

The information below was adapted from OpenStax Biology 30.5

The structure of plant roots, stems, and leaves facilitates the transport of water, nutrients, and products of photosynthesis throughout the plant. The phloem is the tissue primarily responsible for movement of nutrients and photosynthetic produces, and xylem is the tissue primarily responsible for movement of water). Plants are able to transport water from their roots up to the tips of their tallest shoot through the combination of water potential, evapotranspiration, and stomatal regulation – all without using any cellular energy!

Water potential is a measure of the potential energy in water based on potential water movement between two systems. Water potential can be defined as the difference in potential energy between any given water sample and pure water (at atmospheric pressure and ambient temperature). Water potential is denoted by the Greek letter Ψ ( psi ) and is expressed in units of pressure (pressure is a form of energy) called megapascals (MPa). The potential of pure water (Ψ pure H2O ) is defined as zero (even though pure water contains plenty of potential energy, this energy is ignored in this context).

Water potential can be positive or negative, and water potential is calculated from the combined effects of  solute concentration   (s) and  pressure (p) . The equation for this calculation is Ψ

An example of the effect of turgor pressure is the wilting of leaves and their restoration after the plant has been watered. Water is lost from the leaves via transpiration (approaching Ψ p  = 0 MPa at the wilting point) and restored by uptake via the roots.

This video provides an overview of water potential, including solute and pressure potential (stop after 5:05):

And this video describes how plants manipulate water potential to absorb water and how water and minerals move through the root tissues:

Impact of Soil and Environmental Conditions on the Plant Water Potential Gradient

As noted above, Ψ soil  must be > Ψ root  > Ψ stem  > Ψ leaf  > Ψ atmosphere in order for transpiration to occur (continuous movement of water through the plant from the soil to the air without equilibrating. This continuous movement of water relies on a water potential gradient , where water potential decreases at each point from soil to atmosphere as it passes through the plant tissues. However, this gradient can become disrupted if the soil becomes too dry, which can result in both decreased solute potential (due to the same amount of solutes dissolved in a smaller quantity of water) as well as decreased pressure potential in severe droughts (resulting from negative pressure or a “vacuum” in the soil due to loss of water volume). If water potential becomes sufficiently lower in the soil than in the plant’s roots, then water will move out of the plant root and into the soil.

Pathways of Water and Mineral Movement in the Roots

Once water has been absorbed by a root hair, it moves through the ground tissue and along its water potential gradient through one of three possible routes before entering the plant’s xylem:

• the  symplast : “sym” means “same” or “shared,” so symplast is “shared cytoplasm”.  In this pathway, water and minerals move from the cytoplasm of one cell in to the next, via plasmodesmata that physically join different plant cells, until eventually reaching the xylem.
• the  transmembrane  pathway: in this pathway, water moves through water channels present in the plant cell plasma membranes, from one cell to the next, until eventually reaching the xylem.
• the  apoplast : “a” means “outside of,” so apoplast is “outside of the cell”. In this pathway, water and dissolved minerals never move through a cell’s plasma membrane but instead travel through the porous cell walls that surround plant cells.

Water and minerals that move into a cell through the plasma membrane has been “filtered” as it passes through water or other channels within the plasma membrane; however water and minerals that move via the apoplast do not encounter a filtering step until they reach a layer of cells known as the endodermis which separate the vascular tissue (called the stele in the root) from the ground tissue in the outer portion of the root. The endodermis is present only in roots, and serves as a checkpoint for materials entering the root’s vascular system. A waxy substance called suberin is present on the walls of the endodermal cells. This waxy region, known as the Casparian strip , forces water and solutes to cross the plasma membranes of endodermal cells instead of slipping between the cells. This process ensures that only materials required by the root pass through the endodermis, while toxic substances and pathogens are generally excluded.

Movement of Water Up the Xylem Against Gravity

How is water transported up a plant against gravity, when there is no “pump” or input of cellular energy to move water through a plant’s vascular tissue? There are three hypotheses that explain the movement of water up a plant against gravity. These hypotheses are not mutually exclusive, and each contribute to movement of water in a plant, but only one can explain the height of tall trees:

• Root pressure  pushes water up
• Capillary action draws water up within the xylem
• Cohesion-tension pulls water up the xylem

We’ll consider each of these in turn.

Root pressure relies on positive pressure that forms in the roots as water moves into the roots from the soil. Water moves into the roots from the soil by osmosis, due to the low solute potential in the roots (lower Ψs in roots than in soil). This intake o f water in the roots increases Ψp in the root xylem, “pushing” water up. In extreme circumstances, or when stomata are closed at night preventing water from evaporating from the leaves, root pressure results in guttation , or secretion of water droplets from stomata in the leaves. However, root pressure can only move water against gravity by a few meters, so it is not sufficient to move water up the height of a tall tree. 

Capillary action  (or capillarity) is the tendency of a liquid to move up against gravity when confined within a narrow tube (capillary). You can directly observe the effects of capillary action when water forms a meniscus when confined in a narrow tube. Capillarity occurs due to three properties of water:

• Surface tension , which occurs because hydrogen bonding between water molecules is stronger at the air-water interface than among molecules within the water.
• Adhesion , which is molecular attraction between “unlike” molecules. In the case of xylem, adhesion occurs between water molecules and the molecules of the xylem cell walls.
• Cohesion , which is molecular attraction between “like” molecules. In water, cohesion occurs due to hydrogen bonding between water molecules.

On its own, capillarity can work well within a vertical stem for up to approximately 1 meter, so it is not strong enough to move water up a tall tree.

This video provides an overview of the important properties of water that facilitate this movement:

The cohesion-tension  hypothesis is the most widely-accepted model for movement of water in vascular plants. Cohesion-tension combines the process of capillary action with transpiration or the evaporation of water from the plant stomata. Transpiration is ultimately the main driver of water movement in xylem, combined with the effects of capillary action. The cohesion-tension model works like this:

• Transpiration (evaporation) occurs because stomata in the leaves are open to allow gas exchange for photosynthesis. As transpiration occurs, evaporation of water deepens the meniscus of water in the leaf, creating negative pressure (also called tension or suction).
• The tension created by transpiration “pulls” water in the plant xylem, drawing the water upward in much the same way that you draw water upward when you suck on a straw.
• Cohesion (water molecules sticking to other water molecules) causes more water molecules to fill the gap in the xylem as the top-most water is pulled toward end of the meniscus within the stomata.

Transpiration results in a phenomenal amount of negative pressure within the xylem vessels and tracheids, which are structurally reinforced with lignin to cope with large changes in pressure. The taller the tree, the greater the tension forces (and thus negative pressure) needed to pull water up from roots to shoots.

Follow this link to watch this video on YouTube for an overview of the different processes that cause water to move throughout a plant (this video is linked because it cannot be directly embedded within the textbook; if needed, the video url is https://www.youtube.com/watch?v=8YlGyb0WqUw )

Transpiration Energy Source

The term “ transpiration ” has been used throughout this reading in the context of water movement in plants. Here we will define it as: evaporation of water from the plant stomata resulting in the continuous movement of water through a plant via the xylem, from soil to air, without equilibrating.

Transpiration is a passive process with respect to the plant, meaning that ATP is not required to move water up the plant’s shoots. The energy source that drives the process of transpiration is the extreme difference in water potential between the water in the soil and the water in the atmosphere. Factors that alter this extreme difference in water potential can also alter the rate of transpiration in the plant.

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Water Uptake and Transport in Vascular Plants

Why Do Plants Need So Much Water?

If water is so important to plant growth and survival, then why would plants waste so much of it? The answer to this question lies in another process vital to plants — photosynthesis. To make sugars, plants must absorb carbon dioxide (CO 2 ) from the atmosphere through small pores in their leaves called stomata (Figure 1). However, when stomata open, water is lost to the atmosphere at a prolific rate relative to the small amount of CO 2 absorbed; across plant species an average of 400 water molecules are lost for each CO 2 molecule gained. The balance between transpiration and photosynthesis forms an essential compromise in the existence of plants; stomata must remain open to build sugars but risk dehydration in the process.

From the Soil into the Plant

Essentially all of the water used by land plants is absorbed from the soil by roots. A root system consists of a complex network of individual roots that vary in age along their length. Roots grow from their tips and initially produce thin and non-woody fine roots. Fine roots are the most permeable portion of a root system, and are thought to have the greatest ability to absorb water, particularly in herbaceous (i.e., non-woody) plants (McCully 1999). Fine roots can be covered by root hairs that significantly increase the absorptive surface area and improve contact between roots and the soil (Figure 2). Some plants also improve water uptake by establishing symbiotic relationships with mycorrhizal fungi, which functionally increase the total absorptive surface area of the root system.

Roots of woody plants form bark as they age, much like the trunks of large trees. While bark formation decreases the permeability of older roots they can still absorb considerable amounts of water (MacFall et al . 1990, Chung & Kramer 1975). This is important for trees and shrubs since woody roots can constitute ~99% of the root surface in some forests (Kramer & Bullock 1966).

Roots have the amazing ability to grow away from dry sites toward wetter patches in the soil — a phenomenon called hydrotropism. Positive hydrotropism occurs when cell elongation is inhibited on the humid side of a root, while elongation on the dry side is unaffected or slightly stimulated resulting in a curvature of the root and growth toward a moist patch (Takahashi 1994). The root cap is most likely the site of hydrosensing; while the exact mechanism of hydrotropism is not known, recent work with the plant model Arabidopsis has shed some light on the mechanism at the molecular level (see Eapen et al . 2005 for more details).

Roots of many woody species have the ability to grow extensively to explore large volumes of soil. Deep roots (>5 m) are found in most environments (Canadell et al . 1996, Schenk & Jackson 2002) allowing plants to access water from permanent water sources at substantial depth (Figure 3). Roots from the Shepard's tree ( Boscia albitrunca ) have been found growing at depths 68 m in the central Kalahari, while those of other woody species can spread laterally up to 50 m on one side of the plant (Schenk & Jackson 2002). Surprisingly, most arid-land plants have very shallow root systems, and the deepest roots consistently occur in climates with strong seasonal precipitation (i.e., Mediterranean and monsoonal climates).

Through the Plant into the Atmosphere

Flow = Δψ / R ,

which is analogous to electron flow in an electrical circuit described by Ohm's law equation:

i = V / R ,

where R is the resistance, i is the current or flow of electrons, and V is the voltage. In the plant system, V is equivalent to the water potential difference driving flow (Δψ) and i is equivalent to the flow of water through/across a plant segment. Using these plant equivalents, the Ohm's law analogy can be used to quantify the hydraulic conductance (i.e., the inverse of hydraulic R ) of individual segments (i.e., roots, stems, leaves) or the whole plant (from soil to atmosphere).

Upon absorption by the root, water first crosses the epidermis and then makes its way toward the center of the root crossing the cortex and endodermis before arriving at the xylem (Figure 4). Along the way, water travels in cell walls (apoplastic pathway) and/or through the inside of cells (cell to cell pathway, C-C) (Steudle 2001). At the endodermis, the apoplastic pathway is blocked by a gasket-like band of suberin — a waterproof substance that seals off the route of water in the apoplast forcing water to cross via the C-C pathway. Because water must cross cell membranes (e.g., in the cortex and at apoplastic barriers), transport efficiency of the C-C pathway is affected by the activity, density, and location of water-specific protein channels embedded in cell membranes (i.e., aquaporins). Much work over the last two decades has demonstrated how aquaporins alter root hydraulic resistance and respond to abiotic stress, but their exact role in bulk water transport is yet unresolved.

Once in the xylem tissue, water moves easily over long distances in these open tubes (Figure 5). There are two kinds of conducting elements (i.e., transport tubes) found in the xylem: 1) tracheids and 2) vessels (Figure 6). Tracheids are smaller than vessels in both diameter and length, and taper at each end. Vessels consist of individual cells, or "vessel elements", stacked end-to-end to form continuous open tubes, which are also called xylem conduits. Vessels have diameters approximately that of a human hair and lengths typically measuring about 5 cm although some plant species contain vessels as long as 10 m. Xylem conduits begin as a series of living cells but as they mature the cells commit suicide (referred to as programmed cell death), undergoing an ordered deconstruction where they lose their cellular contents and form hollow tubes. Along with the water conducting tubes, xylem tissue contains fibers which provide structural support, and living metabolically-active parenchyma cells that are important for storage of carbohydrates, maintenance of flow within a conduit (see details about embolism repair below), and radial transport of water and solutes.

When water reaches the end of a conduit or passes laterally to an adjacent one, it must cross through pits in the conduit cell walls (Figure 6). Bordered pits are cavities in the thick secondary cell walls of both vessels and tracheids that are essential components in the water-transport system of higher plants. The pit membrane, consisting of a modified primary cell wall and middle lamella, lies at the center of each pit, and allows water to pass between xylem conduits while limiting the spread of air bubbles (i.e., embolism) and xylem-dwelling pathogens. Thus, pit membranes function as safety valves in the plant water transport system. Averaged across a wide range of species, pits account for >50% of total xylem hydraulic resistance. The structure of pits varies dramatically across species, with large differences evident in the amount of conduit wall area covered by pits, and in the porosity and thickness of pit membranes (Figure 6).

After traveling from the roots to stems through the xylem, water enters leaves via petiole (i.e., the leaf stalk) xylem that branches off from that in the stem. Petiole xylem leads into the mid-rib (the main thick vein in leaves), which then branch into progressively smaller veins that contain tracheids (Figure 7) and are embedded in the leaf mesophyll. In dicots, minor veins account for the vast majority of total vein length, and the bulk of transpired water is drawn out of minor veins (Sack & Holbrook 2006, Sack & Tyree 2005). Vein arrangement, density, and redundancy are important for distributing water evenly across a leaf, and may buffer the delivery system against damage (i.e., disease lesions, herbivory, air bubble spread). Once water leaves the xylem, it moves across the bundle sheath cells surrounding the veins. It is still unclear the exact path water follows once it passes out of the xylem through the bundle sheath cells and into the mesophyll cells, but is likely dominated by the apoplastic pathway during transpiration (Sack & Holbrook 2005).

Mechanism Driving Water Movement in Plants

Stephen Hales was the first to suggest that water flow in plants is governed by the C-T mechanism; in his 1727 book Hales states "for without perspiration the [water] must stagnate, notwithstanding the sap-vessels are so curiously adapted by their exceeding fineness, to raise [water] to great heights, in a reciprocal proportion to their very minute diameters." More recently, an evaporative flow system based on negative pressure has been reproduced in the lab for the first time by a ‘synthetic tree' (Wheeler & Stroock 2008).

When solute movement is restricted relative to the movement of water (i.e., across semipermeable cell membranes) water moves according to its chemical potential (i.e., the energy state of water) by osmosis — the diffusion of water. Osmosis plays a central role in the movement of water between cells and various compartments within plants. In the absence of transpiration, osmotic forces dominate the movement of water into roots. This manifests as root pressure and guttation — a process commonly seen in lawn grass, where water droplets form at leaf margins in the morning after conditions of low evaporation. Root pressure results when solutes accumulate to a greater concentration in root xylem than other root tissues. The resultant chemical potential gradient drives water influx across the root and into the xylem. No root pressure exists in rapidly transpiring plants, but it has been suggested that in some species root pressure can play a central role in the refilling of non-functional xylem conduits particularly after winter (see an alternative method of refilling described below).

Disruption of Water Movement

Water transport can be disrupted at many points along the SPAC resulting from both biotic and abiotic factors (Figure 8). Root pathogens (both bacteria and fungi) can destroy the absorptive surface area in the soil, and similarly foliar pathogens can eliminate evaporative leaf surfaces, alter stomatal function, or disrupt the integrity of the cuticle. Other organisms (i.e., insects and nematodes) can cause similar disruption of above and below ground plant parts involved in water transport. Biotic factors responsible for ceasing flow in xylem conduits include: pathogenic organisms and their by-products that plug conduits (Figure 8); plant-derived gels and gums produced in response to pathogen invasion; and tyloses, which are outgrowths produced by living plant cells surrounding a vessel to seal it off after wounding or pathogen invasion (Figure 8).

Abiotic factors can be equally disruptive to flow at various points along the water transport pathway. During drought, roots shrink and lose contact with water adhering to soil particles — a process that can also be beneficial by limiting water loss by roots to drying soils (i.e., water can flow in reverse and leak out of roots being pulled by drying soil). Under severe plant dehydration, some pine needle conduits can actually collapse as the xylem tensions increase (Figure 8).

Water moving through plants is considered meta-stable because at a certain point the water column breaks when tension becomes excessive — a phenomenon referred to as cavitation. After cavitation occurs, a gas bubble (i.e., embolism) can form and fill the conduit, effectively blocking water movement. Both sub-zero temperatures and drought can cause embolisms. Freezing can induce embolism because air is forced out of solution when liquid water turns to ice. Drought also induces embolism because as plants become drier tension in the water column increases. There is a critical point where the tension exceeds the pressure required to pull air from an empty conduit to a filled conduit across a pit membrane — this aspiration is known as air seeding (Figure 9). An air seed creates a void in the water, and the tension causes the void to expand and break the continuous column. Air seeding thresholds are set by the maximum pore diameter found in the pit membranes of a given conduit.

Fixing the Problem

Failure to re-establish flow in embolized conduits reduces hydraulic capacity, limits photosynthesis, and results in plant death in extreme cases. Plants can cope with emboli by diverting water around blockages via pits connecting adjacent functional conduits, and by growing new xylem to replace lost hydraulic capacity. Some plants possess the ability to repair breaks in the water columns, but the details of this process in xylem under tension have remained unclear for decades. Brodersen et al . (2010) recently visualized and quantified the refilling process in live grapevines ( Vitis vinifera L.) using high resolution x-ray computed tomography (a type of CAT scan) (Figure 10). Successful vessel refilling was dependent on water influx from living cells surrounding the xylem conduits, where individual water droplets expanded over time, filled vessels, and forced the dissolution of entrapped gas. The capacity of different plants to repair compromised xylem vessels and the mechanisms controlling these repairs are currently being investigated.

References and Recommended Reading

Agrios, G. N. Plant Pathology . New York, NY: Academic Press, 1997.

Beerling, D. J. & Franks, P. J. Plant science: The hidden cost of transpiration. Nature 464, 495-496 (2010).

Brodersen, C. R. et al . The dynamics of embolism repair in xylem: In vivo visualizations using high-resolution computed tomography Plant Physiology 154 , 1088-1095 (2010).

Brodribb, T. J. & Holbrook, N. M. Water stress deforms tracheids peripheral to the leaf vein of a tropical conifer. Plant Physiology 137 , 1139-1146 (2005)

Canadell, J. et al . Maximum rooting depth of vegetation types at the global scale. Oecologia 108, 583-595 (1996).

Choat, B., Cobb, A. R. & Jansen, S. Structure and function of bordered pits: New discoveries and impacts on whole-plant hydraulic function. New Phytologist 177, 608-626 (2008).

Chung, H. H. & Kramer, P. J. Absorption of water and "P through suberized and unsuberized roots of loblolly pine. Canadian Journal of Forest Research 5, 229-235 (1975).

Eapen, D. et al . Hydrotropism: Root growth responses to water. Trends in Plant Science 10, 44-50 (2005).

Hetherington, A. M. & Woodward, F. I. The role of stomata in sensing and driving environmental change. Nature 424, 901-908 (2003).

Holbrook, N. M. & Zwieniecki, M. A. Vascular Transport in Plants . San Diego, CA: Elsevier Academic Press, 2005.

Javot, H. & Maurel, C. The role of aquaporins in root water uptake. Annals of Botany 90, 1-13 (2002).

Kramer, P. J. & Boyer, J. S. Water Relations of Plants and Soils . New York, NY: Academic Press, 1995.

Kramer, P. J. & Bullock, H. C. Seasonal variations in the proportions of suberized and unsuberized roots of trees in relation to the absorption of water. American Journal of Botany 53, 200-204 (1966).

MacFall, J. S., Johnson, G. A. & Kramer, P. J. Observation of a water-depletion region surrounding loblolly pine roots by magnetic resonance imaging. Proceedings of the National Academy of Sciences of the United States of America 87 , 1203-1207 (1990).

McCully, M. E. Roots in Soil: Unearthing the complexities of roots and their rhizospheres. Annual Review of Plant Physiology and Plant Molecular Biology 50, 695-718 (1999).

McDowell, N. G. et al . Mechanisms of plant survival and mortality during drought: Why do some plants survive while others succumb to drought? New Phytologist 178, 719-739 (2008).

Nardini, A., Lo Gullo, M. A. & Salleo, S. Refilling embolized xylem conduits: Is it a matter of phloem unloading? Plant Science 180, 604-611 (2011).

Pittermann, J. et al . Torus-margo pits help conifers compete with angiosperms. Science 310, 1924 (2005).

Sack, L. & Holbrook, N. M. Leaf hydraulics. Annual Review of Plant Biology 57, 361-381 (2006).

Sack, L. & Tyree, M. T. "Leaf hydraulics and its implications in plant structure and function," in Vascular Transport in Plants , eds. N. M. Holbrook & M. A. Zwieniecki. (San Diego, CA: Elsevier Academic Press, 2005) 93-114.

Schenk, H. J. & Jackson, R. B. Rooting depths, lateral root spreads, and belowground/aboveground allometries of plants in water-limited environments. Journal of Ecology 90, 480-494 (2002).

Sperry, J. S. & Tyree, M. T. Mechanism of water-stress induced xylem embolism. Plant Physiology 88, 581-587 (1988).

Steudle, E. The cohesion-tension mechanism and the acquisition of water by plants roots. Annual Review of Plant Physiological and Molecular Biology 52, 847-875 (2001).

Steudle, E. Transport of water in plants. Environmental Control in Biology 40, 29-37 (2002).

Takahashi, H. Hydrotropism and its interaction with gravitropism in roots. Plant Soil 165 , 301-308 (1994).

Tyree, M. T. & Ewers, F. W. The hydraulic architecture of trees and other woody plants. New Phytologist 119, 345-360 (1991).

Tyree, M. T. & Sperry, J. S. Vulnerability of xylem to cavitation and embolism. Annual Review of Plant Physiology and Molecular Biology 40, 19-38 (1989).

Tyree, M. T. & Zimmerman, M. H. Xylem Structure and the Ascent of Sap . 2nd ed. New York, NY: Springer-Verlag, 2002.

Tyree, M. T. & Ewers, F. The hydraulic architecture of trees and other woody plants. New Phytologist 119, 345-360 (1991).

Wheeler, T. D. & Stroock, A. D. The transpiration of water at negative pressures in a synthetic tree. Nature 455, 208-212 (2008).

Wullschleger, S. D., Meinzer, F. C. & Vertessy, R. A. A review of whole-plant water use studies in trees. Tree Physiology 18, 499-512 (1998).

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23.5 Transport of Water and Solutes in Plants

Learning objectives.

In this section, you will explore the following questions:

• What is water potential, and how is it influenced by solutes, pressures, gravity, and the matric potential?
• How do water potential, evapotranspiration, and stomatal regulation influence how water is transported in plants?
• How are photosynthates transported in plants?

Connection for AP ® Courses

Information in this section applies to concepts we explored in previous chapters by connecting them to the transport of water and solutes through a plant, showing ways that plants take up and transport materials. These concepts include the processes of photosynthesis and cellular respiration, the chemical and physical properties of water, and the coevolution of plants with mutualistic bacteria and fungi. The vascular system of terrestrial plants allows the efficient absorption and delivery of water through the cells that comprise xylem, whereas phloem delivers sugars produced in photosynthesis to all parts of the plant, including the roots for storage. The physical separation of xylem and phloem permits plants to move different nutrients simultaneously from roots to shoots and vice versa. Nearly all plants use related mechanisms of osmoregulation, and we will focus on the transport of water and other nutrients.

You likely remember the concept of water potential (Ψ) from our exploration of diffusion and osmosis in the chapter where we discuss the structure and function of plasma membranes. Water potential is a measure of the differences in potential energy between a water sample with solutes and pure water. Water moves via osmosis from an area of higher water potential (more water molecules, less solute) to an area of lower water potential (less water, more solutes). The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and other factors (matrix effects). Water potential and transpiration influence how water is transported through the xylem.

Carbohydrates synthesized in photosynthesis, primarily sucrose, move from sources to sinks through the plant’s phloem. Sucrose produced in the Calvin cycle is loaded into the sieve-tube elements of the phloem, and the increased solute concentration causes water to move by osmosis from the xylem into the phloem.

Information presented and the examples highlighted in the section support concepts outlined in Big Idea 2 and Big Idea 4 of the AP ® Biology Curriculum Framework. The AP ® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® exam questions. A learning objective merges required content with one or more of the seven science practices.

The Practice Challenge Questions contain additional test questions for this section that will help you prepare for the AP exam. These questions address the following standards: [APLO 2.40][APLO 4.12][APLO 2.1][APLO 2.8][APLO 2.9][APLO 2.41][APLO 1.2][APLO 1.22][APLO 1.25][APLO 2.19][APLO 2.32]

The structure of plant roots, stems, and leaves facilitates the transport of water, nutrients, and photosynthates throughout the plant. The phloem and xylem are the main tissues responsible for this movement. Water potential, evapotranspiration, and stomatal regulation influence how water and nutrients are transported in plants. To understand how these processes work, we must first understand the energetics of water potential.

Water Potential

Plants are phenomenal hydraulic engineers. Using only the basic laws of physics and the simple manipulation of potential energy, plants can move water to the top of a 116-meter-tall tree ( Figure 23.31 a ). Plants can also use hydraulics to generate enough force to split rocks and buckle sidewalks ( Figure 23.31 b ). Plants achieve this because of water potential.

Water potential is a measure of the potential energy in water. Plant physiologists are not interested in the energy in any one particular aqueous system, but are very interested in water movement between two systems. In practical terms, therefore, water potential is the difference in potential energy between a given water sample and pure water (at atmospheric pressure and ambient temperature). Water potential is denoted by the Greek letter Ψ ( psi ) and is expressed in units of pressure (pressure is a form of energy) called megapascals (MPa). The potential of pure water (Ψ w pure H2O ) is, by convenience of definition, designated a value of zero (even though pure water contains plenty of potential energy, that energy is ignored). Water potential values for the water in a plant root, stem, or leaf are therefore expressed relative to Ψ w pure H2O .

The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and factors called matrix effects. Water potential can be broken down into its individual components using the following equation:

where Ψ s , Ψ p , Ψ g , and Ψ m refer to the solute, pressure, gravity, and matric potentials, respectively. “System” can refer to the water potential of the soil water (Ψ soil ), root water (Ψ root ), stem water (Ψ stem ), leaf water (Ψ leaf ) or the water in the atmosphere (Ψ atmosphere ): whichever aqueous system is under consideration. As the individual components change, they raise or lower the total water potential of a system. When this happens, water moves to equilibrate, moving from the system or compartment with a higher water potential to the system or compartment with a lower water potential. This brings the difference in water potential between the two systems (ΔΨ) back to zero (ΔΨ = 0). Therefore, for water to move through the plant from the soil to the air (a process called transpiration), Ψ soil must be > Ψ root > Ψ stem > Ψ leaf > Ψ atmosphere .

Water only moves in response to ΔΨ, not in response to the individual components. However, because the individual components influence the total Ψ system , by manipulating the individual components (especially Ψ s ), a plant can control water movement.

Solute Potential

Solute potential (Ψ s ), also called osmotic potential, is related to the solute concentration (in molarity). That relationship is given by the van 't Hoff equation: Ψ s = –M i RT; where M is the molar concentration of the solute, i is the van 't Hoff factor (the ratio of the amount of particles in the solution to amount of formula units dissolved), R is the ideal gas constant, and T is temperature in Kelvin degrees. The solute potential is negative in a plant cell and zero in distilled water. Typical values for cell cytoplasm are –0.5 to –1.0 MPa. Solutes reduce water potential (resulting in a negative Ψ w ) by consuming some of the potential energy available in the water. Solute molecules can dissolve in water because water molecules can bind to them via hydrogen bonds; a hydrophobic molecule like oil, which cannot bind to water, cannot go into solution. The energy in the hydrogen bonds between solute molecules and water is no longer available to do work in the system because it is tied up in the bond. In other words, the amount of available potential energy is reduced when solutes are added to an aqueous system. Thus, Ψ s decreases with increasing solute concentration. Because Ψ s is one of the four components of Ψ system or Ψ total , a decrease in Ψ s will cause a decrease in Ψ total . The internal water potential of a plant cell is more negative than pure water because of the cytoplasm’s high solute content ( Figure 23.32 ). Because of this difference in water potential water will move from the soil into a plant’s root cells via the process of osmosis. This is why solute potential is sometimes called osmotic potential.

Plant cells can metabolically manipulate Ψ s (and by extension, Ψ total ) by adding or removing solute molecules. Therefore, plants have control over Ψ total via their ability to exert metabolic control over Ψ s .

Visual Connection

• Yes, water level can be equalized by adding solute to the right side of the tube so that water moves toward the left until the water levels are equal.
• No, water level cannot be equalized on both sides of the tubes by adding solutes with no other action.
• Yes, water level can be equalized by adding solute to the left side of the tube so that water moves toward the left until the water levels are equal.
• No, water level cannot be equalized by adding solutes because solutes are always pulled down by gravity, thereby not letting water equalize.

Pressure Potential

Pressure potential (Ψ p ), also called turgor potential, may be positive or negative ( Figure 23.32 ). Because pressure is an expression of energy, the higher the pressure, the more potential energy in a system, and vice versa. Therefore, a positive Ψ p (compression) increases Ψ total , and a negative Ψ p (tension) decreases Ψ total . Positive pressure inside cells is contained by the cell wall, producing turgor pressure. Pressure potentials are typically around 0.6–0.8 MPa, but can reach as high as 1.5 MPa in a well-watered plant. A Ψ p of 1.5 MPa equates to 210 pounds per square inch (1.5 MPa x 140 lb in -2 MPa -1 = 210 lb/in -2 ). As a comparison, most automobile tires are kept at a pressure of 30–34 psi. An example of the effect of turgor pressure is the wilting of leaves and their restoration after the plant has been watered ( Figure 23.33 ). Water is lost from the leaves via transpiration (approaching Ψ p = 0 MPa at the wilting point) and restored by uptake via the roots.

A plant can manipulate Ψ p via its ability to manipulate Ψ s and by the process of osmosis. If a plant cell increases the cytoplasmic solute concentration, Ψ s will decline, Ψ total will decline, the ΔΨ between the cell and the surrounding tissue will decline, water will move into the cell by osmosis, and Ψ p will increase. Ψ p is also under indirect plant control via the opening and closing of stomata. Stomatal openings allow water to evaporate from the leaf, reducing Ψ p and Ψ total of the leaf and increasing ΔΨ between the water in the leaf and the petiole, thereby allowing water to flow from the petiole into the leaf.

Gravity Potential

Gravity potential (Ψ g ) is always negative to zero in a plant with no height. It always removes or consumes potential energy from the system. The force of gravity pulls water downwards to the soil, reducing the total amount of potential energy in the water in the plant (Ψ total ). The taller the plant, the taller the water column, and the more influential Ψ g becomes. On a cellular scale and in short plants, this effect is negligible and easily ignored. However, over the height of a tall tree like a giant coastal redwood, the gravitational pull of –0.1 MPa m -1 is equivalent to an extra 1 MPa of resistance that must be overcome for water to reach the leaves of the tallest trees. Plants are unable to manipulate Ψ g .

Matric Potential

Matric potential (Ψ m ) is always negative to zero. In a dry system, it can be as low as –2 MPa in a dry seed, and it is zero in a water-saturated system. The binding of water to a matrix always removes or consumes potential energy from the system. Ψ m is similar to solute potential because it involves tying up the energy in an aqueous system by forming hydrogen bonds between the water and some other component. However, in solute potential, the other components are soluble, hydrophilic solute molecules, whereas in Ψ m , the other components are insoluble, hydrophilic molecules of the plant cell wall. Every plant cell has a cellulosic cell wall and the cellulose in the cell walls is hydrophilic, producing a matrix for adhesion of water: hence the name matric potential. Ψ m is very large (negative) in dry tissues such as seeds or drought-affected soils. However, it quickly goes to zero as the seed takes up water or the soil hydrates. Ψ m cannot be manipulated by the plant and is typically ignored in well-watered roots, stems, and leaves.

Movement of Water and Minerals in the Xylem

Solutes, pressure, gravity, and matric potential are all important for the transport of water in plants. Water moves from an area of higher total water potential (higher Gibbs free energy) to an area of lower total water potential. Gibbs free energy is the energy associated with a chemical reaction that can be used to do work. This is expressed as ΔΨ.

Transpiration is the loss of water from the plant through evaporation at the leaf surface. It is the main driver of water movement in the xylem. Transpiration is caused by the evaporation of water at the leaf–atmosphere interface; it creates negative pressure (tension) equivalent to –2 MPa at the leaf surface. This value varies greatly depending on the vapor pressure deficit, which can be negligible at high relative humidity (RH) and substantial at low RH. Water from the roots is pulled up by this tension. At night, when stomata shut and transpiration stops, the water is held in the stem and leaf by the adhesion of water to the cell walls of the xylem vessels and tracheids, and the cohesion of water molecules to each other. This is called the cohesion–tension theory of sap ascent.

Inside the leaf at the cellular level, water on the surface of mesophyll cells saturates the cellulose microfibrils of the primary cell wall. The leaf contains many large intercellular air spaces for the exchange of oxygen for carbon dioxide, which is required for photosynthesis. The wet cell wall is exposed to this leaf internal air space, and the water on the surface of the cells evaporates into the air spaces, decreasing the thin film on the surface of the mesophyll cells. This decrease creates a greater tension on the water in the mesophyll cells ( Figure 23.34 ), thereby increasing the pull on the water in the xylem vessels. The xylem vessels and tracheids are structurally adapted to cope with large changes in pressure. Rings in the vessels maintain their tubular shape, much like the rings on a vacuum cleaner hose keep the hose open while it is under pressure. Small perforations between vessel elements reduce the number and size of gas bubbles that can form via a process called cavitation. The formation of gas bubbles in xylem interrupts the continuous stream of water from the base to the top of the plant, causing a break termed an embolism in the flow of xylem sap. The taller the tree, the greater the tension forces needed to pull water, and the more cavitation events. In larger trees, the resulting embolisms can plug xylem vessels, making them non-functional.

• Negative water potential draws water into the root hairs, cohesion and adhesion draw water up the xylem, and transpiration draws water from the leaf.
• Negative water potential draws water into the root hairs, cohesion and adhesion draw water up the phloem, and transpiration draws water from the leaf.
• Water potential decreases from the roots to the top of the plant.
• Water enters the plant through the root hairs and exits through stomata.

Transpiration —the loss of water vapor to the atmosphere through stomata—is a passive process, meaning that metabolic energy in the form of ATP is not required for water movement. The energy driving transpiration is the difference in energy between the water in the soil and the water in the atmosphere. However, transpiration is tightly controlled.

Control of Transpiration

The atmosphere to which the leaf is exposed drives transpiration, but also causes massive water loss from the plant. Up to 90 percent of the water taken up by roots may be lost through transpiration.

Leaves are covered by a waxy cuticle on the outer surface that prevents the loss of water. Regulation of transpiration, therefore, is achieved primarily through the opening and closing of stomata on the leaf surface. Stomata are surrounded by two specialized cells called guard cells, which open and close in response to environmental cues such as light intensity and quality, leaf water status, and carbon dioxide concentrations. Stomata must open to allow air containing carbon dioxide and oxygen to diffuse into the leaf for photosynthesis and respiration. When stomata are open, however, water vapor is lost to the external environment, increasing the rate of transpiration. Therefore, plants must maintain a balance between efficient photosynthesis and water loss.

Plants have evolved over time to adapt to their local environment and reduce transpiration( Figure 23.35 ). Desert plants (xerophytes) and plants that grow on other plants (epiphytes) have limited access to water. Such plants usually have a much thicker waxy cuticle than those growing in more moderate, well-watered environments (mesophytes). Aquatic plants (hydrophytes) also have their own set of anatomical and morphological leaf adaptations.

Xerophytes and epiphytes often have a thick covering of trichomes or of stomata that are sunken below the leaf’s surface. Trichomes are specialized hair-like epidermal cells that secrete oils and substances. These adaptations impede air flow across the stomatal pore and reduce transpiration. Multiple epidermal layers are also commonly found in these types of plants.

Transportation of Photosynthates in the Phloem

Plants need an energy source to grow. In seeds and bulbs, food is stored in polymers (such as starch) that are converted by metabolic processes into sucrose for newly developing plants. Once green shoots and leaves are growing, plants are able to produce their own food by photosynthesizing. The products of photosynthesis are called photosynthates, which are usually in the form of simple sugars such as sucrose.

Structures that produce photosynthates for the growing plant are referred to as sources . Sugars produced in sources, such as leaves, need to be delivered to growing parts of the plant via the phloem in a process called translocation . The points of sugar delivery, such as roots, young shoots, and developing seeds, are called sinks . Seeds, tubers, and bulbs can be either a source or a sink, depending on the plant’s stage of development and the season.

The products from the source are usually translocated to the nearest sink through the phloem. For example, the highest leaves will send photosynthates upward to the growing shoot tip, whereas lower leaves will direct photosynthates downward to the roots. Intermediate leaves will send products in both directions, unlike the flow in the xylem, which is always unidirectional (soil to leaf to atmosphere). The pattern of photosynthate flow changes as the plant grows and develops. Photosynthates are directed primarily to the roots early on, to shoots and leaves during vegetative growth, and to seeds and fruits during reproductive development. They are also directed to tubers for storage.

Translocation: Transport from Source to Sink

Photosynthates, such as sucrose, are produced in the mesophyll cells of photosynthesizing leaves. From there they are translocated through the phloem to where they are used or stored. Mesophyll cells are connected by cytoplasmic channels called plasmodesmata. Photosynthates move through these channels to reach phloem sieve-tube elements (STEs) in the vascular bundles. From the mesophyll cells, the photosynthates are loaded into the phloem STEs. The sucrose is actively transported against its concentration gradient (a process requiring ATP) into the phloem cells using the electrochemical potential of the proton gradient. This is coupled to the uptake of sucrose with a carrier protein called the sucrose-H + symporter.

Phloem STEs have reduced cytoplasmic contents, and are connected by a sieve plate with pores that allow for pressure-driven bulk flow, or translocation, of phloem sap. Companion cells are associated with STEs. They assist with metabolic activities and produce energy for the STEs ( Figure 23.36 ).

Once in the phloem, the photosynthates are translocated to the closest sink. Phloem sap is an aqueous solution that contains up to 30 percent sugar, minerals, amino acids, and plant growth regulators. The high percentage of sugar decreases Ψ s, which decreases the total water potential and causes water to move by osmosis from the adjacent xylem into the phloem tubes, thereby increasing pressure. This increase in total water potential causes the bulk flow of phloem from source to sink ( Figure 23.37 ). Sucrose concentration in the sink cells is lower than in the phloem STEs because the sink sucrose has been metabolized for growth, or converted to starch for storage or other polymers, such as cellulose, for structural integrity. Unloading at the sink end of the phloem tube occurs by either diffusion or active transport of sucrose molecules from an area of high concentration to one of low concentration. Water diffuses from the phloem by osmosis and is then transpired or recycled via the xylem back into the phloem sap.

Science Practice Connection for AP® Courses

Based on water’s molecular properties, create a visual diagram/model to illustrate how water travels up a 300-foot California redwood tree through xylem.

AP ® Biology Investigative Labs: Inquiry-Based, Investigation 11: Transpiration . Design and conduct a series of experiments to investigate the effects of environmental variables on transpiration rates.

Think About It

Desert travelers claim that cactus juice tastes sweeter during the day than at night. Based on your understanding of photosynthesis, transpiration, and the regulation of stomata by guard cells in response to environmental conditions, is there any validity to this claim?

Teacher Support

• The activity is an application of AP ® Learning Objective 2.9 and Science Practices 1.1 and 1.4 because students are creating a representation to model how the cohesive and adhesive properties of water help move water from the environment to the leaves of a tree where water is necessary for photosynthesis.
• The lab investigation is an application of AP ® Learning Objective 2.8 and Science Practice 4.1 and Learning Objective 2.9 and Science Practices 1.1 and 1.4 because students will collect data to determine the effect(s) of environmental variables on the uptake of water and nutrients necessary for photosynthesis via transpiration.
• Think About It: Cactus juice would taste sweeter during the day than at night because photosynthesis is only partially completed at night, ending with the storing of sour-tasting malic acid in vacuoles. During the day, the malic acid is recovered and converted into CO2 and pyruvate, which enters the Calvin cycle, allowing the completion of photosynthesis and the production of sugars. These sugars would allow cactus juice to taste sweeter during the day than at night.
• The Think About It is an application of AP ® Learning Objective 4.8 and Science Practice 3.3 because students are evaluating a question about environmental variables and transpiration rates.

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30. Plant Form and Physiology

Transport of water and solutes in plants, learning objectives.

By the end of this section, you will be able to do the following:

• Define water potential and explain how it is influenced by solutes, pressure, gravity, and the matric potential
• Describe how water potential, evapotranspiration, and stomatal regulation influence how water is transported in plants
• Explain how photosynthates are transported in plants

The structure of plant roots, stems, and leaves facilitates the transport of water, nutrients, and photosynthates throughout the plant. The phloem and xylem are the main tissues responsible for this movement. Water potential, evapotranspiration, and stomatal regulation influence how water and nutrients are transported in plants. To understand how these processes work, we must first understand the energetics of water potential.

Water Potential

Plants are phenomenal hydraulic engineers. Using only the basic laws of physics and the simple manipulation of potential energy, plants can move water to the top of a 116-meter-tall tree ( (Figure) a ). Plants can also use hydraulics to generate enough force to split rocks and buckle sidewalks ( (Figure) b ). Plants achieve this because of water potential.

Figure 1. With heights nearing 116 meters, (a) coastal redwoods (Sequoia sempervirens) are the tallest trees in the world. Plant roots can easily generate enough force to (b) buckle and break concrete sidewalks, much to the dismay of homeowners and city maintenance departments. (credit a: modification of work by Bernt Rostad; credit b: modification of work by Pedestrians Educating Drivers on Safety, Inc.)

Water potential is a measure of the potential energy in water. Plant physiologists are not interested in the energy in any one particular aqueous system, but are very interested in water movement between two systems. In practical terms, therefore, water potential is the difference in potential energy between a given water sample and pure water (at atmospheric pressure and ambient temperature). Water potential is denoted by the Greek letter ψ ( psi ) and is expressed in units of pressure (pressure is a form of energy) called megapascals (MPa). The potential of pure water (Ψ w pure H2O ) is, by convenience of definition, designated a value of zero (even though pure water contains plenty of potential energy, that energy is ignored). Water potential values for the water in a plant root, stem, or leaf are therefore expressed relative to Ψ w pure H2O .

The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and factors called matrix effects. Water potential can be broken down into its individual components using the following equation:

where Ψ s , Ψ p , Ψ g , and Ψ m refer to the solute, pressure, gravity, and matric potentials, respectively. “System” can refer to the water potential of the soil water (Ψ soil ), root water (Ψ root ), stem water (Ψ stem ), leaf water (Ψ leaf ) or the water in the atmosphere (Ψ atmosphere ): whichever aqueous system is under consideration. As the individual components change, they raise or lower the total water potential of a system. When this happens, water moves to equilibrate, moving from the system or compartment with a higher water potential to the system or compartment with a lower water potential. This brings the difference in water potential between the two systems (ΔΨ) back to zero (ΔΨ = 0). Therefore, for water to move through the plant from the soil to the air (a process called transpiration), Ψ soil must be > Ψ root > Ψ stem > Ψ leaf > Ψ atmosphere .

Water only moves in response to ΔΨ, not in response to the individual components. However, because the individual components influence the total Ψ system , by manipulating the individual components (especially Ψ s ), a plant can control water movement.

Solute Potential

Solute potential (Ψ s ), also called osmotic potential, is negative in a plant cell and zero in distilled water. Typical values for cell cytoplasm are –0.5 to –1.0 MPa. Solutes reduce water potential (resulting in a negative Ψ w ) by consuming some of the potential energy available in the water. Solute molecules can dissolve in water because water molecules can bind to them via hydrogen bonds; a hydrophobic molecule like oil, which cannot bind to water, cannot go into solution. The energy in the hydrogen bonds between solute molecules and water is no longer available to do work in the system because it is tied up in the bond. In other words, the amount of available potential energy is reduced when solutes are added to an aqueous system. Thus, Ψ s decreases with increasing solute concentration. Because Ψ s is one of the four components of Ψ system or Ψ total , a decrease in Ψ s will cause a decrease in Ψ total . The internal water potential of a plant cell is more negative than pure water because of the cytoplasm’s high solute content ( (Figure) ). Because of this difference in water potential water will move from the soil into a plant’s root cells via the process of osmosis. This is why solute potential is sometimes called osmotic potential.

Plant cells can metabolically manipulate Ψ s (and by extension, Ψ total ) by adding or removing solute molecules. Therefore, plants have control over Ψ total via their ability to exert metabolic control over Ψs.

Figure 2. In this example with a semipermeable membrane between two aqueous systems, water will move from a region of higher to lower water potential until equilibrium is reached. Solutes (Ψs), pressure (Ψp), and gravity (Ψg) influence total water potential for each side of the tube (Ψtotalright or left), and therefore, the difference between Ψtotal on each side (ΔΨ). (Ψm , the potential due to interaction of water with solid substrates, is ignored in this example because glass is not especially hydrophilic). Water moves in response to the difference in water potential between two systems (the left and right sides of the tube).

Positive water potential is placed on the left side of the tube by increasing Ψ p such that the water level rises on the right side. Could you equalize the water level on each side of the tube by adding solute, and if so, how?

Yes, you can equalize the water level by adding the solute to the left side of the tube such that water moves toward the left until the water levels are equal.

Pressure Potential

Pressure potential (Ψ p ), also called turgor potential, may be positive or negative ( (Figure) ). Because pressure is an expression of energy, the higher the pressure, the more potential energy in a system, and vice versa. Therefore, a positive Ψp (compression) increases Ψ total , and a negative Ψ p (tension) decreases Ψ total . Positive pressure inside cells is contained by the cell wall, producing turgor pressure. Pressure potentials are typically around 0.6–0.8 MPa, but can reach as high as 1.5 MPa in a well-watered plant. A Ψ p of 1.5 MPa equates to 210 pounds per square inch (1.5 MPa x 140 lb in -2 MPa -1 = 210 lb/in -2 ). As a comparison, most automobile tires are kept at a pressure of 30–34 psi. An example of the effect of turgor pressure is the wilting of leaves and their restoration after the plant has been watered ( (Figure) ). Water is lost from the leaves via transpiration (approaching Ψ p = 0 MPa at the wilting point) and restored by uptake via the roots.

A plant can manipulate Ψ p via its ability to manipulate Ψ s and by the process of osmosis. If a plant cell increases the cytoplasmic solute concentration, Ψ s will decline, Ψ total will decline, the ΔΨ between the cell and the surrounding tissue will decline, water will move into the cell by osmosis, and Ψ p will increase. Ψ p is also under indirect plant control via the opening and closing of stomata. Stomatal openings allow water to evaporate from the leaf, reducing Ψ p and Ψ total of the leaf and increasing Ψ between the water in the leaf and the petiole, thereby allowing water to flow from the petiole into the leaf.

Figure 3. When (a) total water potential (Ψtotal) is lower outside the cells than inside, water moves out of the cells and the plant wilts. When (b) the total water potential is higher outside the plant cells than inside, water moves into the cells, resulting in turgor pressure (Ψp) and keeping the plant erect. (credit: modification of work by Victor M. Vicente Selvas)

Gravity Potential

Gravity potential (Ψ g ) is always negative to zero in a plant with no height. It always removes or consumes potential energy from the system. The force of gravity pulls water downwards to the soil, reducing the total amount of potential energy in the water in the plant (Ψ total ). The taller the plant, the taller the water column, and the more influential Ψ g becomes. On a cellular scale and in short plants, this effect is negligible and easily ignored. However, over the height of a tall tree like a giant coastal redwood, the gravitational pull of –0.1 MPa m -1 is equivalent to an extra 1 MPa of resistance that must be overcome for water to reach the leaves of the tallest trees. Plants are unable to manipulate Ψg.

Matric Potential

Matric potential (Ψ m ) is always negative to zero. In a dry system, it can be as low as –2 MPa in a dry seed, and it is zero in a water-saturated system. The binding of water to a matrix always removes or consumes potential energy from the system. Ψ m is similar to solute potential because it involves tying up the energy in an aqueous system by forming hydrogen bonds between the water and some other component. However, in solute potential, the other components are soluble, hydrophilic solute molecules, whereas in Ψ m , the other components are insoluble, hydrophilic molecules of the plant cell wall. Every plant cell has a cellulosic cell wall and the cellulose in the cell walls is hydrophilic, producing a matrix for adhesion of water: hence the name matric potential. Ψ m is very large (negative) in dry tissues such as seeds or drought-affected soils. However, it quickly goes to zero as the seed takes up water or the soil hydrates. Ψ m cannot be manipulated by the plant and is typically ignored in well-watered roots, stems, and leaves.

Movement of Water and Minerals in the Xylem

Solutes, pressure, gravity, and matric potential are all important for the transport of water in plants. Water moves from an area of higher total water potential (higher Gibbs free energy) to an area of lower total water potential. Gibbs free energy is the energy associated with a chemical reaction that can be used to do work. This is expressed as ΔΨ.

Transpiration is the loss of water from the plant through evaporation at the leaf surface. It is the main driver of water movement in the xylem. Transpiration is caused by the evaporation of water at the leaf–atmosphere interface; it creates negative pressure (tension) equivalent to –2 MPa at the leaf surface. This value varies greatly depending on the vapor pressure deficit, which can be negligible at high relative humidity (RH) and substantial at low RH. Water from the roots is pulled up by this tension. At night, when stomata shut and transpiration stops, the water is held in the stem and leaf by the adhesion of water to the cell walls of the xylem vessels and tracheids, and the cohesion of water molecules to each other. This is called the cohesion–tension theory of sap ascent.

Inside the leaf at the cellular level, water on the surface of mesophyll cells saturates the cellulose microfibrils of the primary cell wall. The leaf contains many large intercellular air spaces for the exchange of oxygen for carbon dioxide, which is required for photosynthesis. The wet cell wall is exposed to this leaf internal air space, and the water on the surface of the cells evaporates into the air spaces, decreasing the thin film on the surface of the mesophyll cells. This decrease creates a greater tension on the water in the mesophyll cells ( (Figure) ), thereby increasing the pull on the water in the xylem vessels. The xylem vessels and tracheids are structurally adapted to cope with large changes in pressure. Rings in the vessels maintain their tubular shape, much like the rings on a vacuum cleaner hose keep the hose open while it is under pressure. Small perforations between vessel elements reduce the number and size of gas bubbles that can form via a process called cavitation. The formation of gas bubbles in xylem interrupts the continuous stream of water from the base to the top of the plant, causing a break termed an embolism in the flow of xylem sap. The taller the tree, the greater the tension forces needed to pull water, and the more cavitation events. In larger trees, the resulting embolisms can plug xylem vessels, making them nonfunctional.

Art Connection

Figure 4. The cohesion–tension theory of sap ascent is shown. Evaporation from the mesophyll cells produces a negative water potential gradient that causes water to move upwards from the roots through the xylem.

Which of the following statements is false?

• Negative water potential draws water into the root hairs. Cohesion and adhesion draw water up the xylem. Transpiration draws water from the leaf.
• Negative water potential draws water into the root hairs. Cohesion and adhesion draw water up the phloem. Transpiration draws water from the leaf.
• Water potential decreases from the roots to the top of the plant.
• Water enters the plants through root hairs and exits through stoma.

Transpiration—the loss of water vapor to the atmosphere through stomata—is a passive process, meaning that metabolic energy in the form of ATP is not required for water movement. The energy driving transpiration is the difference in energy between the water in the soil and the water in the atmosphere. However, transpiration is tightly controlled.

Control of Transpiration

The atmosphere to which the leaf is exposed drives transpiration, but also causes massive water loss from the plant. Up to 90 percent of the water taken up by roots may be lost through transpiration.

Leaves are covered by a waxy cuticle on the outer surface that prevents the loss of water. Regulation of transpiration, therefore, is achieved primarily through the opening and closing of stomata on the leaf surface. Stomata are surrounded by two specialized cells called guard cells, which open and close in response to environmental cues such as light intensity and quality, leaf water status, and carbon dioxide concentrations. Stomata must open to allow air containing carbon dioxide and oxygen to diffuse into the leaf for photosynthesis and respiration. When stomata are open, however, water vapor is lost to the external environment, increasing the rate of transpiration. Therefore, plants must maintain a balance between efficient photosynthesis and water loss.

Plants have evolved over time to adapt to their local environment and reduce transpiration ( (Figure) ). Desert plant (xerophytes) and plants that grow on other plants (epiphytes) have limited access to water. Such plants usually have a much thicker waxy cuticle than those growing in more moderate, well-watered environments (mesophytes). Aquatic plants (hydrophytes) also have their own set of anatomical and morphological leaf adaptations.

Figure 5. Plants are suited to their local environment. (a) Xerophytes, like this prickly pear cactus (Opuntia sp.) and (b) epiphytes such as this tropical Aeschynanthus perrottetii have adapted to very limited water resources. The leaves of a prickly pear are modified into spines, which lowers the surface-to-volume ratio and reduces water loss. Photosynthesis takes place in the stem, which also stores water. (b) A. perottetii leaves have a waxy cuticle that prevents water loss. (c) Goldenrod (Solidago sp.) is a mesophyte, well suited for moderate environments. (d) Hydrophytes, like this fragrant water lily (Nymphaea odorata), are adapted to thrive in aquatic environments. (credit a: modification of work by Jon Sullivan; credit b: modification of work by L. Shyamal/Wikimedia Commons; credit c: modification of work by Huw Williams; credit d: modification of work by Jason Hollinger)

Xerophytes and epiphytes often have a thick covering of trichomes or of stomata that are sunken below the leaf’s surface. Trichomes are specialized hair-like epidermal cells that secrete oils and substances. These adaptations impede air flow across the stomatal pore and reduce transpiration. Multiple epidermal layers are also commonly found in these types of plants.

Transportation of Photosynthates in the Phloem

Plants need an energy source to grow. In seeds and bulbs, food is stored in polymers (such as starch) that are converted by metabolic processes into sucrose for newly developing plants. Once green shoots and leaves are growing, plants are able to produce their own food by photosynthesizing. The products of photosynthesis are called photosynthates, which are usually in the form of simple sugars such as sucrose.

Structures that produce photosynthates for the growing plant are referred to as sources. Sugars produced in sources, such as leaves, need to be delivered to growing parts of the plant via the phloem in a process called translocation. The points of sugar delivery, such as roots, young shoots, and developing seeds, are called sinks. Seeds, tubers, and bulbs can be either a source or a sink, depending on the plant’s stage of development and the season.

The products from the source are usually translocated to the nearest sink through the phloem. For example, the highest leaves will send photosynthates upward to the growing shoot tip, whereas lower leaves will direct photosynthates downward to the roots. Intermediate leaves will send products in both directions, unlike the flow in the xylem, which is always unidirectional (soil to leaf to atmosphere). The pattern of photosynthate flow changes as the plant grows and develops. Photosynthates are directed primarily to the roots early on, to shoots and leaves during vegetative growth, and to seeds and fruits during reproductive development. They are also directed to tubers for storage.

Translocation: Transport from Source to Sink

Photosynthates, such as sucrose, are produced in the mesophyll cells of photosynthesizing leaves. From there they are translocated through the phloem to where they are used or stored. Mesophyll cells are connected by cytoplasmic channels called plasmodesmata. Photosynthates move through these channels to reach phloem sieve-tube elements (STEs) in the vascular bundles. From the mesophyll cells, the photosynthates are loaded into the phloem STEs. The sucrose is actively transported against its concentration gradient (a process requiring ATP) into the phloem cells using the electrochemical potential of the proton gradient. This is coupled to the uptake of sucrose with a carrier protein called the sucrose-H + symporter.

Phloem STEs have reduced cytoplasmic contents, and are connected by a sieve plate with pores that allow for pressure-driven bulk flow, or translocation, of phloem sap. Companion cells are associated with STEs. They assist with metabolic activities and produce energy for the STEs ( (Figure) ).

Figure 6. Phloem is comprised of cells called sieve-tube elements. Phloem sap travels through perforations called sieve tube plates. Neighboring companion cells carry out metabolic functions for the sieve-tube elements and provide them with energy. Lateral sieve areas connect the sieve-tube elements to the companion cells.

Once in the phloem, the photosynthates are translocated to the closest sink. Phloem sap is an aqueous solution that contains up to 30 percent sugar, minerals, amino acids, and plant growth regulators. The high percentage of sugar decreases Ψ s, which decreases the total water potential and causes water to move by osmosis from the adjacent xylem into the phloem tubes, thereby increasing pressure. This increase in total water potential causes the bulk flow of phloem from source to sink ( (Figure) ). Sucrose concentration in the sink cells is lower than in the phloem STEs because the sink sucrose has been metabolized for growth, or converted to starch for storage or other polymers, such as cellulose, for structural integrity. Unloading at the sink end of the phloem tube occurs by either diffusion or active transport of sucrose molecules from an area of high concentration to one of low concentration. Water diffuses from the phloem by osmosis and is then transpired or recycled via the xylem back into the phloem sap.

Figure 8. Sucrose is actively transported from source cells into companion cells and then into the sieve-tube elements. This reduces the water potential, which causes water to enter the phloem from the xylem. The resulting positive pressure forces the sucrose-water mixture down toward the roots, where sucrose is unloaded. Transpiration causes water to return to the leaves through the xylem vessels.

Section Summary

Water potential (Ψ) is a measure of the difference in potential energy between a water sample and pure water. The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and matric potential. Water potential and transpiration influence how water is transported through the xylem in plants. These processes are regulated by stomatal opening and closing. Photosynthates (mainly sucrose) move from sources to sinks through the plant’s phloem. Sucrose is actively loaded into the sieve-tube elements of the phloem. The increased solute concentration causes water to move by osmosis from the xylem into the phloem. The positive pressure that is produced pushes water and solutes down the pressure gradient. The sucrose is unloaded into the sink, and the water returns to the xylem vessels.

Art Connections

(Figure) Positive water potential is placed on the left side of the tube by increasing Ψ p such that the water level rises on the right side. Could you equalize the water level on each side of the tube by adding solute, and if so, how?

(Figure) Yes, you can equalize the water level by adding the solute to the left side of the tube such that water moves toward the left until the water levels are equal.

(Figure) Which of the following statements is false?

Show Answer

Review Questions

When stomata open, what occurs?

• Water vapor is lost to the external environment, increasing the rate of transpiration.
• Water vapor is lost to the external environment, decreasing the rate of transpiration.
• Water vapor enters the spaces in the mesophyll, increasing the rate of transpiration.
• Water vapor enters the spaces in the mesophyll, decreasing the rate of transpiration.

Show Solution

Which cells are responsible for the movement of photosynthates through a plant?

• tracheids, vessel elements
• tracheids, companion cells
• vessel elements, companion cells
• sieve-tube elements, companion cells

Water transport, perception, and response in plants

• JPR Symposium
• Toward unveiling plant adaptation mechanisms to environmental stresses
• Published: 11 February 2019
• Volume 132 , pages 311–324, ( 2019 )

Cite this article

• Johannes Daniel Scharwies 1 , 2 &
• José R. Dinneny 1 , 2  

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Sufficient water availability in the environment is critical for plant survival. Perception of water by plants is necessary to balance water uptake and water loss and to control plant growth. Plant physiology and soil science research have contributed greatly to our understanding of how water moves through soil, is taken up by roots, and moves to leaves where it is lost to the atmosphere by transpiration. Water uptake from the soil is affected by soil texture itself and soil water content. Hydraulic resistances for water flow through soil can be a major limitation for plant water uptake. Changes in water supply and water loss affect water potential gradients inside plants. Likewise, growth creates water potential gradients. It is known that plants respond to changes in these gradients. Water flow and loss are controlled through stomata and regulation of hydraulic conductance via aquaporins. When water availability declines, water loss is limited through stomatal closure and by adjusting hydraulic conductance to maintain cell turgor. Plants also adapt to changes in water supply by growing their roots towards water and through refinements to their root system architecture. Mechanosensitive ion channels, aquaporins, proteins that sense the cell wall and cell membrane environment, and proteins that change conformation in response to osmotic or turgor changes could serve as putative sensors. Future research is required to better understand processes in the rhizosphere during soil drying and how plants respond to spatial differences in water availability. It remains to be investigated how changes in water availability and water loss affect different tissues and cells in plants and how these biophysical signals are translated into chemical signals that feed into signaling pathways like abscisic acid response or organ development.

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Acknowledgements

The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR 1565-1555 and in part by a Faculty Scholar grant from the Howard Hughes Medical Institute and the Simons Foundation, both awarded to JRD.

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Scharwies, J.D., Dinneny, J.R. Water transport, perception, and response in plants. J Plant Res 132 , 311–324 (2019). https://doi.org/10.1007/s10265-019-01089-8

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Received : 12 December 2018

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Absorption and transport of water in plants

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TRANSPORT IN PLANTS.

Unit Plant Science.

Transport in Plants Explain the need for transport systems in multicellular plants in terms of size and surface area:volume ratio; Describe, with the aid.

Transport in Plants Explain the need for transport systems in multicellular plants in terms of size and surface- area-to-volume ratio. Describe the distribution.

TRANSPORT in PLANTS.

Transportation of Water

9.2 Transport in angiospermophytes

Leaf Structure and Function

Chapter : Transport in Flowering Plants

Transport in plants.

Transport in Plants.

IB Assessment Statements Define Transpiration Explain how water is carried by the transpirational stream, including structure of xylem vessels,

Transport in Angiospermatophyta

36 Resource Acquisition and Transport in Vascular Plants.

Transport in Vascular Plants Chapter 36. Transport in Plants Occurs on three levels:  the uptake and loss of water and solutes by individual cells 

Question ? u How do plants move materials from one organ to the other ?

Long-Distance Transport in Plants Biology 1001 November 21, 2005.

Chapter 36: Transport in Plants.

Absorption and Transport Chapter 11. Fig. 11-1, p. 164 H2OH2O product of photosynthesis (sucrose) H 2 O vapor H2OH2O mineral ions.

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Transport in Plants | PPT PDF slides | Class 11/Plus 1/CBSE

👉 part 2: plant-water relations, 👉 part 3: long distance transport of water, 👉 part 4: transpiration, xylem transport, 👉 part 5: phloem transport.

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So helpful. Simply superb to understand.thank u so much

Very very helpful

Just awesome no words to say just want to bless the guy who made it had never ever thought that I could even get those concepts cleared by ppt hats off to the guy

Excellent presentation. It meets the requirements of most of the syllabus specifications.

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Study shows how water transport in plants is regulated by cell membranes

In a recent study published in Nature Communications, scientists from the University of Illinois Urbana-Champaign explored how cell membranes can change the behavior of proteins embedded within them.

In a recent study published in Nature Communications, scientists from the University of Illinois Urbana-Champaign explored how cell membranes can change the behavior of proteins embedded within them. Chemical and biomolecular engineering professor Diwakar Shukla led the research.

In the study, researchers focused on a specific protein called aquaporin, which acts as a microscopic water channel in cells. Aquaporins allow water molecules to pass through cell membranes while blocking other substances. Proper regulation of aquaporins is crucial for maintaining cell function, hydration and osmotic balance.

“Traditionally, we thought that a protein’s structure determined its function,” Shukla said. “However, proteins exist in a dynamic cellular environment, which can significantly affect their behavior. Until now, we didn’t fully understand how the lipid bilayer – a component of cell membranes – influences aquaporin function and dynamics.”

The team studied aquaporins because they are present in a wide variety of life forms, from yeast and bacteria to more complex organisms. These proteins play critical roles in human diseases and water regulation in plants, making their regulation important for drug development and conservation efforts. No matter where they are found or what type of membrane they are in, aquaporins must work properly, Shukla said.

The study revealed that the choice of membrane can significantly impact the thermodynamics, kinetics and overall behavior of proteins. By understanding how aquaporins behave in different cell membrane environments, researchers can gain insights into their roles in water conduction in different parts of the plants. Adjusting the local lipid conditions around aquaporins could help enhance their desired functions.

“We have a limited understanding of how plant membrane protein function is regulated due to the lack of extensive structural, sequence and functional datasets,” he said. “This study provides comprehensive insights into the functional regulation of a key plant protein.”

Under the oversight of Shukla, the research was carried out by Anh T.P. Nguyen and Austin T. Weigle while they were chemical and biomolecular engineering students. Nguyen is currently a graduate student at the Massachusetts Institute of Technology. Weigle is a SCINet/AI-CoE Postdoctoral Fellow at the U.S. Department of Agriculture and Agricultural Research Services.

The National Institute of General Medical Sciences of the National Institutes of Health supported this work.

Editor’s Note:

To reach Diwakar Shukla, email [email protected]

The paper “Functional regulation of aquaporin dynamics by lipid bilayer composition” is available online . DOI:10.1038/s41467-024-46027-y

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This story was published March 29, 2024.

What we know about the container ship that crashed into the Baltimore bridge

• The ship that crashed into the Francis Scott Key Bridge on Tuesday was the Singapore-flagged Dali.
• The container ship had been chartered by Maersk, the Danish shipping company. 
• Two people were recovered from the water but six remain missing, authorities said.

A container ship crashed into a major bridge in Baltimore early Tuesday, causing its collapse into the Patapsco River.

A livestream showed vehicles traveling on the Francis Scott Key Bridge just moments before the impact at 1:28 a.m. ET.

Baltimore first responders called the situation a "developing mass casualty event" and a "dire emergency," per The Associated Press.

James Wallace, chief of the Baltimore Fire Department, said in a press conference that two people had been recovered from the water.

One was uninjured, but the other was transported to a local trauma center in a "very serious condition."

Wallace said up to 20 people were thought to have fallen into the river and some six people were still missing.

Richard Worley, Baltimore's police chief, said there was "no indication" the collision was purposeful or an act of terrorism.

Wes Moore, the governor of Maryland, declared a state of emergency around 6 a.m. ET. He said his office was in close communication with Pete Buttigieg, the transportation secretary.

"We are working with an interagency team to quickly deploy federal resources from the Biden Administration," Moore added.

Understanding why the bridge collapsed could have implications for safety, in both the shipping and civil engineering sectors.

The container ship is the Singapore-flagged Dali, which is about 984 feet long, and 157 feet wide, per a listing on VesselFinder.

An unclassified Cybersecurity and Infrastructure Security Agency report said that the ship "lost propulsion" as it was leaving port, ABC News reported.

The crew notified officials that they had lost control and warned of a possible collision, the report said, per the outlet.

The Dali's owner is listed as Grace Ocean, a Singapore-based firm, and its manager is listed as Synergy Marine, which is also headquartered in Singapore.

Shipping news outlet TradeWinds reported that Grace Ocean confirmed the Dali was involved in the collapse, but is still determining what caused the crash.

Related stories

Staff for Grace Ocean declined to comment on the collision when contacted by Business Insider.

"All crew members, including the two pilots have been accounted for and there are no reports of any injuries. There has also been no pollution," Synergy Marine said in a statement.

The company did not respond to a request for further comment from BI.

'Horrified'

Maersk chartered the Dali, with a schedule for the ship on its website.

"We are horrified by what has happened in Baltimore, and our thoughts are with all of those affected," the Danish shipping company said in a statement.

Maersk added: "We are closely following the investigations conducted by authorities and Synergy, and we will do our utmost to keep our customers informed."

Per ship tracking data, the Dali left Baltimore on its way to Colombo, the capital of Sri Lanka, at around 1 a.m., about half an hour before the crash.

The Port of Baltimore is thought to be the largest in the US for roll-on/roll-off ships carrying trucks and trailers.

Barbara Rossi, associate professor of engineering science at the University of Oxford, told BI the force of the impact on one of the bridge's supporting structures "must have been immense" to lead to the collapse.

Dr Salvatore Mercogliano, a shipping analyst and maritime historian at Campbell University, told BI: "It appears Dali left the channel while outbound. She would have been under the control of the ship's master with a Chesapeake Bay pilot onboard to advise the master.

"The deviation out of the channel is probably due to a mechanical issue as the ship had just departed the port, but you cannot rule out human error as that was the cause of the Ever Forward in 2022 just outside of Baltimore."

He was referring to the incident two years ago when the container ship became grounded for a month in Chesapeake Bay after loading up cargo at the Port of Baltimore.

The US Coast Guard found the incident was caused by pilot error, cellphone use, and "inadequate bridge resource management."

Claudia Norrgren, from the maritime research firm Veson Nautical, told BI: "The industry bodies who are here to protect against incidents like this, such as the vessel's flag state, classification society, and regulatory bodies, will step in and conduct a formal investigation into the incident. Until then, it'll be very hard for anyone to truly know what happened on board."

This may not have been the first time the Dali hit a structure.

In 2016, maritime blogs such as Shipwreck Log and ship-tracking site VesselFinder posted videos of what appears to be the stern of the same, blue-hulled container vessel scraping against a quay in Antwerp.

A representative for the Port of Antwerp told BI the Dali did collide with a quay there eight years ago but couldn't "give any information about the cause of the accident."

The Dali is listed as being built in 2015 by Hyundai Heavy Industries in South Korea.

Watch: The shipwreck at the center of a battle between China and the Philippines

• Main content

What we know about Baltimore’s Francis Scott Key Bridge collapse

The Francis Scott Key Bridge in Baltimore collapsed early Tuesday after being hit by a cargo ship, with large parts of the bridge falling into the Patapsco River.

At least eight people fell into the water, members of a construction crew working on the bridge at the time, officials said. Two were rescued, one uninjured and one in serious condition, and two bodies were recovered on Wednesday. The remaining four are presumed dead. The workers are believed to be the only victims in the disaster.

Here’s what we know so far.

Baltimore bridge collapse

How it happened: Baltimore’s Francis Scott Key Bridge collapsed after being hit by a cargo ship . The container ship lost power shortly before hitting the bridge, Maryland Gov. Wes Moore (D) said. Video shows the bridge collapse in under 40 seconds.

Victims: Divers have recovered the bodies of two construction workers , officials said. They were fathers, husbands and hard workers . A mayday call from the ship prompted first responders to shut down traffic on the four-lane bridge, saving lives.

Economic impact: The collapse of the bridge severed ocean links to the Port of Baltimore, which provides about 20,000 jobs to the area . See how the collapse will disrupt the supply of cars, coal and other goods .

Rebuilding: The bridge, built in the 1970s , will probably take years and cost hundreds of millions of dollars to rebuild , experts said.

• Baltimore bridge collapse: Crane arrives at crash site to aid cleanup March 29, 2024 Baltimore bridge collapse: Crane arrives at crash site to aid cleanup March 29, 2024
• Wes Moore envisioned economic revival. Then the Key Bridge collapsed. April 1, 2024 Wes Moore envisioned economic revival. Then the Key Bridge collapsed. April 1, 2024
• Officials studied Baltimore bridge risks but didn’t prepare for ship strike March 29, 2024 Officials studied Baltimore bridge risks but didn’t prepare for ship strike March 29, 2024

Transportation of Water in Plants

Sep 07, 2014

290 likes | 864 Views

Transportation of Water in Plants. By Arsalaan Muhammad 8a E-portfolio Project. So what basically happens:.

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• water molecules
• root pressure
• water moves
• water molecules stick
• long day plant plant

Presentation Transcript

Transportation of Water in Plants • By Arsalaan Muhammad • 8a • E-portfolio Project

So what basically happens: • In every plant, water moves from the ground, through the roots, up the stem, enters the leaves, and finally exits from the plant through the leaves and repeats. In the following slide show, you will find out what how water is transported through a plant. This is fun waiting to be seen!

It all starts at the roots • The water and any other Soluble nutrients disseminate in through the roots to the xylem and then are distributed where needed in the plant. Root hairs help absorb the water and others nutrients in the soil. From here, there is a process called osmosis, which is when water moves from a region of higher concentration to one of lower concentration.

Parts of the roots • The outer layer of the root is the epidermis. This region produces root hairs which project into the soil increasing surface area for uptake of water. • Below the epidermis is the cortex. Cortical cells are highly permeable to water and dissolved solutes. • Below the cortex is a thin layer called the endodermis. The endodermis controls flow of water and minerals within the plant. • At the centre of the root is the stele. This region contains the vascular tissues (the xylem and phloem) surrounded by a layer of cells called the pericycle. The remainder of the stele is the vascular cambium. These cells are able to divide to form new xylem and phloem increasing the size of the roots.

So what are the Xylem and Phloem?

What causes the water to go up the plant? • The answer to this is the Root Pressure and Capillary Action. • Root Pressure: Water moves up through a plant because of root pressure and capillary action. Root pressure occurs during times when transpiration is low but the soil is very moist, and the roots absorb too much water. Because the water accumulates in the plant, a slight root pressure is created that pushes the xylem sap to the tips or edges of leaves, where it forms drops (the process is called guttation). • Capillary action: During capillary action, water rises up the walls of the thin, porous xylem tube as a result of the forces of adhesion, cohesion and surface tension. Xylem tubes are made of cellulose, to which water molecules stick.

Next Part of the Journey- Reaching the Xylem • The water moves across the root from the hair cell to the xylem by three different pathways: • apoplast pathway - water passes through the cell walls and from the wall of one cell to the wall of an adjacent cell • symplast pathway - water moves through the cytoplasm and from the cytoplasm of one cell into the cytoplasm of the next cell via the plasmodesmata- (a narrow thread of cytoplasm that passes through the cell walls of adjacent plant cells and allows communication between them.) • vacuolar pathway - water passes from the vacuole of one cell into the vacuole of the next cell • Water is not completely free to move from the root hair cell to the xylem. • The apoplastic pathway is blocked when it reaches the endodermis because the endodermal cells have a waxy, waterproof layer which prevents the further passage of water through the wall. This waxy layer is called the Casparian strip and is shown in the diagram below. • The Casparian strip allows the endodermis to regulate the quantity of water entering the xylem.

In the xylem • Once in the xylem, water with the minerals that have been deposited in it (as well as occasional organic molecules supplied by the root tissue) move up in the vessels and tracheids. At any level, the water can leave the xylem and pass to supply the needs of other tissues. At the leaves, the xylem passes into the petiole and then into the veins of the leaf. Water leaves the finest veins and enters the cells of the spongy (temporarily stores the sugars and amino acids synthesized in the palisade mesophyll) and palisade layers. Here some of the water may be used in metabolism, but most is lost in transpiration.

Cohesion and Adhesion Theory • Cohesion: The cohesion theory is when water molecules are strongly attracted to each. Cohesion is whereby the driving force of transport is transpiration, that is, the evaporation of water from the leaf surfaces. Water molecules cohere (stick together), and are pulled up the plant by the tension, or pulling force, exerted by evaporation at the leaf surface. • Adhesion is when water molecules adhere or stick to the walls of the xylem. When unequal distribution of ions go across the membrane, solutes move along the concentration gradient. when unequal distribution of ions go across the membrane, solutes move along the concentration gradient. This is called diffusion.

The leaves (main parts) • Leaves have xylem and phloem tubes. • The epidermis is on the outside to protect the leaf. The epidermal layer of a leaf protects the tissues which lie between them and also help in the process of gaseous exchange. • The palisade mesophyll (photosynthesis cell) have chloroplast which have green substance called chlorophyl. • Spongy mesophyll protects the epidermis. They are more rounded and are not as tightly packed as palisades. They contain less chlorophyll. • Stomata's connect to air spaces between mesophyll. Carbon dioxide travels through stomata at the bottom of the leaf. • Light energy which chloroplast creates turns carbon dioxide and water to create glucose and oxygen • Photosynthesis takes place in chloroplasts in leaves which absorbs sunlight. Photosynthesis is the process by which plants convert energy from the sun. • Plants use sugar to make starch, fats and proteins.

Transpiration • Once the water reaches the xylem it moves upwards as a result of transpiration. So basically, transpiration is kind of evaporation. Leaves have openings called stomata which are guard cells which open and close. They only respond to light, meaning that the plant doesn’t transpire as much at night. Stomata's circulate carbon dioxide from the air for photosynthesis to enter the leaf as well as regulate the amount of water in the leaf. In the process, water evaporates from the leaf. Transpiration cools plants and allows large amounts of nutrients and water to shoots. • The formula for photosynthesis: • carbon dioxide + water (+ light energy) → glucose + oxygen = • 6 CO2 + 6 H2O → C6H12O6 + 6 O2

Bibliography • Walker Keenan. "Gymnosperms." 2009. Web. 20 Mar. 2011. <http://sharon-taxonomy2009-p2.wikispaces.com/Gymnosperms>. • "Transport of Water and Minerals in Plants." 16 Dec. 2010. Web. 18 Mar. 2011. <http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/X/Xylem.html>. • Ridwan. "TRANSPORTATION IN PLANTS." Scribd. Web. 23 Mar. 2011. <http://www.scribd.com/doc/4801010/TRANSPORTATION-IN-PLANTS>. • Water Transport in Plants. "Water Transport in Plants." Nick's Pages - Mostly Biology. Web. 23 Mar. 2011. <http://www.nicksnowden.net/Module_2_Biology_pages/water_transport_in_plants.htm>. • "SparkNotes: Plants: Essential Processes: Water Transport." SparkNotes: Today's Most Popular Study Guides. Web. 23 Mar. 2011. <http://www.sparknotes.com/biology/plants/essentialprocesses/section1.html>. • Holbrook, N. M., M. J. Burns, and C. B. Field. "Negative Xylem Pressures in Plants: A Test of the Balancing Pressure Technique." Science 270 (1995): 1183–1192. • Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999. • Taiz, Lincoln, and Eduardo Zeiger. Plant Physiology, 2nd ed. Sunderland, MA: Sinauer Associates, 1998. • "Water Movement in Plants - Biology Encyclopedia - Cells, Body, Function, Process, Used, Structure, Molecules, Energy." Biology Reference. Web. 23 Mar. 2011. <http://www.biologyreference.com/Ve-Z/Water-Movement-in-Plants.html>. • "Water in Plants - Biology Online." Life Science Reference - Biology Online. Web. 23 Mar. 2011. <http://www.biology-online.org/11/8_water_in_plants.htm>.http://www.youtube.com/watch?v=DpFU-NkKUqg • Scully, Lizzy. "How Water Moves Through Plants | EHow.com." EHow | How To Do Just About Everything! | How To Videos & Articles | EHow.com. Web. 23 Mar. 2011. <http://www.ehow.com/how-does_4912679_how-water-moves-through-plants.html>.

Glossary • Abscisic Acid  -  The best known of the inhibitor hormones; inhibits growth and prolongs dormancy. • Acid Growth Hypothesis  -  Explains phototropism by suggesting that increased acidity in the walls of certain cells (stimulated by the hormone auxin) increases their flexibility and expandability, so that more water can diffuse into the cells and cause cell elongation. • Active Transport  -  Movement of substances across cell membranes that requires energy expenditure on the part of the cell; contrasts with passive diffusion, or osmosis. • Apoplast  -  The pathway from the root surface to the core by which water moves along cell walls and through intercellular spaces, bypassing the cells themselves. • Auxin  -  One in a class of plant hormones that stimulates (among other things) cell elongation, secondary tissue growth, and fruit development. • Cytokinin  -  One in a class of plant hormones that promotes cell division and tissue growth. • Day-Neutral Plant  -  Plant in which blooming is not affected by photoperiod, so that flowering occurs independently of the duration of day and night. • Ethylene  -  A plant hormone that controls fruit ripening and promotes senescence (aging). • Florigen  -  Name given to the hypothetical hormone that might control flowering in plants. • Gibberellin  -  One of a class of plant hormones that stimulates stem elongation, germination, and conversion of the embryonic food source into usable sugars. • Gravitotropism  -  Reaction of a plant to gravity; a stem grows against gravity, roots toward gravity. • Hormone  -  A hormone is a chemical that affects the ways in which an organism functions; it is produced in one part of the plant body but, by traveling to target cells throughout the body, affects many other parts as well. • Inhibitor  -  One in a class of plant hormones that inhibits growth and prolongs dormancy in buds and seeds. • Leaf Abscission  -  Hormone-stimulated leaf loss; caused by the formation of a weak, thin-walled abscission layer at the base of the leaf. • Long-Day Plant  -  Plant in which blooming is affected by photoperiod so that flowering occurs when the hours of darkness in a 24-hour photoperiod fall below a certain level. • Osmosis  -  The passive diffusion of water across a membrane. Osmotic concentration refers to the concentration of solutes (dissolved substances) in the water; when the osmotic concentrations of two regions differ, water will flow from the area of low concentration to the area of high concentration. In contrast, the solutes themselves will flow from areas of high osmotic concentration to areas of low osmotic concentration. • Phloem  -  Vascular tissue composed of cells that are living at maturity; transports the products of photosynthesis throughout the plant body. • Photoperiodism  -  An organism's response to the length of day and night within a 24-hour period (photoperiod); in many plants, this phenomenon determines when flowering will occur. • Photosynthesis  -  The process by which plants and other autotrophic organisms convert light energy into vital organic materials. • Phototropism  -  The growth of a plant toward a light source, resulting from the rapid elongation of cells on the dark side of the plant; stimulated by auxin. • Phytochrome  -  Pigment in leaves that allows them to measure the duration of day and night. • Pressure Flow  -  The mechanism by which sugars are transported through the phloem, from sources to sinks; dependent upon the high turgor pressure of sources and the low turgor pressure of sinks. • Root Hair  -  An outgrowth of a plant root that provides an increased surface area for the absorption of water and dissolved minerals from the soil. • Short-Day Plant  -  Plant in which blooming is affected by photoperiod so that flowering occurs when the hours of darkness in a 24-hour photoperiod rise above a certain level. • Sieve Element  -  A living conductive cell of phloem. • Sink  -  Regions of the plant, such as growing tissues, that are in need of nutrients; characterized by low turgor pressure. • Source  -  Nutrient-rich region, such as a leaf, that supplies sugars for the rest of the plant; characterized by high turgor pressure. • Symplast  -  The pathway from the root surface to the core by which water enters the root hair membrane and travels through the cytoplasm of adjacent cells, via channels that connect their contents. • Target Cell  -  A cell that receives hormone signals. • TATC  -  Transpiration-Adhesion-Tension-Cohesion; the mechanism by which scientists theorize that fluids are pulled upward through the xylem (driven by transpiration, the evaporation of water from the leaf, and the cohesion between water molecules). • Thigmotropism  -  Reaction of a plant to touch; results from differential cell elongation. • Transpiration  -  The process by which a plant loses water to its environment through evaporation. • Tropism  -  Long-term growth of a plant toward or away from a stimulus as a result of differential cell elongation. • Turgor Movement  -  Relatively rapid, easily reversible plant movement, occurring in response to a stimulus, that results from changes in turgor pressure in certain plant cells. • Turgor Pressure  -  The force that the contents of a plant cell exert on the cell wall after the osmotic entry of water into the cell. • Vascular System  -  Mechanism of internal water and nutrient transport, made up of the vascular tissues xylem and phloem, that is characteristic of tracheophytes. • Vascular Tissue  -  A conductile component (either xylem or phloem) of the system that transports food and nutrients throughout the plant body. • Water Potential  -  The pressure that causes water to move across a membrane; water always moves naturally from areas of higher water potential to those of lower water potential. • Xylem  -  Vascular tissue composed of cells that are dead at maturity; transports water and dissolved minerals upwards from the roots to the shoot.

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