MODERN BOTANY - P. RAVEN - 1990

SECTION VII. WATER RELATIONS AND TRANSPORT IN PLANTS

CHAPTER 27. WATER AND SOLUTE TRANSPORT IN PLANTS

The ability of a plant to transport organic and inorganic substances, including Water, is essential both for its normal functioning and for maintaining its form and Structure. In the first part of this chapter, we will examine the uptake of water and dissolved substances from the soil to the aerial PARTS OF THE plant. Then, at the end of the chapter, we will discuss how soluble substances and water move from sites of Photosynthesis to the rest of the plant. First, we will focus on The process of Transpiration, as it is the primary driver of water movement within the plant.

Water Movement in Plants

Transpiration

In the early eighteenth century, the English physician S. Hales observed that plants absorb significantly more water than animals. He calculated that, per unit of mass, a single sunflower plant "absorbs" and "transpires" 17 times more water every 24 hours than a human (Fig. 27-1). Indeed, the total amount of water absorbed by a plant is enormous—far exceeding that used by an animal of comparable weight. Animals require less water because it is continually recycled within their bodies as Blood Plasma and other fluids. In plants, more than 90% of the water absorbed by the roots is released into the atmosphere as water vapor (Table 27-1). By definition, this process, known as transpiration, is the loss of water vapor from any part of the plant, although the leaves are the primary Organs of transpiration.

Class="center">Fig. 27-1. Schematic drawing of the sunflower plant used by Hales in his studies on water movement in plants. Hales discovered that most of the water the plant "absorbs" is subsequently lost through "perspiration"

Table 27-1. Water Loss via Transpiration by a Single Plant over a Growing Season

Why do plants lose so much water through transpiration? This question can be answered by examining photosynthesis—the primary process occurring in leaves that supplies nutrients to the entire plant. The energy required for photosynthesis comes from the Sun. Therefore, to maximize photosynthesis, a plant must spread out and expose the largest possible surface area to sunlight. However, photosynthesis requires more than just light; METABOLISM/14.html">Chloroplasts also need carbon dioxide. In most cases, Carbon dioxide is readily available from the surrounding air, but it can only enter Cells (via diffusion) in a dissolved state, as Cell/30.html">The Plasma Membrane is virtually impermeable to carbon dioxide in gaseous form. Consequently, carbon dioxide must come into contact with a moist cell surface. Yet, as soon as water reaches the leaf surface, evaporation begins. Plants have evolved a variety of special adaptations to limit evaporation, but all of them simultaneously restrict the influx of carbon dioxide. In other words, the uptake of carbon dioxide for photosynthesis and the loss of water through transpiration represent an inescapable dilemma in the life of a green plant.

ROOT Water Uptake

The Root System anchors the plant in the soil and, most importantly, satisfies the immense water demands of the leaves. Almost all the water a plant extracts from the soil passes through the youngest Regions of the root. Water uptake occurs directly through the root epidermis. Root hairs, located a few millimeters behind the root tip, provide a massive absorbing surface (Fig. 27-2; Table 27-2). From the root hairs, water moves inward through the cortex, endodermis (the innermost layer of cortical cells), and pericycle to the primary xylem. Once inside the conducting elements of the xylem, water travels up through the root and stem into the leaves.

Fig. 27-2. A. Primary root of a radish seedling (Raphanus sativus) showing root hairs. B. Root hairs surrounded by soil particles that retain water

Table 27-2. Number of Root Hairs per Square Centimeter of Root Surface in Three Plant Species

Water and dissolved substances move through plants via the apoplast (through cell walls), the symplast (from protoplast to protoplast via plasmodesmata), or a combination of both pathways. The primary pathway for water across the root epidermis and cortex is the apoplastic route (Fig. 27-3). However, at the endodermis, water must cross the Plasma Membranes and protoplasts of the tightly packed endodermal cells because their radial and transverse walls contain waterproof Casparian strips (see Chapter 21). The endodermis thus acts as an osmotic barrier between the root cortex and the vascular cylinder.

Fig. 27-3. Main Pathways of movement for water (black line) and inorganic ions (colored line) from the soil through the epidermis and cortex into the xylem, or water-conducting elements of the root. Water moves primarily via the apoplast until it reaches the endodermis, where apoplastic flow is blocked by the Casparian strips. The Casparian strips force water to cross the plasma membranes and protoplasts of the endodermal cells on its way to the xylem. After passing through the plasma membrane on the inner surface of the endodermis, water can resume its apoplastic journey into the cavities of the xylem vessels. Inorganic ions are actively taken up by epidermal cells and then transported symplastically across the cortex to parenchymal cells, from which they are pumped into the xylem elements

During periods of high transpiration, the water surrounding root hairs may be depleted so rapidly that the soil dries out, and water must travel to the root hairs from a distance through tiny soil pores. Overall, roots "find" water as they grow, although they will not grow in dry soil. Under normal conditions, apple tree roots, for example, grow an average of 3 to 9 mm per day, prairie grass roots can grow more than 13 mm per day, and corn roots average 52 to 63 mm per day. Such rapid growth can produce staggering results: a four-month-old rye plant (Secale cereale) develops about 10,000 km of roots and many billions of root hairs.

In addition to supplying the SHOOT with water, the transpiration stream also delivers inorganic ions (Fig. 27-4). Once ions are absorbed by the outer root cells, they are transported across the cortex and secreted into the xylem. Driven by transpiration, these ions are rapidly distributed throughout the plant.

Fig. 27-4. Radioactive potassium (⁴²K) added to soil water demonstrates that the upward transport of water and inorganic ions occurs through the xylem. A strip of waxed paper was inserted between the xylem and phloem to prevent lateral movement of the isotope. The relative amounts of radioactive potassium detected in each stem segment are shown in the table

Root Pressure and Guttation

When transpiration is very low or absent altogether, such as at night, root cells can still pump ions into the xylem. Because the conducting tissue of the root is surrounded by the endodermis, these ions cannot escape from the xylem. As a result, The water potential (see Ch. 4) of the xylem becomes more negative, and by the laws of osmosis, water rushes into the xylem through the surrounding cells. This generates a positive pressure, known as root pressure, which forces water and dissolved ions up the xylem (Fig. 27-5).

Fig. 27-5. Demonstration of root pressure in a de-topped plant. Water absorption by the roots causes mercury to rise in the tube. This method demonstrates pressures of 3 — 5 bar

Dew-like droplets of water on the tips of grass blades in the early morning demonstrate root pressure in action (Fig. 27-6). These droplets are not dew condensing from water vapor in the air; they appear on the leaves through a process called guttation (from the Latin gutta, meaning drop). The droplets are secreted not through Stomata, but through hydathodes—specialized pores located at the tips and margins of leaves (Fig. 27-7). During guttation, water is literally squeezed out of the leaves by root pressure.

Fig. 27-6. Guttation. Water droplets at the leaf tips of barley (Hordeum vulgare) also indicate the presence of root pressure. These droplets do not condense from the water vapor of the surrounding air. They are forced out of the leaf through specialized pores—hydathodes—located at the leaf tips

Fig. 27-7. Longitudinal section through a hydathode of a Saxifraga lingulata leaf. The hydathode consists of terminal tracheids at the end of a vascular bundle, thin-walled parenchyma (epithem) with numerous intercellular spaces, and epidermal pores. The tracheids are in direct contact with the epithem. The epidermal pores are typically modified stomata that have lost The ability to open and close

Root pressure is least effective during the day (when Water movement through the plant is most intense) and is never sufficient to lift water to the top of a tall tree. Moreover, in many plants, including conifers such as pine, root pressure does not develop at all. Thus, it appears to be a byproduct of the ion-pumping mechanism into the xylem and an auxiliary mechanism for water movement in the shoot under specific conditions.

Passive Water Uptake

During periods of intense transpiration, ions accumulated in the root xylem are swept away in the transpirational stream, and osmotic movement across the endodermis decreases. Under these conditions, the roots become a passive absorbing surface through which water is pulled by the bulk flow generated in the transpiring shoots. Some researchers believe that virtually all water uptake by the roots of a transpiring plant occurs through this passive mechanism.

Water Transport

Water enters the plant through the roots and is lost in large quantities through the leaves. How does water move through the plant, often overcoming great vertical distances? This question has occupied generations of botanists.

The primary pathway for water ascent has been precisely established. You can trace this path with a simple experiment. Place a cut stem in water dyed with a harmless stain (it is best to cut the stem underwater to prevent air from entering the xylem conducting elements), and then observe the movement of the liquid into the leaves. The dye will clearly reveal the xylem conducting elements. Experiments with radioactive isotopes confirm that the isotope, and presumably water as well, actually ascends through the xylem vessels (or tracheids). In the experiment shown in Fig. 27-4, the xylem was carefully separated from the phloem. Earlier experiments without such Separation yielded ambiguous results due to lateral movement from the xylem to the phloem. However, experiments show that this lateral movement is not essential for the overall transport of water and mineral nutrients from the soil to the leaf.

Such is the path taken by water, but how is this movement accomplished? Logic suggests two possibilities: water is pushed from below, or it is pulled from above. (A third option—the presence of active pumps, or "hearts," along the pathway—has been proposed from time to time, but is no longer taken seriously by botanists.) The first of these possibilities has already been ruled out. Root pressure, as noted above, does not occur in all plants, and even when present, it cannot lift water to the top of a tall tree. Therefore, we settle on the hypothesis that water is "pulled" through the entire plant (a hypothesis supported by available evidence).

The Cohesion-Adhesion-Tension Mechanism

When water evaporates from The Cell wall surfaces bordering intercellular spaces within the leaf during transpiration, it is replaced by water from the cells diffusing across the plasma membrane. The membrane is freely permeable to water but impermeable to solutes within The Cell. As a result, the solute concentration in the cell increases, and its water potential decreases. A water potential gradient is established between this cell and neighboring, more hydrated cells. These cells, in turn, withdraw water from others until eventually the "chain" reaches a vein, establishing water tension in the xylem. Because of the extremely high cohesive strength of water molecules, this tension extends all the way down the stem to the roots, pulling water out of them, moving it up the xylem, and distributing it among the cells that evaporate it into the atmosphere (Fig. 27-8). As a result of this water loss, the WATER POTENTIAL OF the roots becomes more negative, enhancing their ability to extract water from the soil.

Fig. 27-8. Studies of water transport in ash (Fraxinus) show that an increase in water uptake follows an increase in transpiration. These data confirm that water loss generates the force required for its absorption

The Theory of water transport is known as the cohesion-tension theory because movement depends on the cohesive force of water molecules, which allows the water Column to withstand tension (Fig. 27-9). However, it is better termed the cohesion-adhesion-tension theory, because the adhesion of water molecules to the walls of xylem tracheids and vessels, as well as to leaf and root cell walls, is just as crucial for water ascent as cohesion and tension. The cell walls along which water moves have evolved into surfaces that attract water very efficiently, maximizing water adhesion and creating conditions favorable for its cohesiveness. The system's reliability is further enhanced by the narrow diameter of the xylem conducting elements—vessels and tracheids—through which water moves.

Fig. 27-9. A. A simple physical system illustrating the cohesion-adhesion-tension theory. A porous clay pot is filled with water and attached to the end of a long, thin Glass tube, also filled with water. The lower end of the water-filled tube is immersed in mercury contained in a beaker. Water evaporates through the pores of the pot and is replaced by water drawn from the tube, rising within it as a continuous column. As the water rises, mercury is drawn into the tube, replacing the water. B. Leaf transpiration results in water loss sufficient to generate similar negative pressure. Therefore, the cohesion-adhesion-tension theory is often called the "transpiration pull theory"

There is no doubt that the tensile strength of water is sufficiently high to prevent the separation of its molecules under the tension required to lift water up the xylem of a tall tree. For example, it has been demonstrated that a water column in a fine capillary tube can withstand a tension of -264 bar, whereas lifting water to the top of a coast redwood (Sequoia sempervirens) is calculated to require a tension of only about -20 bar. It has been estimated that the tensile strength of water is extremely high: between 1300 and 1500 bar, and possibly even 15,000 bar!

How can the cohesion-adhesion-tension theory be tested? A direct method of testing is to measure water tension in the xylem. When a branch of a transpiring tree is cut, the water in the vessels snaps back away from the cut edge. By placing the branch in a thick-walled chamber, pressure can be applied until (under magnification) the water column menisci in the vessels are observed to rise back to the cut surface (Fig. 27-10). The pressure required to return the water to the cut is equal in magnitude to the water tension that existed in the branch prior to cutting. The results obtained by this method are in complete agreement with the predictions of the cohesion-adhesion-tension theory.

Fig. 27-10. MEASUREMENT OF WATER tension in the xylem. The branch in which the xylem tension is to be measured is cut and placed into a pressure chamber. When the branch is severed, the xylem sap, which was previously under tension, recedes into the xylem away from the cut surface. Pressure is then applied within the chamber until the sap appears at the cut surface of the stem. Assuming that an equivalent pressure is required to move the sap in both directions, it is concluded that the positive pressure needed to push the sap back to the cut surface equals the tension that existed in the xylem prior to cutting the branch.

A second body of data consistent with this theory demonstrates that water movement begins at the top of the tree. The velocity of the xylem sap in various parts of the tree has been measured using a simple method: a small heating element briefly warms the xylem contents for a few seconds, and a sensitive thermocouple detects the exact moment the heated xylem sap passes a fixed point (Fig. 27-11). As the graph shows, in the morning, sap movement begins first in the branches—since tension develops closer to the leaves—and only subsequently in the trunk. In the evening, the flow velocity decreases first in the branches, as transpirational water loss by the leaves declines, and then in the trunk. In trees with wide vessels (200 — 400 µm in diameter), a midday velocity peak ranging from 16 to 45 m/h is observed (measured at human chest height), whereas in trees with narrow vessels (50 — 150 µm in diameter), the midday peak is lower, ranging from 1 to 6 m/h.

Fig. 27-11. Method for measuring sap flow velocity. A small heating element inserted into the xylem briefly warms the ascending sap for several seconds. A thermocouple placed downstream from the heating element records the passing heat wave. The experimenter measures the time interval between these two events. As seen on the graph, in the morning, the increase in sap flow velocity begins first in the branches (upper curve) and then in the trunk (lower curve). In the evening, the velocity decreases first in the branches and subsequently in the trunk.

A third set of data comes from measurements of diurnal fluctuations in tree trunk diameter (Fig. 27-12). Trunk shrinkage occurs as a result of negative pressure within the water-Conducting pathways of the xylem. Water molecules, adhering to the vessel walls, pull them inward. When transpiration begins in the morning, the upper part of the trunk shrinks first, because water is withdrawn from its xylem before it can be replenished by water from the roots. Afterwards, the diameter of the lower part of the trunk decreases. Later in the day, as transpiration intensity subsides, the upper part of the trunk expands earlier than the lower part.

Fig. 27-12. A dendrometer (left) records subtle diurnal changes in tree trunk diameter. Simultaneous measurements at two different heights show, as illustrated in the graph, that in the morning, trunk shrinkage occurs slightly earlier in the upper section than in the lower section. These data indicate that leaf transpiration "pulls" water out of the trunk before it can be replaced by water from the roots. Shaded bars indicate nighttime.

It is important to emphasize that the energy required for the evaporation of water molecules—and thus for The transport of water and mineral nutrients throughout the plant—is supplied not by the plant itself, but directly by the Sun. Furthermore, this movement is made possible by the exceptional cohesion and adhesion properties of water molecules, which plants exploit so masterfully.

Regulation of Transpiration

When carbon dioxide, essential for photosynthesis, is absorbed by the leaves, water vapor is released into the atmosphere via transpiration, which can pose a significant hazard to the plant. These two processes are inextricably linked; however, plants possess a series of specialized adaptations that minimize water loss while simultaneously favoring carbon dioxide uptake.

Cuticle and Stomata

Leaves are covered by a cuticle that renders their surface largely impermeable to both water and carbon dioxide. Only a minor fraction of the water transpired by a plant escapes through this protective outer layer, and another small portion is lost via lenticels in the bark. The vast majority of transpired water is lost through the stomata (Fig. 27-13). Stomatal transpiration involves two stages: (1) the evaporation of water from the surfaces of cell walls bounding the intercellular air spaces of the leaf; and (2) the diffusion of the resulting water vapor from the intercellular spaces into the atmosphere through the stomatal pores (see Fig. 27-18).

Fig. 27-13. Scanning electron micrographs showing an open stoma in the leaf epidermis of cucumber (Cucumis sativus) (A) and a closed stoma in the leaf epidermis of parsley (Apium petroselinum) (B). Stomata lead into the air Chambers of the leaf, which are surrounded by thin-walled photosynthetic mesophyll cells. These air spaces are saturated with water vapor evaporating from the mesophyll cell surfaces.

Stomata are microscopic Pores in the epidermis. Each stoma is flanked by two guard cells that can alter their shape, thereby opening and closing the stomatal pore. Although stomata also occur on young stems, they are far more abundant on leaves. Their numbers can be quite staggering; for instance, a square centimeter of tobacco leaf surface contains approximately 12,000 stomata. Stomata open into internal air spaces surrounded by thin-walled mesophyll cells. The air within these spaces, which account for 15% to 40% of total leaf volume, is saturated with water vapor evaporating from the moist surfaces of the mesophyll cells. Although stomata occupy only about 1% of the total leaf area, more than 90% of the water transpired by the plant evaporates through them, with the remainder being lost through the cuticle.

Stomatal closure not only prevents the loss of water vapor from the leaf but, as noted above, also cuts off the supply of carbon dioxide. However, a certain amount of carbon dioxide is generated internally through Plant Respiration, and as long as light is available, this endogenous carbon dioxide can be utilized to sustain photosynthesis at a very low rate even when the stomata are closed.

Mechanism of Stomatal Movements

Stomatal movements are driven by changes in turgor pressure within the guard cells. Opening occurs when solute molecules actively accumulate within the guard cells. This accumulation of solutes drives the influx of water into the guard cells, generating a turgor pressure that exceeds that of the surrounding subsidiary cells. Stomatal closure is achieved by the reverse process: as the solute content in the guard cells decreases, water flows out of them and turgor pressure drops. Thus, turgor is established or dissipated as a result of passive Osmotic water movement into or out of the cells along a water potential gradient.

The water potential gradient is established primarily by potassium ions (K⁺). Potassium ions have been detected in the guard cells of open stomata in more than 50 plant species, including CAM plants, whose stomata open at night. Measurements of potassium content in individual guard cells using specialized techniques demonstrate that potassium concentration increases when stomata are open and drops when they are closed. The subsidiary cells act as a reservoir for potassium ions. The potassium gradient between the guard cells and subsidiary cells fluctuates significantly, accompanied by the osmotic Movement of water and consequent changes in turgor (Fig. 27-14).

Fig. 27-14. Changes in potassium concentration within the stomatal complex (guard and subsidiary cells) and adjacent ordinary epidermal cells of dayflower (Commelina communis). The guard cells of Commelina leaves are associated with six subsidiary cells—four lateral and two terminal. A. Potassium concentration in the vacuoles of various cells when stomata are closed. B. Potassium concentration in various cells when stomata are open. Potassium-sensitive microelectrodes were used to determine the ion content of individual cells.

Alongside positively charged K⁺ ions, which are translocated in such large quantities, negatively charged ions (anions) must also move to maintain electrical neutrality within the cell. In this regard, two anions—chloride and malate—play an especially vital role.

The cell wall architecture of guard cells is critical for stomatal movement. During the expansion of paired guard cells, two factors promote their bowing and consequently the opening of the stomatal pore. One of these is the radial orientation of Cellulose microfibrils within the guard cell walls (Fig. 27-15, A). These radial micelles allow the guard cells to elongate while simultaneously preventing them from expanding radially. The second constraint is located at the ends of the guard cells where they connect with one another; their common end walls undergo virtually no change in length during stomatal opening and closing. Consequently, an increase in turgor pressure forces the outer (dorsal) walls of the guard cells to bow outward relative to their common end walls. As this occurs, the radial micelles pull the walls lining the stomatal pore (the ventral walls) along with them, causing the pore to open. Fig. 27-15, B – D illustrates the results of balloon experiments conducted to confirm The Role of radial micelles in stomatal mechanics.

Fig. 27-15. A. Diagram of a pair of guard cells. Radial lines indicate the arrangement of cellulose microfibrils in the guard cell walls. B – D. Schematic model used to study the role of radial micelles in stomatal opening. B. Two partially inflated balloons glued together at their ends. C. The same balloons under higher pressure, showing a narrow slit between them. D. A pair of inflated balloons bound with tape simulating radial micelles. The resulting gap is significantly wider than in the previous case (see C).

Factors Affecting Stomatal Movement

Among the external factors influencing stomatal opening and closing, water loss is of paramount importance. When leaf turgor drops below a certain critical threshold—which varies among plant species—the stomatal aperture narrows. Water loss overrides the Effects of Other factors on stomata; however, stomatal movements can also occur independently of increases or decreases in the plant's water content. A striking example of this is the regular morning opening and evening closing of stomata observed in many species, which takes place even in the absence of changes in plant water supply.

During periods of water deficit, many plants exhibit a sharp increase in Abscisic acid (ABA) content. Applying or feeding ABA to leaves induces stomatal closure within minutes. Furthermore, The Effect of ABA on stomatal movement is readily reversible. Experimental evidence indicates that the efflux of solutes (K⁺) from guard cells begins when ABA—produced in the mesophyll to signal a water shortage—reaches the stomata. The mechanism by which ABA acts on guard cells remains to be fully elucidated.

Other environmental factors affecting stomatal movement include carbon dioxide concentration, light, and Temperature. In most species, an elevation in CO₂ concentration triggers stomatal closure. The magnitude of the CO₂ response depends on the plant species and the severity of the water stress it experiences. In maize (Zea mays), stomata can respond to changes in CO₂ concentration within seconds. It has been demonstrated that the perception of CO₂ levels takes place within the guard cells.

In most species, stomata open in the light and close in the dark. This can be partly explained by the consumption of CO₂ during photosynthesis, which lowers its concentration within the leaf. However, light also exerts a more direct effect on stomata. It has long been known that blue light stimulates stomatal opening independently of CO₂ concentration. For instance, protoplasts of onion (Allium cepa) guard cells swell in the presence of K⁺ when illuminated with blue light. The pigment absorbing blue light (a flavin or flavoprotein localized in the tonoplast and possibly the plasma membrane) stimulates K⁺ uptake by guard cells.

Temperature variations within the normal physiological range (from 10°C to 25°C) have little effect on stomatal movement, whereas temperatures rising above 30–35°C can induce closure. However, this stomatal closure can be prevented by placing the plant in a carbon dioxide-free atmosphere. This indicates that the temperature effect is initially mediated by changes in internal leaf CO₂ concentration. Elevated temperatures accelerate respiration and correspondingly increase carbon dioxide levels within the cells, which may be the actual cause of stomatal closure in response to heating. Many hot-climate plants regularly close their stomata at midday, which can be attributed both to the Effect of temperature on CO₂ concentration and to leaf dehydration resulting from transpirational water loss exceeding water uptake.

Stomata not only respond to environmental cues but also exhibit daily rhythms of opening and closing controlled by endogenous mechanisms—in other words, stomata display circadian rhythms (see Chapter 25).

Although the stomata of most plants are open by day and closed at night, this is not a universal rule. Various succulents, including cacti, pineapples (Ananas comosus), and members of the stonecrop family (Crassulaceae), open their stomata at night when environmental conditions are least favorable for transpiration. The pathway of CO₂ fixation in Crassulaceae is fundamentally similar to the C₄ photosynthetic pathway, as discussed in Chapter 7. At night, when their stomata are open, CAM plants take up carbon dioxide and convert it into organic acids. By day, when the stomata are closed, the carbon dioxide is released from the organic acids and utilized in photosynthesis.

Factors Affecting Transpiration Rate

Although stomatal opening and closing is the primary factor determining The rate of transpiration, A number of other external and internal factors also play a role. Temperature is undoubtedly one of the most critical. The rate of water evaporation roughly doubles for every 10°C rise in temperature. Nevertheless, water evaporation cools the leaf surface, preventing its temperature from rising as rapidly as that of the surrounding air. As noted above, stomata close when temperatures exceed 30–35°C.

Humidity is also of great significance. Water evaporates much more slowly if the air is already saturated with water vapor. Plants growing in a shaded forest, where humidity is typically high, freely expose a large leaf surface area because their main challenge is capturing sufficient light rather than conserving water. Conversely, plants in steppes or other open habitats often feature narrow leaves with a relatively small surface area. They receive as much light as they can possibly utilize, yet constantly face the threat of excessive water loss.

Air currents significantly affect the rate of transpiration. On a hot day, a breeze cools our Skin by sweeping away the water vapor accumulating near the surface, thereby accelerating body evaporation. In the same manner, wind carries water vapor away from the leaf surface. Occasionally, if the air is extremely humid, wind may reduce transpiration by cooling the leaf; however, dry wind drastically increases evaporation. Leaves of plants in open, windswept environments are frequently pubescent (hairy). These trichomes are thought to protect the leaf surface from the drying effects of wind and reduce transpiration rates by stabilizing the boundary layer of air adjacent to the leaf.

It has recently been suggested that the primary role of stomatal sensitivity to CO₂ is to induce partial closure during high wind speeds. Wind sweeps water vapor away from the leaf surface while simultaneously delivering a fresh supply of CO₂. Thus, by closing in response to rising CO₂ concentrations, stomata serve to moderate transpiration.

Transport of Inorganic Nutrients in Plants

Absorption of Inorganic Nutrients

The uptake, or absorption, of inorganic ions occurs through the root epidermis. Current evidence indicates that ions travel primarily symplastically from the epidermis to the endodermis. Ion movement along the symplastic pathway begins with their crossing of the plasma membrane of epidermal cells. Ions then move from the protoplasts of epidermal cells into the Cells of the outer cortical layer via plasmodesmata spanning the epidermal-cortical cell walls (see Fig. 27-3). Radial ion transport continues across the cortical symplast—from protoplast to protoplast via plasmodesmata—and through the endodermis into the parenchymatous cells of the vascular cylinder by diffusion. This movement is likely facilitated by cytoplasmic streaming.

The mechanism by which ions enter the xylem vessels (or tracheids) from the stelar parenchyma cells remains a subject of active debate. It has been proposed that ions passively "leak" from parenchymatous cells into the vessels. However, compelling evidence now indicates that ions are secreted into the vessels from parenchyma cells via active, carrier-mediated membrane transport (see Chapter 4).

Active Uptake of Solutes

The mineral composition of root cells contrasts sharply with that of their surrounding environment. For example, one study demonstrated that potassium ion concentrations in pea root cells (Pisum sativum) are 75 times higher than in the nutrient solution. Similarly, findings from another experiment show that the vacuoles of rutabaga cells (Brassica napus var. napobrassica) contain 10,000 times more potassium than the external solution.

Since solutes cannot diffuse against a concentration gradient, it is evident that mineral nutrients are absorbed via Active Transport. This hypothesis is supported by evidence showing that mineral absorption is an energy-requiring process. If, for instance, roots are deprived of oxygen or poisoned—thereby inhibiting respiration—mineral uptake drops dramatically. Likewise, if a plant is shaded until its carbohydrate reserves are depleted, it ceases to absorb salts and eventually even leaches them back into the soil solution (Fig. 27-16). Consequently, transporting ions from the soil into the xylem vessels requires a two-step active, carrier-mediated membrane transport process: (1) ion uptake across the plasma membrane of epidermal cells; and (2) ion secretion into the vessels by the plasma membrane of stelar parenchyma cells.

Fig. 27-16. The rate of phosphate absorption by maize plants (Zea mays) drops almost to zero after four days of continuous darkness. It begins to rise again once the plants are returned to the light. These and other findings demonstrate that salt uptake by plants is an energy-dependent process.

Transmembrane Potential

Active Ion transport across the plasma membrane can generate a difference in electrical charge across the Two Sides of the membrane. When this occurs, a voltage, termed the transmembrane potential, is established across the membrane. The hydrogen ion H⁺ is one of the principal cations involved in generating the transmembrane potential. As H⁺ is pumped out of the cell, a negative internal potential develops. Once established, the transmembrane potential can profoundly influence subsequent ion movements. For instance, the intracellular negative potential will attract positively charged ions, such as K⁺, while simultaneously repelling negatively charged ions, such as Cl^-.

Transport of Inorganic Nutrients

Once secreted into the xylem vessels, inorganic ions rise rapidly and are distributed throughout the plant by the transpiration stream. Some ions move laterally from the xylem into adjacent root and stem Tissues, while others are transported to the leaves (Fig. 27-17).

Fig. 27-17. Diagram of water, inorganic ion, and assimilate Circulation in a plant. Water and inorganic ions absorbed by the root move upward through the xylem with the transpiration stream. A fraction of these moves laterally into the root and stem tissues, whereas the remainder is transported to growing regions and mature leaves. In the leaves, a significant amount of water and inorganic ions enters the phloem and is exported along with sucrose in the assimilate stream. Growing organs, which have a relatively low capacity for water uptake via transpiration, receive the bulk of their nutrients and water via the phloem. Both water and solutes entering the roots through the phloem can be transferred back to the xylem and recirculate in the transpiration stream. The letter A indicates sites specialized for the absorption and assimilation of raw Materials from the environment. The letters 3 and P designate the loading and unloading sites, respectively, and O marks the main junctions where exchange between the xylem and phloem occurs

Much less is known about the pathways of ion movement within leaves than about their transport in roots. In the leaf, ions are carried with water into the apoplast, i.e., the cell walls. Some ions may remain in the transpiring water and reach the primary sites of water evaporation: the stomata and other epidermal cells. Eventually, most ions enter the protoplasts of leaf cells, presumably via carrier-mediated transport, much as they do in roots. Through the symplast, ions can move into other leaf tissues, including the phloem. Because inorganic ions can also be absorbed in small quantities through the leaves, foliar application of micronutrients directly to foliage has become a standard agricultural practice for certain crops.

Substantial quantities of inorganic ions delivered to the leaves via the xylem exchange with the phloem of leaf Veins and are exported from the leaf along with sucrose in the assimilate stream (Fig. 27-17, see also the Discussion of translocation in the following section). For example, in annual white lupine (Lupinus albus), phloem transport accounts for more than 80% of the nitrogen and sulfur supplied to the fruits, and 70–80% of their phosphorus, potassium, magnesium, and zinc. The uptake of these inorganic ions by developing fruits is undoubtedly coupled with the flow of sucrose through the phloem.

Nutrient recirculation can occur within a plant when ions reaching the roots in the descending assimilate stream are transferred to the ascending transpiration stream in the xylem (Fig. 27-17). Only those ions capable of moving through the phloem, classified as phloem-mobile, can be exported from leaves in appreciable amounts. For instance, K⁺, Cl^-, and H₂PO₄^-2 are readily exported from leaves, whereas Ca²⁺ is not. Elements like calcium are considered phloem-immobile.

Translocation: Movement of Substances Through the Phloem

As described in Chapters 20–22, the xylem and phloem together form a continuous Vascular System that pervades virtually all parts of the plant. While water and inorganic solutes ascend via the xylem in the transpiration stream, sugars produced during photosynthesis move outward from the leaves through the phloem in the assimilate stream (Fig. 27-18) toward utilization sites—such as growing shoot and root apices—and storage sinks—such as fruits, seeds, and the storage parenchyma of stems and roots (see Fig. 27-17).

Fig. 27-18. Diagram of a leaf showing the pathways of water molecules within the transpiration stream. Water flows from the xylem of a minor vein into the mesophyll cells, evaporates from the surfaces of their cell walls, and subsequently diffuses out of the leaf through open stomata (colored lines). Also illustrated are the pathways taken by sugar molecules synthesized during photosynthesis. These exit the mesophyll cells into the phloem of the same vein and enter the assimilate stream. Sugar molecules formed in the palisade cells are believed to move into the spongy parenchyma cells and travel laterally through them toward the phloem (gray lines)

Assimilate transport is widely considered to operate on a source-sink model. The primary sources of assimilates are photosynthesizing leaves, although storage tissues can also serve as significant sources. Any plant part unable to independently meet its own metabolic demands can act as a sink, functioning as a consumer or importer of assimilates. Thus, storage tissues act as sinks when they import assimilates and as sources when they export them.

Source-sink relationships can be relatively simple and direct, as in young seedlings where cotyledons containing nutrient reserves serve as the primary source and growing roots as the principal sink. In older plants, upper, most recently formed leaves typically direct their assimilates mainly toward the shoot apex, whereas lower leaves export them in both directions (Fig. 27-19A). This assimilate distribution pattern shifts markedly during the transition from vegetative to reproductive growth.

Developing fruits are fierce competitors for assimilates, monopolizing supplies from nearby and often more distant leaves, which can stunt or even halt vegetative growth (Fig. 27-19B).

Fig. 27-19. Diagrams of a plant in the vegetative stage (A) and the fruiting stage (B). Arrows indicate the direction of assimilate transport

Evidence for Sugar Transport via the Phloem

The initial Evidence for the role of the phloem in assimilate transport came from observations of ringed (girdled) trees. As noted in Chapter 23, the bark of older stems consists primarily of phloem and lacks xylem. When a ring of bark is removed from a photosynthesizing tree (girdling), the bark above the incision swells, indicating an accumulation of assimilates moving downward through the phloem from the photosynthesizing leaves (Fig. 27-20).

Fig. 21-20. As early as the 17th century, the Italian scientist Marcello Malpighi observed that removing a ring-shaped strip of bark from a stem (A) caused the tissues above the incision to swell (B). He correctly explained this phenomenon as being driven by enhanced growth of bark and wood tissues stimulated by the accumulation of nutrients descending from the leaves and blocked above the ring. Malpighi investigated the effects of girdling at different times of year and found that no Swelling occurred during the winter months

More compelling proof of the phloem's transport function was obtained using radioactive tracers. Previously, before radioisotopes were available, researchers had to cut into plants to introduce Dyes or other chemicals to study transport dynamics. However, the moment sieve tubes are severed, their high turgor (hydrostatic) pressure drops instantly, causing their contents to surge toward the cut surfaces and disrupting the entire system. As discussed in Chapter 20, this phenomenon is responsible for The formation of slime plugs (P-protein plugs) in damaged sieve elements. Today, The Use of radiotracers allows experimentation on intact plants, providing a clear and accurate picture of normal transport. Experiments with radioactive assimilates (such as ¹⁴C-labeled sucrose) have confirmed that these compounds travel through the phloem. Later, such studies provided conclusive evidence that sugars are transported specifically within the sieve tubes of the phloem (see Appendix).

The Use of Aphids in Phloem Research

A wealth of valuable information regarding phloem transport has been gathered from studies utilizing aphids, which are small, plant-sap-sucking insects. Most aphid species feed on phloem sap. They insert their modified mouthparts, or stylets, into a stem or leaf until the tips penetrate the sieve tubes (Fig. 27-21). The turgor pressure within the sieve tube forces sap through the aphid’s digestive tract and out its posterior as droplets of sooty mold or honeydew. If feeding aphids are anesthetized and severed from their stylets, phloem exudate continues to flow from the cut stylets for many hours and can be collected using a micropipette. Analyses of exudate obtained in this manner have demonstrated that sieve-tube sap contains 10% to 25% dry matter, 90% of which in most plants consists of sugars (primarily sucrose). Amino Acids and other nitrogenous compounds are also present in low concentrations (less than 1%).

Fig. 27-21. A. An aphid (Longistigma caryae) feeding on a linden stem (Tilia americana). A droplet of honeydew is visible exuding from the aphid. B. Micrograph showing a fragment of the aphid's modified mouthpart (stylet) embedded within a sieve tube of the secondary phloem of a linden stem

Research involving aphids and radioactive tracers shows that the longitudinal translocation rate of assimilates through the phloem is remarkably high. For instance, in a series of experiments utilizing excised aphid stylets, it was determined that sieve-tube sap flows at a velocity of about 100 cm/h at puncture sites.

Mechanism of phloem Transport: The Pressure-Flow Hypothesis

Over years of research, several hypotheses have been proposed regarding The Mechanism of assimilate transport in phloem sieve tubes. According to the earliest of these, the transport mechanism is based on diffusion accelerated by cytoplasmic streaming. Free diffusion and cytoplasmic streaming, commonly observed in higher plant cells, were considered plausible mechanisms of assimilate movement until it became apparent that the velocity of assimilate transport (typically 50 to 100 cm/h) significantly exceeds the rates of these two processes, which therefore cannot account for long-distance transport through sieve tubes.

Other hypotheses have been advanced to explain the mechanism of phloem transport, but only one of them—the pressure-flow hypothesis—satisfactorily accounts for virtually all experimental data while accommodating the Structural Features of the phloem.

Originally proposed in 1927 by the German plant physiologist Ernst Münch and later modified, the pressure-flow hypothesis undoubtedly provides the simplest and currently most widely accepted explanation for the long-distance Transport of Assimilates through sieve tubes. This explanation is the simplest because it relies exclusively on osmosis as the driving force for assimilate transport.

In brief, the pressure-flow hypothesis states that assimilates are transported from source to sink along a turgor pressure gradient generated as a result of osmosis. The core principle of this hypothesis can be illustrated using a simple physical model consisting of bulbs, or osmotic cells, permeable only to water and connected by glass tubing (Fig. 27-22). Initially, the first bulb (A) contains a more concentrated sugar solution than the second bulb (B). The connected bulbs are immersed in water, and water begins to enter the first bulb by osmosis, increasing its turgor pressure. This pressure is transmitted via the tubing to the second bulb, causing the sugar solution to flow into it and displacing the water. If the second bulb is connected to a third one containing a lower sucrose concentration than the second, the solution will flow from the second bulb to the third (through the same process) and so on indefinitely along the turgor pressure gradient.

Fig. 27-22. Münch model demonstrating the fundamental mechanism of pressure flow. A and B are osmotic cells. A contains a more concentrated sucrose solution than B. C is a glass tube connecting the two cells, and D is a vessel containing water. Water enters A by osmosis, increasing turgor pressure and forcing the sugar solution into B.

In the plant, sucrose produced in the leaf via photosynthesis is actively secreted into the sieve tubes of minor veins (Fig. 27-23). This active process, termed phloem loading, decreases the water potential within the sieve tubes and causes water—supplied to the leaf by the transpiration stream—to enter them by osmosis. As a result of water influx into the donor sieve tubes, sucrose is passively carried by water to a sink, such as a storage root, where sucrose is removed (unloaded) from the sieve tubes. Sucrose removal leads to an increase in the water potential of the sink sieve tubes and the subsequent efflux of water from them. Sucrose may then either be utilized or stored, but the bulk of the water returns to the xylem and is recirculated in the transpiration stream.

Fig. 27-23. Diagram illustrating the mechanism of osmotically generated pressure flow. Circles represent sugar molecules. Sugar is actively loaded into the donor sieve tubes (at the production site). As sugar concentration increases, water potential decreases, and water from the xylem enters the sieve tubes osmotically. At the consumption or storage site (sink), sugar is released (unloaded), its concentration in the sieve tubes drops, resulting in an increase in water potential, and water exits the sieve tubes. As water moves into the donor sieve tubes and out of the sink sieve tubes, sugar molecules are passively carried by water along concentration and hydrostatic pressure gradients between source and sink. Note that unlike the Münch model shown in Fig. 27-22, the sieve tubes located between source and sink are surrounded by selectively permeable membranes—plasma membranes. Consequently, water enters and exits the sieve tubes not only at the source and sink but also all along the pathway. Evidence indicates that only a small fraction (if any) of the water molecules initially entering the donor sieve tubes actually reaches the sink, as they are exchanged for other water molecules that enter the sieve tubes via the phloem apoplast along the transport route.

It should be noted that the pressure-flow hypothesis assigns a passive role to sieve tubes in the passage of the sugar solution through them. Active transport is also involved in solution movement, though not directly in long-distance transport through the sieve tubes, but rather in the loading and, potentially, unloading of sugars and other substances into and out of the sieve tubes in source and sink regions. There is strong evidence that the driving force for sucrose accumulation (phloem loading) at production sites is generated by a proton pump utilizing ATP energy via an ATPase present in the plasma membrane. This operates via a sucrose-proton cotransport (symport) system (see Chapter 4). The metabolic energy required for loading and unloading is expended primarily by companion cells and parenchyma cells surrounding the sieve tubes rather than by the sieve tubes themselves. Until recently, it was assumed that loading occurred via the companion cell plasma membrane, from which sugar was subsequently transferred to the sieve tube through numerous plasmodesmata in their shared wall. However, it is now clear that some sieve tubes are capable of autonomous loading and that active transport is concentrated in their own plasma membranes. Nevertheless, a mature sieve tube depends for most or all of its Energy Requirements on companion cells or adjacent parenchyma cells.

Phloem loading is a selective process. As noted earlier, the primary transported sugar is sucrose; moreover, all sugars contained in sieve-tube sap are non-reducing. Certain amino acids and ions are also selectively loaded into the phloem.

Conclusion

In plants, over 90% of the water absorbed by roots is released into the atmosphere as water vapor. This process is called transpiration, and the bulk of the water transpired by vascular plants evaporates through leaf stomata.

Water absorption occurs primarily through root hairs, which provide an immense absorbing surface. In some plants, when roots absorb water from the soil and transport it to the xylem, the water in the xylem develops a positive pressure known as root pressure. This osmotic absorption depends on The transfer of inorganic ions from the soil into the xylem by living root cells and can lead to a phenomenon called guttation, whereby water is forced out through specialized pores at the tips or margins of leaves. Water predominantly moves via the apoplastic pathway through the epidermis and cortex until it reaches the endodermis, where further apoplastic movement is blocked by Casparian strips. En route to the xylem, water must cross the plasma membranes and protoplasts of endodermal cells.

Water ascends from the roots to the leaves via the xylem. A widely accepted theory for water transport to the tops of tall plants through the xylem is the cohesion-tension-adhesion theory. According to this theory, water in the vessels is under tension because, owing to molecular cohesion, it forms continuous columns that are "pulled" upward as a result of evaporation. Water has been shown to possess sufficient tensile strength to withstand the tension generated in narrow-diameter tubes. THE CONCEPT OF tension in xylem water is further supported by the evidence that water movement in trees begins in the topmost branches and the tree trunk compresses slightly when water starts moving.

The rate of transpiration is influenced by such factors as intercellular carbon dioxide concentration (and atmospheric $ ext{CO}_2$ content around the leaf), light, temperature, atmospheric humidity, air currents, and soil water availability. Most of these factors act upon the stomata. Stomatal opening and closing are controlled by turgor changes in guard cells, which correlate closely with fluctuations in potassium ion concentration within these cells. Abscisic acid and blue light also play roles in stomatal movement. Stomata open when guard cell turgor increases and close when it decreases.

Inorganic substances from soil solutions are available to plants in ionic form. Plants utilize metabolic energy to accumulate essential ions. Most ions are absorbed via active transport, while others cross the plasma membrane passively driven by a water potential established by actively transported ions and their pumps. Inorganic ions travel primarily along the symplastic pathway from the epidermis to the xylem.

The Study of substance translocation through the phloem has been greatly facilitated by the use of aphids and radioactive tracers. Analyses of sieve-tube sap indicate that it contains sugar—primarily sucrose—and small amounts of nitrogenous compounds. The velocity of longitudinal translocation through the phloem far exceeds the normal rate of sucrose diffusion in water, typically ranging from 50 to 100 cm/h.

According to the pressure-flow hypothesis, assimilates move from source to sink along a turgor pressure gradient developed osmotically. Sugars are actively secreted (loaded) into sieve tubes and absorbed (unloaded) from them by companion cells and parenchyma cells located at production and consumption sites, respectively. Sieve tubes play a passive role.

Appendix. Radioactive Labeling and Autoradiography in Plant Research

Radioactive tracers can be employed in various ways to study the synthesis, transport, and metabolism of substances in plants. First of all, any such experiment requires introducing a radioisotope into the plant. Radioactive carbon, for instance, will be absorbed by the plant if its leaves are maintained in an atmosphere containing carbon dioxide labeled with ¹⁴C; radioactive phosphorus will enter the plant if its roots are immersed in a solution containing ions with ³²P.

The duration of the plant's exposure to radioactive material is determined by the specific information researchers aim to obtain. For example, in experiments designed to determine the time required for carbon dioxide incorporation into various photosynthetic products, a series of exposure times must be used. In assays aimed at detecting a specific metabolic product using radioactive substances, exposure duration will be dictated by the reaction times under study.

In whole-plant autoradiography, the plant is rapidly frozen following exposure to a radioactive substance and freeze-dried. The plant is then pressed flat against X-ray film. Radiation emitted by the isotope affects the film adjacent to those parts of the plant where the label is localized. By comparing the flattened plant with the developed film, researchers can determine the localization of the radioactive substance within the plant.

In tissue radioautography (microradioautography), plant tissues subjected to freeze-drying are embedded in paraffin, resin, or a similar material. Very thin sections are then prepared and placed on Microscope slides. The sections are coated with a photographic emulsion or film. Radiation emitted by the isotope exposes the film in areas contacting the parts of the tissue section that contain the radioactive material. After a specific period, the film is developed. By comparing the film and the underlying section under a microscope, the exact localization of the radioactive substance within the plant tissues is revealed.

A. Two leaves of a broad bean plant (Vicia faba) were enclosed in a Plexiglas chamber containing ¹⁴CO₂. The leaves were exposed to the radioactive carbon dioxide in the light for 35 minutes. During this time, ¹⁴CO₂ was incorporated into sugars, which were subsequently translocated to other parts of the plant. Transverse (B) and longitudinal (C) sections of the stem were coated with radioautographic film for 32 days. When the film was developed and compared with the underlying sections, it was revealed that radioactivity (detected as black grains on the film) was concentrated almost exclusively in the sieve tubes.