Lecture Outline for Campbell/Reece Biology, 8th Edition

Chapter 36 Resource Acquisition and Transport in Vascular Plants Lecture Outline Overview: Underground Plants The perennial stone plant (Lithops) lives underground in the Kalahari Desert of southern Africa. Only the tips of two succulent leaves are exposed at the surface. Each leaf tip has a region of clear, lens-like cells that allow light to penetrate to the underground photosynthetic tissues. Stone plants conserve moisture and the high temperatures and light intensity of the desert. The success of plants depends largely on their ability to gather and conserve limiting resources in their environment. Concept 36.1 Land plants acquire resources from both above- and below-ground. Land plants live in two worlds: above-ground, where shoot systems acquire light and CO2 for photosynthesis, and below-ground, where root systems acquire water and minerals from the soil. The algal ancestors of plants obtained water, minerals, and CO2 from the water in which they were completely immersed. Each cell was close to the source of these materials. The earliest land plants were nonvascular, leafless shoots with waxy cuticles and few stomata, anchored by threadlike rhizoids. As land plants evolved, competition for light, water, and nutrients intensified. Taller plants with broad, flat leaves had an advantage in absorbing light. The increased surface area of tall plants, however, resulted in greater evaporation and a need for more water. Larger shoots required more anchorage, which favored the production of branching roots. This morphological solution created a new problem: the need for efficient, long-distance transport of water, minerals, and photosynthetic products between roots and shoots. The evolution of vascular tissue consisting of xylem and phloem made possible the development of extensive root and shoot systems capable of long-distance transport. Xylem transports water and minerals from the roots to the shoots. Phloem transports sugars from the site of production to the regions that need them for growth and metabolism. Plants have evolved many mechanisms for acquiring light from the sun, CO2 from the air, and water from the ground. Land plants must also minimize evaporative loss of water, especially in environments where water is scarce. Shoot architecture is designed to capture light. Stems support leaves and serve as conduits for the transport of water and nutrients. Shoot systems vary in the arrangement and form of leaves, the outgrowth of axillary buds, and the relative growth in stem length and thickness. Leaves range in length from the tiny 1.3-mm leaves of the pygmy weed (Crassula erecta), a native weed of dry, sandy regions in the western United States, to the 20-m leaves of the Raphia palm (Raphia regalis), a native of African rain forests. The largest leaves are generally found in tropical rain forests, and the smallest are usually found in dry or very cold environments where liquid water is scarce and evaporative loss from leaves must be reduced. Phyllotaxy, the way in which leaves are arranged on a stem, is determined by the shoot apical meristem and is specific to each species. A species may have one leaf per node (alternate, or spiral, phyllotaxy), two leaves per node (opposite phyllotaxy), or more (whorled phyllotaxy). Most angiosperms have alternate phyllotaxy, with leaves arranged in an ascending spiral around the stem. Each successive leaf emerges about 137.5 from the site of the previous one, providing each leaf the maximum exposure to light and reducing shading of the lower leaves by those above. The leaf area index is the ratio of the leaf surface of a plant or crop to the surface area of the land on which the plant or crop grows Leaf area index values as high as 7 are common for many mature crops. The addition of leaves results in increased shading of lower leaves, to the point that they respire more than photosynthesize. When this happens, the nonproductive leaves or branches undergo programmed cell death and are shed, a process called self-pruning. Leaf orientation affects light absorption. Some plants have horizontally oriented leaves, whereas grasses have leaves that are vertically oriented. In conditions with low levels of light, horizontal leaves effectively capture sunlight. In grasslands, horizontal orientation may expose upper leaves to levels of light that are too high, resulting in leaf injury and reduced photosynthesis. Plants vary in their branching pattern due in part to trade-offs between growth in height and extent of branching. Plants also vary in stem thickness. Most tall plants require thick stems, which provide greater vascular flow to, and mechanical support for, the leaves. Vines, though tall, have narrow stems and rely on other plants to raise their leaves higher. Root architecture is designed to acquire water and minerals. As plants became less dependent on very moist environments, the evolution of root branching allowed them to acquire more water and nutrients and provided strong anchorage. The tallest plant species are usually gymnosperms or eudicots because of their taproot systems. Taproots and lateral roots provide strong anchorage for the aerial parts of the plant. The fibrous root systems of monocots do not anchor a tall plant as strongly as a taproot system. Physiological mechanisms prevent competition within the root system of a plant. For example, cuttings from the stolons of buffalo grass (Buchloe dactyloides) developed fewer and shorter roots in the presence of cuttings from the same plant than they did in the presence of cuttings from another buffalo grass plant. When cuttings from the same node were separated, they became more alienated over time, eventually relating to each other as “strangers.” The mechanism underlying this ability to distinguish self from nonself is unknown, but one can imagine that avoiding competition between roots of the same plant for the same limited pool of resources would be beneficial. The evolution of mutualistic associations between roots and fungi was important in the successful colonization of land by plants, especially in poorly developed early soils. About 80% of extant land plant species form mycorrhizal associations with soil fungi. Mycorrhizal hyphae endow the fungus and plant roots with an enormous surface area for absorption of water and minerals, particularly phosphate. As much as 3 m of hyphae can extend from each centimeter along a root’s length, reaching a far greater volume of soil than the root alone could penetrate. Concept 36.2 Transport occurs by short-distance diffusion or active transport and by long-distance bulk flow. Transport at the cellular level depends on the selective permeability of membranes. Transport in plants begins with the absorption of resources by plant cells. The selective permeability of a plant cell’s plasma membrane controls the movement of solutes between the cell and the extracellular solution. Molecules tend to move down their electrochemical gradients, the combined effects of the concentration gradient of the solute and the voltage (charge difference) across the membrane. Diffusion across a membrane is called passive transport and occurs without the direct expenditure of metabolic energy by the cell. Active transport is the pumping of solutes across membranes against their electrochemical gradients; it requires an expenditure of energy by the cell. The cell must expend metabolic energy, usually in the form of ATP, to transport solutes “uphill.” Transport proteins embedded in the membrane can speed the movement across the membrane by either active or passive transport. In either case, transport proteins bind selectively to a solute on one side of the membrane and release it on the opposite side. Other transport proteins act as selective channels across the membrane. For example, the membranes of most plant cells have potassium channels that allow potassium ions (K+) to pass, but not similar ions, such as sodium (Na+). Some channels are gated, opening or closing in response to certain environmental or biochemical stimuli. Proton pumps play a central role in transport across plant membranes. The most important active transport protein in the plasma membrane of plant cells is the proton pump. The proton pump hydrolyzes ATP and uses the released energy to pump hydrogen ions (H+) out of the cell. This creates a proton gradient because the H+ concentration is higher outside the cell than inside. It also creates a membrane potential or voltage, a separation of opposite charges across a membrane. The H+ gradient is a form of potential (stored) energy, and the flow of H+ back into the cell can be harnessed to perform cellular work. This potential energy is used to drive the active transport of many different solutes. For example, the membrane potential generated by proton pumps contributes to the uptake of potassium ions (K+) by root cells. The proton gradient also functions in cotransport, in which the downhill passage of one solute (H+) is coupled with the uphill passage of another, such as NO3- or sucrose. Differences in water potential drive water transport in plant cells. The survival of plant cells depends on their ability to balance water uptake and loss. The net uptake or loss of water by a cell occurs by osmosis, the passive transport of water across a membrane. Plant cells have rigid cell walls. As the protoplast (the living part of the cell) expands, the physical pressure of the cell wall pushes back. In a plant cell, the direction of water movement depends on the solute concentration and physical pressure. The combined effects of the solute concentration and pressure are called water potential, represented by the Greek letter . Free water—water that is not bound to solutes or surfaces—moves from regions of higher water potential to regions of lower water potential. For example, if a plant cell is immersed in a solution with higher water potential than the cell, osmotic uptake of water causes the cell to swell. By moving, water can perform work, such as expanding the cell. The word potential in water potential refers to the potential energy that can be released to do work when water moves from a region of higher to a region of lower . Plant biologists measure in units called megapascals (MPa), where 1 MPa is equal to about 10 atmospheres of pressure. An atmosphere is the pressure exerted at sea level by a volume of air extending though the height of the atmosphere—about 1 kg of pressure per square centimeter. The internal pressure of a plant cell is approximately 0.5 MPa, twice the air pressure inside a tire. Both pressure and solute concentration affect water potential. The effects of pressure and solute concentration on water potential are incorporated into the following equation, where P is the pressure potential and S is the solute potential (or osmotic potential): = S + P The solute potential (S) of a solution is the proportional to the number of dissolved solute molecules. Solute potential is also called osmotic potential because solutes affect the direction of osmosis. By definition, the S of pure water is 0. Solutes bind water molecules, reducing the number of free water molecules and lowering the capacity of water to move and do work. Adding solutes always lowers water potential; the S of a solution is always negative. Pressure potential (P) is the physical pressure on a solution and can be positive or negative relative to atmospheric pressure. The water in the dead vessel element cells of xylem may be under negative pressure of less than ?2 MPa. Water in living cells is usually under positive pressure. The cell contents press the plasma membrane against the cell wall, producing turgor pressure. Water potential can be measured. Let’s represent water movement in a U-shaped tube across a selectively permeable membrane. Remember that water moves in the direction of higher water potential to lower water potential. If the right arm of the tube contains a 0.1 M solution (S = -0.23 MPa) and the left arm contains pure water (S = 0), and there is no physical pressure ( P = 0), the water potential is equal to S. Because water moves from regions of higher water potential to regions of lower water potential, the net water movement will be from the left arm of the tube to the right arm. Applying a positive physical pressure of +0.23 MPa to the solution in the right arm raises its water potential from a negative value to 0 MPa ( = -0.23 + 0.23). There will be no net flow of water between this pressurized solution and the compartment of pure water. If we increase P to +0.30 MPa, the solution has a water potential of +0.07 MPa ( = -0.23 + 0.30), and the solution will actually lose water to a compartment containing pure water. Water potential affects the uptake and loss of water in plant cells. In a flaccid cell, P = 0 and the cell is limp. If this cell is placed in a solution with a higher solute concentration (and, therefore, a lower ), water will leave the cell by osmosis. Eventually, the cell will plasmolyze by shrinking and pulling away from its wall. If a flaccid cell is placed in pure water (= 0), the cell will have lower water potential than pure water due to the presence of solutes, and water will enter the cell by osmosis. As the cell begins to swell, it will push against the cell wall, producing turgor pressure. The partially elastic wall will push back until this pressure is great enough to offset the tendency for water to enter the cell because of solutes. When P and S are equal in magnitude but opposite in sign, = 0, and the cell has reached a dynamic equilibrium with the environment, with no further net movement of water in or out. A walled cell with a greater solute concentration than its surroundings is turgid, or firm. Healthy plants are turgid most of the time, and their turgor contributes to support in nonwoody parts of the plant. You can see the effects of turgor loss in wilting, the drooping of leaves and stems as plant cells become flaccid. Aquaporins affect the rate of water transport across membranes. A difference in water potential determines the direction of water movement across membranes, but how do water molecules actually cross the membrane? Both plant and animal membranes have specific transport proteins, aquaporins, that facilitate the passive movement of water across a membrane. Aquaporins do not affect the water potential gradient or the direction of water flow, but rather increase the rate at which water diffuses down its water potential gradient. Evidence is accumulating that the rate of water movement through aquaporins is regulated by phosphorylation of the aquaporin proteins, brought about by increases in cytoplasmic calcium ions or decreases in cytoplasmic pH. Auqaporins may also facilitate the absorption of CO2 by plant cells. The compartmental structure of plant cells provides three routes for short-distance transport. Every plant is separated from neighboring cells by cell walls, consisting of a mesh of polysaccharides through which mineral ions diffuse readily. Ions can diffuse across a tissue entirely through the apoplast, the continuum formed by cell walls, intercellular spaces, and the dead interior of tracheids and vessels. While the thick cell wall helps maintain the cell’s shape, the cell membrane regulates the traffic of material into and out of the protoplast. The cytoplasm of cells is collectively referred to as the symplast. Cytoplasmic channels called plasmodesmata connect the cytoplasm of neighboring cells. The compartmental structure of plant cells provides three routes for short-distance transport within a plant tissue or organ: apoplastic, symplastic, and transmembrane routes. In the apoplastic route, water and solutes can move from one location to another within a root or other organ through the continuum of cell walls without ever entering a cell. In the symplastic route, water and solutes move along the continuum of cytosol within a plant tissue. This route requires only one crossing of a plasma membrane. After entering one cell, solutes and water move from cell to cell via plasmodesmata. In the transmembrane route, water and solutes move out of one cell, across the cell wall, and into the neighboring cell, which may then pass the substances along to the next cell by the same mechanism. This transmembrane route requires repeated crossings of plasma membranes. Bulk flow functions in long-distance transport. Diffusion and active transport are fairly efficient for transport within a cell or between cells. However, diffusion is much too slow for long-distance transport within a plant, such as the movement of water and minerals from roots to leaves. Water and solutes move through xylem vessels and sieve tubes by bulk flow, the movement of a fluid driven by pressure. Within the tracheids and vessel elements of the xylem and within the sieve-tube elements of the phloem, water and dissolved solutes move together in the same direction by bulk flow. The structures of the conducting cells of the xylem and phloem help to make bulk flow possible. Tracheids and vessel elements are dead at maturity and lack cytoplasm. The cytoplasm of sieve-tube elements has few internal organelles. Loss of cytoplasm in a plant’s “plumbing” allows for efficient bulk flow through the xylem and phloem. The perforation plates at the ends of vessel elements and the porous sieve plates connecting sieve-tube elements also enhance bulk flow. Diffusion, active transport, and bulk flow act in concert to transport resources throughout the whole plant. For example, bulk flow due to a pressure difference is the mechanism of the long-distance transport of sugars in the phloem, but active transport of sugar at the cellular level maintains this pressure difference. Concept 36.3 Water and minerals are transported from roots to shoots. Most water and mineral absorption occurs in the cells at the tips of roots. Epidermal cells at the root tip are permeable to water, and many are differentiated into root hairs, modified cells specialized for water absorption. The root hairs absorb the soil solution, which consists of water molecules and dissolved mineral ions that are not bound tightly to soil particles. The soil solution flows through the hydrophilic walls of epidermal cells and travels along the cell walls and the intercellular spaces into the root cortex. Although the soil solution usually has a low mineral concentration, active transport enables roots to accumulate essential minerals, such as K+, to concentrations hundreds of times higher than in the soil. Water and minerals cross the endodermis to reach the vascular cylinder. Water and minerals that pass from the soil into the root cortex cannot be transported to the rest of the plant until they enter the xylem of the stele. The endodermis, the innermost layer of cells in the root cortex, surrounds the stele and regulates the selective passage of minerals from the cortex into the stele tissue. Minerals already in the symplast when they reach the endodermis continue through the plasmodesmata of endodermal cells and pass into the stele. These minerals already crossed a plasma membrane to enter the symplast in the epidermis or cortex. The endodermis, with its Casparian strip, ensures that no minerals can reach the vascular tissue of the root without crossing a selectively permeable plasma membrane. The Casparian strip, located in the transverse and radial walls of each endodermal cell, is a belt made of suberin, a waxy material impervious to water and dissolved minerals. The Casparian strip prevents water and minerals from crossing the endodermis and entering the vascular tissue via the apoplast. Water and minerals that are passively moving through the apoplast must cross the plasma membrane of an endodermal cell and enter the stele via the symplast. The endodermis also prevents solutes that have accumulated in the xylem from leaking back into the soil solution. Water and minerals ascend from roots to shoots through the xylem. Water and minerals from the soil enter the plant through the epidermis of roots, cross the root cortex, and pass into the stele. Xylem sap, the water and dissolved minerals in the xylem, is transported long distances by bulk flow from the stele of roots to veins that branch throughout each leaf. Bulk flow is much faster than diffusion or active transport. Peak velocities in the transport of xylem sap can range from 15 to 45 m/hr for trees with wide vessels. Plants lose an astonishing amount of water by transpiration, the loss of water vapor from leaves and other aerial parts of the plant. A single corn plant transpires 60 L of water during its growing season. The flow of water transported up from the xylem replaces the water lost in transpiration and also carries minerals to the shoot system. The transpiration-cohesion-tension mechanism transports xylem sap against gravity. Xylem sap rises against gravity to reach heights of more than 100 m in the tallest trees. At night, when transpiration is very low or zero, the root cells continue pumping mineral ions into the xylem. The accumulation of minerals in the vascular cylinder lowers the water potential there, generating a positive pressure, called root pressure, that pushes xylem sap. Root pressure causes guttation, the exudation of water droplets that can be seen in the morning on the tips of grass blades or the leaf margins of some plants. In most plants, root pressure is not the major mechanism driving the ascent of xylem sap. At most, root pressure can force water upward only a few meters, and many plants generate no root pressure at all. For the most part, xylem sap is not pushed from below by root pressure but is pulled upward by the leaves themselves. Transpiration provides the pull, and the cohesion of water due to hydrogen bonding transmits the upward pull along the entire length of the xylem to the roots. Stomata on a leaf’s surface lead to internal air spaces that become saturated with water vapor. Most of the time, the air outside the leaf is drier and has a lower water potential than the air inside the leaf. Water vapor diffuses down its water potential gradient, leaving the leaf through the stomata. The loss of water vapor from the leaf translates into a pulling force for the upward movement of water through a plant. The cell wall lining the mesophyll cells acts like a very fine capillary network. Water adheres to the cellulose microfibrils of the cell wall. As water evaporates from the water film that covers the cell walls of mesophyll cells, the air-water interface withdraws between the microfibrils of the cell wall. Because of the high surface tension of water, the curvature of the interface induces a tension, or negative pressure potential, in the water. As more water evaporates from the cell wall, the curvature of the air-water interface increases and the pressure of the water becomes more negative. Water molecules from the more hydrated parts of the leaf are then pulled toward this area to reduce the tension. These pulling forces are transferred to the xylem because each water molecule is cohesively bound to the next by hydrogen bonds. The role of negative pressure potential in transpiration is consistent with the water potential equation because negative pressure potential (tension) lowers water potential. The negative water potential of leaves provides the “pull” in transpirational pull. Cohesion and adhesion facilitate the ascent of xylem sap. The transpirational pull on xylem sap is transmitted all the way from the leaves to the root tips and even into the soil solution. Cohesion and adhesion facilitate this long-distance transport by bulk flow. The cohesion of water due to hydrogen bonding makes it possible to pull a column of xylem sap from above without the water molecules separating. Helping to fight gravity is the strong adhesion of water molecules to the hydrophilic walls of the xylem cells. The upward pull on the cohesive sap creates tension within the xylem. This tension can actually cause a measurable decrease in the diameter of a tree on a warm day. Transpiration puts the xylem under tension all the way down to the root tips, lowering the water potential in the root xylem and pulling water from the soil. Transpirational pull extends down to the roots only through an unbroken chain of water molecules. Cavitation, the formation of water vapor pockets in the xylem vessel, breaks the chain. Cavitation is more common in wide vessels than in tracheids and can occur under drought stress or when xylem sap freezes in water. The air bubbles resulting from cavitation expand and block the water channels of the xylem. Root pressure is used by small plants to refill blocked vessels in spring. Root pressure cannot push water to the top of a tree, however. In trees, a xylem vessel with a water vapor pocket can never function as a water pipe again. The transpirational stream can detour around the water vapor pocket, and secondary growth adds a new layer of xylem vessels each year. Only the youngest, outermost secondary xylem vessels in trees transport water. The older xylem vessels no longer function in water transport but do provide support for the tree. Xylem sap ascends by solar-powered bulk flow: a review. The transpiration-cohesion-tension mechanism transports xylem sap against gravity. Long-distance transport of water from roots to leaves occurs by bulk flow. The movement of fluid is driven by a water potential difference at opposite ends of xylem tissue. The water potential difference is created at the leaf end of the xylem by evaporation of water from leaf cells. Evaporation lowers the water potential at the air-water interface, generating the negative pressure (tension) that pulls water through the xylem. Bulk flow, the mechanism for long-distance transport up xylem vessels, differs from diffusion in some key ways. Bulk flow is driven by differences in P, so S is not a factor. The water potential gradient within the xylem is essentially a pressure gradient. The flow occurs within hollow, dead cells. Unlike osmosis, bulk flow moves the entire solution—not just water or solutes—at much greater speed. The plant expends none of its own metabolic energy to lift xylem sap up to the leaves by bulk flow. The absorption of sunlight drives transpiration by causing water to evaporate from the moist walls of mesophyll cells and by lowering the water potential in the air spaces within a leaf. Thus, the ascent of xylem sap is ultimately solar-powered. Concept 36.4 Stomata help regulate the rate of transpiration. Most leaves have broad surface areas and high surface-to-volume ratios. In many plants, leaf shape is a morphological adaptation to enhance the absorption of light for photosynthesis. Leaf shape can also increase water loss through stomata. To make food, a plant must spread its leaves to the sun and obtain CO2 from air. CO2 diffuses into and O2 diffuses out of the leaf via the stomata. Within the leaf, CO2 enters a honeycomb of air spaces formed by the irregularly shaped spongy mesophyll cells. This internal surface may be 10–30 times larger than the external leaf surface. The large surface area and high surface-to-volume ratio of leaves increase the rate of photosynthesis and also increase water loss through stomata. A plant’s need for water is largely a negative consequence of the shoot system’s need for gas exchange for photosynthesis. By regulating the opening and closing of the stomata, guard cells help to conserve water. Stomata are major pathways for water loss. About 95% of the water that a plant loses escapes through stomata, although these pores account for only 1–2% of the external leaf surface. The waxy cuticle reduces water loss through the remaining leaf surface. Guard cells control the diameter of the stoma by changing shape, thereby widening or narrowing the gap between the two cells. The amount of water lost by a leaf depends on the number of stomata and the average size of their pores. The stomatal density of a leaf may be as high as 20,000 per cm2. Stomatal density is under both genetic and environmental control. Desert plants have lower stomatal densities than do marsh plants. High light intensities and low CO2 levels during leaf development tend to increase stomatal density in many species. A recent British survey found that the stomatal density of many woodland species has decreased since 1927. This is consistent with the dramatic increases in CO2 levels during the 1900s. Guard cells mediate the photosynthesis-transpiration compromise. When guard cells take in water by osmosis, they become more turgid. In most angiosperms, the thickness of cell walls is uneven. Because of the orientation of cellulose microfibrils in the cell wall, the guard cells buckle outward when turgid. The buckling increases the size of the pore between the guard cells. When guard cells lose water and become flaccid, they become less bowed, and the pore closes. Changes in turgor pressure that open and close stomata result primarily from the reversible uptake and loss of potassium ions (K+) by guard cells. Stomata open when guard cells actively accumulate K+ from neighboring epidermal cells. The K+ fluxes across the guard cell membranes are coupled to the generation of membrane potentials by proton pumps. Stomatal opening correlates with active transport of H+ out of guard cells. The resulting voltage (membrane potential) drives K+ into the cell through specific membrane channels. The absorption of K+ decreases water potential in guard cells, leading to an inflow of water by osmosis and increasing cell turgor. Most of the K+ and water are stored in the vacuole, and the vacuolar membrane also plays an important regulatory role. Stomatal closing results from an exodus of K+ from guard cells, leading to osmotic loss of water. Regulation of aquaporins may also be involved in the swelling and shrinking of guard cells by varying the permeability of the membranes to water. Internal and environmental cues contribute to the opening and closing of stomata. In general, stomata are open during the day and closed at night to minimize water loss when it is too dark for photosynthesis to occur. At least three cues contribute to stomatal opening at dawn: light, CO2 depletion, and an internal “clock” in guard cells. Illumination of blue-light receptors in the guard cells stimulates the activity of ATP-powered proton pumps in the plasma membrane, promoting the uptake of K+. The amount of CO2 within the air spaces of the leaf is depleted as photosynthesis begins. An internal “clock” is located in the guard cells. Even in the dark, stomata continue their daily rhythm of opening and closing due to the presence of internal clocks that regulate cyclic processes. The opening and closing cycle of the stomata is an example of a circadian rhythm, cycles that have intervals of approximately 24 hours. Various environmental stresses can cause stomata to close during the daytime. When the plant is suffering a water deficiency, guard cells may lose turgor and close stomata. Abscisic acid, a hormone produced by the roots and leaves in response to water deficiency, signals guard cells to close stomata. While reducing further wilting, closing the stomata also restricts CO2 absorption and slows photosynthesis. Since turgor is necessary for cell elongation, growth ceases. Guard cells control stomatal opening on a moment-to-moment basis, reacting to a cloud or transient shaft of sunlight. In general, transpiration is greatest on sunny, warm, dry, and windy days because these environmental factors increase evaporation. If transpiration cannot pull sufficient water to the leaves, the shoot becomes slightly wilted, as cells lose turgor pressure. Under prolonged drought conditions, leaves can become severely wilted and irreversibly injured. Transpiration also results in evaporative cooling, which can lower the temperature of a leaf by as much as 10–15°C relative to the surrounding air. This cooling prevents the leaf from reaching temperatures that could denature enzymes involved in photosynthesis and other metabolic processes. Xerophytes have evolutionary adaptations that reduce transpiration. Plants adapted to regions with little free moisture, called xerophytes, have various leaf modifications that reduce the rate of transpiration. Many xerophytes, such as cacti, have very small leaves, which reduces excessive water loss; photosynthesis is carried out mainly in their stems. Many xerophytes store water in fleshy stems for use during prolonged drought. Another adaptation to arid habitats is crassulacean acid metabolism (CAM), a specialized form of photosynthesis found in succulent species of the family Crassulaceae and several other families. The leaves of CAM plants take in CO2 at night, allowing the stomata to remain closed during the day when transpiration is greatest. Other species of desert plants avoid drought by completing short life cycles during brief rainy seasons. Concept 36.5 Sugars are transported from leaves and other sources to sites of use or storage. The phloem transports the organic products of photosynthesis throughout the plant via a process called translocation. In angiosperms, the specialized cells of the phloem that function in translocation are the sieve-tube elements. These elements are arranged end to end to form long sieve tubes with porous sieve plates between cells along the tube. Phloem sap is an aqueous solution that flows through sieve tubes. Sugar, primarily the disaccharide sucrose, is the most common solute in phloem sap. Sucrose concentration in sap can be as high as 30% by weight. Sap may also contain minerals, amino acids, and hormones. Phloem translocates its sap from sugar sources to sugar sinks. In contrast to the unidirectional flow of xylem sap from roots to leaves, the direction that phloem sap travels can vary. Sieve tubes always carry food from a sugar source to a sugar sink. A sugar source is a plant organ (especially mature leaves) in which sugar is produced by either photosynthesis or the breakdown of starch. A sugar sink is an organ (such as growing roots, buds, stems, and fruit) that is a net consumer or depository of sugar. Mature leaves are the primary sugar sources. A storage organ, such as a tuber or a bulb, may be either a source or a sink, depending on the season. When the storage organ is stockpiling carbohydrates during the summer, it is a sugar sink. After breaking dormancy in the early spring, the storage organ becomes a source as its starch is broken down to sugar, which is carried away in the phloem to the growing buds of the shoot system. A sugar sink usually receives its sugar from the sources nearest to it. The upper leaves on a branch may send sugar to the growing shoot tip, while the lower leaves of the same branch export sugar to roots. A growing fruit may monopolize the sugar sources that surround it. One sieve tube in a vascular bundle may carry phloem sap in one direction, while sap in a different tube in the same bundle may flow in the opposite direction. The direction of transport in each sieve tube depends only on the locations of the source and sink connected by that tube. Sugar from mesophyll cells or other sources must be loaded into sieve-tube elements before it can be exported to sugar sinks. In some species, sugar moves from mesophyll cells to sieve-tube elements via the symplast, passing through plasmodesmata. In other species, sucrose reaches sieve-tube elements by a combination of symplastic and apoplastic pathways. For example, in corn leaves, sucrose diffuses through the symplast from mesophyll cells into small veins. Much of this sugar moves out of the cells into the apoplast and is accumulated by nearby sieve-tube elements, either directly or through companion cells. In some plants, companion cells (transfer cells) have numerous ingrowths in their walls to increase the cell’s surface area and enhance the transfer of solutes between apoplast and symplast. In many plants, sieve-tube elements and companion cells accumulate sucrose at higher concentrations than those in mesophyll cells. This requires active transport to load the phloem. Proton pumping and cotransport of sucrose and H+ promote the movement of sucrose from mesophyll cells to sieve-tube elements. Downstream, at the sink end of the sieve tube, phloem unloads its sucrose. The mechanism of phloem unloading is highly variable and depends on the plant species and type of organ. Regardless of the mechanism, because the concentration of free sugar in the sink is lower than the concentration in the phloem, sugar molecules diffuse from the phloem into the sink tissues. Water follows by osmosis. Pressure flow is the mechanism of translocation in angiosperms. Phloem sap flows from source to sink at rates as fast as 1 m/hr, faster than can be accounted for by either diffusion or cytoplasmic streaming. Phloem sap moves by bulk flow driven by positive pressure, known as pressure flow. The buildup of pressure at the source end and the reduction of pressure at the sink end cause water to flow from source to sink, carrying sugar along. The pressure flow hypothesis explains why phloem sap always flows from source to sink. Studies using electron microscopes suggest that in nonflowering vascular plants, the pores between phloem may be too small or obstructed to permit pressure flow. It is not clear whether the pressure flow model applies to all vascular plants. Sinks vary in energy demands and capacity to unload sugars. Plants may have more sinks than can be supported by sources. In that case, a plant may abort some flowers, seeds, or fruits in a process of self-thinning. Concept 36.6 The symplasm is highly dynamic. Plant transport is a dynamic process because the transport needs of a plant cell typically change during its development. For example, a leaf may begin as a sugar sink but spend most of its life as a sugar source. Environmental changes may alter plant transport processes. Water stress may activate signal transduction pathways that greatly alter the membrane transport proteins governing the overall transport of water and minerals. Since symplasm is living tissue, it is largely responsible for the dynamic changes in plant transport processes. Plasmodesmata are highly dynamic structures that can change in permeability and number. Plasmodesmata can open or close rapidly in response to changes in turgor pressure, cytoplasmic calcium levels, or cytoplasmic pH. Plasmodesmata may form during cytokinesis but may also form much later. Loss of function is common during differentiation. For example, as a leaf matures from being a sink to a source, phloem unloading ceases as the plasmodesmata either close or are eliminated. Early studies by plant physiologists and pathologists came to differing conclusions regarding the pore sizes of plasmodesmata. Physiologists injected fluorescent probes of different molecular sizes into cells and recorded whether the molecules passed into adjacent cells. Based on these observations, they concluded that the pore sizes were approximately 2.5 nm—too small for macromolecules such as proteins. In contrast, pathologists provided electron micrographs showing evidence of the passage of viral particles with diameters of 10 nm or larger. These discordant findings are explained by the hypothesis that viruses can greatly dilate plasmodesmata. Plant viruses produce viral movement proteins that cause plasmodesmata to dilate, enabling viral RNA to pass between cells. More recent evidence shows that plant cells themselves can dilate plasmodesmata as part of a dynamic communication network. Viruses subvert this network by mimicking the cell’s regulators of plasmodesmata. A high degree of cytoplasmic interconnectedness exists within groups of cells and tissues known as symplastic domains. Proteins and RNAs coordinate development between cells within a symplastic domain. Phloem plays a variety of roles. Electrical signaling has been studied extensively in plants that have rapid leaf movements, such as the sensitive plant (Mimosa pudica) and Venus’ flytrap (Dionaea muscipula). The role of electrical signaling in other species is less clear. Studies have revealed that a stimulus in one part of a plant can trigger an electrical signal in the phloem that affects another part of the plant, eliciting a change in gene transcription, respiration, photosynthesis, phloem unloading, or hormonal levels. Thus, the phloem serves a nerve-like function, allowing for swift electrical communication between widely separated organs. In addition to transporting sugars and conducting electrical signals, the phloem is a “superhighway” for the systemic transport of macromolecules and viruses. Systemic changes are those that spread throughout the body, affecting many or all body systems or organs. Proteins and various types of RNA may enter the sieve tubes through plasmodesmata, allowing systemic communication that helps to integrate the functions of the whole plant. The delivery of a flower-inducing signal from leaves to vegetative meristems is an example of a systemic change. Another example is a defense response to localized infection, in which signals traveling through the phloem activate defense genes in noninfected tissues. The coordinated transport of materials and information is central to plant survival, making optimal use of limited resources. Ultimately, the successful acquisition and conservation of resources and the integrated functioning of the whole plant are the most critical determinants of whether the plant will compete successfully. Lecture Outline for Campbell/Reece Biology, 8th Edition, © Education, Inc. 36-13