Plant Transport

TRANSPORT IN PLANTS Introduction The algal ancestors of plants were completely immersed in water and dissolved minerals. The evolutionary journey onto land involved the differentiation of the plant body into roots, which absorb water and minerals from the soil, and shoots, which are exposed to light and atmospheric CO2. This morphological solution created a new problem: the need to transport materials between roots and shoots. Roots and shoots are bridged by vascular tissues that transport sap throughout the plant body. A. An Overview of Transport Mechanisms in Plants Transport in plants occurs on three levels: • (1) The uptake and loss of water and solutes by individual cells. • (2) Short-distance transport of substances from cell to cell at the level of tissues or organs. • (3) Long-distance transport of sap within xylem and phloem at the level of the whole plant. 1. Transport at the cellular level depends on the selective permeability of membranes 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 concentration gradient, and when this occurs across a membrane it is called passive transport and occurs without the direct expenditure of metabolic energy by the cell. Transport proteins embedded in the membrane can speed movement across the membrane. Some transport proteins bind selectively to a solute on one side of the membrane and release it on the opposite side. Others act as selective channels, providing a selective passageway 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. In active transport, solutes are pumped across membranes against their electrochemical gradients. The cell must expend metabolic energy, usually in the form of ATP, to transport solutes “uphill”—counter to the direction in which the solute diffuses. Transport proteins that simply facilitate diffusion cannot perform active transport. Active transporters are a special class of membrane proteins, each responsible for pumping specific solutes. 2. Proton pumps play a central role in transport across plant membranes The most important active transporter in the plasma membrane of plant cells is the proton pump. It 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 because the proton pump moves positive charges (H+) outside the cell, making the inside of the cell negative in charge relative to the outside. Both the concentration gradient and the membrane potential are forms of potential (stored) energy that can be harnessed to perform cellular work. These are often used to drive the 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. The role of protons pumps in transport is a specific application of the general mechanism called chemiosmosis, a unifying principle in cellular energetics. In chemiosmosis, a transmembrane proton gradient links energy-releasing processes to energy-consuming processes. The ATP synthases that couple H+ diffusion to ATP synthesis during cellular respiration and photosynthesis, function somewhat like proton pumps. However, proton pumps normally run in reverse, using ATP energy to pump H+ against its gradient. 3. 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. In the case of a plant cell, the direction of water movement depends on solute concentration and physical pressure, together called water potential, abbreviated by the Greek letter “psi.” Water will move across a membrane from the solution with the higher water potential to the solution with the lower water potential. For example, if a plant cell is immersed in a solution with a higher water potential than the cell, osmotic uptake of water will cause the cell to swell. By moving, water can perform work. Therefore the potential in water potential refers to the potential energy that can be released to do work when water moves from a region with higher psi to lower psi. Plant biologists measure psi in units called megapascals (abbreviated MPa), where one MPa is equal to about 10 atmospheres of pressure. An atmosphere is the pressure exerted at sea level by an imaginary column of air—about 1 kg of pressure per square centimeter. A car tire is usually inflated to a pressure of about 0.2 MPa and water pressure in home plumbing is about 0.25 MPa. For purposes of comparison, the water potential of pure water in a container open to the atmosphere is zero. The addition of solutes lowers the water potential because the water molecules that form shells around the solute have less freedom to move than they do in pure water. Any solution at atmospheric pressure has a negative water potential. For instance, a 0.1-molar (M) solution of any solute has a water potential of -0.23 MPa. If a 0.1 M solution is separated from pure water by a selectively permeable membrane, water will move by osmosis into the solution. Water will move from the region of higher psi (0 MPa) to the region of lower psi (-0.23 MPa). In contrast to the inverse relationship of psi to solute concentration, water potential is directly proportional to pressure. Physical pressure—pressing the plunger of a syringe filled with water, for example—causes water to escape via any available exit. If a solution is separated from pure water by a selectively permeable membrane, external pressure on the solution can counter its tendency to take up water due to the presence of solutes or even force water from the solution to the compartment with pure water. It is also possible to create negative pressure, or tension as when you pull up on the plunger of a syringe. The combined effects of pressure and solute concentrations on water potential are incorporated into the following equation: psi = psip + psis Where psip is the pressure potential and psis is the solute potential (or osmotic potential). If a 0.1 M solution (psi = -0.23 MPa) is separated from pure water (psi = 0 MPa) by a selectively permeable membrane, then water will move from the pure water to the solution. Application of physical pressure can balance or even reverse the water potential. A negative potential can decrease water potential. Water potential impacts the uptake and loss of water in plant cells. In a flaccid cell, psip = 0 and the cell is not firm. If this cell is placed in a solution with a higher solute concentration (and therefore a lower psi), 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 pure water (psi = 0), the cell will have lower water potential due to the presence of solutes than that in the surrounding solution and water will enter the cell by osmosis. As the cell begins to swell, it will push against the wall, producing a 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 psip and psis are equal in magnitude (but opposite in sign), psi = 0, and the cell reaches 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 will be turgid or firm. Healthy plants are turgid most of the time as turgor contributes to support in nonwoody parts of the plant. 4. Aquaporins affect the rate of water transport across membranes Until recently, most biologists accepted the hypothesis that leakage of water across the lipid bilayer was enough to account for water fluxes across membranes. However, careful measurements in the 1990s indicated that water transport across biological membranes was too specific and too rapid to be explained entirely by diffusion. 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 the rate at which water diffuses down its water potential gradient. This raises the possibility that the cell can regulate the rate of water uptake or loss when its water potential is different from that of its environment. If aquaporins are gated channels, then they may open and close in response to variables, such as turgor pressure, in the cell. 5. Vacuolated plant cells have three major compartments While the thick cell wall helps maintain cell shape, it is the cell membrane, and not the cell wall, that regulates the traffic of material into and out of the protoplast. This membrane is a barrier between two major compartments: the wall and the cytosol. Most mature plants have a third major compartment, the vacuole. The membrane that bounds the vacuole, the tonoplast, regulates molecular traffic between the cytosol and the contents of the vacuole, called the cell sap. Proton pumps in the tonoplast expel H+ from the cytosol to the vacuole. This augments the ability of proton pumps of the plasma membrane to maintain a low cytosolic concentration of H+. In most plant tissues, two of the three cellular compartments are continuous from cell to cell. Plasmodesmata connect the cytosolic compartments of neighboring cells. This cytoplasmic continuum, the symplast, forms a continuous pathway for transport. The walls of adjacent plant cells are also in contact, forming a second continuous compartment, the apoplast. 6. Both the symplast and the apoplast function in transport within tissues and organs Three routes are available for lateral transport, the movement of water and solutes from one location to another within plant tissues and organs. This often occurs along the radial axis of plant organs. In one route, substances 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 same mechanism. This transmembrane route requires repeated crossings of plasma membranes. The second route, via the symplast, requires only one crossing of a plasma membrane. After entering one cell, solutes and water move from cell to cell via plasmodesmata. The third route is along the apoplast, the extracellular pathway consisting of cell wall and extracellular spaces. Water and solutes can move from one location to another within a root or other organ through the continuum of cell walls before ever entering a cell. 7. Bulk flow functions in long-distance transport Diffusion in a solution is fairly efficient for transport over distances of cellular dimensions (less than 100 microns). However, diffusion is much too slow for long-distance transport within a plant—for example, 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. In phloem, for example, hydrostatic pressure generated at one end of a sieve tube forces sap to the opposite end of the tube. In xylem, it is actually tension (negative pressure) that drives long-distance transport. Transpiration, the evaporation of water from a leaf, reduces pressure in the leaf xylem. This creates a tension that pulls xylem sap upward from the roots. Flow rates depend on a pipe’s internal diameter. To maximize bulk flow, the sieve-tube members are almost entirely devoid of internal organelles. Vessel elements and tracheids are dead at maturity. The porous plates that connect contiguous sieve-tube members and the perforated end walls of xylem vessel elements also enhance bulk flow. B. Absorption of Water and Minerals by Roots Water and mineral salts from soil enter the plant through the epidermis of roots, cross the root cortex, pass into the stele, and then flow up xylem vessels to the shoot system. • (1) The uptake of soil solution by the hydrophilic epidermal walls provides access to the apoplast, and water and minerals soak into the cortex along this route. • (2) Minerals and water that cross the plasma membranes of root hairs enter the symplast. • (3) Some water and minerals are transported into cells of the epidermis and cortex and inward via the symplast. • (4) Materials flowing along the apoplastic route are blocked by the waxy Casparian strip at the endoderm. • (5) Endodermal and parenchyma cells discharge water and minerals into their walls. The water and minerals now enter the dead cells of xylem vessels and are transported upward into the shoots. 1. Root hairs, mycorrhizae, and a large surface area of cortical cells enhance water and mineral absorption Much of the absorption of water and minerals occurs near root tips, where the epidermis is permeable to water and where root hairs are located. Root hairs, extensions of epidermal cells, account for much of the surface area of roots. The soil solution flows into the hydrophilic walls of epidermal cells and passes freely along the apoplast into the root cortex, exposing all the parenchyma cells to soil solution and increasing membrane surface area. As the soil solution moves along the apoplast into the roots, cells of the epidermis and cortex take up water and certain solutes into the symplast. Selective transport proteins of the plasma membrane and tonoplast enable root cells to extract essential minerals from the dilute soil solution and concentrate them hundred of times higher than in the soil solution. This selective process enables the cell to extract K+, an essential mineral nutrient, and exclude most Na+. Most plants form partnerships with symbiotic fungi for absorbing water and minerals from soil. “Infected” roots form mycorrhizae, symbiotic structures consisting of the plant’s roots united with the fungal hyphae. Hyphae absorb water and selected minerals, transferring much of these to the host plants. The mycorrhizae create an enormous surface area for absorption and can even enable older regions of the roots to supply water and minerals to the plant. 2. The endodermis functions as a selective sentry between the root cortex and vascular tissue Water and minerals in the root cortex cannot be transported to the rest of the plant until they enter the xylem of the stele. The endodermis, the innermost layers of the root cortex, surrounds the stele and functions as a last checkpoint for the selective passage of minerals from the cortex into the vascular tissue. Minerals already in the symplast continue through the plasmodesmata of the endodermal cells and pass into the stele. Those minerals that reach the endodermis via the apoplast are blocked by the Casparian strip in the walls of each endodermal cell. This strip is a belt of suberin, a waxy material that is impervious to water and dissolved minerals. These materials must cross the plasma membrane of the endodermal cell and enter the stele via the symplast. The endodermis, with its Casparian strip, ensures that no minerals reach the vascular tissue of the root without crossing a selectively permeable plasma membrane. The endodermis acts as a sentry on the cortex-stele border. The last segment in the soil -> xylem pathway is the passage of water and minerals into the tracheids and vessel elements of the xylem. Because these cells lack protoplast, the lumen and the cells walls are part of the apoplast. Endodermal cells and parenchyma cells within the stele discharge minerals into their walls. Both diffusion and active transport are probably involved in the transfer of solutes from the symplast to apoplast, entering the tracheids and xylem vessels. C. Transport of Xylem Sap Xylem sap flows upward to veins that branch throughout each leaf, providing each with water. Plants lose an astonishing amount of water by transpiration, the loss of water vapor from leaves and other aerial parts of the plant. An average-sized maple tree loses more than 200 L of water per hour during the summer. The flow of water transported up from the xylem replaces the water lost in transpiration and also carries minerals to the shoot system. 1. The ascent of xylem sap depends mainly on transpiration and the physical properties of water Xylem sap rises against gravity, without the help of any mechanical pump, to reach heights of more than 100 m in the tallest trees. At night, when transpiration is very low or zero, the root cells are still expending energy to pump mineral ions into the xylem. The accumulation of minerals in the stele lowers water potential there, generating a positive pressure, called root pressure, that forces fluid up the xylem. 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 small, herbaceous dicots. During the night, when transpiration is low, the roots of some plants continue to accumulate ions, and root pressure pushes xylem sap into the shoot system. More water enters leaves than is transpired, and the excess is forced out as guttation fluid. 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 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. The mechanism of transpiration depends on the generation of negative pressure (tension) in the leaf due to the unique physical properties of water. As water transpires from the leaf, water coating the mesophyll cells replaces water lost from the air spaces. The remaining film of liquid water retreats into the pores of the cell walls, attracted by adhesion to the hydrophilic walls. Cohesive forces in the water resist an increase in the surface area of the film. Adhesion to the wall and surface tension cause the surface of the water film to form a meniscus, “pulling on” the water by adhesive and cohesive forces. The water film at the surface of leaf cells has a negative pressure, a pressure less than atmospheric pressure. The more concave the meniscus, the more negative the pressure of the water film. This tension is the pulling force that draws water out of the leaf xylem, through the mesophyll, and toward the cells and surface film bordering the air spaces. The tension generated by adhesion and surface tension lowers the water potential, drawing water from where its potential is higher to where it is lower. Mesophyll cells will lose water to the surface film lining the air spaces, which in turn loses water by transpiration. The water lost via the stomata is replaced by water pulled out of the leaf xylem. 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 of water due to hydrogen bonding makes it possible to pull a column of sap from above without the water separating. Helping to fight gravity is the strong adhesion of water molecules to the hydrophilic walls of the xylem cells. The very small diameter of the tracheids and vessel elements exposes a large proportion of the water to the hydrophilic walls. The upward pull on the cohesive sap creates tension within the xylem This tension can actually cause a 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. This occurs when xylem sap freezes in water. Small plants use root pressure to refill xylem vessels in spring, but trees cannot push water to the top and a 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. The older xylem supports the tree. 2. Xylem sap ascends by solar-powered bulk flow: a review Long-distance transport of water from roots to leaves occurs by bulk flow, the movement of fluid driven by a pressure difference at opposite ends of a conduit, the xylem vessels or chains of tracheids. The pressure difference is generated at the leaf end by transpirational pull, which lowers pressure (increases tension) at the “upstream” end of the xylem. On a smaller scale, gradients of water potential drive the osmotic movement of water from cell to cell within root and leaf tissue. Differences in both solute concentration and pressure contribute to this microscopic transport. In contrast, bulk flow, the mechanism for long-distance transport up xylem vessels, depends only on pressure. Bulk flow moves the whole solution, water plus minerals and any other solutes dissolved in the water. The plant expends none 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 maintaining a high humidity in the air spaces within a leaf. Thus, the ascent of xylem sap is ultimately solar powered. D. The Control of Transpiration 1. Guard cell mediate the photosynthesis-transpiration compromise A leaf may transpire more than its weight in water each day. To keep the leaf from wilting, flows in xylem vessels may reach 75 cm/min. Guard cells, by controlling the size of stomata, help balance the plant’s need to conserve water with its requirements for photosynthesis. To make food, a plant must spread its leaves to the sun and obtain CO2 from air. Carbon dioxide diffuses in and oxygen diffuses out of the leaf via the stomata. Within the leaf, CO2 enters a honeycomb of air spaces formed by the irregularly shape parenchyma cells. This internal surface may be 10 to 30 times greater than the external leaf surface. This structural feature increases exposure to CO2, but it also increases the surface area for evaporation. About 90% of the water that a plant loses escapes through stomata, though these pores account for only 1 - 2 % of the external leaf surface. One gauge of how efficiently a plant uses water is the transpiration-to-photosynthesis ratio, the amount of water lost per gram of CO2 assimilated into organic molecules by photosynthesis. For many plant species, this ration is about 600:1. However, corn and other plants that assimilate atmospheric CO2 by the C4 pathway have transpiration-to-photosynthesis ratios of 300:1 or less. C4 plants are more efficient in assimilating CO2 for each gram of water sacrificed than C3 plants when stomata are partially closed. The transpiration stream also assists in the delivery of minerals and other substances from roots to the shoots and leaves. Transpiration also results in evaporative cooling, which can lower the temperature of a leaf by as much as 10-15 oC compared with the surrounding air. This prevents the leaf from reaching temperatures that could denature enzymes involved in photosynthesis and other metabolic processes. Cacti and other desert succulents, which have low rates of transpiration, can tolerate high leaf temperatures. When transpiration exceeds the delivery of water by xylem, as when the soil begins to dry out, the leaves begin to wilt as the cells lose turgor pressure. The potential rate of transpiration will be greatest on sunny, warm, dry, windy days that increase the evaporation of water. Regulation of the size of the stomatal opening can adjust the photosynthesis-transpiration compromise. Each stoma is flanked by a pair of guard cells which are suspended by other epidermal cells over an air chamber, leading to the internal air space. Guard cells control the diameter of the stoma by changing shape, thereby widening or narrowing the gap between the two cells. When guard cells take in water by osmosis, they become more turgid, and because of the orientation of cellulose microfibrils, the guard cells buckle outward. This increases the gap between cells. When cells lose water and become flaccid, they become less bowed and the space between them 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 into the vacuole. This decreasing water potential in guard cells leads to a flow of water by osmosis and increasing turgor. Stomatal closing results from an exodus of K+ from guard cells, leading to osmotic loss of water. The K+ fluxes across the guard cell membranes are probably passive, being 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. Plant physiologists use a technique called patch clamping to study the regulation of the guard cell’s proton pumps and K+ channels. In patch clamping a very tiny “patch” of membrane is isolated on a micropipette. The micropipette functions as an electrode to record ion fluxes across the tiny patch of membrane, focusing on a single kind of ion through selective channels or pumps. In general, stomata are open during the day and closed at night to minimize water loss when it is too dark for photosynthesis. At least three cues contribute to stomatal opening at dawn. First, blue-light receptors in the guard cells stimulate the activity of ATP-powered proton pumps in the plasma membrane, promoting the uptake of K+. Also, photosynthesis in guard cells (the only epidermal cells with chloroplasts) may provide ATP for the active transport of hydrogen ions. A second stimulus is depletion of CO2 within air spaces of the leaf as photosynthesis begins. A third cue in stomatal opening is an internal clock located in the guard cells. Even in the dark, stomata will 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 day. When the plant is suffering a water deficiency, guard cells may lose turgor. Abscisic acid, a hormone produced by the mesophyll cells in response to water deficiency, signals guard cells to close stomata. While reducing further wilting, it also slows photosynthesis. High temperatures, by stimulating CO2 production by respiration, and excessive transpiration may combine to cause stomata to close briefly during mid-day. 2. Xerophytes have evolutionary adaptations that reduce transpiration Plants adapted to arid climates, called xerophytes, have various leaf modifications that reduce the rate of transpiration. Many xerophytes have small, thick leaves, reducing leaf surface area relative to leaf volume. A thick cuticle gives some of these leaves a leathery consistency. During the driest months, some desert plants shed their leaves, while others (such as cacti) subsist on water stored in fleshy stems during the rainy season. In some xerophytes, the stomata are concentrated on the lower (shady) leaf surface. They are often located in depressions (“crypts”) that shelter the pores from the dry wind. Trichomes (“hairs”) also help minimize transpiration by breaking up the flow of air, keeping humidity higher in the crypt than in the surrounding atmosphere. An elegant adaptation to arid habitats is found in ice plants and other succulent species of the family Crassulaceae and in representatives of many other families. These assimilate CO2 by an alternative photosynthetic pathway, crassulacean acid metabolism (CAM). Mesophyll cells in CAM plants store CO2 in organic acids during the night and release the CO2 from these organic acid during the day. This CO2 is used to synthesize sugars by the conventional (C3) photosynthetic pathway, but the stomata can remain closed during the day when transpiration is most severe. E. Translocation of Phloem Sap 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 members. These are arranged end to end to form long sieve tubes with porous cross-walls between cells along the tube. Phloem sap is an aqueous solution in which sugar, primarily the disaccharide sucrose in most plants, is the most prevalent solute. It may also contain minerals, amino acids, and hormones. 1. 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 is variable. In general, sieve tubes carry food from a sugar source to a sugar sink. A sugar source is a plant organ (especially mature leaves) in which sugar is being produced by either photosynthesis or the breakdown of starch. A sugar sink is an organ (such as growing roots, shoots, or fruit) that is a net consumer or storer of sugar. 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. Other solutes, such as minerals, are also transported to sinks along with sugar. 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, whereas the lower leaves of the same branch export sugar to roots. 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 members before it can be exported to sugar sinks. In some species, sugar moves from mesophyll cells to sieve-tube members via the symplast. In other species, sucrose reaches sieve-tube members 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 in the vicinity of sieve-tube members and companion cells. Companion cells pass the sugar they accumulate into the sieve-tube members via plasmodesmata. In some plants, companion cells (transfer cells) have numerous ingrowths in their wall to increase the cell’s surface area and these enhance the transfer of solutes between apoplast and symplast. In corn and many other plants, sieve-tube members accumulate sucrose at concentrations two to three times higher than those in mesophyll cells. This requires active transport to load the phloem. Proton pumps generate an H+ gradient, which drives sucrose across the membrane via a cotransport protein that couples sucrose transport with the diffusion of H+ back into the cell. Downstream, at the sink end of the sieve tube, phloem unloads its sucrose. The mechanism of phloem unloading is highly variable and depends on plant species and type of organ. Regardless of mechanism, because the concentration of free sugar in the sink is lower than in the phloem, sugar molecules diffuse from the phloem into the sink tissues. Water follows by osmosis. 2. Pressure flow is the mechanism of translocation in angiosperms Phloem sap flows from source to sink at rates as great as 1 m/hr, faster than can be accounted for by either diffusion or cytoplasmic streaming. Phloem sap moves by bulk flow driven by pressure. Higher levels of sugar at the source lowers the water potential and causes water to flow into the tube. Removal of sugar at the sink increases the water potential and causes water to flow out of the tube. The difference in hydrostatic pressure drives phloem sap from the source to the sink • (1) Loading of sugar into the sieve tube at the source reduces the water potential inside the sieve-tube members and causes the uptake of water. • (2) This absorption of water generates hydrostatic pressure that forces the sap to flow along the tube. • (3) The pressure gradient is reinforced by unloading of sugar and loss of water from the tube at the sink. • (4) For leaf-to-root translocation, xylem recycles water from sink to source. The pressure flow model explains why phloem sap always flows from sugar source to sugar sink, regardless of their locations in the plant. Researchers have devised several experiments to test this model, including an innovative experiment that exploits natural phloem probes: aphids that feed on phloem sap. The closer the aphid’s stylet is to a sugar source, the faster the sap will flow out and the greater its sugar concentration. In our study of how sugar moves in plants, we have seen examples of plant transport on three levels. At the cellular level across membranes, sucrose accumulates in phloem cells by active transport. At the short-distance level within organs, sucrose migrates from mesophyll to phloem via the symplast and apoplast. At the long-distance level between organs, bulk flow within sieve tubes transports phloem sap from sugar sources to sugar sinks. Interestingly, the transport of sugar from the leaf, not photosynthesis, limits plant yields.