36                    Transport in Plants
                From Tarzan of the Apes to George of the Jungle, in legions of c...

                                                                           through the plan...
TRANSPORT IN PLANTS            703

36.2 Water Potential, Solute
Potential, and Pressure Potential
Water potential (ψ) is ...

36.3 The Proton Pump             (a)                                                     (b)...
TRANSPORT IN PLANTS             705

        36.4 Apoplast and Symplast Plant cell walls and

             Wall extensions                                              Experiments ruled...
TRANSPORT IN PLANTS            707

                                                                        which was lost...
708    CHAPTER THIRT Y-SIX                                                            RESEARCH METHOD

EXPERIMENT                                                                              TRANSPORT IN PLANTS             70...

Closed stomata prevent water loss, but also exclude CO2             the stoma. Negatively ch...
EXPERIMENT                                                                              TRANSPORT IN PLANTS            711...

with masses of a fibrous protein, suggesting that sieve tube       Plasmodesmata and mat...

Transport of Water and Minerals in the Xylem                          2. Water potential

10. The fibrous protein in sieve tube elements                  2. Phloem transports mate...
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  1. 1. 36 Transport in Plants From Tarzan of the Apes to George of the Jungle, in legions of comic strips and adventure movies, heroes have swung through the forest canopy on lianas—twining jungle vines. And when these heroes’ ex- ertions left them thirsty, they took a machete, chopped open the lianas, and drank the water found in the hollow stems. Lianas are a realistic source of water, in fact as well as fiction. Like all plants, these vines are continually moving water, along with dissolved solutes, from place to place in their bodies. The water and minerals in a plant’s xylem must be transported from the roots to the entire shoot system, all the way to the highest leaves and apical buds. Similarly, carbohydrates produced by photosynthesis in all the leaves, including the highest, must be translocated in the phloem to all the living nonphotosynthetic parts of the Hollywood Vines Although the famous swinging-through-the-jungle scenes in the plant, such as roots, tubers, and internal stem tissues. Before we consider the mech- Tarzan movies of the 1940s were pure fic- anisms underlying these processes, we should have some idea about the magnitude tion and the vines the actors used were of what they accomplish. Let’s consider two questions: How much water is trans- rope props, the use of lianas—heavy, twin- ported? And how high can water be transported? ing vines of the tropical rain forests—as a source of drinking water is quite plausible. In answer to the first question, consider the following example: A single maple tree 15 meters tall is estimated to have some 177,000 leaves, with a total leaf surface area of 675 square me- ters—half again the area of a basketball court. During a summer day, that tree loses 220 liters of water per hour to the atmosphere by evaporation from the leaves. To prevent wilting, the xylem needs to transport 220 liters of water from the roots to the leaves every hour. (By comparison, a 50-gallon drum holds 189 liters.) The second question can be rephrased: How tall are the tallest trees? The tallest gym- nosperms, the coast redwoods—Sequoia semper- virens—exceed 110 meters in height, as do the tallest angiosperms, the Australian Eucalyptus regnans. Any successful explanation of water transport in the xylem must account for the transport of water to these great heights. In this chapter, we will consider the uptake and transport of water and minerals by plants, the control of evaporative water loss from leaves, and the translocation of substances in the phloem.
  2. 2. 702 CHAPTER THIRT Y-SIX through the plant into the xylem without crossing at least one Uptake and Movement of Water and Solutes plasma membrane, we will focus first on osmosis. Then we Terrestrial plants must obtain both water and mineral nutri- will examine the uptake of mineral ions and follow the path- ents from the soil, usually by way of their roots. The roots, in way by which both water and minerals move through the turn, obtain carbohydrates and other important materials root to gain entry to the xylem. from the leaves (Figure 36.1). We learned in Chapter 8 that water is one of the ingredients required for carbohydrate pro- duction by photosynthesis in the leaves. Water is also essen- Water moves through a membrane by osmosis tial for transporting solutes both upward and downward, for Osmosis, the movement of water through a membrane in ac- cooling the plant, and for developing the internal pressure cordance with the laws of diffusion, was described in Chap- that supports the plant body. Plants lose large quantities of ter 5 (see Figure 5.8). The solute potential (osmotic potential) water to evaporation, and this water must be continually re- of a solution is a measure of the effect of dissolved solutes on placed. the osmotic behavior of the solution. The following statement How do leaves high in a tree obtain water from the soil? presents an opportunity for confusion, so study it carefully: What are the mechanisms by which water and mineral ions The greater the solute concentration of a solution, the enter the plant body through the roots and ascend as sap in more negative its solute potential, and the greater the the xylem? Because neither water nor minerals can move tendency of water to move into it from another solution of lower solute concentration (and less negative solute potential). O2 CO2 For osmosis to occur, the two solutions must be separated by a membrane permeable to water but relatively impermeable H2O to the solute. Recall, too, that osmosis is a passive process— energy is not directly required. Unlike animal cells, plant cells are surrounded by a rela- tively rigid cell wall. As water enters a plant cell, the entry of more water is increasingly resisted by an opposing pressure potential (called turgor pressure in plants), owing to the rigid- ity of the wall. As more and more water enters, the pressure potential becomes greater and greater. H2O, carbohydrates, etc. Pressure potential is a hydraulic pressure analogous to the air pressure in an automobile tire; it is a mechanical pressure that can be measured with a pressure gauge. Plant cells do not burst when placed in pure water; instead, water enters by osmosis until the pressure potential exactly balances the solute potential. At this point, the cell is turgid; that is, it has a significant pressure potential. The overall tendency of a solution to take up water from pure water, across a membrane, is called its water potential, represented as ψ, the Greek letter psi (pronounced “sigh”) (Figure 36.2). The water potential of a solution is simply the H2O and sum of its (negative) solute potential (ψs) and its (usually pos- dissolved itive) pressure potential (ψp): minerals ψ = ψs + ψp For pure water open to the atmosphere and therefore under no applied pressure, all three of these parameters are zero. We can measure solute potential, pressure potential, and water potential in megapascals (MPa), a unit of pressure. (At- mospheric pressure, “one atmosphere,” is about 0.1 MPa, or 36.1 The Pathways of Water and Solutes in the Plant Water trav- els from the soil to the atmosphere, and it circulates within the plant, 14.7 pounds per square inch; typical pressure in an automo- carrying important solutes with it. bile tire is about 0.2 MPa.)
  3. 3. TRANSPORT IN PLANTS 703 36.2 Water Potential, Solute Potential, and Pressure Potential Water potential (ψ) is the tendency of a 3 Water entering the tube solution to take up water from pure dilutes the solution, making water. Its water potential is the sum of 2 Because of the difference in ψ its ψs less negative. As the between the solution and the solution rises in the tube, the solute potential (ψs) and the pres- distilled water, water moves ψp = 0.15 pressure potential (ψp) sure potential (ψp). For pure water under no applied pressure, all three of from the beaker into the tube. ψs = –0.15 builds up until it balances the ψs. This pressure these parameters are equal to zero. ψ = 0 corresponds to turgor pressure in plants. At ψ = 0 equilibrium, ψ in the solution is equal to ψ in the beaker. 1 The solution in the tube has a negative solute potential (ψs) due to the presence of ψp = 0 dissolved solutes; its ψp = 0; thus its ψ is negative. The ψs = –0.4 beaker contains distilled ψ = –0.4 water (ψ = 0). The two liquids 4 A piston resists the entry of are not at equilibrium. water, as does the wall of a ψ = 0 plant cell. The solution in the tube is not diluted, so Membrane its ψs does not change. However, the system is not Piston initially at equilibrium. ψp = 0.4 Enough water squeezes in ψs = –0.4 to raise ψp until equilibrium is reached, with equal ψ = 0 water potentials. In all cases in which water moves between two solutions ψ = 0 separated by a membrane, the following rule applies: Water always moves across a selectively permeable membrane toward the region of lower (more negative) water potential. Uptake of mineral ions requires Osmotic phenomena are of great importance to plants. membrane transport proteins The structure of many plants is maintained by the pressure Mineral ions, which carry electric charges, generally cannot potential of their cells; if the pressure potential is lost, the move across a membrane unless they are aided by transport plant wilts. Within living tissues, the movement of water proteins (explained in Chapter 5). When the concentration of from cell to cell follows a gradient of water potential. Over these charged ions in the soil is greater than that in the plant, longer distances, in unobstructed tubes such as xylem ves- ion channels and carrier proteins can move them into the sels and phloem sieve tubes, the flow of water and dissolved plant by facilitated diffusion, which is a passive process. The solutes is driven by a gradient of pressure potential. The concentrations of most ions in the soil solution, however, are movement of a solution due to a difference in pressure po- lower than those required inside the plant. Thus the plant tential between two parts of a plant is called bulk flow. must take up these ions against a concentration gradient—a process that requires energy. Electric charge differences also play a role in the uptake of Aquaporins facilitate the movement of water mineral ions. Movement of a negatively charged ion into a across membranes negatively charged region is movement against an electrical Aquaporins are membrane channel proteins through which gradient and requires energy. The combination of concentra- water can move without interacting with the hydrophobic tion and electrical gradients is called an electrochemical gradi- environment of the membrane’s phospholipid bilayer. These ent. Uptake against an electrochemical gradient is active trans- proteins, important in both plants and animals, allow water port, an energy-requiring process, which depends on cellular to move rapidly from environment to cell and from cell to respiration for a supply of ATP. Active transport, of course, cell. The permeability of some aquaporins can be regulated, requires specific transport proteins. changing the rate of osmosis across the membrane. However, Unlike animals, plants do not have a sodium–potassium water movement through aquaporins is always passive, so pump for active transport. Rather, plants have a proton the direction of water movement is unchanged by alterations pump, which uses energy obtained from ATP to move pro- in aquaporin permeability. tons out of the cell against a proton concentration gradient
  4. 4. 704 CHAPTER THIRT Y-SIX 36.3 The Proton Pump (a) (b) (c) in Active Transport of K+ and Cl– The buildup of 1 A proton pump generates 2 The difference in 3 Symport couples the diffusion of differences in H+ concentration electric charge causes H+ to the transport (against an hydrogen ions (H+) trans- and electric charge across the cations such as K+ to electrochemical gradient) of ported outside the cell by membrane. enter the cell. anions such as Cl− into the cell. the proton pump (a) drives the movement of both Outside H+ cations (b) and anions H+ H+ H+ H+ H+ of cell H+ H+ (c) into the cell. H+ Cl– H+ H+ K+ H+ H+ Plasma membrane H+ H+ Proton Potassium Symport pump channel protein ATP ADP + Pi Inside H+ K+ H+ Cl– of cell H+ (Figure 36.3a). Because protons (H+) are positively charged, Water and ions pass to the xylem by way of their accumulation outside the cell has two results: the apoplast and symplast Mineral ions enter and move through plants in various ways. The region outside the cell becomes positively charged Where water is moving by bulk flow, dissolved minerals are with respect to the region inside. carried along in the stream. Both water and minerals also A proton concentration gradient develops across the move by diffusion. At certain sites, where plasma mem- plasma membrane. branes are being crossed, some mineral ions are moved by Each of these results has consequences for the movement of active transport. One such site is the surface of a root hair, other ions. Because of the charge difference across the mem- where mineral ions first enter the cells of the plant. Later, brane, there is increased movement of cations (positively within the stele, the ions must cross another plasma mem- charged ions), such as potassium (K+), into the cell through brane before entering the nonliving vessels and tracheids of their membrane channels. These ions move into the now the xylem. more negatively charged interior of the cell by facilitated dif- The movement of ions across membranes can also result fusion (Figure 36.3b). In addition, the proton concentration in the movement of water. Water moves into a root because gradient can be harnessed to drive secondary active trans- the root has a more negative water potential than does the port, in which anions (negatively charged ions) such as chlo- soil solution. Water moves from the cortex of the root into the ride (Cl–) are moved into the cell against an electrochemical stele (which is where the vascular tissues are located) because gradient by a symport protein that couples their movement the stele has a more negative water potential than does the with that of H+ (Figure 36.3c). In sum, there is a vigorous traf- cortex. fic of ions across plant cell membranes, involving specific Water and minerals from the soil may pass through the membrane transport proteins and both active and passive dermal and ground tissues to the stele via two pathways: the processes. apoplast and the symplast. The apoplast (from the Greek, The proton pump and the coordinated activities of other apo-, “away from”; -plast, “living material”) consists of the membrane transport proteins cause the interior of a plant cell cell walls, which lie outside the plasma membranes, and the to be very negative with respect to the exterior. Such a dif- intercellular spaces (spaces between cells) that are common ference in charge across a membrane is called a membrane to many tissues. The apoplast is a continuous meshwork potential. Biologists can measure the membrane potential of through which water and dissolved substances can flow or a plant cell with microelectrodes, just as they can measure diffuse without ever having to cross a membrane (Figure similar charge differences in nerve cells and other animal 36.4). Movement of materials through the apoplast is thus cells (see Chapter 44). Most plant cells maintain a membrane unregulated—until it reaches the endodermis, as we will potential of at least –120 millivolts (mV). soon discuss.
  5. 5. TRANSPORT IN PLANTS 705 36.4 Apoplast and Symplast Plant cell walls and Water and ions travel through Water and ions cross a intercellular spaces constitute the apoplast. The sym- cell walls and intercellular plasma membrane to plast comprises the living cells, which are connected spaces in the apoplast. enter the symplast path. by plasmodesmata. To enter the symplast, water and solutes must pass through a plasma membrane. No such selective barrier limits movement through the apoplast. Root hair Epidermis Cortex Plasmodesmata Casparian Plasma strip membrane Endodermis Stele The remainder of the plant body is the symplast (from the Pericycle Greek, sym-, “together with”). The symplast is the portion of Tracheary the plant body enclosed by membranes—the continuous cy- elements toplasm of the living cells, connected by plasmodesmata (see Figure 36.4). The selectively permeable plasma membranes of the cells control access to the symplast, so movement of cells determine which mineral ions pass into the stele, and at water and dissolved substances into the symplast is tightly what rates. regulated. Once they have passed the endodermal barrier, water and Water and minerals can pass from the soil solution minerals leave the symplast and enter the apoplast of the through the apoplast as far as the endodermis, the innermost stele. Parenchyma cells in the pericycle or xylem can aid this layer of the root cortex. The endodermis is distin- guished from the rest of the ground tissue by the pres- ence of Casparian strips. These waxy, suberin-im- pregnated regions of the endodermal cell wall form a water-repelling (hydrophobic) belt around each en- dodermal cell where it is in contact with other endo- dermal cells. The hydrophobic Casparian strips act as a seal that prevents water and ions from moving be- tween the cells (Figure 36.5). Casparian strips prevent water in To bypass the Casparian The Casparian strips of the endodermis thus com- the apoplast from passing between strips, water must enter the endodermal cells into the stele. pletely separate the apoplast of the cortex from the the living cells and access the stele via the symplast. apoplast of the stele. However, they do not obstruct Plasmodesmata the outer or inner faces of the endodermal cells. Ac- cordingly, water and ions can enter the stele only by way of the symplast—that is, by entering and passing through the cytoplasm of the endodermal cells. Thus Endodermis transport proteins in the plasma membranes of these Pericycle 36.5 Casparian Strips Casparian strips in the endodermis of the (stele) cortex are impregnated with the water-repelling substance suberin. These strips separate the apoplast in the cortex from the apoplast in the stele.
  6. 6. 706 CHAPTER THIRT Y-SIX Wall extensions Experiments ruled out xylem transport by pumping action of living cells Some of the earliest attempts to explain the rise of sap in the xylem were based on a hypothetical pumping action by liv- Mitochondria ing cells in the stem, which might push the sap upward. However, experiments conducted and published in 1893 by Plasmodesma the German botanist Eduard Strasburger definitively ruled Transfer cell out such models. Strasburger worked with trees about 20 meters tall. He Sieve tube sawed through the trunk of each tree at its base and plunged element the cut end into a bucket containing a solution of a poison, such as picric acid. The solution rose through the trunk, as was readily evident from the progressive death of the bark higher and higher up. When the solution reached the leaves, the leaves died, too, at which point the movement of the so- Parenchyma cell lution stopped (as shown by the liquid level in the bucket, which stopped dropping). 36.6 A Transfer Cell Three walls of this transfer cell in a pea leaf This simple experiment established three important have knobby extensions that face the cells from which the transfer cell imports solutes. A transfer cell exports the solutes to the neigh- points: boring sieve tube element. Living, “pumping” cells were not responsible for the up- ward movement of the solution, because the solution itself killed all living cells with which it came in contact. The leaves played a crucial role in transport. As long as process. Some of these parenchyma cells, called transfer cells, they were alive, the solution continued to move upward; are structurally modified for transporting mineral ions from when the leaves died, movement ceased. their cytoplasm (part of the symplast) into their cell walls The movement was not caused by the roots, because the (part of the apoplast). The cell wall that receives the trans- trunk had been completely separated from the roots. ported ions has many knobby extensions projecting into the transfer cell, increasing the surface area of the cell’s plasma membrane, the number of transport proteins, and thus the rate of transport (Figure 36.6). Transfer cells also have many Root pressure does not account for xylem transport mitochondria that produce the ATP needed to power the ac- In spite of Strasburger’s observations, some plant physiolo- tive transport of mineral ions. gists hypothesized that xylem transport was based on root As mineral ions move into the solution in the cell walls, pressure—pressure exerted by the root tissues that would the water potential in the apoplast becomes more negative; force liquid up the xylem. The basis for root pressure is a thus water moves out of the cells and into the apoplast by os- higher solute concentration, and accordingly a more nega- mosis. In other words, active transport of ions moves the ions tive water potential, in the xylem sap than in the soil solu- directly, and water follows passively. The end result is that tion. This water potential draws water into the stele; once water and minerals end up in the xylem, where they consti- there, the water has nowhere to go but up, so it rises in the tute the xylem sap. How do the water and materials move on vessels and tracheids. from the xylem of the root system? There is good evidence that root pressure exists—for ex- ample, the phenomenon of guttation, in which liquid water is forced out through openings at the margins of leaves (Fig- Transport of Water and Minerals in the Xylem ure 36.7). Guttation occurs only under conditions of high at- So far in this chapter we’ve described the movement of wa- mospheric humidity and plentiful water in the soil, which oc- ter and minerals into plant roots and their entry into the root cur most commonly at night. Root pressure is also the source xylem. Now we will consider how xylem sap moves through of the sap that oozes from the cut stumps of some plants, the remainder of the plant. Let’s first consider some early such as Coleus, when their tops are cut off. ideas about the ascent of xylem sap and then turn to our cur- Root pressure, however, cannot account for the ascent of rent understanding of how it works. We’ll describe the ex- sap in trees. Root pressure seldom exceeds 0.1–0.2 MPa (1–2 periments that ruled out some early models as well as some atmospheres). If root pressure were driving sap up the xylem, evidence in support of the current model. we would observe a positive pressure potential in the xylem
  7. 7. TRANSPORT IN PLANTS 707 which was lost. The tension in the mesophyll draws water from the xylem of the nearest vein into the apoplast sur- rounding the mesophyll cells. The removal of water from the veins, in turn, establishes tension on the entire column of wa- ter contained within the xylem, so that the column is drawn upward all the way from the roots. The ability of water to be pulled upward through tiny tubes results from the remarkable cohesion of water—the ten- dency of water molecules to stick to one another through hy- drogen bonding. The narrower the tube, the greater the ten- sion the water column can withstand without breaking. The integrity of the column is also maintained by the adhesion of water to the xylem walls. In the tallest trees, such as a 110- meter redwood, the difference in pressure potential between 36.7 Guttation Root pressure is responsible for forcing water through openings in the margins of this strawberry leaf. 1 Water vapor diffuses Leaf Stoma out of the stomata. at all times. In fact, as we are about to see, the xylem sap in most trees is under tension—has 2 Water evaporates from a negative pressure potential—when it is as- mesophyll cell walls. cending. Furthermore, as Strasburger had al- ready shown, materials can be transported up- Mesophyll 3 Tension pulls water ward in the xylem even when the roots have cell from the veins into been removed. If the roots are not pushing the the apoplast of the Vein mesophyll cells. xylem sap upward, what causes it to rise? 4 Tension pulls the water Stem column upward and The transpiration–cohesion–tension outward in the xylem mechanism accounts for xylem transport Xylem of veins in the leaves. The obvious alternative to pushing is pulling: The leaves pull the xylem sap upward. The evaporative loss of water from the leaves gen- 5 Tension pulls the erates a pulling force (tension) on the water in water column the apoplast of the leaves. Hydrogen bonding upward in the between water molecules makes the sap in the xylem of the root and stem. xylem cohesive enough to withstand the ten- sion and rise by bulk flow. Let’s see how this Root process works. 6 Water molecules The concentration of water vapor in the at- form a cohesive mosphere is lower than that in the leaf. Because column from the roots to the of this difference, water vapor diffuses from the leaves. intercellular spaces of the leaf, through open- ings called stomata, to the outside air. This 7 Water moves process is called transpiration (Figure 36.8). H2O into the stele by Within the leaf blade, water evaporates from osmosis. Root hair Xylem the moist walls of the mesophyll cells and en- ters the intercellular spaces. The force gener- ated by the evaporation of water from the mes- 36.8 The Transpiration–Cohesion–Tension Mechanism Transpiration causes evapo- ration from mesophyll cell walls, generating tension on the xylem. Cohesion among ophyll cell walls creates a tension that draws water molecules in the xylem transmits the tension from the leaf to the root, causing more water into the cell walls, replacing that water to move from the soil to the atmosphere.
  8. 8. 708 CHAPTER THIRT Y-SIX RESEARCH METHOD 1 By applying just 2 …so that xylem sap the top and the bottom of the column may be as great as 3 Sap enough pressure… is pushed back to MPa. The cohesion of water in the xylem is great enough to the cut surface of a withstand even that great a tension. plant sample,… In summary, the key elements of water transport in the 3 …a scientist can xylem are determine the tension on the sap Transpiration, the evaporation of water from the leaves in the living plant. Tension in the xylem sap resulting from transpiration Cohesion in the xylem sap from the leaves to the roots Pressure This transpiration–cohesion–tension mechanism requires gauge Gas pressure no work (that is, no expenditure of energy) on the part of the plant. At each step between soil and atmosphere, water moves passively toward a region with a more negative wa- Pressure release valve ter potential. Dry air has the most negative water potential (–95 MPa at 50% relative humidity), and the soil solution has the least negative water potential (between –0.01 and –3 MPa). Xylem sap has a water potential more negative than 36.9 A Pressure Bomb The amount of tension on the sap in differ- ent types of plants can be measured with this device. that of cells in the cortex of the root, but less negative than that of mesophyll cells in the leaf. Mineral ions contained in the xylem sap rise passively with water as it ascends from root to leaf. In this way the nu- tritional needs of the shoot are met. Some of the mineral ele- the xylem sap is pushed back to the cut surface. When the ments brought to the leaves are subsequently redistributed sap first becomes visible again at the cut surface, the pressure to other parts of the plant by way of the phloem, but the ini- in the bomb is recorded. This pressure is equal in magnitude tial delivery from the roots is through the xylem. but opposite in sign to the tension (negative pressure poten- In addition to promoting the transport of minerals, tran- tial) originally present in the xylem (Figure 36.9). spiration contributes to temperature regulation. As water Scholander used the pressure bomb to study dozens of evaporates from mesophyll cells, heat is taken up from the plant species, from diverse habitats, growing under a variety cells, and the leaf temperature drops. This cooling effect of of conditions. In all cases in which xylem sap was ascending, evaporation (so evident in the cooling of our skin when we it was found to be under tension. The tension disappeared in sweat) is important in enabling plants to live in hot environ- some of the plants at night, when transpiration ceased. In de- ments. A farmer can hold a leaf between thumb and forefin- veloping vines, the xylem sap was under no tension until ger to estimate its temperature; if the leaf doesn’t feel cool, leaves formed. Once leaves developed, transport in the that means that transpiration is not occurring, so it must be xylem began, and tensions were recorded. time to water. Suppose you wanted to measure tensions in the xylem at various heights in a tall tree to confirm that the tensions are sufficient to account for the rate at which sap is moving up A pressure bomb measures tension in the xylem sap the trunk. How would you obtain stem samples for meas- The transpiration–cohesion–tension model can be true only urement? Per Scholander used surveying instruments to de- if the column of sap in the xylem is under tension (has a neg- termine the heights of particular twigs, then had a sharp- ative pressure potential). The most elegant demonstrations shooter shoot the identified twigs from the tree with a of this tension, and of its adequacy to account for the ascent high-powered rifle. As the twigs fell to the ground, Scholan- of xylem sap in tall trees, were performed by the biologist Per der quickly inserted them in the pressure bomb and recorded Scholander, who measured tension in stems with an instru- their xylem tension. In every case, the differences in tensions ment called a pressure bomb. at different heights were great enough to keep the xylem sap Consider a stem in which the xylem sap is under tension. ascending. If the stem is cut, the sap pulls away from the cut, into the The rate at which the sap ascends is not the same at all stem. This behavior indicates that the pressure in the intact times. No flow of xylem sap takes place at night, when there xylem is lower than that of the atmosphere. Now the stem is is little or no transpiration. By day, when the sap is ascending, quickly placed in the pressure bomb, in which the pressure the rate of ascent depends on several factors. One is the con- may be raised. The cut surface remains outside the bomb. As centration of K+ in the sap: The rate of flow increases as the K+ gas pressure is applied to the plant parts within the bomb, concentration increases (Figure 36.10). Other factors, such as
  9. 9. EXPERIMENT TRANSPORT IN PLANTS 709 Question: How does K+ affect xylem flow rate? METHOD dioxide. This poses a problem: How can the leaf balance its Two xylem-containing flaps need to retain water with its need to obtain CO2 for photo- were created on a tobacco plant. synthesis? One was connected to a source Plants have evolved an elegant compromise in the form of pure water (the control). The other flap could be connected of stomata (singular, stoma), or pores, in the epidermis of to either pure water or to a their leaves. A pair of specialized epidermal cells, called solution containing a known concentration of K+. guard cells, controls the opening and closing of each stoma (Figure 36.11a). When the stomata are open, CO2 can enter the leaf by diffusion—but water vapor is lost in the same way. H 2O K+ solution (control) or H2O (a) RESULTS The addition of the K+ solution dramatically increased the flow rate. The rate returned to the control level when the K+ solution was replaced by pure water. The experimental trace spiked immediately after the injection of K+ solution. 2 Relative flow rate Return to H2O alone 1.5 K+ solution (b) injected H2O Cl– K+ 1 In the presence of light, 1 potassium ions are actively The control trace transported into the guard H2O (control) (H2O only) showed cells from epidermal cells. little variation in 0 1000 2000 3000 4000 flow rate. Time (seconds) Guard cells Conclusion: K+ increases the rate of flow in the xylem. 2 Higher internal K+ and Cl– concentrations give guard cells a more negative 36.10 Potassium Ions Speed Transport in the Xylem This experi- water potential, causing ment showed that the rate of fluid ascending through the xylem them to take up water and spiked when a solution with a known concentration of potassium stretch, opening the ions was injected. Repeating the experiment with solutions of differ- stoma. ent concentrations of K+ showed that the higher the K+ concentra- tion, the greater the flow rate. 3 In the absence of light, as H2O K+ Cl– K+ diffuses passively out of temperature, light intensity, and wind velocity affect the tran- the guard cells, water follows by osmosis, the spiration rate, and hence the rate of sap flow, more directly. guard cells go limp, and Although transpiration provides the impetus for the trans- the stoma closes. port of water and minerals in the xylem, it also results in the loss of tremendous quantities of water from the plant. How do plants control this loss? 36.11 Stomata (a) A scanning electron micrograph of an open Transpiration and the Stomata stoma formed by two sausage-shaped guard cells. (b) Potassium ion The epidermis of leaves and stems minimizes transpirational concentrations affect the water potential of the guard cells, control- ling the opening and closing of stomata. Negatively charged ions water loss by secreting a waxy cuticle, which is impermeable accompanying K+ maintain electrical balance and contribute to the to water. However, the cuticle is also impermeable to carbon changes in solute potential that open and close the stomata.
  10. 10. 710 CHAPTER THIRT Y-SIX Closed stomata prevent water loss, but also exclude CO2 the stoma. Negatively charged chloride ions and organic ions from the leaf. also move into and out of the guard cells along with the Most plants open their stomata only when the light inten- potassium ions, maintaining electrical balance and contribut- sity is sufficient to maintain a moderate rate of photosynthe- ing to the change in the solute potential of the guard cells. sis. At night, when darkness precludes photosynthesis, the stomata remain closed; no CO2 is needed at this time, and water is conserved. Even during the day, the stomata close if Transpiration from crops can be decreased water is being lost at too great a rate. Stomata are the “referees” of a compromise between the ad- The stoma and guard cells in Figure 36.11a are typical of mission of CO2 for photosynthesis and the loss of water by eudicots. Monocots typically have specialized epidermal cells transpiration. Farmers would like their crops to transpire less, associated with their guard cells. The principle of operation, thus reducing the need for irrigation. Similarly, nurseries and however, is the same for both monocot and eudicot stomata. gardeners would like to be able to reduce the amount of wa- In what follows, we describe the regulation and mechanism ter lost by plants that are to be transplanted, because trans- of stomatal opening and the normal cycle of opening and planting often damages the roots, causing the plant to wilt or closing. die. What they need is a good antitranspirant: a compound that can be applied to plants, reducing water loss from the stomata without producing disastrous side effects by exces- The guard cells control the size of the stomatal opening sively limiting CO2 uptake. Light causes the stomata of most plants to open, admitting Abscisic acid and its commercial chemical analogs have CO2 for photosynthesis. Another cue for stomatal opening is been found to work as antitranspirants in small-scale tests, the level of CO2 in the intercellular spaces inside the leaf. A but their high cost has precluded commercial use. What low level favors opening of the stomata, thus allowing the about making plants more sensitive to their own abscisic uptake of more CO2. acid? The guard cells of transgenic plants with a mutant al- Water stress is a common problem for plants, especially lele of the era gene are highly sensitive to abscisic acid and on hot, sunny, windy days. Plants have a protective response hence resistant to wilting during drought stress. to these conditions, which uses the water potential of the A totally different type of antitranspirant temporarily seals mesophyll cells as a cue. Even when the CO2 level is low and off the leaves from the atmosphere. Growers use a variety of the sun is shining, if the mesophyll is too dehydrated—that compounds, most of which form polymeric films around is, if the water potential of the mesophyll is too negative— leaves, to form a barrier to evaporation. These compounds the mesophyll cells release a plant hormone called abscisic cause undesirable side effects, however, and can be used only acid. Abscisic acid acts on the guard cells, causing them to for short periods of time. Their most common use is in the close the stomata and prevent further drying of the leaf. This transplanting of nursery stock. response reduces the rate of photosynthesis, but it protects In the absence of antitranspirants, stomata are normally the plant. open during daylight hours, allowing CO2 to be fixed and These processes are regulated by control of the K+ concen- converted to the products of photosynthesis. Next we’ll see tration in the guard cells. Blue light, absorbed by a pigment how these products are delivered to other parts of the plant, in the guard cell plasma membrane, activates a proton pump, supporting growth of those parts. which actively transports protons (H+) out of the guard cells and into the surrounding epidermis. The resulting proton gra- dient drives the accumulation of K+ (Figure 36.11b; also re- Translocation of Substances in the Phloem view Figure 36.3) in the guard cell. The increasing internal Photosynthesis takes place in the mesophyll cells and, in C4 concentration of K+ makes the water potential of the guard plants, in the bundle sheath cells of the leaf (see Figure 8.16). cells more negative. Water enters the guard cells by osmosis, The products of photosynthesis (primarily carbohydrates) increasing their pressure potential. The arrangement of the diffuse to the nearest small vein, where they are actively cellulose microfibrils in their cell walls causes the guard cells transported into sieve tube elements. to respond to this increase by changing their shapes so that a Substances in the phloem move from sources to sinks. A gap—the stoma—appears between them. source is an organ (such as a mature leaf or a storage root) The stoma closes by the reverse process when active trans- that produces (by photosynthesis or by digestion of stored re- port ceases in response to the absence of blue light or the pres- serves) more sugars than it requires. A sink is an organ (such ence of abscisic acid. Potassium ions diffuse passively out of as a root, a flower, a developing fruit or tuber, or an imma- the guard cells, water follows by osmosis, the pressure po- ture leaf) that consumes sugars for its own growth and stor- tential decreases, and the guard cells sag together and seal off age needs. Sugars (primarily sucrose), amino acids, some
  11. 11. EXPERIMENT TRANSPORT IN PLANTS 711 Question: Are organic solutes translocated in the xylem or in the phloem? To investigate translocation, plant physiologists needed to METHOD RESULTS obtain samples of pure sieve tube sap from individual sieve Remove bark to Organic solutes accumulate tube elements. This difficult task was simplified when it was girdle the tree. in the phloem above the discovered that a common garden pest, the aphid, feeds on girdle, causing swelling. plants by drilling into a sieve tube. An aphid inserts its spe- cialized feeding organ, called a stylet, into a stem until the stylet enters a sieve tube (Figure 36.13a). The pressure within the sieve tube is greater than that in the surrounding plant tissues or outside the plant, so the nutritious sieve tube sap is forced through the stylet and into the aphid’s digestive Time Bark tract. So great is the pressure that sugary liquid is forced through the insect’s body and out the anus (Figure 36.13b). Wood Plant physiologists use aphids to collect sieve tube sap. When liquid appears on the aphid’s abdomen, indicating that Conclusion: Organic solutes are translocated in the phloem, not in the the insect has connected with a sieve tube, the physiologist xylem. quickly freezes the aphid and cuts its body away from the 36.12 Girdling Blocks Translocation in the Phloem By removing stylet, which remains in the sieve tube element. For hours, a ring of bark (containing the phloem), Malpighi blocked the translo- sieve tube sap continues to exude from the cut stylet, where it cation of organic solutes in a tree. may be collected for analysis. Chemical analysis of sieve tube sap collected in this manner reveals the contents of a single sieve tube element over time. Physiologists can also infer the rates at which different substances are translocated by meas- minerals, and a variety of other solutes are translocated be- uring how long it takes for radioactive tracers administered to tween sources and sinks in the phloem. a leaf to appear at stylets at different distances from the leaf. How do we know that such organic solutes are translo- These methods have allowed us to understand how, at cated in the phloem, rather than in the xylem? Just over 300 times, different substances might move in opposite directions years ago, the Italian scientist Marcello Malpighi performed in the phloem of a stem. Experiments with aphid stylets have a classic experiment in which he removed a ring of bark (con- shown that all the contents of any given sieve tube element taining the phloem) from the trunk of a tree—that is, he gir- move in the same direction. Thus, bidirectional translocation dled the tree (Figure 36.12). The bark in the region above the can be understood in terms of different sieve tubes conduct- girdle swelled over time. We now know that the swelling re- ing sap in opposite directions. These and other experiments sulted from the accumulation of organic solutes that came from higher up the tree and could no longer continue downward because of the dis- (a) (b) Longistigma caryae ruption of the phloem. Later, the bark below the girdle died because it no longer received sugars Sieve tube from the leaves. Eventually the roots, and then element the entire tree, died. Any explanation of the translocation of or- ganic solutes must account for a few important observations: Translocation stops if the phloem tissue is killed by heating or other methods; thus the The aphid‘s stylet mechanism must be different from that of has successfully transport in the xylem. penetrated the sieve tube. Translocation often proceeds in both direc- tions—up the stem and down the stem— Sap droplet simultaneously. Translocation is inhibited by compounds that 36.13 Aphids Collect Sieve Tube Sap (a) Aphids feed on sap drawn from a sieve tube, which they penetrate with a modified feeding organ, the stylet. (b) inhibit respiration and thus limit the ATP Pressure inside the sieve tube forces sap through the aphid’s digestive tract, from supply in the source. which it can be harvested.
  12. 12. 712 CHAPTER THIRT Y-SIX Phloem 2 Source cells load sucrose sieve Source into phloem sieve tubes, Xylem tube reducing their water led to the general adoption of the pressure cell potential… 1 Transpiration flow model as an explanation for translocation pulls water up in the phloem. xylem vessels. H2O H2O Sucrose 3 …so water is taken up The pressure flow model appears to account for from xylem vessels by translocation in the phloem osmosis. H2O During sieve tube element development, the tonoplast and 4 Internal pressure much of the cytosol breaks down, allowing the contents of the Sink cell differences drive the sap central vacuole to combine with much of the cytosol to form down the sieve tube to the sieve tube sap. The sap flows under pressure through the sink cells. sieve tubes, moving from one sieve tube element to the next H2O Sucrose by bulk flow through the sieve plates, without crossing a membrane. We need to understand how this pressure is gen- erated in order to understand translocation in the phloem. 5 Sucrose is unloaded into Two steps in translocation require metabolic energy: sink cells… 6 …and water moves Transport of sucrose and other solutes into the sieve back to xylem vessels. tubes at sources, called loading Removal of the solutes, called unloading, where the sieve 36.14 The Pressure Flow Model Combined pressure tubes enter sinks potential and water potential differences drive the bulk flow of sieve tube sap from a source to a sink. According to the pressure flow model of translocation in the phloem, sucrose is actively transported into sieve tube el- ements at a source, giving those cells a greater sucrose con- centration than the surrounding cells. Water therefore enters Other mechanisms have been proposed to account for the sieve tube elements by osmosis. The entry of this water translocation in sieve tubes, but some have been disproved, causes a greater pressure potential at the source end of the and none of the rest has as much support as the pressure sieve tube, so that the entire fluid content of the sieve tube is flow model. pushed toward the sink end of the tube— in other words, the Two essential requirements must be met in order for the sap moves by bulk flow in response to a pressure gradient pressure flow model to be valid: (Figure 36.14). In the sink, the sucrose is unloaded by active The sieve plates must be unobstructed, so that bulk flow transport, maintaining the gradients of solute potential and from one sieve tube element to the next is possible. water potential needed for movement. There must be an effective method for loading sucrose The pressure flow model of translocation in the phloem is and other solutes into the phloem in source tissues and contrasted with the transpiration–cohesion–tension model of removing them in sink tissues. xylem transport in Table 36.1. Let’s see whether these requirements are met. The pressure flow model has been ARE THE SIEVE PLATES CLOGGED OR OPEN? Early electron micro- experimentally tested scopic studies of phloem samples cut from plants produced The pressure flow model was first proposed more than half results that seemed to contradict the pressure flow model. a century ago, but some of its features are still being debated. The pores in the sieve plates always appeared to be plugged 36.1 Mechanisms of Sap Flow in Plant Vascular Tissues XYLEM PHLOEM Driving force for bulk flow Transpiration from leaves Active transport of sucrose at source Site of bulk flow Non-living vessel elements and tracheids (cohesion) Living sieve tube elements Pressure potential in sap Negative (pull from top; tension) Positive (push from source; pressure)
  13. 13. TRANSPORT IN PLANTS 713 with masses of a fibrous protein, suggesting that sieve tube Plasmodesmata and material transfer between cells sap could not flow freely. But what is the function of that Many substances move from cell to cell within the symplast fibrous protein? by way of plasmodesmata (see Figure 35.7). Among their One possibility is that this protein is usually distributed other roles, plasmodesmata participate in the loading and un- more or less at random throughout the sieve tube elements loading of sieve tube elements. The mechanisms vary among until the sieve tube is damaged; then the sudden surge of sap plant species, but the story in tobacco plants is a common toward the cut surface carries the protein into the pores, one. In tobacco, sugars and other solutes in source tissues en- blocking them and preventing the loss of valuable nutrients. ter companion cells by active transport from the apoplast and In other words, perhaps the protein does not block the pores move on to the sieve tube elements through plasmodesmata. unless the phloem is damaged. How might this hypothesis In sink tissues, plasmodesmata connect sieve tube elements, be tested? Could phloem for microscopic observation be ob- companion cells, and the cells that will receive and use the tained without causing the sap to surge to the cut surface? transported compounds. One way to prevent the surge of the sap is to freeze plant Plasmodesmata undergo developmental changes as an tissue rapidly before cutting it. Another way is to let the tis- immature sink leaf matures into a mature source leaf. Plas- sue wilt so that there is no pressure in the phloem before cut- modesmata in sink tissues favor rapid unloading: They are ting. When these methods are used, the sieve plates are not more abundant, and they allow the passage of larger mole- clogged by the protein. Thus, the first condition of the pres- cules. Plasmodesmata in source tissues are few in number. sure flow model is met. It was long thought that only substances with molecular weights of less than 1,000 could fit through a plasmodesma. HOW DO NEIGHBORING CELLS LOAD AND UNLOAD THE SIEVE TUBE Then biologists discovered that cells infected with tobacco mo- ELEMENTS ? If the pressure flow model is correct, there saic virus (TMV) could allow molecules with molecular must be mechanisms for loading sugars and other solutes weights of as much as 20,000 to exit. We now know that TMV into the phloem in source regions and for unloading them encodes a “movement protein” that produces this change in in sink regions. Such mechanisms exist in all plants. the permeability of the plasmodesmata—and that the plants Sugars and other solutes produced in the mesophyll pass themselves normally produce at least one such movement pro- from cell to cell in the leaf and eventually enter the sieve tein. Even large molecules such as proteins and RNAs, with tubes of the phloem. In some plants these substances leave molecular weights up to at least 50,000, can thus move be- the mesophyll cells and enter the apoplast, sometimes with tween living plant cells. We will see some consequences of this the help of transfer cells. Then specific sugars and amino movement of macromolecules through plasmodesmata in later acids are actively transported into cells of the phloem, thus chapters. Biologists are exploring possible ways to regulate the reentering the symplast. This passage through the apoplast permeability, number, and form of plasmodesmata as a means and back into the symplast allows the selection of substances of modifying traffic in the plant. Such modifications might, for to be translocated by forcing them to pass through a selec- example, allow the diversion of more of a grain crop’s photo- tively permeable membrane. In many plants, solutes reenter synthetic products into the seeds, increasing the crop yield. the symplast at the companion cells, which then transfer the solutes to the adjacent sieve tube elements. Chapter Summary A form of secondary active transport loads sucrose into the companion cells and sieve tubes. Sucrose is carried across Uptake and Movement of Water and Solutes the plasma membrane from apoplast to symplast by su- Plant roots take up water and minerals from the soil. Review Figure 36.1 crose–proton symport; thus the entry of sucrose and protons Water moves through biological membranes by osmosis, is strictly coupled. For this symport to work, the apoplast always moving toward cells with a more negative water poten- must have a high concentration of protons; these protons are tial. The water potential of a cell or solution is the sum of the supplied by a primary active transport system, the proton solute potential and the pressure potential. All three parameters are expressed in megapascals (MPa). Review Figure 36.2 pump. The protons then diffuse back into the cell through the Mineral uptake requires transport proteins. Some minerals symport protein, bringing sucrose with them. enter the plant by facilitated diffusion; others enter by active In sink regions, the solutes are actively transported out of transport. A proton pump facilitates the active transport of the sieve tube elements and into the surrounding tissues. many mineral ions across membranes in plants. Review Figure 36.3 This unloading serves two purposes: It helps maintain the Water and minerals pass from the soil to the xylem by way of gradient of solute potential and hence of pressure potential the apoplast and symplast. In the root, water and minerals can in the sieve tubes, and it promotes the buildup of sugars and move from the cortex into the stele only by way of the symplast starch to high concentrations in storage regions, such as de- because Casparian strips in the endodermis block their move- ment through the apoplast. Review Figures 36.4, 36.5. See veloping fruits and seeds. Web/CD Activity 36.1
  14. 14. 714 CHAPTER THIRT Y-SIX Transport of Water and Minerals in the Xylem 2. Water potential Early experiments established that xylem sap does not move a. is the difference between the solute potential and the via the pumping action of living cells. pressure potential. b. is analogous to the air pressure in an automobile tire. Root pressure is responsible for guttation and for the oozing c. is the movement of water through a membrane. of s0ap from cut stumps, but it cannot account for the ascent of d. determines the direction of water movement between cells. xylem sap in trees. e. is defined as 1.0 MPa for pure water under no applied Water transport in the xylem is the result of the combined pressure. effects of transpiration, cohesion, and tension. Evaporation from the leaf produces tension in the mesophyll cells, which pulls a 3. Which statement about aquaporins is not true? column of water—held together by cohesion—up through the a. They are membrane transport proteins. xylem from the root. Dissolved minerals are carried passively in b. Water movement through aquaporins is always active. the water. Review Figure 36.8 c. The permeability of some aquaporins is subject to regulation. Evaporation of water cools the leaves, but a plant cannot d. They are found in both animals and plants. afford to lose too much water. e. They enable water to pass through the phospholipid Support for the transpiration–cohesion–tension model of bilayer without encountering a hydrophobic environment. water transport comes from studies using a pressure bomb. Review Figure 36.9 4. Which statement about proton pumping across the plasma membrane of plants is not true? The role of transport in the xylem depends on several factors, a. It requires ATP. including the K+ concentration. Review Figure 36.10 b. The region inside the membrane becomes positively Transpiration and the Stomata charged with respect to the region outside. c. It enhances the movement of K+ ions into the cell. Transpirational water loss is minimized by the waxy cuticle d. It pushes protons out of the cell against a proton of the leaves. concentration gradient. Stomata allow a compromise between water retention and e. It can drive the secondary active transport of negatively carbon dioxide uptake. charged ions. A pair of guard cells controls the size of the stomatal open- 5. Which statement is not true? ing. A proton pump, activated by blue light, pumps protons a. The symplast is a meshwork consisting of the (connected) from the guard cells to surrounding epidermal cells, setting up living cells. a proton gradient that drives the active transport of potassium b. Water can enter the stele without entering the symplast. ions into the cells. Water follows osmotically, swelling the cells c. The Casparian strips prevent water from moving between and opening the stomata. endodermal cells. Carbon dioxide and water levels in the leaf also affect stom- d. The endodermis is a cell layer in the cortex. atal opening. Review Figure 36.11 e. Water can move freely in the apoplast without entering cells. Translocation of Substances in the Phloem 6. In the xylem, a. the products of photosynthesis travel down the stem. Products of photosynthesis, as well as some minerals, are b. living, pumping cells push the sap upward. translocated through sieve tubes in the phloem by way of living c. the motive force is in the roots. sieve tube elements. Review Figure 36.12 d. the sap is often under tension. Translocation in the phloem can proceed in both directions in e. the sap must pass through sieve plates. the stem, although in a single sieve tube it goes only one way. Translocation requires a supply of ATP. 7. Which of the following is not part of the transpiration- cohesion-tension mechanism? Translocation in the phloem is explained by the pressure flow a. Water evaporates from the walls of mesophyll cells. model: The difference in solute concentration between sources b. Removal of water from the xylem exerts a pull on the and sinks creates a difference in pressure potential along the water column. sieve tubes, resulting in bulk flow. Review Figure 36.14, c. Water is remarkably cohesive. Table 36.1. See Web/CD Tutorial 36.1 d. The wider the tube, the greater the tension its water The validity of the pressure flow model is supported by the column can withstand. facts that the sieve plates are normally unobstructed, allowing e. At each step, water moves to a region with a more bulk flow, and that the neighboring cells load organic solutes strongly negative water potential. into the sieve tube elements in source regions and unload them in sink regions. 8. Stomata a. control the opening of guard cells. The distribution and properties of plasmodesmata differ b. release less water to the environment than do other parts between source and sink tissues. It may become possible to of the epidermis. regulate plasmodesma permeability in crop plants. c. are usually most abundant on the upper epidermis of a leaf. d. are covered by a waxy cuticle. e. close when water is being lost at too great a rate. Self-Quiz 9. Which statement about phloem transport is not true? 1. Osmosis a. It takes place in sieve tubes. a. requires ATP. b. It depends on mechanisms for loading solutes into the b. results in the bursting of plant cells placed in pure water. phloem in sources. c. can cause a cell to become turgid. c. It stops if the phloem is killed by heat. d. is independent of solute concentrations. d. A high pressure potential is maintained in the sieve tubes. e. continues until the pressure potential equals the water e. In sinks, solutes are actively transported into sieve tube potential. elements.
  15. 15. TRANSPORT IN PLANTS 715 10. The fibrous protein in sieve tube elements 2. Phloem transports material from sources to sinks. What is a. may plug leaks when a plant is damaged. meant by “source” and “sink”? Give examples of each. b. clogs the sieve plates at all times. 3. What is the minimum number of plasma membranes a water c. never clogs the sieve plates. molecule would have to cross in order to get from the soil d. serves no known function. solution to the atmosphere by way of the stele? To get from e. provides the motive force for transport in the phloem. the soil solution to a mesophyll cell in a leaf? 4. Transpiration exerts a powerful pulling force on the water For Discussion column in the xylem. When would you expect transpiration to proceed most rapidly? Why? Describe the source of the 1. Epidermal cells protect against excess water loss. How do pulling force. they perform this function?