chap36

From Tarzan of the Apes to George of the Jungle, in legions of comicstrips and adventure movies, heroes have swung through the forestcanopy 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 continuallymoving 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 tothe 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 theplant, such as roots, tubers, and internal stem tissues. Before we consider the mech-anisms underlying these processes, we should have some idea about the magnitudeof what they accomplish. Let’s consider two questions: How much water is trans-ported? And how high can water be transported?

In answer to the first question, consider thefollowing example: A single maple tree 15 meterstall 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 ofwater per hour to the atmosphere by evaporationfrom the leaves. To prevent wilting, the xylemneeds to transport 220 liters of water from theroots to the leaves every hour. (By comparison, a50-gallon drum holds 189 liters.)

The second question can be rephrased: Howtall are the tallest trees? The tallest gym-nosperms, the coast redwoods—Sequoia semper-virens—exceed 110 meters in height, as do thetallest angiosperms, the Australian Eucalyptusregnans. Any successful explanation of watertransport in the xylem must account for thetransport of water to these great heights.

In this chapter, we will consider the uptakeand transport of water and minerals by plants,the control of evaporative water loss from leaves,and the translocation of substances in thephloem.

Transport in Plants

Hollywood Vines Although the famousswinging-through-the-jungle scenes in theTarzan movies of the 1940s were pure fic-tion and the vines the actors used wererope props, the use of lianas—heavy, twin-ing vines of the tropical rain forests—as asource of drinking water is quite plausible.

36

702 CHAPTER THIRT Y-SIX

Uptake and Movement of Water and Solutes

Terrestrial plants must obtain both water and mineral nutri-ents from the soil, usually by way of their roots. The roots, inturn, obtain carbohydrates and other important materialsfrom the leaves (Figure 36.1). We learned in Chapter 8 thatwater is one of the ingredients required for carbohydrate pro-duction by photosynthesis in the leaves. Water is also essen-tial for transporting solutes both upward and downward, forcooling the plant, and for developing the internal pressurethat supports the plant body. Plants lose large quantities ofwater to evaporation, and this water must be continually re-placed.

How do leaves high in a tree obtain water from the soil?What are the mechanisms by which water and mineral ionsenter the plant body through the roots and ascend as sap inthe xylem? Because neither water nor minerals can move

through the plant into the xylem without crossing at least oneplasma membrane, we will focus first on osmosis. Then wewill examine the uptake of mineral ions and follow the path-way by which both water and minerals move through theroot to gain entry to the xylem.

Water moves through a membrane by osmosisOsmosis, the movement of water through a membrane in ac-cordance with the laws of diffusion, was described in Chap-ter 5 (see Figure 5.8). The solute potential (osmotic potential)of a solution is a measure of the effect of dissolved solutes onthe osmotic behavior of the solution. The following statementpresents an opportunity for confusion, so study it carefully:

� The greater the solute concentration of a solution, themore negative its solute potential, and the greater thetendency of water to move into it from another solutionof lower solute concentration (and less negative solutepotential).

For osmosis to occur, the two solutions must be separated bya membrane permeable to water but relatively impermeableto 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 ofmore water is increasingly resisted by an opposing pressurepotential (called turgor pressure in plants), owing to the rigid-ity of the wall. As more and more water enters, the pressurepotential becomes greater and greater.

Pressure potential is a hydraulic pressure analogous to theair pressure in an automobile tire; it is a mechanical pressurethat can be measured with a pressure gauge. Plant cells donot burst when placed in pure water; instead, water entersby osmosis until the pressure potential exactly balances thesolute potential. At this point, the cell is turgid; that is, it hasa significant pressure potential.

The overall tendency of a solution to take up water frompure 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 thesum of its (negative) solute potential (ψs) and its (usually pos-itive) pressure potential (ψp):

ψ = ψs + ψp

For pure water open to the atmosphere and therefore underno applied pressure, all three of these parameters are zero.

We can measure solute potential, pressure potential, andwater potential in megapascals (MPa), a unit of pressure. (At-mospheric pressure, “one atmosphere,” is about 0.1 MPa, or14.7 pounds per square inch; typical pressure in an automo-bile tire is about 0.2 MPa.)

H2O and dissolvedminerals

H2O, carbohydrates, etc.

H2O

CO2

O2

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,carrying important solutes with it.

In all cases in which water moves between two solutionsseparated by a membrane, the following rule applies:

� Water always moves across a selectively permeablemembrane toward the region of lower (more negative)water potential.

Osmotic phenomena are of great importance to plants.The structure of many plants is maintained by the pressurepotential of their cells; if the pressure potential is lost, theplant wilts. Within living tissues, the movement of waterfrom cell to cell follows a gradient of water potential. Overlonger distances, in unobstructed tubes such as xylem ves-sels and phloem sieve tubes, the flow of water and dissolvedsolutes is driven by a gradient of pressure potential. Themovement of a solution due to a difference in pressure po-tential between two parts of a plant is called bulk flow.

Aquaporins facilitate the movement of water across membranesAquaporins are membrane channel proteins through whichwater can move without interacting with the hydrophobicenvironment of the membrane’s phospholipid bilayer. Theseproteins, important in both plants and animals, allow waterto move rapidly from environment to cell and from cell tocell. The permeability of some aquaporins can be regulated,changing the rate of osmosis across the membrane. However,water movement through aquaporins is always passive, sothe direction of water movement is unchanged by alterationsin aquaporin permeability.

Uptake of mineral ions requires membrane transport proteins

Mineral ions, which carry electric charges, generally cannotmove across a membrane unless they are aided by transportproteins (explained in Chapter 5). When the concentration ofthese charged ions in the soil is greater than that in the plant,ion channels and carrier proteins can move them into theplant by facilitated diffusion, which is a passive process. Theconcentrations of most ions in the soil solution, however, arelower than those required inside the plant. Thus the plantmust take up these ions against a concentration gradient—aprocess that requires energy.

Electric charge differences also play a role in the uptake ofmineral ions. Movement of a negatively charged ion into anegatively charged region is movement against an electricalgradient and requires energy. The combination of concentra-tion and electrical gradients is called an electrochemical gradi-ent. Uptake against an electrochemical gradient is active trans-port, an energy-requiring process, which depends on cellularrespiration for a supply of ATP. Active transport, of course,requires specific transport proteins.

Unlike animals, plants do not have a sodium–potassiumpump for active transport. Rather, plants have a protonpump, which uses energy obtained from ATP to move pro-tons out of the cell against a proton concentration gradient

TRANSPORT IN PLANTS 703

ψ = 0

ψp = 0ψs = –0.4

ψ = –0.4

Membrane

Piston

ψ = 0

ψp = 0.15ψs = –0.15

ψ = 0

ψ = 0

ψs = –0.4ψp = 0.4

ψ = 0

The solution in the tube has a negative solute potential (ψs) due to the presence of dissolved solutes; its ψp = 0; thus its ψ is negative. The beaker contains distilled water (ψ = 0). The two liquids are not at equilibrium.

1

Water entering the tube dilutes the solution, making its ψs less negative. As the solution rises in the tube, pressure potential (ψp) builds up until it balances the ψs. This pressure corresponds to turgor pressure in plants. Atequilibrium, ψ in the solution is equal to ψ inthe beaker.

3

A piston resists the entry of water, as does the wall of a plant cell. The solution in the tube is not diluted, so its ψs does not change. However, the system is not initially at equilibrium. Enough water squeezes in to raise ψp until equilibrium is reached, with equalwater potentials.

4

Because of the difference in ψ between the solution and the distilled water, water moves from the beaker into the tube.

2

36.2 Water Potential, SolutePotential, and Pressure PotentialWater potential (ψ) is the tendency of asolution to take up water from purewater. Its water potential is the sum ofthe solute potential (ψs) and the pres-sure potential (ψp). For pure waterunder no applied pressure, all three ofthese parameters are equal to zero.

(Figure 36.3a). Because protons (H+) are positively charged,their accumulation outside the cell has two results:

� The region outside the cell becomes positively chargedwith respect to the region inside.

� A proton concentration gradient develops across theplasma membrane.

Each of these results has consequences for the movement ofother ions. Because of the charge difference across the mem-brane, there is increased movement of cations (positivelycharged ions), such as potassium (K+), into the cell throughtheir membrane channels. These ions move into the nowmore negatively charged interior of the cell by facilitated dif-fusion (Figure 36.3b). In addition, the proton concentrationgradient can be harnessed to drive secondary active trans-port, in which anions (negatively charged ions) such as chlo-ride (Cl–) are moved into the cell against an electrochemicalgradient by a symport protein that couples their movementwith that of H+ (Figure 36.3c). In sum, there is a vigorous traf-fic of ions across plant cell membranes, involving specificmembrane transport proteins and both active and passiveprocesses.

The proton pump and the coordinated activities of othermembrane transport proteins cause the interior of a plant cellto be very negative with respect to the exterior. Such a dif-ference in charge across a membrane is called a membranepotential. Biologists can measure the membrane potential ofa plant cell with microelectrodes, just as they can measuresimilar charge differences in nerve cells and other animalcells (see Chapter 44). Most plant cells maintain a membranepotential of at least –120 millivolts (mV).

Water and ions pass to the xylem by way of the apoplast and symplast

Mineral ions enter and move through plants in various ways.Where water is moving by bulk flow, dissolved minerals arecarried along in the stream. Both water and minerals alsomove by diffusion. At certain sites, where plasma mem-branes are being crossed, some mineral ions are moved byactive transport. One such site is the surface of a root hair,where mineral ions first enter the cells of the plant. Later,within the stele, the ions must cross another plasma mem-brane before entering the nonliving vessels and tracheids ofthe xylem.

The movement of ions across membranes can also resultin the movement of water. Water moves into a root becausethe root has a more negative water potential than does thesoil solution. Water moves from the cortex of the root into thestele (which is where the vascular tissues are located) becausethe stele has a more negative water potential than does thecortex.

Water and minerals from the soil may pass through thedermal and ground tissues to the stele via two pathways: theapoplast and the symplast. The apoplast (from the Greek,apo-, “away from”; -plast, “living material”) consists of thecell walls, which lie outside the plasma membranes, and theintercellular spaces (spaces between cells) that are commonto many tissues. The apoplast is a continuous meshworkthrough which water and dissolved substances can flow ordiffuse without ever having to cross a membrane (Figure36.4). Movement of materials through the apoplast is thusunregulated—until it reaches the endodermis, as we willsoon discuss.


Outsideof cell

Plasma membrane

Insideof cell

(a)

(b)

Potassiumchannel

Protonpump

+

Symportprotein

K+

K+

Cl–

Cl–

(c)

A proton pump generates differences in H+ concentration and electric charge across the membrane.

The difference in electric charge causes cations such as K+ to enter the cell.

Symport couples the diffusion of H+ to the transport (against an electrochemical gradient) of anions such as Cl− into the cell.

Pi

ATP ADP

H+

H+

H+ H+

H+

H+

H+

H+

H+H+

H+ H+H+

H+

H+

H+

1 32

H+

H+

36.3 The Proton Pump in Active Transport of K+

and Cl– The buildup ofhydrogen ions (H+) trans-ported outside the cell bythe proton pump (a) drivesthe movement of bothcations (b) and anions (c) into the cell.

The remainder of the plant body is the symplast (from theGreek, sym-, “together with”). The symplast is the portion ofthe plant body enclosed by membranes—the continuous cy-toplasm of the living cells, connected by plasmodesmata (seeFigure 36.4). The selectively permeable plasma membranesof the cells control access to the symplast, so movement ofwater and dissolved substances into the symplast is tightlyregulated.

Water and minerals can pass from the soil solutionthrough the apoplast as far as the endodermis, the innermostlayer 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 awater-repelling (hydrophobic) belt around each en-dodermal cell where it is in contact with other endo-dermal cells. The hydrophobic Casparian strips act asa seal that prevents water and ions from moving be-tween the cells (Figure 36.5).

The Casparian strips of the endodermis thus com-pletely separate the apoplast of the cortex from theapoplast of the stele. However, they do not obstructthe outer or inner faces of the endodermal cells. Ac-cordingly, water and ions can enter the stele only byway of the symplast—that is, by entering and passingthrough the cytoplasm of the endodermal cells. Thustransport proteins in the plasma membranes of these

cells determine which mineral ions pass into the stele, and atwhat rates.

Once they have passed the endodermal barrier, water andminerals leave the symplast and enter the apoplast of thestele. Parenchyma cells in the pericycle or xylem can aid this


Water and ions cross a plasma membrane to enter the symplast path.

Water and ions travel through cell walls and intercellular spaces in the apoplast.

Plasma membrane

Epidermis

Cortex

Stele

Endodermis

Pericycle

Tracheary elements

Plasmodesmata

Root hair

Casparianstrip

36.4 Apoplast and Symplast Plant cell walls andintercellular spaces constitute the apoplast. The sym-plast comprises the living cells, which are connectedby plasmodesmata. To enter the symplast, water and

solutes must pass through a plasma membrane. No suchselective barrier limits movement through the apoplast.

Pericycle(stele)

Endodermis

Casparian strips prevent water in the apoplast from passing between the endodermal cells into the stele.

To bypass the Casparian strips, water must enter the living cells and access the stele via the symplast.

Plasmodesmata

36.5 Casparian Strips Casparian strips in the endodermis of thecortex are impregnated with the water-repelling substance suberin.These strips separate the apoplast in the cortex from the apoplast inthe stele.

process. Some of these parenchyma cells, called transfer cells,are structurally modified for transporting mineral ions fromtheir cytoplasm (part of the symplast) into their cell walls(part of the apoplast). The cell wall that receives the trans-ported ions has many knobby extensions projecting into thetransfer cell, increasing the surface area of the cell’s plasmamembrane, the number of transport proteins, and thus therate of transport (Figure 36.6). Transfer cells also have manymitochondria that produce the ATP needed to power the ac-tive transport of mineral ions.

As mineral ions move into the solution in the cell walls,the water potential in the apoplast becomes more negative;thus water moves out of the cells and into the apoplast by os-mosis. In other words, active transport of ions moves the ionsdirectly, and water follows passively. The end result is thatwater and minerals end up in the xylem, where they consti-tute the xylem sap. How do the water and materials move onfrom the xylem of the root system?

Transport of Water and Minerals in the Xylem

So far in this chapter we’ve described the movement of wa-ter and minerals into plant roots and their entry into the rootxylem. Now we will consider how xylem sap moves throughthe remainder of the plant. Let’s first consider some earlyideas about the ascent of xylem sap and then turn to our cur-rent understanding of how it works. We’ll describe the ex-periments that ruled out some early models as well as someevidence in support of the current model.

Experiments ruled out xylem transport by pumping action of living cells

Some of the earliest attempts to explain the rise of sap in thexylem were based on a hypothetical pumping action by liv-ing cells in the stem, which might push the sap upward.However, experiments conducted and published in 1893 bythe German botanist Eduard Strasburger definitively ruledout such models.

Strasburger worked with trees about 20 meters tall. Hesawed through the trunk of each tree at its base and plungedthe cut end into a bucket containing a solution of a poison,such as picric acid. The solution rose through the trunk, aswas readily evident from the progressive death of the barkhigher and higher up. When the solution reached the leaves,the leaves died, too, at which point the movement of the so-lution stopped (as shown by the liquid level in the bucket,which stopped dropping).

This simple experiment established three importantpoints:

� Living, “pumping” cells were not responsible for the up-ward movement of the solution, because the solutionitself killed all living cells with which it came in contact.

� The leaves played a crucial role in transport. As long asthey were alive, the solution continued to move upward;when the leaves died, movement ceased.

� The movement was not caused by the roots, because thetrunk had been completely separated from the roots.

Root pressure does not account for xylem transportIn spite of Strasburger’s observations, some plant physiolo-gists hypothesized that xylem transport was based on rootpressure—pressure exerted by the root tissues that wouldforce liquid up the xylem. The basis for root pressure is ahigher solute concentration, and accordingly a more nega-tive water potential, in the xylem sap than in the soil solu-tion. This water potential draws water into the stele; oncethere, the water has nowhere to go but up, so it rises in thevessels and tracheids.

There is good evidence that root pressure exists—for ex-ample, the phenomenon of guttation, in which liquid wateris forced out through openings at the margins of leaves (Fig-ure 36.7). Guttation occurs only under conditions of high at-mospheric humidity and plentiful water in the soil, which oc-cur most commonly at night. Root pressure is also the sourceof the sap that oozes from the cut stumps of some plants,such as Coleus, when their tops are cut off.

Root pressure, however, cannot account for the ascent ofsap in trees. Root pressure seldom exceeds 0.1–0.2 MPa (1–2atmospheres). If root pressure were driving sap up the xylem,we would observe a positive pressure potential in the xylem


Plasmodesma

Mitochondria

Wall extensions

Transfer cell

Sieve tubeelement

Parenchyma cell

36.6 A Transfer Cell Three walls of this transfer cell in a pea leafhave knobby extensions that face the cells from which the transfercell imports solutes. A transfer cell exports the solutes to the neigh-boring sieve tube element.

at all times. In fact, as we are about to see, thexylem sap in most trees is under tension—hasa negative pressure potential—when it is as-cending. Furthermore, as Strasburger had al-ready shown, materials can be transported up-ward in the xylem even when the roots havebeen removed. If the roots are not pushing thexylem sap upward, what causes it to rise?

The transpiration–cohesion–tensionmechanism accounts for xylem transportThe obvious alternative to pushing is pulling:The leaves pull the xylem sap upward. Theevaporative loss of water from the leaves gen-erates a pulling force (tension) on the water inthe apoplast of the leaves. Hydrogen bondingbetween water molecules makes the sap in thexylem cohesive enough to withstand the ten-sion and rise by bulk flow. Let’s see how thisprocess works.

The concentration of water vapor in the at-mosphere is lower than that in the leaf. Becauseof this difference, water vapor diffuses from theintercellular spaces of the leaf, through open-ings called stomata, to the outside air. Thisprocess is called transpiration (Figure 36.8).Within the leaf blade, water evaporates fromthe moist walls of the mesophyll cells and en-ters the intercellular spaces. The force gener-ated by the evaporation of water from the mes-ophyll cell walls creates a tension that drawsmore water into the cell walls, replacing that

which was lost. The tension in the mesophyll draws waterfrom the xylem of the nearest vein into the apoplast sur-rounding the mesophyll cells. The removal of water from theveins, in turn, establishes tension on the entire column of wa-ter contained within the xylem, so that the column is drawnupward all the way from the roots.

The ability of water to be pulled upward through tinytubes 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. Theintegrity of the column is also maintained by the adhesion ofwater to the xylem walls. In the tallest trees, such as a 110-meter redwood, the difference in pressure potential between


Root

Stem

Leaf

Xylem

XylemRoot hairH2O

Water molecules form a cohesive column from the roots to the leaves.

6

Water moves into the stele by osmosis.

7

Water vapor diffuses out of the stomata.

1

Tension pulls the water column upward and outward in the xylemof veins in the leaves.

4

Stoma

Vein

Mesophyllcell

Tension pulls water from the veins into the apoplast of the mesophyll cells.

3

Water evaporates from mesophyll cell walls.

2

Tension pulls the water column upward in the xylem of the root and stem.

5

36.7 Guttation Root pressure is responsible for forcing waterthrough openings in the margins of this strawberry leaf.

36.8 The Transpiration–Cohesion–Tension Mechanism Transpiration causes evapo-ration from mesophyll cell walls, generating tension on the xylem. Cohesion amongwater molecules in the xylem transmits the tension from the leaf to the root, causingwater to move from the soil to the atmosphere.

the top and the bottom of the column may be as great as 3MPa. The cohesion of water in the xylem is great enough towithstand even that great a tension.

In summary, the key elements of water transport in thexylem are

� Transpiration, the evaporation of water from the leaves� Tension in the xylem sap resulting from transpiration� Cohesion in the xylem sap from the leaves to the roots

This transpiration–cohesion–tension mechanism requiresno work (that is, no expenditure of energy) on the part of theplant. At each step between soil and atmosphere, watermoves passively toward a region with a more negative wa-ter potential. Dry air has the most negative water potential(–95 MPa at 50% relative humidity), and the soil solution hasthe least negative water potential (between –0.01 and –3MPa). Xylem sap has a water potential more negative thanthat of cells in the cortex of the root, but less negative thanthat of mesophyll cells in the leaf.

Mineral ions contained in the xylem sap rise passivelywith 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-ments brought to the leaves are subsequently redistributedto other parts of the plant by way of the phloem, but the ini-tial delivery from the roots is through the xylem.

In addition to promoting the transport of minerals, tran-spiration contributes to temperature regulation. As waterevaporates from mesophyll cells, heat is taken up from thecells, and the leaf temperature drops. This cooling effect ofevaporation (so evident in the cooling of our skin when wesweat) is important in enabling plants to live in hot environ-ments. A farmer can hold a leaf between thumb and forefin-ger to estimate its temperature; if the leaf doesn’t feel cool,that means that transpiration is not occurring, so it must betime to water.

A pressure bomb measures tension in the xylem sapThe transpiration–cohesion–tension model can be true onlyif the column of sap in the xylem is under tension (has a neg-ative pressure potential). The most elegant demonstrationsof this tension, and of its adequacy to account for the ascentof xylem sap in tall trees, were performed by the biologist PerScholander, who measured tension in stems with an instru-ment called a pressure bomb.

Consider a stem in which the xylem sap is under tension.If the stem is cut, the sap pulls away from the cut, into thestem. This behavior indicates that the pressure in the intactxylem is lower than that of the atmosphere. Now the stem isquickly placed in the pressure bomb, in which the pressuremay be raised. The cut surface remains outside the bomb. Asgas pressure is applied to the plant parts within the bomb,

the xylem sap is pushed back to the cut surface. When thesap first becomes visible again at the cut surface, the pressurein the bomb is recorded. This pressure is equal in magnitudebut opposite in sign to the tension (negative pressure poten-tial) originally present in the xylem (Figure 36.9).

Scholander used the pressure bomb to study dozens ofplant species, from diverse habitats, growing under a varietyof conditions. In all cases in which xylem sap was ascending,it was found to be under tension. The tension disappeared insome of the plants at night, when transpiration ceased. In de-veloping vines, the xylem sap was under no tension untilleaves formed. Once leaves developed, transport in thexylem began, and tensions were recorded.

Suppose you wanted to measure tensions in the xylem atvarious heights in a tall tree to confirm that the tensions aresufficient to account for the rate at which sap is moving upthe trunk. How would you obtain stem samples for meas-urement? Per Scholander used surveying instruments to de-termine the heights of particular twigs, then had a sharp-shooter shoot the identified twigs from the tree with ahigh-powered rifle. As the twigs fell to the ground, Scholan-der quickly inserted them in the pressure bomb and recordedtheir xylem tension. In every case, the differences in tensionsat different heights were great enough to keep the xylem sapascending.

The rate at which the sap ascends is not the same at alltimes. No flow of xylem sap takes place at night, when thereis little or no transpiration. By day, when the sap is ascending,the rate of ascent depends on several factors. One is the con-centration of K+ in the sap: The rate of flow increases as the K+

concentration increases (Figure 36.10). Other factors, such as


By applying just enough pressure… Sap

Pressure gauge

Pressure release valve

…so that xylem sap is pushed back to the cut surface of a plant sample,…

…a scientist can determine the tension on the sap in the living plant.

1 2

3

RESEARCH METHOD

Gas pressure

36.9 A Pressure Bomb The amount of tension on the sap in differ-ent types of plants can be measured with this device.

temperature, light intensity, and wind velocity affect the tran-spiration rate, and hence the rate of sap flow, more directly.

Although transpiration provides the impetus for the trans-port of water and minerals in the xylem, it also results in theloss of tremendous quantities of water from the plant. Howdo plants control this loss?

Transpiration and the Stomata

The epidermis of leaves and stems minimizes transpirationalwater loss by secreting a waxy cuticle, which is impermeableto water. However, the cuticle is also impermeable to carbon

dioxide. This poses a problem: How can the leaf balance itsneed to retain water with its need to obtain CO2 for photo-synthesis?

Plants have evolved an elegant compromise in the formof stomata (singular, stoma), or pores, in the epidermis oftheir leaves. A pair of specialized epidermal cells, calledguard cells, controls the opening and closing of each stoma(Figure 36.11a). When the stomata are open, CO2 can enterthe leaf by diffusion—but water vapor is lost in the same way.


Rel

ativ

e fl

ow r

ate

0 1000 2000 3000 4000Time (seconds)

1

1.5

2

H2O (control)

Return toH2O alone

K+ solutionor H2O

H2O (control)

Question: How does K+ affect xylem flow rate?

EXPERIMENT

Conclusion: K+ increases the rate of flow in the xylem.

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.

METHOD

Two xylem-containing flaps were created on a tobacco plant. One was connected to a source of pure water (the control). The other flap could be connected to either pure water or to a solution containing a known concentration of K+.

The control trace (H2O only) showed little variation in flow rate.

The experimental trace spiked immediately after the injection of K+ solution.

RESULTS

K+ solutioninjected

36.10 Potassium Ions Speed Transport in the Xylem This experi-ment showed that the rate of fluid ascending through the xylemspiked when a solution with a known concentration of potassiumions was injected. Repeating the experiment with solutions of differ-ent concentrations of K+ showed that the higher the K+ concentra-tion, the greater the flow rate.

In the presence of light, potassium ions are actively transported into the guard cells from epidermal cells.

Higher internal K+ and Cl– concentrations give guard cells a more negative water potential, causing them to take up water and stretch, opening the stoma.

In the absence of light, as K+ diffuses passively out of the guard cells, water follows by osmosis, the guard cells go limp, and the stoma closes.

Guard cells

K+

(b)

(a)

Cl–H2O

K+Cl–H2O

1

2

3

36.11 Stomata (a) A scanning electron micrograph of an openstoma formed by two sausage-shaped guard cells. (b) Potassium ionconcentrations affect the water potential of the guard cells, control-ling the opening and closing of stomata. Negatively charged ionsaccompanying K+ maintain electrical balance and contribute to thechanges in solute potential that open and close the stomata.

Closed stomata prevent water loss, but also exclude CO2from the leaf.

Most plants open their stomata only when the light inten-sity is sufficient to maintain a moderate rate of photosynthe-sis. At night, when darkness precludes photosynthesis, thestomata remain closed; no CO2 is needed at this time, andwater is conserved. Even during the day, the stomata close ifwater is being lost at too great a rate.

The stoma and guard cells in Figure 36.11a are typical ofeudicots. Monocots typically have specialized epidermal cellsassociated with their guard cells. The principle of operation,however, is the same for both monocot and eudicot stomata.In what follows, we describe the regulation and mechanismof stomatal opening and the normal cycle of opening andclosing.

The guard cells control the size of the stomatal openingLight causes the stomata of most plants to open, admittingCO2 for photosynthesis. Another cue for stomatal opening isthe level of CO2 in the intercellular spaces inside the leaf. Alow level favors opening of the stomata, thus allowing theuptake of more CO2.

Water stress is a common problem for plants, especiallyon hot, sunny, windy days. Plants have a protective responseto these conditions, which uses the water potential of themesophyll cells as a cue. Even when the CO2 level is low andthe sun is shining, if the mesophyll is too dehydrated—thatis, if the water potential of the mesophyll is too negative—the mesophyll cells release a plant hormone called abscisicacid. Abscisic acid acts on the guard cells, causing them toclose the stomata and prevent further drying of the leaf. Thisresponse reduces the rate of photosynthesis, but it protectsthe plant.

These processes are regulated by control of the K+ concen-tration in the guard cells. Blue light, absorbed by a pigmentin the guard cell plasma membrane, activates a proton pump,which actively transports protons (H+) out of the guard cellsand into the surrounding epidermis. The resulting proton gra-dient drives the accumulation of K+ (Figure 36.11b; also re-view Figure 36.3) in the guard cell. The increasing internalconcentration of K+ makes the water potential of the guardcells more negative. Water enters the guard cells by osmosis,increasing their pressure potential. The arrangement of thecellulose microfibrils in their cell walls causes the guard cellsto respond to this increase by changing their shapes so that agap—the stoma—appears between them.

The stoma closes by the reverse process when active trans-port ceases in response to the absence of blue light or the pres-ence of abscisic acid. Potassium ions diffuse passively out ofthe guard cells, water follows by osmosis, the pressure po-tential decreases, and the guard cells sag together and seal off

the stoma. Negatively charged chloride ions and organic ionsalso move into and out of the guard cells along with thepotassium ions, maintaining electrical balance and contribut-ing to the change in the solute potential of the guard cells.

Transpiration from crops can be decreasedStomata are the “referees” of a compromise between the ad-mission of CO2 for photosynthesis and the loss of water bytranspiration. Farmers would like their crops to transpire less,thus reducing the need for irrigation. Similarly, nurseries andgardeners would like to be able to reduce the amount of wa-ter lost by plants that are to be transplanted, because trans-planting often damages the roots, causing the plant to wilt ordie. What they need is a good antitranspirant: a compoundthat can be applied to plants, reducing water loss from thestomata without producing disastrous side effects by exces-sively limiting CO2 uptake.

Abscisic acid and its commercial chemical analogs havebeen found to work as antitranspirants in small-scale tests,but their high cost has precluded commercial use. Whatabout making plants more sensitive to their own abscisicacid? The guard cells of transgenic plants with a mutant al-lele of the era gene are highly sensitive to abscisic acid andhence resistant to wilting during drought stress.

A totally different type of antitranspirant temporarily sealsoff the leaves from the atmosphere. Growers use a variety ofcompounds, most of which form polymeric films aroundleaves, to form a barrier to evaporation. These compoundscause undesirable side effects, however, and can be used onlyfor short periods of time. Their most common use is in thetransplanting of nursery stock.

In the absence of antitranspirants, stomata are normallyopen during daylight hours, allowing CO2 to be fixed andconverted to the products of photosynthesis. Next we’ll seehow these products are delivered to other parts of the plant,supporting growth of those parts.

Translocation of Substances in the Phloem

Photosynthesis takes place in the mesophyll cells and, in C4plants, in the bundle sheath cells of the leaf (see Figure 8.16).The products of photosynthesis (primarily carbohydrates)diffuse to the nearest small vein, where they are activelytransported into sieve tube elements.

Substances in the phloem move from sources to sinks. Asource is an organ (such as a mature leaf or a storage root)that produces (by photosynthesis or by digestion of stored re-serves) more sugars than it requires. A sink is an organ (suchas a root, a flower, a developing fruit or tuber, or an imma-ture leaf) that consumes sugars for its own growth and stor-age needs. Sugars (primarily sucrose), amino acids, some


minerals, and a variety of other solutes are translocated be-tween sources and sinks in the phloem.

How do we know that such organic solutes are translo-cated in the phloem, rather than in the xylem? Just over 300years ago, the Italian scientist Marcello Malpighi performeda classic experiment in which he removed a ring of bark (con-taining the phloem) from the trunk of a tree—that is, he gir-dled the tree (Figure 36.12). The bark in the region above thegirdle swelled over time. We now know that the swelling re-sulted from the accumulation of organic solutesthat came from higher up the tree and could nolonger continue downward because of the dis-ruption of the phloem. Later, the bark below thegirdle died because it no longer received sugarsfrom the leaves. Eventually the roots, and thenthe entire tree, died.

Any explanation of the translocation of or-ganic solutes must account for a few importantobservations:

� Translocation stops if the phloem tissue iskilled by heating or other methods; thus themechanism must be different from that oftransport in the xylem.

� Translocation often proceeds in both direc-tions—up the stem and down the stem—simultaneously.

� Translocation is inhibited by compounds thatinhibit respiration and thus limit the ATPsupply in the source.

To investigate translocation, plant physiologists needed toobtain samples of pure sieve tube sap from individual sievetube elements. This difficult task was simplified when it wasdiscovered that a common garden pest, the aphid, feeds onplants by drilling into a sieve tube. An aphid inserts its spe-cialized feeding organ, called a stylet, into a stem until thestylet enters a sieve tube (Figure 36.13a). The pressure withinthe sieve tube is greater than that in the surrounding planttissues or outside the plant, so the nutritious sieve tube sapis forced through the stylet and into the aphid’s digestivetract. So great is the pressure that sugary liquid is forcedthrough the insect’s body and out the anus (Figure 36.13b).

Plant physiologists use aphids to collect sieve tube sap.When liquid appears on the aphid’s abdomen, indicating thatthe insect has connected with a sieve tube, the physiologistquickly freezes the aphid and cuts its body away from thestylet, which remains in the sieve tube element. For hours,sieve tube sap continues to exude from the cut stylet, where itmay be collected for analysis. Chemical analysis of sieve tubesap collected in this manner reveals the contents of a singlesieve tube element over time. Physiologists can also infer therates at which different substances are translocated by meas-uring how long it takes for radioactive tracers administered toa leaf to appear at stylets at different distances from the leaf.

These methods have allowed us to understand how, attimes, different substances might move in opposite directionsin the phloem of a stem. Experiments with aphid stylets haveshown that all the contents of any given sieve tube elementmove in the same direction. Thus, bidirectional translocationcan be understood in terms of different sieve tubes conduct-ing sap in opposite directions. These and other experiments


METHOD RESULTS

Remove bark to girdle the tree.

Organic solutes accumulate in the phloem above the girdle, causing swelling.

Bark

Wood

Time

EXPERIMENT

Conclusion: Organic solutes are translocated in the phloem, not in the xylem.

Question: Are organic solutes translocated in the xylem or in the phloem?

36.12 Girdling Blocks Translocation in the Phloem By removinga ring of bark (containing the phloem), Malpighi blocked the translo-cation of organic solutes in a tree.

The aphid‘s stylet has successfully penetrated the sieve tube.

Sieve tube element

(a) (b) Longistigma caryae

Sap droplet

36.13 Aphids Collect Sieve Tube Sap (a) Aphids feed on sap drawn from asieve tube, which they penetrate with a modified feeding organ, the stylet. (b)Pressure inside the sieve tube forces sap through the aphid’s digestive tract, fromwhich it can be harvested.

led to the general adoption of the pressureflow model as an explanation for translocationin the phloem.

The pressure flow model appears to account fortranslocation in the phloemDuring sieve tube element development, the tonoplast andmuch of the cytosol breaks down, allowing the contents of thecentral vacuole to combine with much of the cytosol to formthe sieve tube sap. The sap flows under pressure through thesieve tubes, moving from one sieve tube element to the nextby bulk flow through the sieve plates, without crossing amembrane. We need to understand how this pressure is gen-erated in order to understand translocation in the phloem.

Two steps in translocation require metabolic energy:

� Transport of sucrose and other solutes into the sievetubes at sources, called loading

� Removal of the solutes, called unloading, where the sievetubes enter sinks

According to the pressure flow model of translocation inthe 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 entersthe sieve tube elements by osmosis. The entry of this watercauses a greater pressure potential at the source end of thesieve tube, so that the entire fluid content of the sieve tube ispushed toward the sink end of the tube— in other words, thesap moves by bulk flow in response to a pressure gradient(Figure 36.14). In the sink, the sucrose is unloaded by activetransport, maintaining the gradients of solute potential andwater potential needed for movement.

The pressure flow model of translocation in the phloem iscontrasted with the transpiration–cohesion–tension model ofxylem transport in Table 36.1.

The pressure flow model has been experimentally testedThe pressure flow model was first proposed more than halfa century ago, but some of its features are still being debated.

Other mechanisms have been proposed to account fortranslocation in sieve tubes, but some have been disproved,and none of the rest has as much support as the pressureflow model.

Two essential requirements must be met in order for thepressure flow model to be valid:

� The sieve plates must be unobstructed, so that bulk flowfrom one sieve tube element to the next is possible.

� There must be an effective method for loading sucroseand other solutes into the phloem in source tissues andremoving them in sink tissues.

Let’s see whether these requirements are met.

ARE THE SIEVE PLATES CLOGGED OR OPEN? Early electron micro-scopic studies of phloem samples cut from plants producedresults that seemed to contradict the pressure flow model.The pores in the sieve plates always appeared to be plugged


H2O

H2O

H2O

H2O

Sucrose

Sucrose

Sink cell

SourcecellXylem

Phloemsieve tube

Transpiration pulls water up xylem vessels.

1

Source cells load sucrose into phloem sieve tubes, reducing their water potential…

2

…so water is taken up from xylem vessels by osmosis.

3

Internal pressure differences drive the sap down the sieve tube to sink cells.

4

Sucrose is unloaded into sink cells…

5

…and water moves back to xylem vessels.

6

36.14 The Pressure Flow Model Combined pressurepotential and water potential differences drive the bulk flowof sieve tube sap from a source to a sink.

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)

36.1

with masses of a fibrous protein, suggesting that sieve tubesap could not flow freely. But what is the function of thatfibrous protein?

One possibility is that this protein is usually distributedmore or less at random throughout the sieve tube elementsuntil the sieve tube is damaged; then the sudden surge of saptoward the cut surface carries the protein into the pores,blocking them and preventing the loss of valuable nutrients.In other words, perhaps the protein does not block the poresunless the phloem is damaged. How might this hypothesisbe tested? Could phloem for microscopic observation be ob-tained without causing the sap to surge to the cut surface?

One way to prevent the surge of the sap is to freeze planttissue rapidly before cutting it. Another way is to let the tis-sue wilt so that there is no pressure in the phloem before cut-ting. When these methods are used, the sieve plates are notclogged by the protein. Thus, the first condition of the pres-sure flow model is met.

HOW DO NEIGHBORING CELLS LOAD AND UNLOAD THE SIEVE TUBE

ELEMENTS? If the pressure flow model is correct, theremust be mechanisms for loading sugars and other solutesinto the phloem in source regions and for unloading themin sink regions. Such mechanisms exist in all plants.

Sugars and other solutes produced in the mesophyll passfrom cell to cell in the leaf and eventually enter the sievetubes of the phloem. In some plants these substances leavethe mesophyll cells and enter the apoplast, sometimes withthe help of transfer cells. Then specific sugars and aminoacids are actively transported into cells of the phloem, thusreentering the symplast. This passage through the apoplastand back into the symplast allows the selection of substancesto be translocated by forcing them to pass through a selec-tively permeable membrane. In many plants, solutes reenterthe symplast at the companion cells, which then transfer thesolutes to the adjacent sieve tube elements.

A form of secondary active transport loads sucrose intothe companion cells and sieve tubes. Sucrose is carried acrossthe plasma membrane from apoplast to symplast by su-crose–proton symport; thus the entry of sucrose and protonsis strictly coupled. For this symport to work, the apoplastmust have a high concentration of protons; these protons aresupplied by a primary active transport system, the protonpump. The protons then diffuse back into the cell through thesymport protein, bringing sucrose with them.

In sink regions, the solutes are actively transported out ofthe sieve tube elements and into the surrounding tissues.This unloading serves two purposes: It helps maintain thegradient of solute potential and hence of pressure potentialin the sieve tubes, and it promotes the buildup of sugars andstarch to high concentrations in storage regions, such as de-veloping fruits and seeds.

Plasmodesmata and material transfer between cellsMany substances move from cell to cell within the symplastby way of plasmodesmata (see Figure 35.7). Among theirother roles, plasmodesmata participate in the loading and un-loading of sieve tube elements. The mechanisms vary amongplant species, but the story in tobacco plants is a commonone. In tobacco, sugars and other solutes in source tissues en-ter companion cells by active transport from the apoplast andmove on to the sieve tube elements through plasmodesmata.In sink tissues, plasmodesmata connect sieve tube elements,companion cells, and the cells that will receive and use thetransported compounds.

Plasmodesmata undergo developmental changes as animmature sink leaf matures into a mature source leaf. Plas-modesmata in sink tissues favor rapid unloading: They aremore abundant, and they allow the passage of larger mole-cules. Plasmodesmata in source tissues are few in number.

It was long thought that only substances with molecularweights of less than 1,000 could fit through a plasmodesma.Then biologists discovered that cells infected with tobacco mo-saic virus (TMV) could allow molecules with molecularweights of as much as 20,000 to exit. We now know that TMVencodes a “movement protein” that produces this change inthe permeability of the plasmodesmata—and that the plantsthemselves normally produce at least one such movement pro-tein. Even large molecules such as proteins and RNAs, withmolecular weights up to at least 50,000, can thus move be-tween living plant cells. We will see some consequences of thismovement of macromolecules through plasmodesmata in laterchapters. Biologists are exploring possible ways to regulate thepermeability, number, and form of plasmodesmata as a meansof modifying traffic in the plant. Such modifications might, forexample, allow the diversion of more of a grain crop’s photo-synthetic products into the seeds, increasing the crop yield.

Chapter Summary

Uptake and Movement of Water and Solutes� Plant roots take up water and minerals from the soil. Review Figure 36.1� Water moves through biological membranes by osmosis,always moving toward cells with a more negative water poten-tial. The water potential of a cell or solution is the sum of thesolute potential and the pressure potential. All three parametersare expressed in megapascals (MPa). Review Figure 36.2� Mineral uptake requires transport proteins. Some mineralsenter the plant by facilitated diffusion; others enter by activetransport. A proton pump facilitates the active transport ofmany mineral ions across membranes in plants. Review Figure 36.3� Water and minerals pass from the soil to the xylem by way ofthe apoplast and symplast. In the root, water and minerals canmove from the cortex into the stele only by way of the symplastbecause Casparian strips in the endodermis block their move-ment through the apoplast. Review Figures 36.4, 36.5. SeeWeb/CD Activity 36.1


Transport of Water and Minerals in the Xylem� Early experiments established that xylem sap does not movevia the pumping action of living cells.� Root pressure is responsible for guttation and for the oozingof s0ap from cut stumps, but it cannot account for the ascent ofxylem sap in trees.� Water transport in the xylem is the result of the combinedeffects of transpiration, cohesion, and tension. Evaporation fromthe leaf produces tension in the mesophyll cells, which pulls acolumn of water—held together by cohesion—up through thexylem from the root. Dissolved minerals are carried passively inthe water. Review Figure 36.8� Evaporation of water cools the leaves, but a plant cannotafford to lose too much water.� Support for the transpiration–cohesion–tension model ofwater transport comes from studies using a pressure bomb.Review Figure 36.9� The role of transport in the xylem depends on several factors,including the K+ concentration. Review Figure 36.10

Transpiration and the Stomata� Transpirational water loss is minimized by the waxy cuticleof the leaves.� Stomata allow a compromise between water retention andcarbon dioxide uptake.� A pair of guard cells controls the size of the stomatal open-ing. A proton pump, activated by blue light, pumps protonsfrom the guard cells to surrounding epidermal cells, setting upa proton gradient that drives the active transport of potassiumions into the cells. Water follows osmotically, swelling the cellsand opening the stomata.� Carbon dioxide and water levels in the leaf also affect stom-atal opening. Review Figure 36.11

Translocation of Substances in the Phloem� Products of photosynthesis, as well as some minerals, aretranslocated through sieve tubes in the phloem by way of livingsieve tube elements. Review Figure 36.12� Translocation in the phloem can proceed in both directions inthe stem, although in a single sieve tube it goes only one way.Translocation requires a supply of ATP.� Translocation in the phloem is explained by the pressure flowmodel: The difference in solute concentration between sourcesand sinks creates a difference in pressure potential along thesieve tubes, resulting in bulk flow. Review Figure 36.14, Table 36.1. See Web/CD Tutorial 36.1� The validity of the pressure flow model is supported by thefacts that the sieve plates are normally unobstructed, allowingbulk flow, and that the neighboring cells load organic solutesinto the sieve tube elements in source regions and unload themin sink regions.� The distribution and properties of plasmodesmata differbetween source and sink tissues. It may become possible to regulate plasmodesma permeability in crop plants.

Self-Quiz1. Osmosis

a. requires ATP.b. results in the bursting of plant cells placed in pure water.c. can cause a cell to become turgid.d. is independent of solute concentrations.e. continues until the pressure potential equals the water

potential.

2. Water potentiala. is the difference between the solute potential and the

pressure potential.b. is analogous to the air pressure in an automobile tire.c. is the movement of water through a membrane.d. determines the direction of water movement between cells.e. is defined as 1.0 MPa for pure water under no applied

pressure.

3. Which statement about aquaporins is not true?a. They are membrane transport proteins.b. Water movement through aquaporins is always active.c. The permeability of some aquaporins is subject to

regulation.d. They are found in both animals and plants.e. They enable water to pass through the phospholipid

bilayer without encountering a hydrophobic environment.

4. Which statement about proton pumping across the plasmamembrane of plants is not true?a. It requires ATP.b. The region inside the membrane becomes positively

charged with respect to the region outside.c. It enhances the movement of K+ ions into the cell.d. It pushes protons out of the cell against a proton

concentration gradient.e. It can drive the secondary active transport of negatively

charged ions.

5. Which statement is not true?a. The symplast is a meshwork consisting of the (connected)

living cells.b. Water can enter the stele without entering the symplast.c. The Casparian strips prevent water from moving between

endodermal cells.d. The endodermis is a cell layer in the cortex.e. Water can move freely in the apoplast without entering cells.

6. In the xylem,a. the products of photosynthesis travel down the stem.b. living, pumping cells push the sap upward.c. the motive force is in the roots.d. the sap is often under tension.e. the sap must pass through sieve plates.

7. Which of the following is not part of the transpiration-cohesion-tension mechanism?a. Water evaporates from the walls of mesophyll cells.b. Removal of water from the xylem exerts a pull on the

water column.c. Water is remarkably cohesive.d. The wider the tube, the greater the tension its water

column can withstand.e. At each step, water moves to a region with a more

strongly negative water potential.

8. Stomataa. control the opening of guard cells.b. release less water to the environment than do other parts

of the epidermis.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.

9. Which statement about phloem transport is not true?a. It takes place in sieve tubes.b. It depends on mechanisms for loading solutes into the

phloem in sources.c. It stops if the phloem is killed by heat.d. A high pressure potential is maintained in the sieve tubes.e. In sinks, solutes are actively transported into sieve tube

elements.


10. The fibrous protein in sieve tube elementsa. may plug leaks when a plant is damaged.b. clogs the sieve plates at all times.c. never clogs the sieve plates.d. serves no known function.e. provides the motive force for transport in the phloem.

For Discussion1. Epidermal cells protect against excess water loss. How do

they perform this function?

2. Phloem transports material from sources to sinks. What ismeant by “source” and “sink”? Give examples of each.

3. What is the minimum number of plasma membranes a watermolecule would have to cross in order to get from the soilsolution to the atmosphere by way of the stele? To get fromthe soil solution to a mesophyll cell in a leaf?

4. Transpiration exerts a powerful pulling force on the watercolumn in the xylem. When would you expect transpirationto proceed most rapidly? Why? Describe the source of thepulling force.


chap36

Education

transport of water

liters of water

movement of water

movetendency of water

source of drinking water

realistic source of

entry ofmore water

plants xylem