Plant Water Relations.pdf

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PlantWater RelationsJohn B Passioura, CSIRO Plant Industry, Canberra, Australia

Plantwater relations concerns how plants control the hydration of their cells, including

the collection of water from the soil, its transport within the plant, and its loss by

evaporation from the leaves.

Introduction

Well-watered plants are turgid. Their cells, which areenclosed in a strong but slightly elastic wall, are distendedby an internal pressure that may be as high as 1 MPa, fivetimes the pressure in a car tyre and 10 times the pressure ofthe atmosphere. Plants perform best when they are turgid.Many of the structures of higher plants serve to maintaintheir cells sufficiently hydrated to function to grow, tophotosynthesize, to respire even though most of these

cells are in the shoots of the plants and so are not onlyremote from their supply of water in the soil, but are alsoexposed to a dry environment.

A well-hydrated leaf may transpire several times its ownvolume of water during a day. Water evaporates from wetcell walls into the internal gas spaces of the leaf. It thenflows away as vapour, largely through stomata, which arevariable pores in the surface of the leaf. The loss is anunavoidable consequence of the stomata being open, asthey must be to allow carbon dioxide to enter the leaf. Therelative humidity inside a leaf is typicallygreater than99%,and thus there is usually a large difference of absolutehumidity across the stomata that induces rapid diffusion of

water vapour out of the leaf.Although a leaf may lose much water by evaporation, its

net loss of water is usually small. Evaporation from cellwalls creates in them a large suction that replenishes waterby drawing it from the soil, principally via the plantsvascular system, but also through flow across cells in rootsand leaf.

Properties of Water

The physico-chemical properties of water have shaped

many of the processes in plants and, indeed, in allorganisms. Several of these properties, together withselected values, are listed in Table 1.

One of the most important features of water is that itforms strong hydrogen bonds. These greatly influenceseveral biologically important bulk properties. Water hasunusually large latent heats of evaporation and freezing,which help plants cope with frosts or heat loads. It hasgreat cohesive strength, which enables it to withstand thevery large tensions that develop in the xylem and thus to

maintain continuity of liquid water throughout the planIt has a large surface tension at an airwater interfacewhich creates a strong skin that ensures that suitably smapores in soil or plant remain filled with water even if thwater is under great tension, as is often the case.

On the molecular scale, hydrogen bonds ensure thamacromolecules such as proteins and DNA are surrounded by a shell of water molecules that acts as a spatiabuffer between the macromolecules, preventing them fromadhering to each other and thereby precipitating. This shealso penetrates interstices in such molecules, thus helpinto maintain the three-dimensional structures on whictheir reactions depend.

Water is polar: despite being electrically neutral, it has slight excess of electrons on one side of each molecule andslight deficit on the other. Its polar nature, coupled with iability to form hydrogen bonds, makes it a very goosolvent for ions and small organic molecules such a

sugars. Molar concentrations of these are possible, anoften occur, thereby enabling cells to generate osmotipressures of several MPa, a requirement for plants tremain turgid in a saline or droughted environment. Thpolar nature is especially important in enabling fatty acidto organize themselves into the membranes that bind celland organelles. Fatty acids have a nonpolar hydrocarbotail attached to a polar carboxylic acid group. Thesnonpolar tails, eschewing an aqueous environment, line uto form two sheets that are appressed together to form lipid bilayer, with the polar heads presenting a hydrophilsurface on each side.

Other biologically important properties of water in

clude: its viscosity, which influences how fast water flows iresponse to a pressure gradient; and its ability to dissociatinto hydrogen- and hydroxyl ions, which is central to thpH scale.

As with all materials above absolute zero temperaturwater and solute molecules are in thermal motion. Thmotion ensures that gradients in solute concentration otemperature are eventually dissipated by diffusion.

Article Contents

Secondary article

. Introduction

. Properties of Water

. Water at Equilibrium: Water Potential and Its

Components

. Water in Motion: Drivers of Flow

. Water in the Soil

. Measurement of Water Status in Soil and Plant

. Movement of Water through Soil and Plant to the

Atmosphere

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Water at Equilibrium: Water Potentialand Its Components

Although plants are never at true equilibrium, owing totheir incessant biochemical activity, they are often closeenough, so far as water is concerned, for it to be useful todescribe the status of their water in terms of classicalequilibrium thermodynamics. The chemical potential ofwater, which is uniform in a system at equilibrium, is thedifference in Gibbs free energy per mole (the energy

available to do work) between the water of interest andpure water at standard temperature and pressure. It hasunits of J mol2 1. Because of the importance of pressure inthe functioning of plants, plant physiologists have chosento express chemical potential in terms of pressure, whichhas the dimensions of force/area or energy/volume, andwhose SI unit is the Pascal (Pa). They have thereforeintroduced the term water potential, denoted by, whichis the chemical potential divided by the volume of one moleof water.

The notion of water potential can be applied to anysample of water, whether inside a cell, in the cell wall, inxylem vessels, or in the soil. Water potential thus defined is

always negative in plants, for by convention it is zero inpure water at atmospheric pressure, and the addition ofany solutes or the imposition of suction (negative hydro-static pressure) for drawing water into a plant necessarilylowers the water potential. It can be divided into twocomponents, hydrostatic pressure (P) and osmotic pres-sure (P), according to eqn [1].

= P2P [1]

Osmotic pressure

Osmotic pressure is a measure of the attraction of solutefor water. It is defined as the hydrostatic pressurethat musbe applied to a solution to prevent water flowing into when it is separated from pure water by a membrane thaallows the passage of water, but not of solute. It is

colligative property, that is, its value depends simply on thnumber of solute molecules present. To a good approximation, at least for dilute solutions, the relation betweeosmotic pressure and solute concentration is described bvant Hoffs law (eqn [2]).

P= cRT [2

In this equation c is the sum of molar concentrations of asolute species (mol L2 1), R is the universal gas constan(0.0083 L MPa mol2 1 K2 1, or 8.3 J mol2 1K2 1) and Tthe absolute temperature (K). For example, a tenth molasolution of sucrose has an osmotic pressure of 0.25 MPa a258C.

The influence of gravity

The concept of water potential excludes effects of graviton the energy status of water, but in some circumstances, itall trees, or when dealing with the flow of water in soigravity must be included. Where the effect is important itappropriate to define total water potential,F (Pa), whichthe sum of the water potential,, and a gravitational termas in eqn [3].

F=+rgh = P2P+rgh [3

Here r (kgm2 3) is the density of water,g (9.8ms2 2)isth

acceleration due to gravity, andh (m) is the height (relativto a given reference) in the gravitational field.F is constanin a system at equilibrium with respect to water even wheheight varies. The gravitational term, rgh, increases b9.8 kPa for each metre increase in height. Hence, in system at equilibrium, the hydrostatic pressure, P, falls bapproximately 10 kPa for each metre increase in height.

Positive and negative hydrostatic pressures

Hydrostatic pressure, P, is usually expressed as gaugepressure, the difference from the normal atmospherpressure of approximately 100 kPa absolute pressuretha

is, a gauge pressure of zero equals an absolute pressure o100 kPa. Large positive values of hydrostatic pressureinduced by osmosis, are typical in plants cells, where theare often called turgor pressure. Large negative values aralso common, both in plants and the soil they are growinin. These large negative values arise because of capillareffects the attraction between water and hydrophilsurfaces, such as cell walls and soil, coupled with thsurface tension at an air/water interface. For a cylindricapore of radius a (m), the impact of this interaction on th

Table 1 Some physico-chemical properties of water and

aqueous solutions

Property Value and units

Cohesive strength 4 25MPa

Surface tension 0.073 N m21 at208C

Viscosity 0.0018 Pa s at 08C

0.0010Pa s at 208CDiffusion coefficient ofsmall solutes

in water

$ 1 1029 m2s21

Molar volume (pure water at 208C) 1.81 1025 m3 mol2 1

Latent heat of evaporation at 208C 2.5 MJ kg21

Latent heat of melting 0.34 MJ kg21

Saturated vapour pressure of pure

water

0.61 kPaat 08C

1.23 kPa at 108C

2.34 kPa at 208C

4.24 kPa at 308C

7.38 kPa at 408C

PlantWater Relations

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pressure in the adjacent water is given by the LaplaceYoung equation.

P

a

HereDP is the drop in pressure across the meniscus, and

g is the surface tension of water, which is about 0.75 N m2

1

at room temperature, so that DP is approximately 0.15/a(Pa). Thus a fully developed (hemispherical) meniscus in acylindrical pore of radius, say, 15mm would have a pressuredrop across it of 10 kPa. The pressure, P, in the waterwould therefore be 210 kPa if the surrounding gas were atatmospheric pressure. Water would rise to a height ofabout 1 m (from eqn [3]) in a capillary tube of this size(Figure1), or, more generally, to a height of 15 102 6/a m.

It is such capillary action that generates the suctions inthe water-filled pores in soil or cell walls. The pores in cellwalls are especially small (of the order of 5 nm), and aretherefore able to develop very large suctions without

emptying, as in severely water-stressed plants.

The connection between water potential andthe vapour pressure of water

Water andair in a closed container at constant temperaturewill develop a steady saturated vapour pressure in the gasphase. The vapour pressure above a salt solution is lowerthan that above pure water, because the water moleculesare attracted to the solutes, and are therefore less likely toescape from the surface of the liquid to enter the gas phase.

The saturated vapour pressure is related to waterpotential as shown in eqn [5].

c RT

Vln

p

p

8>>:

9>>;

Here R is the gas constant (8.3 J mol2 1 K2 1), T is theabsolute temperature (K), Vis the volume of one mole ofwater, ps is the saturated vapour pressure above the testsample, and p0 is that above pure water. The equationshows that is very sensitive to small changes in ps. Forexample, when ps /p050.99, 5 21.35 MPa at 208C,which is close to the limit in the soil at which roots canextract water. Thus the vapour pressure of water is close to

saturation in moist soil and in all but the most severelywater-stressed plants.

Water in Motion: Drivers of Flow

Water flows in three different ways: by osmosis acrosssemipermeable membranes, that is, membranes that allowthe passage of water but not of solutes; by diffusion inliquid or gas; and in bulk.

Osmotic flow across semipermeable membranes driven by differences in water potential. While thmolecular mechanisms for this flow are still not fullunderstood, the process is well characterized phenomenologically, and is essential to the creation of turgopressure. A commonly used general transport equatio

for the flow of water, Jv (m s2

1), across a membrane igiven by eqn [6].

Jv = Lp(DP2sDP) [6

In this equation Lp (m s2 1 Pa2 1) is the hydrauli

conductance of the membrane, and DP and DP are thdifferences across the membrane of hydrostatic pressurand osmotic pressure, respectively. s is the reflectiocoefficient, which is a measure of how impermeable thmembrane is to the solute; it is dimensionless, and rangefrom zero to one, with zero signifying that the membranecompletely permeable to the solute, and one signifying thathe membrane is completely impermeable. The value ofL

depends on the activity of aquaporins.Diffusive flow results from gradients in concentratio

and the random thermal motion of molecules. It is rapiover small distances but is very slow over large. Thcharacteristic time (s) taken for a concentration gradient tdisappear is approximately L2/D, where L (m) is thdistance over which the variation is initially present and D(m2 s2 1) is the diffusion coefficient of the solutes. Smasolutes, such as potassium ions or amino acids, havdiffusion coefficients of about 1 1029 m2 s2 1. Thus, th

1 m

P= 0

Water

Hydrophilictube of radius15 m

P= 10 kPa

Meniscus

Figure 1 Capillary action. Surface tension generates an upward pullonthewaterinthecapillarytube.Thewaterrisestoaheightof1minatubeof radius15 mm,or moregenerallyto a heightof 15 1026/am (eqns [

and [4]) for a tube of radius a (m).


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characteristic time for diffusion within a cell of diameter10mm is only about 0.1 s, whereas that for diffusion in acompartment of diameter 100 mm (say, through the soil ina plant pot) is about 107 s, which is many weeks.

Bulk flow is driven by gradients in hydrostatic pressure.It is much faster than diffusive flow over large distancesbecause the molecules are all travelling in the same

direction and hence their movement is cooperative. Thisis the flow that occurs in xylem vessels, in the interstices ofcell walls, and in water-filled pores in soil. The resistance tosuch flow depends very strongly on the size of the channels.The wider the channel, the greater is the scope forcooperative flow. Bulk flow in a cylindrical tube of radiusa is proportional to a4, of which a2 accounts for the areaavailable for flow and the other factor a2 accounts for thefact thatsuch flow is intrinsically easier in wide tubes. Widevessels, therefore, have much more carrying capacity thannarrow ones. The dependence of flow rate on radius isdescribed by Poiseuilles law (eqn [7]).

Q a

P

L

In this equation Q (m3 s21) is the flow rate through atube of radius a (m) and length L (m), DP (Pa) is thedifference in pressure across the length of the tube, and Z(Pa s) is the viscosity of water.

As water in the transpiration stream moves from the soilto the roots, through the plant, and out through thestomata, all three types of flow are involved at variousstages. A good example of the relative impacts of thediffusive and bulk mechanisms is givenby the flow of waterin the phloem from the leaves to the roots, which runscounter to the transpiration stream.This flow is against theoverall gradient in water potential and is, in energeticterms, uphill. It is powered by the loading of solutes,principally sucrose, into the phloem in the leaves, whichthere generate a large local hydrostatic pressure. This inturn induces bulk flow of the sucrose solution towards theroots, where the sucrose is unloaded. The gradient inosmotic pressure of the sucrose exceeds the opposinggradient in turgor pressure, but because bulk flow is somuch faster than diffusive flow for a given gradient overdistances greater than about 1 mm, the solution as a wholemoves against the gradient in water potential.

Water in the Soil

Soil is porous, and holds water in its pores by capillarity(eqn [4]). The distribution of pore sizes in the soil, togetherwith its depth, determine how much water the soil can holdforuse byplants. In practice, poreslargerthanabout 15mmradius drain so fast within about a day after rain mayhave filled them that they do not contribute much to the

reservoir. They drain because gravity generates a suctiothat the capillary effects cannot withstand: for example, apores greater than 15mm in radius would be empty in parcel of soil that is in equilibrium with a water table 1 mbelow it (eqns [3] and [4] and accompanying discussionThey drain quickly because flow is so much faster in largpores than in small (eqn [7]).

At the other end of the suction scale, plants cannoextract much water from pores smaller than about 200 nmin diameter. The pressure in such pores is about

21.5 MPa, and from that has to be subtracted largdifferences in suction in the soil, betweenbulk soil and roosurface, if water is to move to the roots at reasonable ratethrough such small pores. Leaves typically experiencwater potentials lower than about 22.5 MPa in succircumstances, low enough for most plants to be wilting.

Flow of water through the soil

Horizontal flow of water through soil is induced b

gradients in hydrostatic pressure, P. The rate of flow,(m s2 1), depends on the gradient inP according to Darcylaw (eqn [8]).

F KP

x

Here x (m) is distance and K(m2 s21 Pa2 1) is hydrauliconductivity. K varies enormously, by about a millionfold, over the range of water content available to plantsThe range is so large because water flows much more easilin large pores than in small (eqn [7]) and as the soil dries thlargest water-filled pores drain first.

Vertical flow of water (drainage or capillary rise), whicinvolves doing work against gravity, is driven by gradientin hydraulic head, the sum ofP and rgh (cf. eqn [3]).

Measurement of Water Statusin Soil and Plant

Techniques for measuring water status in soil or plandepend in the main on one of two principles: measuring thvapourpressure of the sample of soil or plant; or measurinthe pressure of water in pores. The former measures wate

potential, with the help of eqn [5]; the latter, hydrostatipressure.

The vapour pressure technique (psychrometry) relies otwo principles. One is that the saturated vapour pressurdepends markedly on temperature (Table 1). The other ithat the temperature of a thermocouple junction can bcontrolled by passing an electric current through it (thPeltier effect). In practice, one places a sample of soil oplant (for example, a leaf disc) in a small closecompartment at a very precisely controlled temperature




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root canbe treated as a cylinder to which water flows downa gradient of pressure in the soil water, according to eqn [8]suitably developed to account for flow with cylindricalgeometry. General conclusions are that the potentialuptake rate exceeds the plants demand for water if thelength of active root per unit volume of soil exceeds about102 3 m m2 3, and if the soilwater suction is less than about300 kPa (when K, the hydraulic conductivity in the soil, isadequately large). With fewer roots and drier soil, a limit issoon reached in the soils ability to supply water to therootsurface. The stomata must then close, so that transpiration

does not exceed the supply, otherwise the plant willdesiccate.

Radialflow through a root to the xylem is partly throughthe cell walls. Impervious walls block the way at theendodermis and possibly hypodermis, thus ensuring thatthe water must enter the symplasm in the root cortex andleave it in the xylem parenchyma cells next to the vessels.The resistance to water flow across the membranesinvolved is large, and often is responsible for most of thefall in water potential between the soil and the evaporatingsurfaces of the leaves, at least in well-watered plants.

Longitudinal flow in the xylemXylem sap of a transpiring plant is usually under tension,with pressure typically in the range of20.5 to 21.5 MPain well-watered plants. Providing that the sap remains inthis metastable state, the xylem usually offers littlehydraulic resistance. The cohesive strength of water is solarge (Table 1) that tensile failure seems never to occur.However, the (porous) walls surrounding the vessels arenot perfect at preventing the ingress of air, and in somecircumstances gas bubbles (embolisms) will form and

block the flow of water. In desert plants especiallypropensity to embolize may have evolved to enable thplants to sacrifice, progressively, small branches, so thathe transpiring leaf area is reduced to levels that the rootcan service.

From the xylem to the substomatalcavities in the leaf

For many years the prevailing view was that the transpiration stream travelled through the apoplasm (cell walls another compartments outside the cell membranes) to thsubstomatal cavities, where water evaporated and diffuseout of the leaf through the stomata. There is nowcompelling evidence that most of the water enters thsymplasm (the interconnectedcell contents) verysoon afteit leaves the vessels, and that solutes in the xylem sap mabe substantially excluded as the water enters the symplasmAs with radial flow in the roots, the passage of wateracros

cell membranes results in a substantial fall in watepotential.

Stomatal control of transpiration

The rate of flow out of the leaf, E (mol m22 s2 1), thtranspiration rate, is driven by diffusion as vapour througthe stomata. It is controlled by the conductance of thstomata to water vapour, g (mol m2 2 s2 1), according teqn [9].

E g

p p

p

Here pi and pe (Pa) are the vapour pressures inside anoutside the leaf, respectively, andpa (Pa) is the atmospheripressure. For a given value ofg, the water potential of thleaf has almost no influence on E, because within the usuarange of water potential (zero to 23 MPa)the insideof thleaf is so close to being saturated with water vapour (eq[5]) that pi is essentially independent of water potential.

Hence, insofar as water potential influences transpiration rate, it does so by affecting the stomata, which closwhen water potential is low. Stomatal conductance is als

influenced by many other factors, including light leveconcentration of carbon dioxide and hormonal levelwhich are affected by adverse conditions in the soil such asalinity, dryness and compaction.

While stomatal conductance is the plants main means ocontrolling transpiration rate on an hourly or daily basiat time scales of a week or more the development oshedding of leaf area is an equally powerful means ocontrolling overall water use, and that, too, is stronglinfluenced by water relations.

Observed surface

Pressure seal

Pressure chamber

Pressure inlet

Leaf

Figure 3 Pressure chamber for measuring the water potential of a leaf.The leaf is cut from a plant and quickly placed in thechamber, with a small

piece of it protruding through the pressure seal. As with the pressure plate(Figure 2), enough gas pressure is applied to the chamber to bring thexylem sap to the point of bleeding from the cut surface. Because

transpiration has stopped, the xylem sap is close to equilibrium with thecells of theleaf,and theappliedbalancingpressureis equaland opposite to

that of the original pressure in the equilibrated xylem sap.




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Plant Water Relations.pdf