Plant Water Plant Water Relations Relations

Prof. Dr. Muhammad Prof. Dr. Muhammad AshrafAshraf

What are Water What are Water Relations?Relations?

A field of study in which one can observe A field of study in which one can observe plant and environmental interactions with plant and environmental interactions with respect to waterrespect to water OROR

Study of all mechanisms related to uptake of Study of all mechanisms related to uptake of water from soil by plants, its translocation water from soil by plants, its translocation from root to shoot and evaporation through from root to shoot and evaporation through stomatastomata

Movement of water and other substances Movement of water and other substances from soil to plant roots across membranes, from soil to plant roots across membranes, throughout the plant and between the plant throughout the plant and between the plant and its environment (Salisbury, 1992)and its environment (Salisbury, 1992)

Water relations of a single Water relations of a single cellcell

Nature of cellular water or distribution of water in Nature of cellular water or distribution of water in cellscells

Water is continuously present throughout the plant bodyWater is continuously present throughout the plant body Some water is held in the micro-capillariesSome water is held in the micro-capillaries Water exists in two systems Water exists in two systems

– ApoplastApoplast– symplastsymplast

Cell wall water (matric Cell wall water (matric water) Apoplastic water) Apoplastic waterwater 5-40% of cell water occurs in the walls depending on the age, 5-40% of cell water occurs in the walls depending on the age,

thickness and composition of the wallsthickness and composition of the walls 50% of cell wall volume is water50% of cell wall volume is water In thick leaves, walls are thick – low water is held in the walls by In thick leaves, walls are thick – low water is held in the walls by

matric forces including H-bonding to various constituentsmatric forces including H-bonding to various constituents The wall contains cellulose microfibrils, pectic substances, proteins The wall contains cellulose microfibrils, pectic substances, proteins

and OH and COOH groups which absorb water by H-bonding. Water and OH and COOH groups which absorb water by H-bonding. Water is also held in inter-micro-fibrillar spaces (matric water or imbibed is also held in inter-micro-fibrillar spaces (matric water or imbibed water)water)

Water in the Water in the CytoplasmCytoplasm In meristematic tissues, vacuole is In meristematic tissues, vacuole is

small so most water occurs in the small so most water occurs in the cytoplasm. However, in mature cells cytoplasm. However, in mature cells cytoplasm is as a thin layer and cytoplasm is as a thin layer and consists of 5 to 10% of the cell waterconsists of 5 to 10% of the cell water

VacuoleVacuole 50-80% or more of the cell water 50-80% or more of the cell water

occurs in the vacuoles. Cell sap occurs in the vacuoles. Cell sap consists of 2% solid and 98% water. consists of 2% solid and 98% water. This water contains solutes mainly This water contains solutes mainly sugars, salts and sometimes organic sugars, salts and sometimes organic acids. OP = -1.0 to -3.0 MPa.acids. OP = -1.0 to -3.0 MPa.

In leaves of Eucalyptus 50% water in In leaves of Eucalyptus 50% water in vacuolevacuole

In wheat roots 80% or more in vacuolesIn wheat roots 80% or more in vacuoles

Cell MembraneCell Membrane Water is held by means of dipolar and H-bond.Water is held by means of dipolar and H-bond. The inner membrane spaces within proteins and lipids The inner membrane spaces within proteins and lipids

are occupied by water moleculesare occupied by water molecules Water is very dense so that they form semi-crystalline Water is very dense so that they form semi-crystalline

structure.structure. Membrane surface is also covered by one molecular Membrane surface is also covered by one molecular

thick layer of waterthick layer of water Water moves to the region of low water potential or Water moves to the region of low water potential or

low energylow energy

Water relations of Water relations of plantplant

HISTORY of Plant Water HISTORY of Plant Water RelationsRelations

Tang and Wang (1941) first used the Tang and Wang (1941) first used the term “water potential” to explain cell term “water potential” to explain cell water relationswater relations

Then it was used by Owen (1952) to Then it was used by Owen (1952) to explain DPD (diffusion pressure deficit) explain DPD (diffusion pressure deficit) which is equivalent to water potential which is equivalent to water potential

DPD = OP -TPDPD = OP -TP Slatyer from Australia and Taylor from Slatyer from Australia and Taylor from

Utah (1960) recognized this termUtah (1960) recognized this term

Acceptance of the water potential concept was slow Acceptance of the water potential concept was slow because of the confusion regarding terminology, the lack of because of the confusion regarding terminology, the lack of convenient methods for measuring it, and the inadequate convenient methods for measuring it, and the inadequate training of plant physiologists in physical chemistry training of plant physiologists in physical chemistry (Kramer 1995). (Kramer 1995).

As a result, plant water status seldom was measured As a result, plant water status seldom was measured during the second quarter of the 20during the second quarter of the 20thth century. century.

Development of thermocouple psychrometers (Monteith Development of thermocouple psychrometers (Monteith and Owen, 1958; Richards and Ogata, 1958; Spanner, and Owen, 1958; Richards and Ogata, 1958; Spanner, 1951) and pressure equilibration by Scholander and his 1951) and pressure equilibration by Scholander and his colleagues (1964,1965) made measurement of water colleagues (1964,1965) made measurement of water potential relatively easy, and they are the measurements potential relatively easy, and they are the measurements used most often today to characterize plant water status.used most often today to characterize plant water status.

Chemical PotentialChemical Potential Chemical potential (a thermodynamic term) is Chemical potential (a thermodynamic term) is the the

amount of energy per mole of a substance to do workamount of energy per mole of a substance to do work

Chemical potential of water is water potential and it is a Chemical potential of water is water potential and it is a measure of the free energy per unit volume available for measure of the free energy per unit volume available for reaction or movementreaction or movement

a quantity that determines the transport of matter from a quantity that determines the transport of matter from one phase to another: a component will flow from one one phase to another: a component will flow from one phase to another when the chemical potential of the phase to another when the chemical potential of the component is greater in the first phase than in the component is greater in the first phase than in the second.second.

Chemical potential depends upon concentration, Chemical potential depends upon concentration, pressure, electric potential and gravity. e.g. molecules pressure, electric potential and gravity. e.g. molecules at high temp move toward low temp regime.at high temp move toward low temp regime.

Different Definitions of Water Different Definitions of Water PotentialPotential

Water potential is the chemical potential (Free energy) Water potential is the chemical potential (Free energy) of water in a system expressed in units of pressure and of water in a system expressed in units of pressure and compared to water potential of pure water i.e. 0compared to water potential of pure water i.e. 0

Free energy of water (water potential) in plants relates Free energy of water (water potential) in plants relates to creating and breaking molecular bonds, moving ions to creating and breaking molecular bonds, moving ions through the cellular organelles or moving water from soil through the cellular organelles or moving water from soil to root and leaf through different cells and xylemto root and leaf through different cells and xylem

Chemical Potential of water divided by partial molal Chemical Potential of water divided by partial molal volume of watervolume of water

It is the difference between matrically bound, It is the difference between matrically bound, pressurised or osmotically held water and pure waterpressurised or osmotically held water and pure water

Chemical Potential and Chemical Potential and Water PotentialWater Potential

ΨΨww = DPD = OP - TP = DPD = OP - TPΨΨw w = = μμww - - μμºº

ww = = RT ln e/ RT ln e/ººee VVw w VVww

μμww = Chemical potential of water = Chemical potential of waterμμºº

ww = Chemical potential of pure water = Chemical potential of pure waterR = 0.00831 kg MPa molR = 0.00831 kg MPa mol-1-1 K K-1 -1 or 0.00831 kJ molor 0.00831 kJ mol-1-1 K K-1 -1

T = Absolute temperature (K)T = Absolute temperature (K)K = 273 + K = 273 + ººCCe = vapor pressure of water in a systeme = vapor pressure of water in a systemeeºº = vapor pressure of pure water = vapor pressure of pure water VVww =partial molal volume of water (e.g. volume of 1 mole of water =partial molal volume of water (e.g. volume of 1 mole of water

is 18 cmis 18 cm33 mol mol-1-1))

Factors affecting Chemical Factors affecting Chemical potentialpotential

Chemical activity (Solute conc.) Chemical activity (Solute conc.) temperature contributes to chemical temperature contributes to chemical

activity (molecules with high temp will activity (molecules with high temp will move toward low temp regime)move toward low temp regime)

electrical potential – only important for electrical potential – only important for charged substancescharged substances

pressure – elastic cell walls allow plant cells pressure – elastic cell walls allow plant cells to develop significant hydrostatic pressure to develop significant hydrostatic pressure

gravitational pull – only applicable in tall gravitational pull – only applicable in tall treestrees

Units of Water Units of Water PotentialPotential

1 MPa = 10 bars1 MPa = 10 bars 1 bar = 0.987 atm = 101 bar = 0.987 atm = 1066 dynes cm dynes cm-2-2 = 10 = 1066

ergs cmergs cm-3-3 1 MPa = 1 kJ kg1 MPa = 1 kJ kg-1-1 = 1 J g = 1 J g-1-1

Water Potential-Water Potential-ExampleExample

w plant = s + p + m

- 0.8 MPa = - 0.9 + 0.3 - 0.2

w soil = s + m

- 0.6 MPa = - 0.2 + - 0.4

Water PotentialWater Potential

0-1-2-3 1 2 3

Water Potential-Water Potential-MagnitudeMagnitude

w = 0 MPa Pure Water

w = 0 to -1 MPaPlant/Cell in good condition

w < -2 MPaPlant/Cell under water stress

w = -1 to -2 MPaPlant/Cell under mild water stress

w ≈ -6 MPa Desert soils

Water Potential-FluxWater Potential-Flux

Water will flow from sites of high w (close to zero) to sites of low w (more negative):

-0.3 MPa -1 MPa -2 MPa -30 MPa

Soil Root Stem Leaf Air

Cell growth

Protein synthesis

Stomatal opening

Photosynthesis

Respiration

Pro/sugar accum.

Transport

0 MPa-1 MPa-2 MPa

Water Deficit-EffectsWater Deficit-Effects

-0.4-1.2 -0.8-1.6

25

50

75

% o

f max

imum

RespirationEnlargement

Photosynthesis

Water Potential (MPa)

Water Deficit-EffectsWater Deficit-Effects

-0.4-1.2 -0.8-1.6

0.3

0.6

0.9

Rat

e of

elo

ngat

ion

(um

s-

1 )

StemRoot

Leaf

Water Potential (MPa) Westgate and Boyer (1985)

Components of Water Components of Water PotentialPotential

The components of water The components of water potential are osmotic potential, potential are osmotic potential, turgor potential, matric potential, turgor potential, matric potential, and gravitational potentialand gravitational potential

ΨΨww = = ΨΨs s + + ΨΨpp + + ΨΨmm+ + ΨΨg g

Values of Water Values of Water PotentialPotential Water potential of pure water at Water potential of pure water at

standard atmospheric conditions is “0” standard atmospheric conditions is “0” (the maximum)(the maximum)

Water potential of a system is always Water potential of a system is always negativenegative

More solute concentration, more More solute concentration, more negative will be the water potentialnegative will be the water potential

It is zero when cell is fully turgidIt is zero when cell is fully turgid Water potential becomes positive when Water potential becomes positive when

pressurized or compressedpressurized or compressed

Water PotentialWater Potential

Solution ASolution AUnconfined systemUnconfined systemΨΨww = = ΨΨs s + + ΨΨpp

-10 = -10 + 0-10 = -10 + 0-10 = -10-10 = -10ΨΨw w = -10= -10

Solution ASolution AConfined systemConfined systemΨΨww = = ΨΨs s + + ΨΨpp

-30 = -30 + 0 (Initial stage)-30 = -30 + 0 (Initial stage)-10 = -30 + 20-10 = -30 + 20-10 = -10-10 = -10ΨΨw w = -10= -10

Osmotic PotentialOsmotic Potential Osmotic Pressure is a pressure that a solution Osmotic Pressure is a pressure that a solution

develops to increase its chemical potential to develops to increase its chemical potential to that of pure water or it is a hydrostatic pressure that of pure water or it is a hydrostatic pressure when applied to a solution prevents the influx of when applied to a solution prevents the influx of waterwater

Is based on concentration of solutes in waterIs based on concentration of solutes in water Is potential developed by solutes in a system with Is potential developed by solutes in a system with

which influx of water occurswhich influx of water occurs Is always negativeIs always negative Higher solute concentration, more negative will Higher solute concentration, more negative will

be the value of osmotic potentialbe the value of osmotic potential Is denoted by Is denoted by ψψss An isolated solution has no osmotic pressure but An isolated solution has no osmotic pressure but

it does have an osmotic potential. it does have an osmotic potential.

Van’t Hoff EquationVan’t Hoff Equation

ΨΨs s = - = -mmiRT = -CiRT = -nRT/ViRT = -CiRT = -nRT/V

i = ionization constanti = ionization constant C = concentrationC = concentration m = m = molalitymolality n = number of solutesn = number of solutes R = Gas constantR = Gas constant T = Absolute temperatureT = Absolute temperature

Effect of Temperature on Effect of Temperature on Osmotic Potentials of Same Osmotic Potentials of Same SolutionSolution

ΨΨs s = - = -mmiRTiRT Osmotic potential of 1 molal glucose solution at 30 Osmotic potential of 1 molal glucose solution at 30 ooCC

ΨΨs s = - (1.0 mol kg= - (1.0 mol kg-1-1) 1.0 x (0.00831 kg MPa mol) 1.0 x (0.00831 kg MPa mol-1-1 K K-1-1) x (273+30) K) x (273+30) K

= -2.518 MPa at 30 = -2.518 MPa at 30 ooCC Osmotic potential of 1 molal glucose solution at 0 Osmotic potential of 1 molal glucose solution at 0 ooCC

ΨΨs s = - (1.0 mol kg= - (1.0 mol kg-1-1) 1.0 x (0.00831 kg MPa mol) 1.0 x (0.00831 kg MPa mol-1-1 K K-1-1) x (273+0) K) x (273+0) K

= -2.269 MPa at 0 = -2.269 MPa at 0 ooCC

Turgor PressureTurgor Pressure Turgor pressure is produced by the diffusion of Turgor pressure is produced by the diffusion of

water into protoplasts enclosed in walls which water into protoplasts enclosed in walls which resist expansionresist expansion

Turgor pressure is hydrostatic pressure of Turgor pressure is hydrostatic pressure of water that is exerted on the liquid by the walls water that is exerted on the liquid by the walls of a turgid cell (pressure per unit area of liquid)of a turgid cell (pressure per unit area of liquid)

Is denoted by Is denoted by ψψPP Is zero in open vesselIs zero in open vessel Is –ve in xylem of transpiring plant while it is Is –ve in xylem of transpiring plant while it is

positive in guttating plantspositive in guttating plants Is zero when cell is flaccidIs zero when cell is flaccid

Turgor pressureTurgor pressure pushes the plasma pushes the plasma membrane against the cell wall of plant, membrane against the cell wall of plant, bacteria, and fungi cells as well as those of bacteria, and fungi cells as well as those of protist cells which have cell walls. This protist cells which have cell walls. This pressure, pressure, turgidityturgidity, is caused by the osmotic , is caused by the osmotic flow of water from area of low solute flow of water from area of low solute concentration outside of the cell into the cell's concentration outside of the cell into the cell's vacuole, which has a higher solute vacuole, which has a higher solute concentration. Healthy plant cells are turgid concentration. Healthy plant cells are turgid and plants rely on turgidity to maintain and plants rely on turgidity to maintain rigidity.rigidity.

Matric PotentialMatric Potential Matric potential is due to the adhesive Matric potential is due to the adhesive

characteristics of water when in contact characteristics of water when in contact with surface or large macromoleculeswith surface or large macromolecules

Potential developed due to water held in Potential developed due to water held in microcapillaries or bound on surfaces of microcapillaries or bound on surfaces of cell wallscell walls

Matric water increases as the cell water Matric water increases as the cell water decreasesdecreases

Is negligible at high tissue hydrationIs negligible at high tissue hydration If tissue hydration is low (60%), it should If tissue hydration is low (60%), it should

be considered (Nobel et al., 1992)be considered (Nobel et al., 1992)

Gravitational PotentialGravitational Potential It is the potential that is It is the potential that is

developed due to gravitational developed due to gravitational pullpull

It is also negligible in crop plants It is also negligible in crop plants while in tall trees it influences the while in tall trees it influences the water potential water potential

Soil water potentialSoil water potential ψψTT = ψ = ψmm + ψ + ψss + ψ + ψpp + ψ + ψzz

ΨΨmm, , matric potential resulting from the combined effects of capillarity matric potential resulting from the combined effects of capillarity and adsorptive forces within the soil matrix (Value –ve)and adsorptive forces within the soil matrix (Value –ve)

ΨΨss, , Solute potential resulting from solutes present in water (Value –ve)Solute potential resulting from solutes present in water (Value –ve) ΨΨpp, , pressure potential i.e. hydrostatic pressure exerted by unsupported pressure potential i.e. hydrostatic pressure exerted by unsupported

(free) water that tends to saturate the soil. Soil ψp is always positive (free) water that tends to saturate the soil. Soil ψp is always positive below a water table, or zero at or above the water table.below a water table, or zero at or above the water table.

ΨΨzz, , gravitational potential is simply the vertical distance from a gravitational potential is simply the vertical distance from a reference level to the point of interestreference level to the point of interest

IfIf we consider no effect of gravity while considering soil water we consider no effect of gravity while considering soil water potential at the same point so the overall equation will be potential at the same point so the overall equation will be

ΨΨww = ψ = ψmm + ψ + ψss + ψ + ψpp

Soil water potential is measured by Soil water potential is measured by PsychrometerPsychrometer

Soil matric potential by TensiometerSoil matric potential by Tensiometer Soil pressure potential by Soil pressure potential by

PiezometerPiezometer Soil solute potential by EC or Soil solute potential by EC or

OsmometerOsmometer