Plant water relations

Gaylon S. Campbell, Ph.D.

Decagon Devices and Washington State University

Plants fundamental dilemma

 Biochemistry requires a highly hydrated environment (> -3 MPa)  Atmospheric environment provides CO 2 and light but is dry (-100 MPa)

Water potential

 Describes how tightly water is bound in the soil  Describes the availability of water for biological processes  Defines the flow of water in all systems (including SPAC)

Water flow in the Soil Plant Atmosphere Continuum (SPAC)

Low water potential Boundary layer conductance to water vapor flow Stomatal conductance to water vapor flow Root conductance to liquid water flow High water potential

Indicators of plant water stress

Leaf stomatal conductance Soil water potential Leaf water potential

Indicator #1: Leaf water potential

  Ψ leaf is potential of water in leaf outside of cells (only matric potential) The water outside cells is in equilibrium with the water inside the cell, so, Ψ cell = Ψ leaf

Leaf water potential

  Turgid leaf: Ψ leaf = Ψ cell = turgor pressure (Ψ p ) + osmotic potential (Ψ o ) of water inside cell Flaccid leaf: Ψ component) leaf = Ψ cell = Ψ o (no positive pressure

Measuring leaf water potential

    There is no direct way to measure leaf water potential Equilibrium methods used exclusively  Liquid equilibration methods between sample and area of known water potential across semi permeable barrier Create equilibrium Pressure chamber   Vapor equilibration methods vapor equilibrium with sample Thermocouple psychrometer Dew point potentiameter Measure humidity air in

Liquid equilibration: pressure chamber

   Used to measure leaf water potential (ψ leaf ) Equilibrate pressure inside chamber with suction inside leaf   Sever petiole of leaf Cover with wet paper towel   Seal in chamber Pressurize chamber until moment sap flows from petiole Range: 0 to -6 MPa  

leaf



P

Pressure Chamber

Two commercial pressure chambers

Vapor equilibration: chilled mirror dewpoint hygrometer     Lab instrument Measures both soil and plant water potential in the dry range  Can measure Ψ leaf Insert leaf disc into sample chamber  Measurement accelerated by abrading leaf surface with sandpaper Range: -0.1 MPa to -300 MPa

Pressure chamber –

in situ

comparison

Vapor equilibration: potential

in situ

leaf water      Field instrument Measures Ψ leaf Clip on to leaf (must have good seal) Must carefully shade clip Range: -0.1 to -5 MPa

Leaf water potential as an indicator of plant water status

   Maximize crop production (table grapes) Schedule deficit irrigation (wine grapes)   Many annual plants will shed leaves rather than lower threshold Non-irrigated potatoes  Most plants will regulate stomatal conductance below threshold

Case study #1 Washington State University apples

  Researchers used pressure chamber to monitor leaf water potential of apple trees One set well-watered  One set kept under water stress   Results ½ as much vegetative growth – less pruning  Same amount of fruit production  Higher fruit quality  Saved irrigation water

Indicator #2: Stomatal conductance

  Describes gas diffusion through plant stomata Plants regulate stomatal aperture in response to environmental conditions  Described as either a conductance or resistance   Conductance is reciprocal of resistance 1/resistance

 

Stomatal conductance

Can be good indicator of plant water status Many plants regulate water loss through stomatal conductance

Fick's Law for gas diffusion

L a E C R

E



C L R L



C a



R a

Evaporation (mol m -2 s -1 ) Concentration (mol mol -1 ) Resistance (m 2 s mol -1 ) leaf air

stomatal resistance of the leaf Boundary layer resistance of the leaf

Do stomata control leaf water loss?

Bange (1953)  Still air: boundary layer resistance controls  Moving air: stomatal resistance controls

Obtaining resistances (or conductances)  Boundary layer conductance depends on wind speed, leaf size and diffusing gas  Stomatal conductance is measured with a leaf porometer

Measuring stomatal conductance – 2 types of leaf porometer

 Dynamic - rate of change of vapor pressure in chamber attached to leaf  Steady state - measure the vapor flux and gradient near a leaf

Dynamic porometer

 Seal small chamber to leaf surface  Use pump and desiccant to dry air in chamber  Measure the time required for the chamber humidity to rise some preset amount Stomatal conductance is proportional to: 

C v



t

ΔC v = change in water vapor concentration Δt = change in time

Delta T dynamic diffusion porometer

Steady state porometer

 Clamp a chamber with a fixed diffusion path to the leaf surface  Measure the vapor pressure at two locations in the diffusion path  Compute stomatal conductance from the vapor pressure measurements and the known conductance of the diffusion path  No pumps or desiccants

Steady state porometer

leaf h 1 R 1

C vL R vs

 

C v

1

R

1 

C v

1 

C v

2

R

2 R 2 h 2

R vs

 1

h

2  

h

1

h

1

R

2 

R

1 atmosphere R vs = stomatal resistance to vapor flow sensors Teflon filter

Decagon steady state porometer

Environmental effects on stomatal conductance: Light  Stomata normally close in the dark  The leaf clip of the porometer darkens the leaf, so stomata tend to close  Leaves in shadow or shade normally have lower conductances than leaves in the sun  Overcast days may have lower conductance than sunny days

Environmental effects on stomatal conductance: Temperature

 High and low temperature affects photosynthesis and therefore conductance  Temperature differences between sensor and leaf affect all diffusion porometer readings. All can be compensated if leaf and sensor temperatures are known

 Environmental effects on stomatal conductance: Humidity Stomatal conductance increases with humidity at the leaf surface  Porometers that dry the air can decrease conductance  Porometers that allow surface humidity to increase can increase conductance .

 Environmental effects on stomatal conductance: CO 2 Increasing carbon dioxide concentration at the leaf surface decreases stomatal conductance.

 Photosynthesis cuvettes could alter conductance, but porometers likely would not  Operator CO 2 could affect readings

What can I do with a porometer?

   Water use and water balance Use conductance with Fick’s law to determine crop transpiration rate Develop crop cultivars for dry climates/salt affected soils    Study effects of environmental conditions Schedule irrigation   Optimize herbicide uptake Study uptake of ozone and other pollutants

Case study #2 Washington State University wheat

   Researchers using steady state porometer to create drought resistant wheat cultivars Evaluating physiological response to drought stress (stomatal closing) Selecting individuals with optimal response

Case study #3 Chitosan study

  Evaluation of effects of Chitosan on plant water use efficiency Chitosan induces stomatal closure   Leaf porometer used to evaluate effectiveness 26 – 43% less water used while maintaining biomass production

Case Study 4: Stress in wine grapes Stomatal Conductance (mmol m -2 s -1 ) 0.0

-2.0

-4.0

-6.0

-8.0

-10.0

-12.0

-14.0

-16.0

-18.0

-20.0

y = 0.0204x - 12.962

R² = 0.5119

Indicator #3: Soil water potential   Defines the supply part of the supply/demand function of water stress “field capacity” = -0.03 MPa   “permanent wilting point” -1.5 MPa We discussed how to measure soil water potential earlier

Applications of soil water potential    Irrigation management Deficit irrigation  Lower yield but higher quality fruit  Wine grapes  Fruit trees No water stress – optimal yield

Appendix: Lower limit water potentials Agronomic Crops

Summary

 Leaf water potential, stomatal conductance, and soil water potential can all be powerful tools to assess plant water status   Knowledge of how plants are affected by water stress are important Ecosystem health  Crop yield  Produce quality

Appendix: Water potential measurement technique matrix

Method Tensiometer

(liquid equilibration)

Pressure chamber

(liquid equilibration)

in situ soil psychrometer

(vapor equilibration) water potential of plant tissue (leaves) matric plus osmotic potential in soil

in situ leaf psychrometer

(vapor equilibration)

Dewpoint hygrometer

(vapor equilibration) water potential of plant tissue (leaves)

Heat dissipation

(solid equilibration)

Electrical properties

(solid equilibration)

Measures

soil matric potential

Principle

internal suction balanced against matric potential through porous cup external pressure balanced against leaf water potential same as sample changer psychrometer same as sample changer psychrometer matric plus osmotic potential of soils, leaves, solutions, other materials matric potential of soil matric potential of soil measures

h r

of vapor equilibrated with sample. Uses Kelvin equation to get water potential ceramic thermal properties empirically related to matric potential ceramic electrical properties empirically related to matric potential

Range (MPa)

+0.1 to -0.085

Precautions

cavitates and must be refilled if minimum range is exceeded 0 to -6 0 to -5 0 to -5 -0.1 to -300 -0.01 to -30 -0.01 to -0.5

sometimes difficult to see endpoint; must have fresh from leaf; same as sample changer psychrometer same as sample changer; should be shaded from direct sun; must have good seal to leaf laboratory instrument. Sensitive to changes in ambient room temperature.

Needs individual calibration Gypsum sensors dissolve with time. EC type sensors have large errors in salty soils