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 CO2 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

High water potential

Boundary layer conductance to water vapor flow

Root conductance to liquid water flow

Stomatal conductance to water vapor flow

Indicators of plant water stress

Soil water potential

Leaf stomatal conductance

Leaf water potential

Indicator #1: Leaf water potential Ψleaf 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, Ψcell = Ψleaf

Leaf water potential Turgid leaf: Ψleaf = Ψcell = turgor pressure (Ψp) +

osmotic potential (Ψo) of water inside cell Flaccid leaf: Ψleaf = Ψcell = Ψo (no positive pressure

component)

Measuring leaf water potential

There is no direct way to measure leaf water potential

Equilibrium methods used exclusively Liquid equilibration methods - Create equilibrium

between sample and area of known water potential across semi-permeable barrier Pressure chamber

Vapor equilibration methods - Measure humidity air in vapor equilibrium with sample Thermocouple psychrometer Dew point potentiameter

Liquid equilibration: pressure chamber

Used to measure leaf water potential (ψleaf)

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

Chamber PressurePleaf

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 Ψleaf

Insert leaf disc into sample chamber Measurement accelerated by

abrading leaf surface withsandpaper

Range: -0.1 MPa to -300 MPa

Pressure chamber – in situ comparison

Vapor equilibration: in situ leaf water potential

Field instrument Measures Ψleaf

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 Can be an indicator of water stress in

perennial crops Maximize crop production (table grapes) Schedule deficit irrigation (wine grapes)

Many annual plants will shed leaves rather than allow leaf water potential to change past a lower threshold Non-irrigated potatoes

Most plants will regulate stomatal conductance before allowing leaf water potential to change 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

E Evaporation (mol m-2 s-1)

C Concentration (mol mol-1)

R Resistance (m2 s mol-1)L leafa air

aL

aL

RR

CCE

Boundary layer resistance of the leaf

stomatal resistance of the leafrvs

Cvt

Cva

rva

Cvs

Do stomata control leaf water loss?

Still air: boundary layer resistance controls

Moving air: stomatal resistance controls

Bange (1953)

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

t

Cv

ΔCv = change in water vapor concentrationΔt = change in time

Stomatal conductance is proportional to:

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

sensors

Teflon filter

R2

R1

h1

h2

1212

1

2

21

1

1

1RR

hh

hR

R

CC

RR

CC

vs

vv

vs

vvL

atmosphere

Rvs = stomatal resistance to vapor flow

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: CO2

Increasing carbon dioxide concentration at the leaf surface decreases stomatal conductance.

Photosynthesis cuvettes could alter conductance, but porometers likely would not

Operator CO2 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

Determine plant water stress in annual and perennial species 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 cultivarsEvaluating 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

y = 0.0204x - 12.962R² = 0.5119

-20.0

-18.0

-16.0

-14.0

-12.0

-10.0

-8.0

-6.0

-4.0

-2.0

0.0

0 50

10

0

15

0

20

0

25

0

30

0

35

0

40

0

45

0

50

0

Mid

-day

Le

af W

ater

Pot

entia

l (ba

rs)

Stomatal Conductance (mmol m-2 s-1)

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 irrigationLower yield but higher quality fruitWine grapesFruit 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

Method Measures Principle Range (MPa) Precautions

Tensiometer(liquid equilibration)

soil matric potential internal suction balanced against matric potential through porous cup

+0.1 to -0.085 cavitates and must be refilled if minimum range is exceeded

Pressure chamber(liquid equilibration)

water potential of plant tissue (leaves)

external pressure balanced against leaf water potential

0 to -6 sometimes difficult to see endpoint; must have fresh from leaf;

in situ soil psychrometer(vapor equilibration)

matric plus osmotic potential in soil

same as sample changer psychrometer

0 to -5 same as sample changer psychrometer

in situ leaf psychrometer(vapor equilibration)

water potential of plant tissue (leaves)

same as sample changer psychrometer

0 to -5 same as sample changer; should be shaded from direct sun; must have good seal to leaf

Dewpoint hygrometer(vapor equilibration)

matric plus osmotic potential of soils, leaves, solutions, other materials

measures hr of vapor equilibrated with sample. Uses Kelvin equation to get water potential

-0.1 to -300 laboratory instrument. Sensitive to changes in ambient room temperature.

Heat dissipation(solid equilibration)

matric potential of soil ceramic thermal properties empirically related to matric potential

-0.01 to -30 Needs individual calibration

Electrical properties(solid equilibration)

matric potential of soil ceramic electrical properties empirically related to matric potential

-0.01 to -0.5 Gypsum sensors dissolve with time. EC type sensors have large errors in salty soils

Appendix: Water potential measurement technique matrix