Plant water relations

Plant water relationsDouglas R. Cobos, 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 and xylem conductance to liquid water flow

Stomatal conductance to water vapor flow

Indicators of plant water stress

Soil water potential

Leaf stomatal conductance

Leaf/stem water potential

Indicator #1: Plant 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)

Original indicator of leaf water potential

Measuring plant 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: -0.05 MPa to -300 MPa

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

In situ stem water potential psychrometer

Ψstem less dynamic than Ψleaf May be better indicator of plant water status

Continuous measurement Thermal insulation needed Range similar to leaf psychrometer

Pressure chamber vs. in situ comparison

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 (fruit trees)

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

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 All plants regulate water loss through

stomatal conductance

Do stomata control leaf water loss?

Still air: boundary layer resistance controls water loss

Moving air: stomatal resistance controls water loss

Bange (1953)

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

tCv

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

How does the SC-1 measure stomatal conductance?

1

1 11

dvapor

leafs

gFCCg

212 CCgF dvapor

More information on the theory of operation is available.

Leaf

Humidity Sensors Humidity Sensors

Filter

CLeaf

D1

C1

C2

D2

gs

gd1

gd2

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

Case study: Washington State University wheatResearchers using steady state

porometer to create drought resistant wheat cultivarsEvaluating physiological response to

drought stress (stomatal closing)Selecting individuals with optimal

response

Case Study: Stomatal conductance vs. leaf water potential in 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 50 100

150

200

250

300

350

400

450

500

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

Lower limit water potentials Agronomic Crops

Take-home points Three primary methods to asses plant

water status Plant water potential Stomatal conductance Soil water potential

Each provides slightly different information, but all have their place in research

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 (leaf/stem)

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

Measures rh of vapor equilibrated with sample, using wet bulb depression.

-0.1 to -5 Must avoid sample temperature drift during measurement

in situ leaf psychrometer(vapor equilibration)

leaf water potential same as in situ soil psychrometer

-0.1 to -5 same as soil psychrometer; should be shaded from direct sun; must have good seal to leaf

In situ stem psychrometer(vapor equilibration)

stem water potential same as in situ soil psychrometer

-0.1 to -5 Same as soil psychrometer; must completely insulate from temperature change

Dewpoint hygrometer(vapor equilibration)

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

Measures rh of vapor equilibrated with sample, using dew point technique.

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

Heat dissipation(solid equilibration)

soil matric potential ceramic thermal properties empirically related to matric potential

-0.01 to -30 Needs individual calibration; accuracy not good pas -0.5 MPa

Electrical properties(solid equilibration)

soil matric potential 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: Soil and Plant water potential measurement technique matrix