Measuring Stomatal Conductance

Colin 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 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 water potential

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 conductanceCan be good indicator of plant water statusMany 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 surfaceUse pump and desiccant to dry air in

chamberMeasure 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

Null balance porometer: LI-1600

€

E =fCvaA

€

rvs =A

f

1

hr−1

⎛

⎝ ⎜

⎞

⎠ ⎟

How does the SC-1 measure stomatal conductance?

1

1 1

1

dvapor

leafs

gF

CCg

212 CCgF dvapor

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 #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

Porometer Comparisons:LI-1600 vs SC-1 – Dried Silica Gel

Porometer Comparison: LI-1600 vs. SC-1 – After 30 min use

LI-1600 vs. SC-1 – Log-based comparison

LI-1600 vs. SC-1 – Reading difference with mean conductance

AP-4 vs. SC-1 Measured conductance

AP-4 vs. SC-1 Reading difference vs. mean conductance

Case study: Chitosan study

Evaluation of effects of Chitosan on plant water use efficiencyChitosan induces stomatal closure Leaf porometer used to evaluate

effectiveness26 – 43% less water used while

maintaining biomass production

Case Study: 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)

Summary Stomatal conductance can be a powerful

tool 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