1. Measuring Stomatal Conductance
Colin S. Campbell, Ph.D.
Decagon Devices and Washington State University

2. Plants fundamental dilemma
Biochemistry requires a highly hydrated environment (> -3 MPa)
Atmospheric environment provides CO2 and light but is dry (-100 MPa)

3. 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)

4. 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 and xylem conductance to liquid water flow
High water potential

5. Indicators of plant water stress
Leaf stomatal conductance
Soil water potential
Leaf water potential

6. 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

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

8. 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 leaf
a air

9. Cvt rvs Cvs rva Cva stomatal resistance of the leaf
Boundary layer resistance
of the leaf

10. Do stomata control leaf water loss?
Bange (1953)
Still air: boundary layer resistance controls
Moving air: stomatal resistance controls

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

12. 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

13. 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:
ΔCv = change in water vapor concentration
Δt = change in time

14. Delta T dynamic diffusion porometer

15. Null balance porometer: LI-1600

16. How does the SC-1 measure stomatal conductance?

17. Decagon steady state porometer

18. 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

19. 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

20. 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.

21. 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

22. 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

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

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

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

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

27. AP-4 vs. SC-1 Measured conductance

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

29. Case study: 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

30. Case Study: Stress in wine grapes

31. 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

32. Appendix: Water potential measurement
technique matrix
Method
Measures
Principle
Range (MPa)
Precautions
Tensiometer
(liquid equilibration)
soil matric potential
internal suction balanced against matric potential through porous cup
+0.1 to
cavitates and must be refilled if minimum range is exceeded
Pressure chamber
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
in situ leaf psychrometer
same as sample changer; should be shaded from direct sun; must have good seal to leaf
Dewpoint hygrometer
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
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