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

Indicators of plant water stress Leaf stomatal conductance Soil water potential 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 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 leaf a air

Cvt rvs Cvs rva Cva 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: ΔCv = change in water vapor concentration Δt = change in time

Delta T dynamic diffusion porometer

Null balance porometer: LI-1600

How does the SC-1 measure stomatal conductance?

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 cultivars Evaluating 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 efficiency Chitosan induces stomatal closure Leaf porometer used to evaluate effectiveness 26 – 43% less water used while maintaining biomass production

Case Study: Stress in wine grapes

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

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 -0.085 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