Plant water relations

1. Plant water relations Douglas R. Cobos, 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. 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

7. 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)

8. 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

9. 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

10. Two commercial pressure chambers

11. 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

12. Pressure chamber – in situ comparison

13. 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

14. 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

15. 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

16. 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

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

18. 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

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

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

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

22. 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

23. 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

24. Delta T dynamic diffusion porometer

25. 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

26. How does the SC-1 measure stomatal conductance? More information on the theory of operation is available.

27. Decagon steady state porometer

28. 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

29. 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

30. 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.

31. 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

32. 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

33. 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

34. Case Study 4: Stress in wine grapes

35. 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

36. Applications of soil water potential • Irrigation management • Deficit irrigation • Lower yield but higher quality fruit • Wine grapes • Fruit trees • No water stress – optimal yield

37. Lower limit water potentials Agronomic Crops

38. 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

39. Appendix: Water potential measurement technique matrix