1. Decagon Devices and Washington State University
Plant water relations
Gaylon 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 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 with sandpaper
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?
Bange (1953)
Still air: boundary layer resistance controls
Moving air: stomatal resistance controls

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. Steady state porometer
leaf R1 h1 R2
sensors h2
Teflon
filter
atmosphere
Rvs = stomatal resistance to vapor flow

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. What can I do with a porometer?
Water use and water balance
Use conductance with Fick’s law to determine crop transpiration rate
Develop crop cultivars for dry climates/salt affected soils
Determine plant water stress in annual and perennial species
Study effects of environmental conditions
Schedule irrigation
Optimize herbicide uptake
Study uptake of ozone and other pollutants

33. 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

34. 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

35. Case Study 4: Stress in wine grapes

36. Indicator #3: Soil water potential
Defines the supply part of the supply/demand function of water stress
“field capacity” = MPa
“permanent wilting point” -1.5 MPa
We discussed how to measure soil water potential earlier

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

38. Appendix: Lower limit water potentials Agronomic Crops

39. 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

40. 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