This document discusses plant water relations and various methods for measuring water potential and stress indicators in plants and soil. It describes how plants face a dilemma between their need for a hydrated environment and the dry atmospheric conditions. Water potential describes the availability of water, and it defines water flow through the soil-plant-atmosphere continuum. Key indicators of plant water stress discussed are leaf water potential, stomatal conductance, and soil water potential. Methods for measuring these indicators like pressure chambers, porometers, and soil moisture sensors are also outlined.

1. Plant water relations
Dr. Umair Riaz
Soil and Water Testing Laboratory for
Research Bahawalpur-63100, Pakistan
umairbwp3@gmail.com

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

5. Indicators of plant water stress
Soil water potential
Leaf stomatal conductance
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
ChamberPressurePleaf 

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
aL
aL
RR
CC
E




19. Boundary layer resistance
of the leaf
stomatal resistance of the leaf
rvsCvtCva
rvaCvs

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
t
Cv


ΔCv = change in water vapor concentration
Δt = change in time
Stomatal conductance is proportional to:

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
sensors
Teflon
filter
R2
R1h1
h2
12
12
1
2
21
1
1
1
RR
hh
h
R
R
CC
RR
CC
vs
vv
vs
vvL








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
y = 0.0204x - 12.962
R² = 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
100
150
200
250
300
350
400
450
500
Mid-day
LeafWaterPotential(bars)
Stomatal Conductance (mmol m-2 s-1)

36. 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

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. 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