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
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
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
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:
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
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
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
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