Plant water relations
Douglas R. Cobos, 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
Leaf/stem water potential
Soil water potential
Indicator #1: Plant 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
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)
Original indicator of leaf water
potential
Measuring plant 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 semipermeable barrier
 Pressure chamber
 Vapor equilibration methods vapor equilibrium with sample
 Thermocouple psychrometer
 Dew point potentiameter
Measure humidity air in
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
  leaf  PPressure
Chamber
Two commercial pressure chambers
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: -0.05 MPa to -300 MPa
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
In situ stem water potential
psychrometer

Ψstem less dynamic than Ψleaf




May be better indicator of plant water status
Continuous measurement
Thermal insulation needed
Range similar to leaf psychrometer
Pressure chamber vs. in situ comparison
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 (fruit trees)
 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
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)
Stomatal conductance
 Can be good indicator of plant water status
 All plants regulate water loss through stomatal
conductance
Do stomata control leaf water loss?
 Still air: boundary layer
resistance controls
water loss
 Moving air: stomatal
resistance controls
water loss
Bange (1953)
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:
Cv
t
ΔCv = change in water vapor concentration
Δt = change in time
Delta T dynamic diffusion porometer
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
How does the SC-1 measure
stomatal conductance?
CLeaf
Fvapor  g d 2 C 1  C 2 
gs
Leaf
D1
gd1
C1
D2
Humidity Sensors
gd2
C2
Filter
gs 
C
1
leaf
 C1 
Fvapor
More information on the theory of operation is available.

1
g d1
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: 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
Case Study: Stomatal conductance vs. leaf
water potential in grapes
0.0
Mid-day
Leaf Water Potential (bars)
-2.0
-4.0
-6.0
-8.0
-10.0
-12.0
-14.0
-16.0
-18.0
-20.0
y = 0.0204x - 12.962
R² = 0.5119
500
450
400
350
300
250
200
150
100
50
0
Stomatal Conductance (mmol m -2 s-1)
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
Applications of soil water potential
 Irrigation management
 Deficit irrigation
 Lower yield but higher quality fruit
 Wine grapes
 Fruit trees
 No water stress – optimal yield
Lower limit water potentials Agronomic Crops
Take-home points
 Three primary methods to asses plant water
status
 Plant water potential
 Stomatal conductance
 Soil water potential
 Each provides slightly different information, but
all have their place in research
Appendix: Soil and Plant water potential measurement
technique matrix
Method
Measures
Principle
Range (MPa)
+0.1 to -0.085
Precautions
Tensiometer
(liquid equilibration)
soil matric potential
internal suction balanced
against matric potential
through porous cup
cavitates and must be refilled if
minimum range is exceeded
Pressure chamber
(liquid equilibration)
water potential of plant
tissue (leaf/stem)
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
Measures rh of vapor
equilibrated with sample,
using wet bulb depression.
-0.1 to -5
Must avoid sample temperature drift
during measurement
in situ leaf
psychrometer
(vapor equilibration)
leaf water potential
same as in situ soil
psychrometer
-0.1 to -5
same as soil psychrometer; should be
shaded from direct sun; must have
good seal to leaf
In situ stem
psychrometer
(vapor equilibration)
stem water potential
same as in situ soil
psychrometer
-0.1 to -5
Same as soil psychrometer; must
completely insulate from temperature
change
Dewpoint hygrometer
(vapor equilibration)
matric plus osmotic
potential of soils, leaves,
solutions, other
materials
Measures rh of vapor
equilibrated with sample,
using dew point technique.
-0.1 to -300
laboratory instrument; sensitive to
changes in ambient room temperature.
Heat dissipation
(solid equilibration)
soil matric potential
ceramic thermal properties
empirically related to matric
potential
-0.01 to -30
Needs individual calibration;
accuracy not good pas -0.5 MPa
Electrical properties
(solid equilibration)
soil matric potential
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