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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 CO 2 and light but is dry (-100 MPa ). Water potential. - PowerPoint PPT Presentation
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Plant water relationsDouglas 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
High water potential
Boundary layer conductance to water vapor flow
Root and xylem conductance to liquid water flow
Stomatal conductance to water vapor flow
Indicators of plant water stress
Soil water potential
Leaf stomatal conductance
Leaf/stem 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 semi-permeable barrier Pressure chamber
Vapor equilibration methods - Measure humidity air in vapor equilibrium with sample Thermocouple psychrometer Dew point potentiameter
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
Chamber PressurePleaf
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 withsandpaper
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
tCv
ΔCv = change in water vapor concentrationΔt = change in time
Stomatal conductance is proportional to:
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?
1
1 11
dvapor
leafs
gFCCg
212 CCgF dvapor
More information on the theory of operation is available.
Leaf
Humidity Sensors Humidity Sensors
Filter
CLeaf
D1
C1
C2
D2
gs
gd1
gd2
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 wheatResearchers using steady state
porometer to create drought resistant wheat cultivarsEvaluating physiological response to
drought stress (stomatal closing)Selecting individuals with optimal
response
Case Study: Stomatal conductance vs. leaf water potential in grapes
y = 0.0204x - 12.962R² = 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
Le
af W
ater
Pot
entia
l (ba
rs)
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 irrigationLower yield but higher quality fruitWine grapesFruit 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
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 (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
Appendix: Soil and Plant water potential measurement technique matrix