plant water relations
DESCRIPTION
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 PresentationTRANSCRIPT
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Plant water relationsDouglas R. Cobos, Ph.D.
Decagon Devices and Washington State University
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Plants fundamental dilemma
Biochemistry requires a highly hydrated environment (> -3 MPa)
Atmospheric environment provides CO2 and light but is dry (-100 MPa)
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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)
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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
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Indicators of plant water stress
Soil water potential
Leaf stomatal conductance
Leaf water potential
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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
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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)
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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
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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
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Two commercial pressure chambers
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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: -0.1 MPa to -300 MPa
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Pressure chamber – in situ comparison
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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
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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
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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
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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
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Stomatal conductance Can be good indicator of plant water status Many plants regulate water loss through stomatal
conductance
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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 leafa air
aL
aL
RRCCE
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Boundary layer resistance of the leaf
stomatal resistance of the leafrvs
Cvt
Cva
rva
Cvs
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Do stomata control leaf water loss?
Still air: boundary layer resistance controls
Moving air: stomatal resistance controls
Bange (1953)
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Obtaining resistances (or conductances)
Boundary layer conductance depends on wind speed, leaf size and diffusing gas
Stomatal conductance is measured with a leaf porometer
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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
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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:
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Delta T dynamic diffusion porometer
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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
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Steady state porometerleaf
sensors
Teflon filter
R2
R1
h1
h2
1212
1
2
21
1
1
1 RRhhhR
RCC
RRCC
vs
vv
vs
vvL
atmosphere
Rvs = stomatal resistance to vapor flow
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Decagon steady state porometer
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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
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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
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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.
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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
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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
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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
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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
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Case Study 4: Stress in wine 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)
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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
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Applications of soil water potential Irrigation management
Deficit irrigation Lower yield but higher quality fruit Wine grapes Fruit trees
No water stress – optimal yield
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Appendix: Lower limit water potentials Agronomic Crops
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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
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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