Plant water relations

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Plant water relations

• Douglas R. Cobos, Ph.D.
• Decagon Devices and Washington State University

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Plants fundamental dilemma

• Biochemistry requires a highly hydrated
  environment (gt -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
Boundary layer conductance to water vapor flow
Stomatal conductance to water vapor flow
Root conductance to liquid water flow
High water potential
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Indicators of plant water stress
Leaf stomatal conductance
Soil water potential
Leaf water potential
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Indicator 1 Leaf water potential

• ?leaf 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, ?cell ?leaf

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Leaf water potential

• Turgid leaf ?leaf ?cell turgor pressure
  (?p) osmotic potential (?o) of water inside
  cell
• Flaccid leaf ?leaf ?cell ?o (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 (?leaf)
• 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

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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 ?leaf
• 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 ?leaf
• 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 leaf
• a air

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stomatal resistance of the leaf
Boundary layer resistance of the leaf
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Do stomata control leaf water loss?
Bange (1953)

• Still air boundary layer resistance controls
• Moving air stomatal resistance controls

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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

Stomatal conductance is proportional to
?Cv change in water vapor concentration ?t
change in time
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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 porometer
leaf
R1
h1
R2
sensors
h2
Teflon filter
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 Ficks 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
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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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Appendix Water potential measurement technique
matrix
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