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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.

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plant water relations

Plant water relations

Douglas R. Cobos, Ph.D.

Decagon Devices and Washington State University

plants fundamental dilemma
Plants fundamental dilemma
  • Biochemistry requires a highly hydrated environment (> -3 MPa)
  • Atmospheric environment provides CO2 and light but is dry (-100 MPa)
water potential
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
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
Indicators of plant water stress

Leaf stomatal conductance

Soil water potential

Leaf/stem water potential

indicator 1 plant 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
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)
measuring plant 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
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
vapor equilibration chilled mirror dewpoint hygrometer
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
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
In situ stem water potential psychrometer
  • Ψstemless dynamic than Ψleaf
    • May be better indicator of plant water status
  • Continuous measurement
  • Thermal insulation needed
  • Range similar to leaf psychrometer
leaf water potential as an indicator of plant water status
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
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
Stomatal conductance
  • Can be good indicator of plant water status
  • All plants regulate water loss through stomatal conductance
do stomata control leaf water loss
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
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
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:

ΔCv = change in water vapor concentration

Δt = change in time

steady state 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
How does the SC-1 measure stomatal conductance?

More information on the theory of operation is available.

environmental effects on stomatal conductance light
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
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
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 co 2
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
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
indicator 3 soil water potential
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
Applications of soil water potential
  • Irrigation management
    • Deficit irrigation
      • Lower yield but higher quality fruit
      • Wine grapes
      • Fruit trees
    • No water stress – optimal yield
take home points
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