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Transpiration. Transpiration. the release of water vapor by plants to the atmosphere “is not an essential or an active physiological function of plants” a largely passive response to the “unquenchably thirsty” atmosphere.

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Presentation Transcript
transpiration1
Transpiration
  • the release of water vapor by plants to the atmosphere
  • “is not an essential or an active physiological function of plants”
  • a largely passive response to the “unquenchably thirsty” atmosphere
slide4
Stoma in a tomato leaf shown via colorized scanning electron microscope image.

A stoma in cross section

Images from http://en.wikipedia.org/wiki/Stoma

the soil plant atmosphere continuum
The soil-plant-atmosphere continuum
  • 1 bar = 100 kPa = 1020 cm H2O
  • -100 bar = 93% relative humidity at 20C
  • -1000 bar = 48% relative humidity at 20C
  • The largest drop in water potential generally occurs between the leaves and the atmosphere
water status of plants
Water status of plants
  • If transpiration exceeds root water uptake
    • the plant begins to wilt
    • the water potential inside the plant drops
    • transpiration decreases
    • common under high evaporative demand
  • If high evaporative demand is relieved
    • root water uptake can exceed transpiration
    • plant turgor is restored
root water uptake
Root water uptake
  • limited by hydraulic conductivity, or
  • limited by the water potential gradient between soil and root
  • root water uptake lowers the conductivity and increases the gradient, until
  • the soil adjacent to the root is in equilibrium with the root
  • then the conductivity and gradients both decrease and uptake declines
water use efficiency
Water use efficiency
  • a ratio of biomass accumulation to water consumed during a given time span
  • accumulation can be expressed as:
    • CO2 assimilation
    • above-ground biomass
    • harvested biomass
water use efficiency cont
Water use efficiency (cont.)
  • water consumed can be expressed as:
    • transpiration
    • evapotranspiration
    • total water supply
  • time scale can be:
    • instantaneous
    • seasonal
    • annual
reading assignment
Reading assignment
  • Sinclair, T.R., C.B. Tanner, and J.M. Bennet. 1984. Water-use efficiency in crop production. BioScience 34:36-40.

http://www.jstor.org/stable/10.2307/1309424

ratio of assimilation to transpiration
Ratio of assimilation to transpiration
  • Mc = mole weight CO2
  • Mw = mole weight of H2O
  • e*L = saturation vapor pressure at leaf temperature
  • e = vapor pressure of the atmosphere
  • Pa = partial pressure of CO2 in atmosphere
  • Pi = partial pressure of CO2 in leaf
  • ra = aerodynamic boundary layer resistance
  • rs = stomatal resistance
  • prime notations signify resistance for CO2 rather than H20
ratio of assimilation to transpiration1
Ratio of assimilation to transpiration
  • e*L = saturation vapor pressure at leaf temperature
  • e = vapor pressure of the atmosphere
  • Pa = partial pressure of CO2 in atmosphere
  • c = 1-Pi/Pa = 0.3 for C3 plants and 0.7 for C4 plants
ratio of biomass to transpiration
Ratio of biomass to transpiration
  • B = above-ground biomass
  • e*a = saturation vapor pressure at air temperature
  • e = vapor pressure of the atmosphere
      • overbar represents daily mean during periods of transpiration
  • kd = constant for a given species at fixed Pa
slide17
Tolk, J.A., and T.A. Howell. 2009. Transpiration and yield relationships of grain sorghum grown in a field environment. Agron. J. 101:657-662.
ratio of yield to evapotranspiration
Ratio of yield to evapotranspiration
  • E = evaporation from the soil, plant, and residue
  • ET = evapotranspiration
  • H = harvest index (yield/biomass)
  • assumes relatively constant seasonal conditions
yield versus evapotranspiration
Yield versus evapotranspiration
  • plot Y versus ET
  • slope is transpirational water use efficiency
  • intercept is an estimate of evaporative losses
slide20
Hochman, Z., D. Holzworth, and J.R. Hunt. 2009. Potential to improve on-farm wheat yield and WUE in Australia. Crop and Pasture Science 60:708-716.
slide21
Pairwise growing season rainfall amount and wheat grain yield for 93 years across 18 counties in central-western Oklahoma. (Patrignani et al., 2012)
reading assignment1
Reading assignment
  • Soil temperature and heat flow
    • p. 215 - 218
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