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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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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


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 microscope

  • 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 microscope

  • 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 microscope

  • 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 microscope

  • 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.) microscope

  • water consumed can be expressed as:

    • transpiration

    • evapotranspiration

    • total water supply

  • time scale can be:

    • instantaneous

    • seasonal

    • annual


Reading assignment
Reading assignment microscope

  • 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 microscope

  • 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 microscope

  • 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 microscope

  • 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


  • Tolk microscope , 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 microscope

    • 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 microscope

    • plot Y versus ET

    • slope is transpirational water use efficiency

    • intercept is an estimate of evaporative losses


    Hochman microscope , 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.


    Pairwise microscope 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 microscope

    • Soil temperature and heat flow

      • p. 215 - 218


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