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Transpiration

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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Transpiration

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  1. Transpiration

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. Transpiration rates for corn

  8. 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

  9. Water use efficiency (cont.) • water consumed can be expressed as: • transpiration • evapotranspiration • total water supply • time scale can be: • instantaneous • seasonal • annual

  10. 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

  11. 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

  12. 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

  13. 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

  14. 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.

  15. 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

  16. Yield versus evapotranspiration • plot Y versus ET • slope is transpirational water use efficiency • intercept is an estimate of evaporative losses

  17. 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.

  18. Pairwise growing season rainfall amount and wheat grain yield for 93 years across 18 counties in central-western Oklahoma. (Patrignani et al., 2012)

  19. Reading assignment • Soil temperature and heat flow • p. 215 - 218

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