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

Images from Stoma in a tomato leaf shown via colorized scanning electron microscope image. A stoma in cross section

The soil-plant-atmosphere continuum 1 bar = 100 kPa = 1020 cm H 2 O -100 bar = 93% relative humidity at 20  C 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 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 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

Transpiration rates for corn

Water use efficiency a ratio of biomass accumulation to water consumed during a given time span accumulation can be expressed as: – CO 2 assimilation – above-ground biomass – harvested biomass

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

Reading assignment Sinclair, T.R., C.B. Tanner, and J.M. Bennet Water-use efficiency in crop production. BioScience 34:

Ratio of assimilation to transpiration M c = mole weight CO 2 M w = mole weight of H 2 O e* L = saturation vapor pressure at leaf temperature e = vapor pressure of the atmosphere P a = partial pressure of CO 2 in atmosphere P i = partial pressure of CO 2 in leaf r a = aerodynamic boundary layer resistance r s = stomatal resistance prime notations signify resistance for CO 2 rather than H 2 0

Ratio of assimilation to transpiration e* L = saturation vapor pressure at leaf temperature e = vapor pressure of the atmosphere P a = partial pressure of CO 2 in atmosphere c = 1-P i /P a = 0.3 for C 3 plants and 0.7 for C 4 plants

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 k d = constant for a given species at fixed P a

Tolk, J.A., and T.A. Howell Transpiration and yield relationships of grain sorghum grown in a field environment. Agron. J. 101:

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 plot Y versus ET slope is transpirational water use efficiency intercept is an estimate of evaporative losses

Hochman, Z., D. Holzworth, and J.R. Hunt Potential to improve on-farm wheat yield and WUE in Australia. Crop and Pasture Science 60:

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

Soil temperature and heat flow – p Reading assignment