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

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

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 H2O

• -100 bar = 93% relative humidity at 20C

• -1000 bar = 48% relative humidity at 20C

• 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:– CO2 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. 1984.

Water-use efficiency in crop production. BioScience 34:36-40.

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

Ratio of assimilation to transpiration

''*

),,(sa

sa

L

ia

w

cL rr

rreePP

MMiTAWUE

Mc = mole weight CO2

Mw = mole weight of H2Oe*L = saturation vapor pressure at leaf temperaturee = vapor pressure of the atmospherePa = partial pressure of CO2 in atmospherePi = partial pressure of CO2 in leafra = aerodynamic boundary layer resistancers = stomatal resistanceprime notations signify resistance for CO2 rather than H20

Ratio of assimilation to transpiration

eePciTAWUEL

aL

*6.1),,(

e*L = saturation vapor pressure at leaf temperature

e = vapor pressure of the atmospherePa = partial pressure of CO2 in atmospherec = 1-Pi/Pa = 0.3 for C3 plants and 0.7 for C4

plants

Ratio of biomass to transpiration

eekdTBWUEa

d

*),,(

B = above-ground biomasse*a = saturation vapor pressure at air temperaturee = vapor pressure of the atmosphere

overbar represents daily mean during periods of transpiration

kd = constant for a given species at fixed Pa

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

eekH

ETEsETYWUE

a

d

*1),,(

E = evaporation from the soil, plant, and residue

ET = evapotranspirationH = harvest index (yield/biomass)

assumes relatively constant seasonal conditions

Yield versus evapotranspiration

ee

kHEETYa

d

*

• 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. 2009. Potential to improve on-farm wheat yield and WUE in Australia. Crop and Pasture Science 60:708-716.

0 100 200 300 400 500 600 700 8000

500

1000

1500

2000

2500

3000

3500

4000

Growing Season Rainfall (mm)

Gra

in Y

ield

(kg

ha-1

)

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. 215 - 218

Reading assignment