transpiration
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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. - PowerPoint PPT PresentationTRANSCRIPT
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