Transcript Transpiration - Soil Physics

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
Stoma in a tomato leaf shown via
colorized scanning electron microscope
image.
Images from http://en.wikipedia.org/wiki/Stoma
A stoma in cross section
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
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
M c Pa  Pi  ra  rs 
WUE ( A, T , i ) L 
'
'
M w e *L e  ra  rs

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 transpiration
Pa
WUE ( A, T , i ) L  1.6c
e *L e
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
WUE ( B, T , d ) 
kd
ea*  e
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, 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
kd
E 

WUE (Y , ET , s )  1 
H *
 ET  ea  e
E = evaporation from the soil, plant, and
residue
ET = evapotranspiration
H = harvest index (yield/biomass)
assumes relatively constant seasonal conditions
Yield versus evapotranspiration
Y  ET  E H
kd
e e
*
a
• 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.
4000
3500
-1
Grain Yield (kg ha )
3000
2500
2000
1500
1000
500
0
0
100
200
300
400
500
600
700
800
Growing Season Rainfall (mm)
Pairwise growing season rainfall amount and wheat grain yield for 93 years across 18 counties in
central-western Oklahoma. (Patrignani et al., 2012)
Reading assignment
• Soil temperature and heat flow
– p. 215 - 218