Control of Stomata1 Conductance
24 1
and
The denominator reduces the value of Vm,25 or Rd,25 rapidly at
temperatures above 41 or 55" C, respectively.
It is, of course, not practical to use this photosynthesis model for
hand calculations. It can be a simple matter, though, to make a computer
program which solves these equations. Table 14.1 gives values for the
model parameters, as supplied by Collatz et al. (1991).
Figure 14.5 shows the temperature response predicted by the model,
Fig. 14.6 shows the light response, and Fig. 14.7 shows the CO2 response.
It is important to note, in the table and in all of the figures, that our values
are per unit total leaf surface area. Collatz et al. (1991) and most other
researchers compute photosynthesis on a per unit projected leaf area basis.
Our values are therefore half those typically found in the photosynthesis
literature.
14.9 Control of Stomatal Conductance
We already mentioned the observation of Wong et al. (1979), who found
that stomata tend to open or close to maintain a constant internal C02
concentration. Stomata must therefore be sensitive to changes in environmental C02 concentration, opening when it decreases and closing when
it increases. Others have observed that stomata also open or close in response to the vapor deficit of the air (Lange, et al. 1971). High vapor
TABLE 14.1. Values of parameters and constants used in the photosynthesis model. The values of b, Rd, and Vm are half those given by Collatz
et al. (1991) because we assume the surface area of the leaf to be the total
surface area, rather than the projected area.
Symbol Value
Units
Temp. Description
Coef.
b
0.003 mol rn-' s-'
intercept, B-B model
em
0.08
moVmol
maximum quantum efficiency
K c
300
pmol/mol
0.074 Michaelis constant for C02
KO
300
mmoVmol
0.01 8 inhibition constant for 0 2
m
5.6
slope parameter for B-B model
Rd
1.5
pmol rn-' s-I
0.088 day respiration
v",
100
pmol m-2 s-'
0.088 Rubisco capacity
CC,
340
pmol/mol
ambient C02 mole fraction
COO
210
mmoVmol
oxygen mole fraction
a~
0.8
leaf absorptivity for PAR
k?
0.98
colimitation factor
t
2.6
mmoVpmol
-0.056 C02/02 specificity ratio
0
0.95
colimitation factor
24 1
and
The denominator reduces the value of Vm,25 or Rd,25 rapidly at
temperatures above 41 or 55" C, respectively.
It is, of course, not practical to use this photosynthesis model for
hand calculations. It can be a simple matter, though, to make a computer
program which solves these equations. Table 14.1 gives values for the
model parameters, as supplied by Collatz et al. (1991).
Figure 14.5 shows the temperature response predicted by the model,
Fig. 14.6 shows the light response, and Fig. 14.7 shows the CO2 response.
It is important to note, in the table and in all of the figures, that our values
are per unit total leaf surface area. Collatz et al. (1991) and most other
researchers compute photosynthesis on a per unit projected leaf area basis.
Our values are therefore half those typically found in the photosynthesis
literature.
14.9 Control of Stomatal Conductance
We already mentioned the observation of Wong et al. (1979), who found
that stomata tend to open or close to maintain a constant internal C02
concentration. Stomata must therefore be sensitive to changes in environmental C02 concentration, opening when it decreases and closing when
it increases. Others have observed that stomata also open or close in response to the vapor deficit of the air (Lange, et al. 1971). High vapor
TABLE 14.1. Values of parameters and constants used in the photosynthesis model. The values of b, Rd, and Vm are half those given by Collatz
et al. (1991) because we assume the surface area of the leaf to be the total
surface area, rather than the projected area.
Symbol Value
Units
Temp. Description
Coef.
b
0.003 mol rn-' s-'
intercept, B-B model
em
0.08
moVmol
maximum quantum efficiency
K c
300
pmol/mol
0.074 Michaelis constant for C02
KO
300
mmoVmol
0.01 8 inhibition constant for 0 2
m
5.6
slope parameter for B-B model
Rd
1.5
pmol rn-' s-I
0.088 day respiration
v",
100
pmol m-2 s-'
0.088 Rubisco capacity
CC,
340
pmol/mol
ambient C02 mole fraction
COO
210
mmoVmol
oxygen mole fraction
a~
0.8
leaf absorptivity for PAR
k?
0.98
colimitation factor
t
2.6
mmoVpmol
-0.056 C02/02 specificity ratio
0
0.95
colimitation factor