Optimum Leaf Form
243
Intercellular C62 Concentration (p moilmol)
FIGURE 14.7. Photosynthesis as a function of C02 concentration at three light
levels.
deficits (low humidities) tend to close stomata. It is not, of course, the
external environment that provides the direct response, but the internal
environment of the leaf. This, in turn, is controlled by external environment as well as the transpiration and assimilation rate of the leaf. Collatz
et al. (1991) were able to combine all of these effects into an empirical
model that looks relatively simple:
where h, and Ccs are the humidity and C02 concentration at the leaf
surface, A, is the net assimilation rate, and m and b are constants determined from gas exchange studies. While Eq. (14.28) looks simple, it
is in reality quite complex because the entire photosynthesis model determines the value of A,; the air vapor pressure, leaf temperature, and
transpiration rate determine the value of the surface humidity, and the
ambient CO;! concentration, assimilation rate, and boundary layer conductance determine Ccs. Since conductance determines assimilation rate,
and assimilation rate determines conductance, Eqs. (14.14), (14.24), and
(14.28) (with all of the equations that go into them) must be solved simultaneously to determine the assimilation of the leaf. Again, this is not
something that can be done easily by hand, but can be done with a computer. The interesting thing is that, after all of the work of solving these
equations, the results are almost identical to those shown in Figs. 14.5 to
14.7. Using the values in Table 14.1, the internal C02 concentration is
controlled at 250 pmollmol.
243
Intercellular C62 Concentration (p moilmol)
FIGURE 14.7. Photosynthesis as a function of C02 concentration at three light
levels.
deficits (low humidities) tend to close stomata. It is not, of course, the
external environment that provides the direct response, but the internal
environment of the leaf. This, in turn, is controlled by external environment as well as the transpiration and assimilation rate of the leaf. Collatz
et al. (1991) were able to combine all of these effects into an empirical
model that looks relatively simple:
where h, and Ccs are the humidity and C02 concentration at the leaf
surface, A, is the net assimilation rate, and m and b are constants determined from gas exchange studies. While Eq. (14.28) looks simple, it
is in reality quite complex because the entire photosynthesis model determines the value of A,; the air vapor pressure, leaf temperature, and
transpiration rate determine the value of the surface humidity, and the
ambient CO;! concentration, assimilation rate, and boundary layer conductance determine Ccs. Since conductance determines assimilation rate,
and assimilation rate determines conductance, Eqs. (14.14), (14.24), and
(14.28) (with all of the equations that go into them) must be solved simultaneously to determine the assimilation of the leaf. Again, this is not
something that can be done easily by hand, but can be done with a computer. The interesting thing is that, after all of the work of solving these
equations, the results are almost identical to those shown in Figs. 14.5 to
14.7. Using the values in Table 14.1, the internal C02 concentration is
controlled at 250 pmollmol.