Simple Assimilation Models
237
respiration and the composition of dry matter (fraction of carbohydrates,
proteins, or lipids). The most stable conversion efficiencies are likely to
be mol C02 (mol photons)-'. Typical daily conversion efficiencies in
these units are 0.01 to 0.03 mol C02 (mol photons)-'. This is sometimes
referred to as canopy light use efficiency. The conversion efficiency approach is used to estimate daily, monthly, or seasonal assimilation. One
of the factors that is known to affect conversion efficiency (e) on a daily
basis is the fraction of incident radiation that is diffuse versus solar beam;
with diffuse radiation being more efficient.
Monteith and others have pointed out that using accumulated dry mattter and intercepted radiation amounts to relating two variables that are
accumulated sums. Summing any two sets of numbers, even randomnumbers, induces a high correlation, similar to that shown in Fig. 14.4. The
fact that we get nice straight lines is therefore not necessarily an indication of a causal relationship between the two quantities. It is known from
other information, though, that light and photosynthesis are causally related, so this induced correlation may add, rather than detract from the
model since it makes the model very robust. The real question is whether
the model is useful for prediction of dry matter production. This depends
on how conservative e is. A number of experiments have shown that e is
very conservative in situations where water, nutrients, and temperature do
not limit plant growth. Equation (14.13) is therefore useful for predicting
maximum productivity. When stresses limit growth, it is often possible
to quantify their effect either in terms of a reduction in conversion efficiency, e, or a decrease in interception, fs. This allows experiments
carried out under different conditions of light availability to be compared
or normalized.
The Monteith model focuses on light as the limiting substrate for
photosynthesis. Another simple model can be derived by considering
gas exchange. The net carbon assimilation for a leaf can be computed
from:
where gc is the conductance of the boundary layer and surface (stomata) for C02, Cca is the atmospheric C02 concentration (around
350 pmoVmol) and Cci is the C02 concentration in the intercellular
spaces of the leaf. The subscript n on the assimilation rate means the
net assimilation rate. Wong, et al. (1979) found that Cci is maintained
at a fairly constant value in light. Genotypes vary in the values they
maintain, but the main variation is between C3 and C4 species. In C3
species values around 280 pmol/mol are common, while in C4 the values
are around 130 pmoVmo1. Photorespiration therefore maintains a much
larger intercellular concentration in C3 leaves.
Water vapor diffuses through the same stornatal pores as C02, so any
assimilation is accompanied by transpiration. The rate of transpiration
can be computed from Eq. (14.10), (with the As canceled). Taking the
237
respiration and the composition of dry matter (fraction of carbohydrates,
proteins, or lipids). The most stable conversion efficiencies are likely to
be mol C02 (mol photons)-'. Typical daily conversion efficiencies in
these units are 0.01 to 0.03 mol C02 (mol photons)-'. This is sometimes
referred to as canopy light use efficiency. The conversion efficiency approach is used to estimate daily, monthly, or seasonal assimilation. One
of the factors that is known to affect conversion efficiency (e) on a daily
basis is the fraction of incident radiation that is diffuse versus solar beam;
with diffuse radiation being more efficient.
Monteith and others have pointed out that using accumulated dry mattter and intercepted radiation amounts to relating two variables that are
accumulated sums. Summing any two sets of numbers, even randomnumbers, induces a high correlation, similar to that shown in Fig. 14.4. The
fact that we get nice straight lines is therefore not necessarily an indication of a causal relationship between the two quantities. It is known from
other information, though, that light and photosynthesis are causally related, so this induced correlation may add, rather than detract from the
model since it makes the model very robust. The real question is whether
the model is useful for prediction of dry matter production. This depends
on how conservative e is. A number of experiments have shown that e is
very conservative in situations where water, nutrients, and temperature do
not limit plant growth. Equation (14.13) is therefore useful for predicting
maximum productivity. When stresses limit growth, it is often possible
to quantify their effect either in terms of a reduction in conversion efficiency, e, or a decrease in interception, fs. This allows experiments
carried out under different conditions of light availability to be compared
or normalized.
The Monteith model focuses on light as the limiting substrate for
photosynthesis. Another simple model can be derived by considering
gas exchange. The net carbon assimilation for a leaf can be computed
from:
where gc is the conductance of the boundary layer and surface (stomata) for C02, Cca is the atmospheric C02 concentration (around
350 pmoVmol) and Cci is the C02 concentration in the intercellular
spaces of the leaf. The subscript n on the assimilation rate means the
net assimilation rate. Wong, et al. (1979) found that Cci is maintained
at a fairly constant value in light. Genotypes vary in the values they
maintain, but the main variation is between C3 and C4 species. In C3
species values around 280 pmol/mol are common, while in C4 the values
are around 130 pmoVmo1. Photorespiration therefore maintains a much
larger intercellular concentration in C3 leaves.
Water vapor diffuses through the same stornatal pores as C02, so any
assimilation is accompanied by transpiration. The rate of transpiration
can be computed from Eq. (14.10), (with the As canceled). Taking the