Simple Assimilation Models
14.6 Photosynthesis
The assimilation of carbon by leaves follows the general reaction
c02 + H20 + light -+ CHzO + 0 2
where CH20 is intended to represent carbohydrate such as sucrose or
starch. Assimilation involves many chemical reactions which occur inside
the chloroplasts in leaf mesophyll cells and are catalyzed by numerous
enzymes. The substrates for assimilation are C02, water, and light. The
carbon dioxide comes from the atmosphere and diffuses into the leaf
through the stomata. The water is available in excess within the leaf, since
the biochemical reactions occur within the highly hydrated cell. Light
is from the sun and is the photosynthetically active radiation PAR) discussed in Ch. 10. Besides carbohydrate, oxygen is an important byproduct
of photosynthesis.
The leaf environment supplies the C02 and the light for photosynthesis and controls the temperature of the leaf. The enzymes that catalyze the
photosynthetic reactions are all strongly temperature dependent, so leaf
temperature can play an important role in determining the assimilation
rate for a leaf. We are primarily interested in knowing how assimilation responds to environment, but this requires some understanding of
the biochemistry, since the biochemistry and the environment interact so
strongly in determining how much assimilation can occur.
Most plant species fall into one of two major groupings with respect
to carbon assimilation. In the most common group the primary product
of photosynthesis is a three carbon sugar, so these species are called C3.
A less common photosynthetic mechanism is present in tropical grasses
such as maize and sugar cane. In these, the fist product of photosynthesis
is a four carbon compound. These species are therefore called C4. Carbon
dioxide and oxygen compete for the same enzyme in C3 species resulting
in the loss of some of the C02 in a process called photorespiration. The
fixing of C02 into the four carbon compound in C4 species concentrates
the carbon dioxide and minimizes photorespiration. The concentration
of C02 inside the stomata of leaves is therefore much lower in C4 than
C3 species typically resulting in higher photosynthetic rates and higher
water use efficiencies.
14.7 Simple Assimilation Models
Two general approaches have been used to derive models relating
assimilation to environment. One is more empirical and the other
more mechanistic. Both are useful for understanding and predicting
leaf-environment interaction.
The simpler model applies mainly to plant communities. Monteith
(1977) observed that when biomass accumulation by a plant community
is plotted as a function of the accumulated solar radiation intercepted by
the community, the result was a straight line. Figure 14.4 shows
14.6 Photosynthesis
The assimilation of carbon by leaves follows the general reaction
c02 + H20 + light -+ CHzO + 0 2
where CH20 is intended to represent carbohydrate such as sucrose or
starch. Assimilation involves many chemical reactions which occur inside
the chloroplasts in leaf mesophyll cells and are catalyzed by numerous
enzymes. The substrates for assimilation are C02, water, and light. The
carbon dioxide comes from the atmosphere and diffuses into the leaf
through the stomata. The water is available in excess within the leaf, since
the biochemical reactions occur within the highly hydrated cell. Light
is from the sun and is the photosynthetically active radiation PAR) discussed in Ch. 10. Besides carbohydrate, oxygen is an important byproduct
of photosynthesis.
The leaf environment supplies the C02 and the light for photosynthesis and controls the temperature of the leaf. The enzymes that catalyze the
photosynthetic reactions are all strongly temperature dependent, so leaf
temperature can play an important role in determining the assimilation
rate for a leaf. We are primarily interested in knowing how assimilation responds to environment, but this requires some understanding of
the biochemistry, since the biochemistry and the environment interact so
strongly in determining how much assimilation can occur.
Most plant species fall into one of two major groupings with respect
to carbon assimilation. In the most common group the primary product
of photosynthesis is a three carbon sugar, so these species are called C3.
A less common photosynthetic mechanism is present in tropical grasses
such as maize and sugar cane. In these, the fist product of photosynthesis
is a four carbon compound. These species are therefore called C4. Carbon
dioxide and oxygen compete for the same enzyme in C3 species resulting
in the loss of some of the C02 in a process called photorespiration. The
fixing of C02 into the four carbon compound in C4 species concentrates
the carbon dioxide and minimizes photorespiration. The concentration
of C02 inside the stomata of leaves is therefore much lower in C4 than
C3 species typically resulting in higher photosynthetic rates and higher
water use efficiencies.
14.7 Simple Assimilation Models
Two general approaches have been used to derive models relating
assimilation to environment. One is more empirical and the other
more mechanistic. Both are useful for understanding and predicting
leaf-environment interaction.
The simpler model applies mainly to plant communities. Monteith
(1977) observed that when biomass accumulation by a plant community
is plotted as a function of the accumulated solar radiation intercepted by
the community, the result was a straight line. Figure 14.4 shows