Plants and Plant Communities
Optimum Leaf Form
It is clear from Fig. 14.5 that there exists an optimum temperature for
photosynthesis which appears to vary with irradiance. The optimum temperature varies from species to species and can even depend on the
temperature under which the leaf is grown. For the particular leaf represented by Fig. 14.5 the optimum temperature is a little above 20" C
at low irradiance and increases to about 30" C at high irradiance. From
the analysis we did earlier in this chapter, we know that leaf temperature is, to some extent, under the control of the plant. Narrow leaves
tend to stay closer to air temperature than broad leaves and leaves with
high evaporation rates can maintain temperatures well below air temperature when vapor deficits are high. Large leaves with high radiation loads
and low evaporation rates can reach temperatures considerably above air
temperature. With this range of possibilities available, the question arises
whether plants evolve leaf shapes and responses to stress which maximize
photosynthesis. In environments where water is a limiting resource for
production, one could also ask whether plant design or behavior adapts
to maximize production per unit water use, or water use efficiency.
We can not know whether plants maximize photosynthesis or water
use efficiency, but we can investigate what size and orientation of leaves
would give maximum photosynthesis or water use efficiency, and then see
if leaves in that environment have that size or orientation. A few cases
appear to be straightforward. The alpine cushion plants, which would be
below the optimum temperature for photosynthesis most of the time ifthey
were at air temperature, clearly benefit by radiative heating of the leaves.
Their growth habit appears to be an adaptation to maximize temperature in
the sun. Desert perennials, which maintain leaves throughout the summer
with limited water supplies, in environments where air temperature is at
or above the photosynthetic optimum, would benefit from minimizing
daytime leaf temperature. Their small leaves appear to be an adaptation
to keep leaves as close to air temperature as possible without evaporating
large amounts of water. A number of these adaptations, and their energetic
consequences, have been analyzed by Taylor (1975). At least for extreme
environments, species native to those environments appear to evolve leaf
shapes that tend to be optimum.
Leaf orientation is another interesting topic for investigation. Leaves
of many species droop or roll when they are water stressed. This can
reduce the radiation load on the leaf, decreasing leaf temperature and
transpiration. On the other hand, leaves of sunflower, peanut, and many
other species follow the sun, tending to increase the irradiance of the leaf.
One interesting species, in this regard, is the prairie compass plant (Jurik
et al., 1990). This plant grows in hot, dry environments. Leaves grow so
that the flat surfaces of leaf blades face east-west to maximize radiation
interception in the morning and evening and minimize it during midday.
Vapor deficits are maximum during midday and leaf temperatures are
higher than optimum for photosynthesis. The main assimilation times
Optimum Leaf Form
It is clear from Fig. 14.5 that there exists an optimum temperature for
photosynthesis which appears to vary with irradiance. The optimum temperature varies from species to species and can even depend on the
temperature under which the leaf is grown. For the particular leaf represented by Fig. 14.5 the optimum temperature is a little above 20" C
at low irradiance and increases to about 30" C at high irradiance. From
the analysis we did earlier in this chapter, we know that leaf temperature is, to some extent, under the control of the plant. Narrow leaves
tend to stay closer to air temperature than broad leaves and leaves with
high evaporation rates can maintain temperatures well below air temperature when vapor deficits are high. Large leaves with high radiation loads
and low evaporation rates can reach temperatures considerably above air
temperature. With this range of possibilities available, the question arises
whether plants evolve leaf shapes and responses to stress which maximize
photosynthesis. In environments where water is a limiting resource for
production, one could also ask whether plant design or behavior adapts
to maximize production per unit water use, or water use efficiency.
We can not know whether plants maximize photosynthesis or water
use efficiency, but we can investigate what size and orientation of leaves
would give maximum photosynthesis or water use efficiency, and then see
if leaves in that environment have that size or orientation. A few cases
appear to be straightforward. The alpine cushion plants, which would be
below the optimum temperature for photosynthesis most of the time ifthey
were at air temperature, clearly benefit by radiative heating of the leaves.
Their growth habit appears to be an adaptation to maximize temperature in
the sun. Desert perennials, which maintain leaves throughout the summer
with limited water supplies, in environments where air temperature is at
or above the photosynthetic optimum, would benefit from minimizing
daytime leaf temperature. Their small leaves appear to be an adaptation
to keep leaves as close to air temperature as possible without evaporating
large amounts of water. A number of these adaptations, and their energetic
consequences, have been analyzed by Taylor (1975). At least for extreme
environments, species native to those environments appear to evolve leaf
shapes that tend to be optimum.
Leaf orientation is another interesting topic for investigation. Leaves
of many species droop or roll when they are water stressed. This can
reduce the radiation load on the leaf, decreasing leaf temperature and
transpiration. On the other hand, leaves of sunflower, peanut, and many
other species follow the sun, tending to increase the irradiance of the leaf.
One interesting species, in this regard, is the prairie compass plant (Jurik
et al., 1990). This plant grows in hot, dry environments. Leaves grow so
that the flat surfaces of leaf blades face east-west to maximize radiation
interception in the morning and evening and minimize it during midday.
Vapor deficits are maximum during midday and leaf temperatures are
higher than optimum for photosynthesis. The main assimilation times