Gas Exchange and Growth
171
8.8.3 Temperature Effects
Air temperature influences growth and gas exchange differently (Fig. 8.8).
Typically, the metabolic responses to temperature have an optimum value
which not only varies with species but is different for different processes
such as leaf and root growth, stomatal aperture, photosynthesis, or respiration (Berry and Raison 1981; Ong and Baker 1985). Traditionally, the
decrease in growth at high temperatures is attributed to a higher optimum
temperature for respiration than for net photosynthesis, thus decreasing
the daily net carbon gain. For example, the importance of warm nights
in reducing plant biomass production has been emphasized by several
authors.
The decrease in A at temperatures above or below To results from
decreases in quantum yield and light-saturated photosynthesis. At temperatures above To there is a "down-regulation" of Rubisco which is apparently
dependent on changes in the activation state of the enzyme (We is and Berry
1988). These responses are not static but may vary as a result of acclimation.
For example, Slatyer (1977) showed ca. lOoC change in temperature optima
for photosynthesis due to acclimation to low and high temperatures in
Eucalyptus pauciflora. In Eucalyptus globulus, Pereira et al. (1986) showed
a shift in the temperature response of leaf respiration corresponding to
seasonal changes in temperature, e. g., respiration rates at 20°C were lower
in hot summer months than in cooler winter and spring months (Fig. 8.9).
The physiological and molecular aspects of acclimation of metabolism to
changing temperatures are out of the scope of this work but they are under
intense scrutiny in the present.
The temperature response of canopy growth does not coincide with metabolic responses. Canopy growth depends on the rate and duration of leaf
expansion, the number of leaves and branches formed per unit time, and the
death rate of leaves. For example, the rate of leaf expansion often has
temperature optima (To) higher than the temperature that maximizes final
leaf size because the duration of leaf expansion decreases with temperature
(Ong and Baker 1985). Leaf production rates and branching also increase at
higher temperatures (Quinby et al. 1973). By increasing the rate of canopy
development, higher temperatures result in the increase in the value of D or
f [Eq. (8)] even though the value of £ may not change (Marshall et al. 1991).
However, as development is also accelerated, leaf longevity decreases at
high temperatures. The end result may be that D (or green leaf area
duration and consequently f) may not be improved at higher temperatures.
This is illustrated by Lupinus albus plants grown at higher temperatures that
produced 37% more leaves but had ca. 34% reduction in D in comparison
to the plants grown at lower temperatures because of the accelerated rate of
senescence in the warmer regime (M. Chaves pers. comm.). On the other
hand, at temperatures substantially below the optimum for growth, plants
171
8.8.3 Temperature Effects
Air temperature influences growth and gas exchange differently (Fig. 8.8).
Typically, the metabolic responses to temperature have an optimum value
which not only varies with species but is different for different processes
such as leaf and root growth, stomatal aperture, photosynthesis, or respiration (Berry and Raison 1981; Ong and Baker 1985). Traditionally, the
decrease in growth at high temperatures is attributed to a higher optimum
temperature for respiration than for net photosynthesis, thus decreasing
the daily net carbon gain. For example, the importance of warm nights
in reducing plant biomass production has been emphasized by several
authors.
The decrease in A at temperatures above or below To results from
decreases in quantum yield and light-saturated photosynthesis. At temperatures above To there is a "down-regulation" of Rubisco which is apparently
dependent on changes in the activation state of the enzyme (We is and Berry
1988). These responses are not static but may vary as a result of acclimation.
For example, Slatyer (1977) showed ca. lOoC change in temperature optima
for photosynthesis due to acclimation to low and high temperatures in
Eucalyptus pauciflora. In Eucalyptus globulus, Pereira et al. (1986) showed
a shift in the temperature response of leaf respiration corresponding to
seasonal changes in temperature, e. g., respiration rates at 20°C were lower
in hot summer months than in cooler winter and spring months (Fig. 8.9).
The physiological and molecular aspects of acclimation of metabolism to
changing temperatures are out of the scope of this work but they are under
intense scrutiny in the present.
The temperature response of canopy growth does not coincide with metabolic responses. Canopy growth depends on the rate and duration of leaf
expansion, the number of leaves and branches formed per unit time, and the
death rate of leaves. For example, the rate of leaf expansion often has
temperature optima (To) higher than the temperature that maximizes final
leaf size because the duration of leaf expansion decreases with temperature
(Ong and Baker 1985). Leaf production rates and branching also increase at
higher temperatures (Quinby et al. 1973). By increasing the rate of canopy
development, higher temperatures result in the increase in the value of D or
f [Eq. (8)] even though the value of £ may not change (Marshall et al. 1991).
However, as development is also accelerated, leaf longevity decreases at
high temperatures. The end result may be that D (or green leaf area
duration and consequently f) may not be improved at higher temperatures.
This is illustrated by Lupinus albus plants grown at higher temperatures that
produced 37% more leaves but had ca. 34% reduction in D in comparison
to the plants grown at lower temperatures because of the accelerated rate of
senescence in the warmer regime (M. Chaves pers. comm.). On the other
hand, at temperatures substantially below the optimum for growth, plants