Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

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T.D. Sharkey and S.M. Schrader
cotton, which normally grows in warm environments, stomatal conductance can be so
high that leaf temperature is substantially below air temperature. In fact, it has been
specifically shown for cotton that the greater the transpirational cooling, the greater the
yield (Radin et al., 1994; Lu et al., 1997) proving that heat-stress-induced reductions of
photosynthesis limit overall yield. Other studies have also linked reduced crop yields
to moderate heat stress of leaves that reduces photosynthesis (Al-Khatib and Paulsen,
1990; Guilioni et al., 2003).
Some important effects of energy balance can be seen in the oak leaf temperatures
shown in Figure 1. During the day, leaves at the top of the tree (black dots) can be
over 10°C above air temperature because of the radiant heat load, while leaves at the
bottom of the tree (light gray dots) are slightly below air temperature because of latent
heat loss. At night, leaves at the top of the tree lose radiant heat to the sky and so are
cooler than leaves at the bottom of the tree because leaves at the bottom of the tree
exchange radiant heat with other leaves and with the ground, both of which are warmer
than the sky.
12
Leaf Temp - Air Temp, o C
10
8
6
4
2
0
- 2
- 4
10:00 20:00 6:00 16:00 2:00 12:00
Time, Eastern Daylight Time starting July 10
Figure 1. Difference between leaf and air temperature for two days for three oak (Quercus
rubra) leaves at the top (black dots) middle (dark gray) or bottom (light gray) of the canopy
Leaves exposed to full sunlight will carry out most photosynthesis but also be
subject to the rapid, high temperature spikes shown in Figure 1. We have started calling
this heat flecks, analogous to (and sometimes simultaneous with) sunflecks (Pearcy et
al., 1996). Because leaves are often the hottest parts of plants, high temperature stress
is especially relevant to photosynthesis. Moreover, plant growth often increases with
temperature to well above 30°C while photosynthesis of individual leaves typically has
a temperature optimum of 30°C or less. At low temperature growth may be more directly