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

High Temperature Stress
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Kelvin. A substantial amount of energy can be lost by infrared emission from leaves
but leaves also receive infrared energy from their surroundings. If the surroundings are
the same temperature as the leaf, the net energy flux will be zero. However, on a sunny
day with no clouds, the leaf is emitting infrared radiation to the sky and receiving
infrared radiation from the sky. The effective temperature of the sky on a clear day can
be less than 0°C and so the radiation balance in the thermal infrared wavelengths can
dissipate over one half of the sunlight energy absorbed by leaves. The worst situation
occurs on a partly cloudy day if the sun shines through a gap in the clouds. Clouds
have a much warmer effective temperature than does the clear sky and so the energy
gained from the sunlight is not as easily dissipated through infrared energy loss. The
energy plants receive from sunlight beyond what is dissipated by thermal radiation
must be dissipated by a combination of sensible heat loss (see above) and latent heat
loss (see below).
1.3. Latent Heat
A substantial amount of energy is required for evaporation of water. Evaporation of
water from the wet surfaces inside of leaves can dissipate a large amount of energy to
balance the energy input from sunlight. Under unusual conditions, latent heat loss can
exceed radiant heat gain, resulting in leaf temperatures below air temperature. Sensible
heat exchange will then be positive, requiring additional heat loss by evaporation.
Cotton is one of few plants that transpire enough water to substantially cool its leaves
below air temperature. The large water loss cools the cotton leaves, in some cases by
more then 5°C (Radin et al., 1994; Wise et al., 2004) and improves yield (Cornish et al.,
1991), demonstrating that high leaf temperature limits yield in cotton.
Because latent heat loss is an important component of the energy balance of leaves,
high temperature stress can be a consequence of drought, if plants do not have water
available for transpirational cooling.
1.4. Conditions of High Temperature Stress
In light of the energy balance considerations, three conditions of high temperature
stress can be described: (1) when air temperature is high, plants will tend to come to the
high temperature by sensible heat transfer, (2) at the soil surface, where sunlight can
cause the temperature to be substantially above air temperature (Campbell and Norman,
1998) and (3) in leaves where the substantial radiant heating by sunlight and the low
heat capacity of leaves conspire to heat leaves very rapidly to as much as 15°C above
air temperature. Using fine-wire thermocouples it has been shown that leaves with low
transpiration rates, such as oak leaves, suffer frequent high temperature episodes
(Singsaas and Sharkey, 1998; Hanson et al., 1999; Singsaas et al., 1999). However, most
crops transpire more than the oak trees measured by Singsaas et al. (1999) and in