Agriculture Reference
In-Depth Information
stress. SA accumulation has been observed for instance in heat stressed
mustard (Dat et al., 1998b) and SA application can improve plant toler-
ance to subsequent heat stress (Dat et al., 1998a). The role of SA in this
process is not fully understood and is apparently not linked to the induc-
tion of HSPs biosynthesis, but to the induction of oxidative responses. The
application of exogenous SA can mimic temperature acclimation, similar
to heat shock generated oxidative stress. However, there might be a stabi-
lizing role for SA in HSP gene activating transcription factor complexes
(Jurivich et al., 1992). ROS on the other hand, serve as fast messengers
inside the cell, activating multiple downstream responses and increased
ROS generation has been observed during heat shock in plants (Dat et al.,
1998b).
With the advent of genetic engineering it is now possible to clone and
transfer the genes of interest including HSP to induce tolerance to high
temperature.
13.5.8 COLD AND FREEZING STRESS
Chilling stress is a suboptimal temperature, where the plant faces reduced
enzyme activity and maybe water availability, however, the temperature
is above the freezing point of water. Cold stress affects mainly metabolic
processes, impairing enzyme reactions, substrate diffusion rates and mem-
brane transport properties. Thereby some reactions are more affected by
the cold then others. In particular, the dark reaction of photosynthesis and
oxidative phosphorylation seem to be sensitive to chilling. The discrep-
ancy between the speeds of biochemical reactions can cause ROS accumu-
lation in the chloroplast and mitochondrion (Scheller and Haldrup, 2005).
One early response to cold and osmotic stress is the rapid Ca2+ influx into
the cell. Physical alterations in the cellular structure may cause this Ca2+
influx by activation of Ca2+ channels and initiate downstream Ca2+ de-
pendent signaling pathways (Xiong et al., 2002).
When temperature drops below zero degrees, plants can experience
freezing stress. Intracellular freezing of water can damage the protoplast
membrane structure, mechanically injure and finally kill the cells by the
expanding ice crystals. During extracellular freezing, the protoplasm of
the plant becomes severely dehydrated when water is transferred to the ice
crystals in the intercellular spaces. Freezing acclimation can be achieved
