Geoscience Reference
In-Depth Information
carriers and channel-forming proteins that regulate the trans-
port of ions and other solutes. High temperatures cause an
increase in the fluidity of membrane lipids and a decrease in
the strength of hydrogen bonds and electrostatic interactions
between polar groups of proteins within the aqueous phase
of the membrane. High temperatures thus modify membrane
composition and structure, and can cause leakage of ions. High
temperatures can also lead to a loss of the three-dimensional
structure required for correct function of enzymes or structural
cellular components, thereby leading to loss of proper enzyme
structure and activity.
temperature
effects on
photosynthesis
Photosynthesis and respiration are both inhibited by tempera-
ture stress. Typically, photosynthetic rates are inhibited by high
temperatures to a greater extent than respiratory rates. Although
chloroplast enzymes such as rubisco, rubiscoactivase, NADP-
G3P dehydrogenase and PEP carboxylase become unstable at
high temperatures, the temperatures at which these enzymes
began to denature and lose activity are distinctly higher than
the temperatures at which photosynthetic rates begin to decline.
This would indicate that the early stages of heat injury to pho-
tosynthesis are more directly related to changes in membrane
properties and to uncoupling of the energy transfer mecha-
nisms in chloroplasts.
This imbalance between photosynthesis and respiration is
one of the main reasons for the deleterious effects of high tem-
peratures. On an individual plant, leaves growing in the shade
have a lower-temperature compensation point than leaves that
are exposed to the sun (and heat). Reduced photosynthate pro-
duction may also result from stress-induced stomatal closure,
reduction in leaf canopy area and regulation of assimilate
partitioning.
Acclimatisation
of plant at high
temperatures
High-temperature acclimation involves a considerable reorgan-
isation of the thylakoid membrane, including adaptive changes
of lipid composition. They contribute to the optimum physi-
cal state of the membrane (microviscosity, permeability, etc.).
During heat acclimation, the threshold temperature at which
fluidity still maintains the native membrane structure and
function, rises (Raison et al. 1982). The more saturated lipid
species decrease, noticeably the thylakoid membrane mobil-
ity at elevated temperatures, thus keeps a well-arranged lateral
movement of electrons carried between the photosystems. The
achieved thermotolerance of the majority of thermosensible
light reactions is probably the result of both the lipid-induced
