Geoscience Reference
In-Depth Information
(Berry and Bjorkman 1980). The damage due to heat stress
includes a wide range of structural and functional changes.
Their effect on growth and survival depends on the intensity
and duration of heat stress. A long period at a moderately
high temperature may be as injurious as a brief exposure to an
extreme temperature.
It is considered that the primary site of damage is associated
with components of the photosynthetic system located in the thy-
lakoid membranes, most probably photosystem II (PSII) (Berry
and Bjorkman 1980). The PSII complex is a pigment-protein
complex that utilises light energy to drive the transport of elec-
trons and the oxidation of water to oxygen. It is believed that
increasing temperature first leads to a blockage of PSII reaction
centres and then to a dissociation of the antenna pigment protein
complexes from the central core of the PSII (Sundby et al. 1986).
Separation of the light-harvesting complex II (LHCII) from the
core centre induces destacking of the grana and temperature-
induced migration of the reaction centre (PSIIb) or LHCII (state
transition) to the non-appressed region, which would have con-
sequences for the energy distribution between PSI and PSII. It
has been found that moderately high temperatures stimulate PSI
activity
in vivo
and
in vitro
(Sayed et al. 1989). This stimulation
appears to be associated with an increased capacity for cyclic
electron flow around PSI, which could be an adaptive process,
producing ATP under conditions when PSII activity is severely
diminished. This ATP synthesis could be important for the
survival of plants and necessary for repair of stress-damaged
processes, as suggested by Janssen et al. (1992). It is also well
known that increasing the temperature at which plants are grown
causes an upward shift of the optimal temperature of photosyn-
thesis in numerous species and renders the photosynthetic appa-
ratus more tolerant to heat stress (Berry and Bjorkman 1980).
This phenomenon is termed acclimation. Acclimation to a new
growth temperature is not instant but requires a certain time
period. Under stress, organisms undergo first of all destabilisa-
tion followed by normalisation and stability enhancement when
limits of tolerance are not exceeded and adaptive capacity is not
overtaxed (Larcher 1987).
14.7
Oxygen may protect during acclimation to
various stresses
Although the oxygen evolving complex associated with PSII
results in the light-dependent evolution of oxygen, O
2
can also
