PHOTOSYNTHESIS, PHOTOINHIBITION AND LOW-TEMPERATURE ACCLIMATION IN COLD TOLERANT PLANTS

Cold acclimation requires adjustment to a combination of light and low temperature, conditions which are potentially photoinhibitory. The ph otosynthetic response of plants to low temperature is dependent upon t ime of exposure and the developmental history of the leaves. Exposure of fully expanded leaves of winter cereals to short-term, low temperat ure shifts inhibits whereas low temperature growth stimulates electron transport capacity and carbon assimilation. However, the photosynthet ic response to low temperature is clearly species and cultivar depende nt. Winter annuals and algae which actively grow and develop at low te mperature and moderate irradiance acquire a resistance to irradiance 5 - to 6-fold higher than their growth irradiance. Resistance to short-t erm photoinhibition (hours) in winter cereals is a reflection of the i ncreased capacity to keep Q(A) oxidized under high light conditions an d low temperature. This is due to an increased capacity for photosynth esis. These characteristics reflect photosynthetic acclimation to low growth temperature and can be used to predict the freezing tolerance o f cereals. It is proposed that the enhanced photosynthetic capacity re flects an increased flux of fixed carbon through to sucrose in source tissue as a consequence of the combined effects of increased storage o f carbohydrate as fructans in the vacuole of leaf mesophyll cells and an enhanced export to the crown due to its increased sink activity. Lo ng-term exposure (months) of cereals to low temperature photoinhibitio n indicates that this reduction of photochemical efficiency of PS II r epresents a stable, long-term down regulation of PS II to match the en ergy requirements for CO2 fixation. Thus, photoinhibition in vivo shou ld be viewed as the capacity of plants to adjust photosynthetically to the prevailing environmental conditions rather than a process which n ecessarily results in damage or injury to plants. Not all cold toleran t, herbaceous annuals use the same mechanism to acquire resistance to photoinhibition. In contrast to annuals and algae, overwintering everg reens become dormant during the cold hardening period and generally re main susceptible to photoinhibition. It is concluded that the photosyn thetic response to low temperatures and susceptibility to photoinhibit ion are consequences of the overwintering strategy of the plant specie s.