PHOTOSYNTHETIC RESPONSE DO LIGHT AND TEMPERATURE - PSII EXCITATION PRESSURE AND REDOX SIGNALING

In studies that attempt to delineate the mechanisms of photosynthetic adjustment to temperature, one typically compares the structure and fu nction of the photosynthetic apparatus of plants and algae grown to th e same developmental state at a high temperature with those grown at a low temperature bur at a constant irradiance and photoperiod. Thus, a basic tenet of such comparative studies presupposes that any changes in the structure and function of the photosynthetic apparatus must ref lect a direct response to growth temperature. Here we summarize recent data from green algae, cyanobacteria and plants which indicate that t his basic assumption is incorrect. We establish that photosynthetic ad justment to low temperature mimes photosynthetic adjustment to high li ght. More important, we show that the common photosynthetic response t o temperature and light can be rationalized in terms of photosystem II excitation pressure which is a measure of the redox state of Q(A). It reflects the redox poise of intersystem electron transport and photos ynthetic carbon metabolism. Thus, modulation of PSII excitation pressu re reflects a change in feedback limited photosynthesis due to alterat ions in Pi availability and limited triose phosphate utilization. Cons equently, increased resistance to photoinhibition is observed in green algae, cyanobacteria and plants under any growth condition whereby th e organisms must adjust to feedback limited photosynthesis, and thus, high PSII excitation pressure. We suggest that the redox poise of inte rsystem electron transport and/or photosynthetic carbon metabolism may represent a general redox sensing/signalling mechanism for photosynth etic adjustment to various environmental stimuli.