PHOTOINHIBITION IN THE ANTARCTIC MOSS GRIMMIA-ANTARCTICI CARD WHEN EXPOSED TO CYCLES OF FREEZING AND THAWING

Freezing and thawing of the endemic moss species Grimmia antarctici Ca rd, caused photoinhibition. When snow cover was removed from moss in t he field, resulting in exposure to fluctuating temperatures and light conditions, photoinhibition, measured as a reduction in the ratio of v ariable to maximum chlorophyll a fluorescence (F-v/F-m), was observed. The extent of photoinhibition was highly variable and appeared to be reversible during periods of warmer temperatures, A series of controll ed laboratory studies found that the light conditions that prevail bet ween freezing and thawing events influenced the recovery from photoinh ibition observed during freezing and thawing, with low light condition s facilitating the greatest rates of recovery, After four cycles of fr eezing and thawing, recovery from photoinhibition in hydrated moss was achieved within 12 h of transfer to 5 degrees C and 15 mu mol quanta m(-2) s(-1). These results favour the hypothesis that photoinhibition observed during freezing represents a protective process involving the down-regulation of photosystem II when photosynthetic carbon assimila tion is limited by low temperatures.