| Summary: | Tolerance of antarctic moss to freezing and thawing stress was investigated using chlorophyll a fluorescence. Freezing in darkness caused reductions in F√F (ratio of variable to maximum fluorescence) and F (initial fluorescence) that were reversible upon thawing. Reductions in F√F and F during freezing in darkness indicate a reduction in the potential efficiency of photosystem 11 that may be due to conformational changes in pigment-protein complexes due to desiccation associated with freezing. The absorption of light during freezing further reduced F√F and F but was also reversible. Using dithiothreitol (DTT), which inhibits the formation of the carotenoid zeaxanthin, we found reduced fluorescence quenching during freezing and reduced concentrations of zeaxanthin and antheraxanthin after freezing in DTT-treated moss. Reduced concentrations of zeaxanthin and antheraxanthin in DTT-treated moss were partially associated with reductions in nonphotochemical fluorescence quenching. The reversible photoinhibition observed in antarctic moss during freezing indicates the existence of processes that protect from photoinhibitory damage in environments where freezing temperatures occur in conjunction with high solar radiation levels. These processes may limit the need for repair cycles that require temperatures favorable for enzyme activity.
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