REVERSIBLE PHOTOINHIBITION IN ANTARCTIC MOSS DURING FREEZING AND THAWING

Tolerance of antarctic moss to freezing and thawing stress was investi gated using chlorophyll a fluorescence. Freezing in darkness caused re ductions in F-v/F-m (ratio of variable to maximum fluorescence) and F- o(initial fluorescence) that were reversible upon thawing. Reductions in F-v/F-m and F-o during freezing in darkness indicate a reduction in the potential efficiency of photosystem II that may be due to conform ational changes in pigment-protein complexes due to desiccation associ ated with freezing. The absorption of light during freezing further re duced F-v/F-m and F-o but was also reversible. Using dithiothreitol (D TT), which inhibits the formation of the carotenoid zeaxanthin, we fou nd reduced fluorescence quenching during freezing and reduced concentr ations of zeaxanthin and antheraxanthin after freezing in DTT-treated moss. Reduced concentrations of zeaxanthin and antheraxanthin in DTT-t reated moss were partially associated with reductions in nonphotochemi cal fluorescence quenching. The reversible photoinhibition observed in antarctic moss during Greeting indicates the existence of processes t hat protect from photoinhibitory damage in environments where freezing temperatures occur in conjunction with high solar radiation levels. T hese processes may limit the need for repair cycles that require tempe ratures favorable for enzyme activity.