Protection of photosystem II against UV-A and UV-B radiation in the cyanobacterium Plectonema boryanum: The role of growth temperature and growth irradiance

Plectonema boryanum UTEX 485 cells were grown at 29 degreesC and 150 mu mol m(-2) s(-1) photosynthetically active radiation (PAR) and exposed to PAR c ombined with ultraviolet-A radiation (UV-A) at 15 degreesC. This induced a time-dependent inhibition of photosystem II (PSII) photochemistry measured as a decrease of the chlorophyll a fluorescence ratio, F-v/F-m to 50% after 2 h of W-A treatment compared to nontreated control cells. Exposure of the same cells to PAR combined with W-A + ultraviolet-B radiation (UV-B) cause d only a 30% inhibition of PSII photochemistry relative to nontreated cells . In contrast, UV-A and UV-A + UV-B irradiation of cells cultured at 15 deg reesC and 150 mu mol m(-2) s(-1) had minimal effects on the F-v/F-m values. However, cells grown at 15 degreesC and lower PAR irradiance (6 mu mol m(- 2) s(-1)) exhibited similar inhibition patterns of PSII photochemistry as c ontrol cells. The decreased sensitivity of PSII photochemistry of P. boryan um grown at 15 degreesC and 150 mu mol m(-2) s(-1) to subsequent exposure t o UV radiation relative to either control cells or cells grown at low tempe rature but low irradiance was correlated with the following: (1) a reduced efficiency of energy transfer to PSII reaction centers; (2) higher levels o f a carotenoid tentatively identified as myxoxanthophyll; (3) the accumulat ion of scytonemin and mycosporine amino acids; and (4) the accumulation of ATP-dependent caseinolytic proteases. Thus, acclimation of P, boryanum at l ow temperature and moderate irradiance appears to confer significant resist ance to UV-induced photoinhibition of PSII. The role of excitation pressure in the induction of this resistance to UV radiation is discussed.