Protection of Photosystem II Against UV-A and UV-B Radiation in the Cyanobacterium Plectonema boryanum: The Role of Growth Temperature and Growth Irradiance¶ (original) (raw)

Plectonema boryanum UTEX 485 cells were grown at 29؇C and 150 mol m Ϫ2 s Ϫ1 photosynthetically active radiation (PAR) and exposed to PAR combined with ultraviolet-A radiation (UV-A) at 15؇C. This induced a timedependent 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 UV-A treatment compared to nontreated control cells. Exposure of the same cells to PAR combined with UV-A ؉ ultraviolet-B radiation (UV-B) caused 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؇C and 150 mol m Ϫ2 s Ϫ1 had minimal effects on the F v /F m values. However, cells grown at 15؇C and lower PAR irradiance (6 mol m Ϫ2 s Ϫ1 ) exhibited similar inhibition patterns of PSII photochemistry as control cells. The decreased sensitivity of PSII photochemistry of P. boryanum grown at 15؇C and 150 mol m Ϫ2 s Ϫ1 to subsequent exposure to UV radiation relative to either control cells or cells grown at low temperature but low irradiance was correlated with the following: (1) a reduced efficiency of energy transfer to PSII reaction centers; (2) higher levels of a carotenoid tentatively identified as myxoxanthophyll; (3) the accumulation of scytonemin and mycosporine amino acids; and (4) the accumulation of ATP-dependent caseinolytic proteases. Thus, acclimation of P. boryanum at low temperature and moderate irradiance appears to confer significant resistance to UV-induced photoinhibition of PSII. The role of excitation pressure in the induction of this resistance to UV radiation is discussed.