Protection of Photosystem II Against UV‐A and UV‐B Radiation in the Cyanobacterium Plectonema boryanum: The Role of Growth Temperature and Growth Irradiance¶

Abstract Plectonema boryanum UTEX 485 cells were grown at 29°C and 150 μmol m−2 s−1 photosynthetically active radiation (PAR) and exposed to PAR combined with ultraviolet-A radiation (UV-A) at 15°C. This induced a time-dependent inhibition of photosystem II (PSII) photochemistry measured as a decrease of the chlorophyll a fluorescence ratio, Fv/Fm, 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 μmol m−2 s−1 had minimal effects on the Fv/Fm values. However, cells grown at 15°C and lower PAR irradiance (6 μmol m−2 s−1) 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 μmol m−2 s−1 to subsequent exposure to UV radiation relati...