title: 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 degrees C and 150 mumol m-2 s-1 photosynthetically active radiation (PAR) and exposed to PAR combined with ultraviolet-A radiation (UV-A) at 15 degrees 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 degrees C and 150 mumol m-2 s-1 had minimal effects on the Fv/Fm values. However, cells grown at 15 degrees C and lower PAR irradiance (6 mumol 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 degrees C and 150 mumol m-2 s-1 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.
authors.affiliation: Department of Plant Sciences, University of Western Ontario, London, Ontario, Canada.
authors.forename: A G
authors.initials: AG
authors.lastname: Ivanov
authors.forename: E
authors.initials: E
authors.lastname: Miskiewicz
authors.forename: A K
authors.initials: AK
authors.lastname: Clarke
authors.forename: B M
authors.initials: BM
authors.lastname: Greenberg
authors.forename: N P
authors.initials: NP
authors.lastname: Huner
mesh_terms: D000458:Cyanobacteria / Q000254:growth & development / Q000528:radiation effects*; D045322:Photosynthetic Reaction Center Complex Proteins / Q000528:radiation effects*; D013696:Temperature; D014466:Ultraviolet Rays
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins