WAVELENGTH DEPENDENCE OF PHOTOINHIBITION IN THE GREEN-ALGA CHLORELLA-VULGARIS

Chlorella vulgaris was exposed to various treatments containing high p hotosynthetically active irradiance (PAR, 400-700 nm) plus increasing amounts of various ranges of ultraviolet radiation (270-400 nm). Upon exposure to all radiation treatments, the efficiency of photosynthetic oxygen evolution and the yield of chlorophyll (Chi) fluorescence show ed first-order type decreases with time. Inhibition of photosynthesis depended upon the spectral composition of radiation. PAR alone had the least photoinhibitory effect on oxygen evolution and variable fluores cence; the inhibition increased with addition of UV-A to PAR, further at PAR+W-A+UV-B, and was largest in treatments containing UV-C. Inhibi tion of photosynthesis was associated with quenching of DCMU-enhanced Chi fluorescence, but not with changes in the shape of absorption spec tra or fluorescence excitation spectra. Recovery of fluorescence induc tion after exposure was also strongly dependent upon the spectral comp osition of the radiation stress. Full recovery of F-V/F-M occurred wit hin 30 min after exposure to PAR alone, showing that the quenching pro cesses induced during this stress were reversible and presumably photo protective. The rate of recovery was slowed as the cut-off of the radi ation spectrum was shifted to shorter UV wavelengths, indicative of in creasing damage to photosynthetic components.