Inhibition of photosynthesis by solar radiation in Dunaliella salina: Relative efficiencies of UV-B, UV-A and PAR

Inhibition of photosynthesis after exposure to solar radiation was investigated in the marine green alga Dunaliella salina by monitoring photosynthetic optimal quantum yield F-v/F-m and efficiency of oxygen production. Samples were exposed to solar radiation in Ancient Korinth, Greece (37 degrees 58' N, 23 degrees 0' E) in August 1994. Within 30 min, F-v/F-m and efficiency of oxygen production decreased with similar kinetics with increasing exposure time. The inhibition, however, diminished when ultraviolet radiation was progressively excluded by means of colour filter glasses. Samples exposed for 3 h showed complete or partial recovery of photosynthesis, with almost the same rate under all irradition conditions. The fit of the experimental data with an analytical model describing inhibition of photosynthesis as a function of a linear combination of the photon fluence in the UV-B, UV-A and PAR allows one to estimate the relative mean effectiveness for inhibition by the three spectral ranges [about 2 x 10(-4), 4 x 10(-6) and 2 x 10(-7) (mu mol photons m(-2))(-1) for UV-B, UV-A and PAR, respectively].