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High light-induced changes of chlorophyll fluorescence during 2 hours are analyzed in cyanobacteria Aphanizomenon elenkinii AICB 709. The light of 800 µmol.m^-2.s^-1 produced the increasing of F0 and Fm in the first part of light exposure. Fv and the maximal and effective quantum yields decreased after 30 min. of light exposure. The opening state and the ratio of the reaction centers decreased. The light of 1500 µmol.m^-2.s^-1 induced the rising of F0, Fm and Fv and the decreasing of maximal and effective quantum yields, relative to control. The decreasing of photochemical coefficients means that the fraction of the opened reaction centers and their ratio also were significantly diminished. The light of 2100 µmol.m^-2.s^-1 determined the increasing of F0 in the first 15 min, and the decreasing of Fm, Fv, quantum yields and photochemical coefficients. The quantum yield of non-regulated energy dissipation Y(NO) enhanced in every light intensity that was used. Changes in fluorescence chlorophyll are caused by structural changes in the antenna and the PS II reaction centers due to photoinhibition produced by high light. The fast recovery of the photosynthetic parameters was obtained in the samples exposed to high light intensity.

Keywords: carotenoid, chlorophyll a, coefficient of photochemical quenching, effective quantum yield, minimal and maximal fluorescence, maximal quantum yield, phycobilisomes, quantum yield of non-regulated energy dissipation, recovery, saturation pulse method.