[Photoinduced reactivation of photosystem II in Chlorella after prolong incubation without light]

The light-dependent reactivation of photosystem II in Chlorella pyrenoidosa Chick, CALU-175 cells, inactivated with supraoptimal temperatures (40-43 degrees C) in the dark or during heterotrophic growth was studied. It was shown that the inactivation of photosystem II after incubation in the dark at 41-42 degrees C, which showed up in the suppression of relative yield of variable chlorophyll fluorescence Fv due to an increase in yield F0 could be completely reversed by light. The inactivation of photosystem II at 43 degrees C in the dark could not be reversed by subsequent irradiation. In this case, the suppression of Fv/Fm was related not only to the growth of F0 but also with the decrease in Fm. The light dependences of the rate and extent of reactivation of yield Fv after heterotrophic growth or incubation of chlorella at 41 degrees C in the dark completely coincided. The full light-induced reactivation of photosystem II took place as the rate of photoinduced electron transport reached the rate of nonphotochemical reduction of plastoquinone in the dark. These results suggest that the light-reversed inactivation of photosystem II after heterotrophic growth or incubation at 41 degrees C in the dark is due to the redox-interaction of the primary quinone acceptor with plastoquinone reduced by the electron flux from the substrates of chlororespiration.