PHOTOINHIBITION AND LAW OF RECIPROCITY IN PHOTOSYNTHETIC REACTIONS OFSYNECHOCYSTIS SP PCC-6803

The effect of irradiance (neutron, alpha, X- and visible electromagnet ic rays) on biological systems (survivals of organs, tissue cultures, breakage of DNA) has been widely studied in radiology. The biological effect of radiation depends on the dose (D = I.t; I and t are the inte nsity and time of irradiance, respectively). In photosynthesis light i s a substrate for the photoelectronic conversion and can be toxic to p hotosystem II (PSII) as well. This toxicity is expressed by the degrad ation of the reaction centre D1 protein and, as a consequence, the los s of PSII activity of wild type and mutant Synechocystis cells being s ubjected to high light. The aim of the present study was to examine wh ether the photoinhibition of PSII follows the reciprocity law characte ristic of a dose response. We measured the change of steady state leve l of room temperature fluorescence at 685 nm (due to the antenna and r eaction center chlorophylls), variable chlorophyll fluorescence and ox ygen evolution of wild type Synechocystis 6803 cells after exposure to high light intensities (1200-8000 mu mol.m(-2).s(-1)) for different d urations (0 - 20 min). We observed that the log of degree of PSII inac tivation as a function of light intensity gave a straight line, indica ting the validity of the reciprocity law. Average cross-sectional area s for inactivation were found to be 0.066 m(2)/mol (steady state oxyge n evolution), 0.12 m(2)/mol (steady state level of F-685nm), and 0.25 m(2)/mol (induction of F-v/F-max).