CHANGES IN PHOTOSYSTEM STOICHIOMETRY IN RESPONSE TO ENVIRONMENTAL-CONDITIONS FOR CELL-GROWTH OBSERVED WITH THE CYANOPHYTE SYNECHOCYSTIS PCC-6714

Changes in photosystem stoichiometry in response to shift of environme nts for cell growth other than light regime were studied with the cyan ophyte Synechocystis PCC 6714 in relation to the change induced by lig ht-quality shift. Following two environment-shifts were examined: the shift of molecular form of inorganic carbon source for photosynthesis from CO2 to HCO3- (CO2 stress) and the increase in salinity of the med ium with NaCl (0.5 M) (Na+ stress). Both CO2 and Na+ stresses induced the increase in PSI abundance resulting in a higher PSI/PSII stoichiom etry. CO2 stress was found to elevate simultaneously Cyt c oxidase act ivity (V-max). The feature was the same as that caused by light-qualit y shift from preferential excitation of PSI to PSII (light stress) tho ugh the enhancement by either stress was smaller than that by light st ress. Under our experimental conditions, PSI/PSII stoichiometry appear ed to increase at a fairly constant rate to the basal level even when the basal level had been differently determined by the light-quality. Enhancing rates for PSI/PSII stoichiometry and for Cyt c oxidase activ ity were also similar to each other. Since the two stresses affect the thylakoid electron transport similarly to the shift of light-quality, we interpreted our results as follows: three environmental stresses, CO2, Na+, and light stresses, cause changes in electron turnover capac ity of PSI and Cyt c oxidase under a similar, probably a common, mecha nism for monitoring redox state of thylakoid electron transport system .