COORDINATED CHANGES IN THE REDOX STATE OF PHOTOSYSTEM-I AND PHOTOSYSTEM-II IN SUNFLOWER LEAVES AT DIFFERENT IRRADIANCES

Rates of CO2 uptake, photochemical, and nonphotochemical quenching of chlorophyll fluorescence and absorption changes at 830 nm, which refle ct redox changes in P700 (the primary election donor of photosystem I) , were measured simultaneously in sunflower (Helianthus annuus L.) lea ves at different light intensities. Two components could be distinguis hed in the light-response curve of nonphotochemical quenching. The fir st component, which saturated at low irradiance, could be related to t he state 2-state 1 transition of the chloroplast pigment system, where as the second component, which saturated at high irradiance, could be related to the development of ''energy-dependent'' quenching associate d with the rise of the transmembrane proton gradient. Similar light-re sponse curves of nonphotochemical quenching were observed in barley (H ordeum vulgare L,), spinach (Spinacea oleracea L.) and wheat (Triticum aestivum L.) leaves. The amount of reduced primary acceptor (QA) in p hotosystem LI and oxidized primary donor in photosystem I(PS I) increa sed with irradiance intensity. The dependence between changes in these two parameters was nearly proportional. The accumulation of reduced p rimary electron donors in PS II was also directly proportional to the extent of ''energy-dependent'' quenching of chlorophyll fluorescence. Our data indicate a high correlation between changes in the redox stat e of PS I and II in intact leaves induced by alterations in environmen tal conditions.