ATP FORMATION, OXYGEN EVOLUTION, AND CAROTENOID CONVERSIONS IN CHLOROPLASTS ILLUMINATED WITH SHORT LIGHT-FLASHES - A NEW HYPOTHESIS FOR THEEVOLUTION OF MOLECULAR-OXYGEN

Experiments using a chemiluminescent method based on the luciferin-luc iferase system have shown that chloroplasts illuminated by short light flashes form ATP with each flash, beginning with the first flash. Oxy gen evolution was assayed by highly sensitive amperometric titration. Evolution was found to occur at intervals of four flashes with a maxim um during the third flash and with some evolution on the second flash in normal chloroplasts, or at the first flash in chloroplasts enriched in violaxanthin. Violaxanthin concentration was increased by incubati ng the plants in the dark for one day prior to isolation of the chloro plasts. Changes in carotenoid composition, determined by thin-layer ch romatography and high-pressure liquid chromatography, were found to co rrelate with the periodicity of oxygen evolution, began from the first flash, and involved 5 to 10% of the total carotinoids. The results ob tained in this work as well as some data from the literature lead us t o propose a new hypothesis for oxygen evolution. Under this hypothesis , oxygen is released from hydroxyl radicals which are formed during AT P synthesis in photosystem II, with the participation of an oxygen-evo lving complex containing carotenoids and manganese.