Absorbance changes of C-550 and cytochrome b(559), and fluorescence-yield changes were measured during irradiation of chloroplasts at -196 degrees . The photo-reduction of C-550 proceeded more rapidly than the photo-oxidation of cytochrome b(559), and the fluorescence-yield change had similar kinetics to the cytochrome b(559) change. The fluorescence yield of chloroplasts exposed to a 16-musec flash at -196 degrees did not increase during the flash, but increased in the dark after the flash. Both of these experiments indicate that the fluorescence yield follows the dark reduction of the primary electron donor of Photosystem II, not the photoreduction of the acceptor. This explanation would also account for the recent results of Mauzerall [Proc. Nat. Acad. Sci. USA (1972) 69, 1358-1362] showing that the fluorescence yield of chloroplasts at room temperature requires about 20 musec to reach a maximum after a very brief flash.
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