title: Evidence for a double hit process in photosystem II based on fluorescence studies.
abstract: 1. The amplitudes of the fast (0-20 microseconds) and slow (20 microseconds-2 ms) fluorescence rise induced by a 2 microseconds flash have been measured as a function of the energy of the flash in chloroplasts inhibited by 3(3,4-dichlorophenyl)-1, 1-dimethylurea. The saturation curve for the slow rise shows a characteristic lag which is not observed for the fast fluorescence rise. This lag indicates that Photosystem II centers undergo a double hit process which implies that (a), each photocenter includes two acceptors Q1 and Q2; (B), after the first hit, oxidized chlorophyll Chl+ is reduced by a secondary acceptor Y in a time shor compared to the duration of the flash; (c), after the second hit, Chl+ is reduced by another secondary donor, D. 2. According to Den Haan et al. (1974) Biochim. Biophys. Acta 368, 409-421), hydroxylamine destroys the secondary donor responsible for the fast reduction of Chl+. In the presence of 3 mM hydroxylamine, only the secondary donor D is functional and a flash induses mainly a single hit process. 3. The saturation curves for the fast and the slow rises have been studied in the presence of 3(3,4-dichlorophenyl)-1, 1-dimethylurea for a second actinic flash given 2.5 s after a first saturating one. The large decrease in the half-saturating energy indicates the existence of efficient energy transfer occuring between potosynthetic units. 4. Two alternate hypotheses are discussed (a) in which D is an auxiliary donor and (b) in which D is included in the main electron transfer chain.
mesh_terms: D002734:Chlorophyll / Q000378:metabolism*; D002736:Chloroplasts / Q000378:metabolism; D004579:Electron Transport / Q000187:drug effects; D004734:Energy Metabolism; D005292:Ferricyanides / Q000378:metabolism; D005453:Fluorescence; D006898:Hydroxylamines / Q000494:pharmacology; D008956:Models, Chemical; D010084:Oxidation-Reduction; D010777:Photochemistry; D010788:Photosynthesis / Q000187:drug effects; D011789:Quantum Theory
chemical_list: D005292:Ferricyanides; D006898:Hydroxylamines; D002734:Chlorophyll