Title: Quenching of chlorophyll fluorescence and primary photochemistry in chloroplasts by dibromothymoquinone
Abstract: The quenching action of dibromothymoquinone on fluorescence and on primary photochemistry was examined in chloroplasts at −196 °C. Both the initial (F0) and final (FM) levels of fluorescence as well as the fluorescence of variable yield (Fv = FM − F0) were quenched at −196 °C to a degree which depended on the concentration of dibromothymoquinone added prior to freezing. The initial rate of photoreduction of C-550 at — 196 °C, which was assumed to be proportional to maximum yield for primary photochemistry, ϕPo, was also decreased in the presence of dibromothymoquinone. Simple theory predicts that the ratio FVFM should equal ϕPo. Excellent agreement was found in a comparison of relative values of ϕPo with relative values of FVFM at various degrees of quenching by dibromothymoquinone. These results are taken to indicate that F0 and FV are the same type of fluorescence, both emanating from the bulk chlorophyll of Photosystem II. Dibromothymoquinone appears to create quenching centers in the bulk chlorophyll of Photosystem II which compete with the reaction centers for excitation energy. The rate constant for the quenching of excitation energy by dibromothymoquinone is directly proportional to the concentration of the quencher. Rate constants for the de-excitation of excited chlorophyll molecules by fluorescence, kF, by nonradiative decay processes, kD, by photochemistry, kP, and by the specific quenching of dibromothymoquinone, kQ, were calculated assuming the absolute yield of fluorescence at F0 to be either 0.02 or 0.05.
Publication Year: 1975
Publication Date: 1975-01-01
Language: en
Type: article
Indexed In: ['crossref', 'pubmed']
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Cited By Count: 1020
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