Abstract
The light-dependent quenching of the chlorophyll a fluorescence at room temperature by N-methylphenazinium methyl sulfate (PMS) was investigated with isolated chloroplasts inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Other investigators have considered this quenching to be a consequence of the formation of a high energy membrane state related to photophosphorylation.It was found that the fluorescence quenching was accompanied by a reversible bleaching of PMS which could not be attributed to its photo-reduction. Both fluorescence quenching and PMS bleaching, and their dark reversal, were similarly affected by the experimental conditions. In particular, they were only slightly sensitive to the presence of uncouplers of photophosphorylation. However, bleaching and fluorescence quenching were strongly inhibited by uncouplers when the creation of a proton gradient across the thylakoid membrane was made possible by a presence of some PMS in its reduced form.It is suggested that the bleaching of PMS resulted from its binding to the thylakoid membranes when charges became available during conformational changes as a consequence of the light reaction in photo-system I. The same conformational changes were apparently responsible for the fluorescence quenching, but a large pH gradient across the membranes was not essential.