[Quenching of chlorophyll in vivo by m-dinitrobenzene]

The quenching of fluorescence and inhibition of photochemical activities by m-dinitrobenzene have been studied in unicellular algae and chloroplasts. The complementary area [formula: see text] is decreased in the same fashion as the maximum amplitude of the variable fluorescence delta phi(M), suggesting the invariance of S properly normalized by delta phi(M). A photochemical type inhibition for all photochemical activities (oxygen evolution, Photoreaction II and I) is observed in a concentration range higher than that required to quench delta phi(M). The ratio of the photochemical rate in limiting light to the O2 burst elicited by a flash is constant whatever the level of inhibition. The pattern of oscillation of O2 burst during a sequence of flashes is also unmodified, the amplitude only being decreased. In order to explain these results, it is assumed that dinitrobenzene (DNB) has a quenching effect both on the center-chlorophyll and the collector-chlorophyll of the System II photosynthetic units; when the external quencher is only acting on the collector, the trapping efficiency for the center is unmodified, but, when the center is turned into its inactive form by the photochemical reaction, the fluorescence of the collector is quenched. It is shown that the rule of invariance of the normalized complementary area applies to this type of quenching; accordingly, the zero level of the System II fluorescence, within the constant part, (cf. Lavorel, J. et Joliot, P. (1972) Biophys. J. 12, 815-831) should lie close to the 0 level (dark-adapted state).