Quenching of chlorophyll fluorescence by quinones in algae and chloroplasts

Abstract

1. A number of quinones and substituted quinones quenched strongly chlorophyll fluorescence in Swiss chard chloroplasts and in the intact algae, Ulva lobata and Porphyra perforata. Quenching occurred in the absence as well as in the presence of 3(3,4-dichlorophenyl)-1,1-dimethylurea. 2. Among the quinones found to have a high quenching activity were 2,3,5,6-tetramethylbenzoquinone, 2-methyl-1,4-naphthoquinone, 5-hydroxy-1,4-naphthoquinone, phenanthrenequinone, and 1,2-dihydroxyanthraquinone. Reduced quinones had no or little quenching activity. 3. The quinones tested quenched less strongly the initial fluorescence, observed immediately upon illumination, than the subsequent rise of fluorescence yield during illumination. For the most active compounds, the concentrations needed for 50% quenching of the initial and the subsequent increase of fluorescence were about 70 and 15 μM, respectively. 4. The kinetics of fluorescence quenching at different light intensities and concentrations of quencher and absence of stimulation of O 2 evolution indicate that the quenchers do not stimulate photosynthetic electron transport but interact directly with chlorophyll molecules of photosystem 2 by formation of traps for the excitation energy. 5. In agreement with the assumption that the site of action of the quinones is near system 2, a number of these compounds inhibited light-induced cytochrome reduction in Porphyra in vivo. However, for most compounds no quantitative relation was found between the extent of inhibition of cytochrome reduction or of the Hill reaction in chloroplasts and the activity in quenching chlorophyll fluorescence.