DELTA-PH-DEPENDENT PHOTOSYSTEM-II FLUORESCENCE QUENCHING INDUCED BY SATURATING, MULTITURNOVER PULSES IN RED ALGAE

We have previously shown that in the red alga Rhodella violacea, expos ure to continuous row intensities of light 2 (green light) or near-sat urating intensities of white light induces a Delta pH-dependent PSII f luorescence quenching. In this article we further characterize this fl uorescence quenching by using white, saturating, multiturnover pulses. Even though the pulses are necessary to induce the Delta pH and the q uenching, the development of the tatter occurred in darkness and requi red several tens of seconds. In darkness or in the light in the presen ce of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, the dissipation of the quenching was very slow (more than 15 min) due to a low consum ption of the Delta pH, which corresponds to an inactive ATP synthase. In contrast, under far-red illumination or in the presence of 3-(3,4-d ichlorophenyl)-1,1'-dimethylurea (only in light), the fluorescence que nching relaxed in a few seconds. The presence of N,N'-dicyclohexyl car bodiimide hindered this relaxation. We propose that the quenching rela xation is related to the consumption of Delta pH by ATP synthase, whic h remains active under conditions favoring pseudolinear and cyclic ele ctron transfer.