THE ORIGINS OF NONPHOTOCHEMICAL QUENCHING OF CHLOROPHYLL FLUORESCENCEIN PHOTOSYNTHESIS - DIRECT QUENCHING BY P680(-II ENRICHED MEMBRANES AT LOW PH() IN PHOTOSYSTEM)

In most plants and algae, a down-regulation of photosynthesis under '' excess'' light conditions occurs which is associated with a quenching of chlorophyll a fluorescence. This nonphotochemical quenching of chlo rophyll a fluorescence most likely arises from a mechanism which prote cts photosystem II from excessive excitation and resulting photoinhibi tion. In this report, nonphotochemical quenching of variable chlorophy ll a fluorescence was induced by low pH in photosystem II enriched spi nach thylakoid membranes. The origin of quenching was investigated wit h picosecond fluorescence decay spectroscopy in samples suspended in b uffers ranging from pH 6.5 to pH 4.0. The yield of a relatively slow ( approximately 1.5 ns) fluorescence decay process associated with the p hotosystem II reaction center decreased with decreasing pH. There were no significant changes in the yield of faster decay components associ ated with photosystem II antenna chlorophyll a processes. These result s suggest a reaction center based rather than antenna chlorophyll base d mechanism for nonphotochemical quenching in these preparations. Meas urements of the photosystem II absorption cross section revealed no de crease in the functional antenna size at low pH which also supports a reaction center quenching mechanism. The kinetics of electron transfer in photosystem II were investigated using a pump probe spectrometer w hich measured simultaneously the flash-induced absorbance change at 82 0 nm (formation of oxidized photosystem II reaction center pigment, P6 80(+)) and the variable fluorescence yield (formation of reduced photo system II, electron acceptor, Q(A)(-)) A large increase in the lifetim e of P680(+) at low pH was correlated with fluorescence quenching. Aft er flash excitation of photosystem II the loss of fluorescence quenchi ng occurred with the same kinetics as the reduction of P680(+). In con flict with reaction center based quenching mechanisms based on charge recombination between P680(+) and Q(A)(-), the oxidation rate of Q(A)( -) was unaffected by low pH and under all conditions occurred at a slo wer rate than the reduction of P680(+). Our data are discussed in term s of a model for low pH dependent nonphotochemical quenching in photos ystem II based on direct quenching by P680(+).