Synechococcus sp. PCC 7942 (Anacystis nidulans R2) contains two forms of the Photosystem II reaction centre protein D1, which differ in 25 of 360 amino acids. D1: 1 predominates under low light but is transiently replaced by D1:2 upon shifts to higher light. Mutant cells containing only D1:1 have lower photochemical energy capture efficiency and decreased resistance to photoinhibition, compared to cells containing D1:2. We show that when dark-adapted or under low to moderate light, cells with D1:1 have higher non-photochemical quenching of PS II fluorescence (higher qN) than do cells with D1:2. This is reflected in the 77 K chlorophyll emission spectra, with lower Photosystem II fluorescence at 697-698 nm in cells containing D1:1 than in cells with D1:2. This difference in quenching of Photosystem II fluorescence occurs upon excitation of both chlorophyll at 435 nm and phycobilisomes at 570 nm. Measurement of time-resolved room temperature fluorescence shows that Photosystem II fluorescence related to charge stabilization is quenched more rapidly in cells containing D1:1 than in those with D1:2. Cells containing D1:1 appear generally shifted towards State II, with PS II down-regulated, while cells with D1:2 tend towards State I. In these cyanobacteria electron transport away from PS II remains non-saturated even under photoinhibitory levels of light. Therefore, the higher activity of D1:2 Photosystem II centres may allow more rapid photochemical dissipation of excess energy into the electron transport chain. D1:1 confers capacity for extreme State II which may be of benefit under low and variable light.

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 chlorophyll a fluorescence most likely arises from a mechanism which protects photosystem II from excessive excitation and resulting photoinhibition. In this report, nonphotochemical quenching of variable chlorophyll a fluorescence was induced by low pH in photosystem II enriched spinach thylakoid membranes. The origin of quenching was investigated with picosecond fluorescence decay spectroscopy in samples suspended in buffers 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 photosystem II reaction center decreased with decreasing pH. There were no significant changes in the yield of faster decay components associated with photosystem II antenna chlorophyll a processes. These results suggest a reaction center based rather than antenna chlorophyll based mechanism for nonphotochemical quenching in these preparations. Measurements of the photosystem II absorption cross section revealed no decrease 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 which measured simultaneously the flash-induced absorbance change at 820 nm (formation of oxidized photosystem II reaction center pigment, P680+) and the variable fluorescence yield (formation of reduced photosystem II, electron acceptor, QA-). A large increase in the lifetime of P680+ at low pH was correlated with fluorescence quenching. After flash excitation of photosystem II the loss of fluorescence quenching occurred with the same kinetics as the reduction of P680+. In conflict with reaction center based quenching mechanisms based on charge recombination between P680+ and QA-, the oxidation rate of QA- was unaffected by low pH and under all conditions occurred at a slower rate than the reduction of P680+. Our data are discussed in terms of a model for low pH dependent nonphotochemical quenching in photosystem II based on direct quenching by P680+.