Bruce D, Samson G, Carpenter C
Department of Biological Sciences, Brock University, St. Catharines, Ontario, Canada.
Biochemistry. 1997 Jan 28;36(4):749-55. doi: 10.1021/bi962216c.
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+.
在大多数植物和藻类中，在“过剩”光照条件下会发生光合作用的下调，这与叶绿素a荧光猝灭有关。叶绿素a荧光的这种非光化学猝灭很可能源于一种保护光系统II免受过度激发及由此产生的光抑制的机制。在本报告中，通过低pH值在富含光系统II的菠菜类囊体膜中诱导了可变叶绿素a荧光的非光化学猝灭。使用皮秒荧光衰减光谱法在pH值范围为6.5至4.0的缓冲液中悬浮的样品中研究了猝灭的起源。与光系统II反应中心相关的相对缓慢（约1.5纳秒）的荧光衰减过程的产率随pH值降低而降低。与光系统II天线叶绿素a过程相关的较快衰减成分的产率没有显著变化。这些结果表明，在这些制剂中，非光化学猝灭是基于反应中心而非天线叶绿素的机制。光系统II吸收截面的测量表明，在低pH值下功能天线大小没有减小，这也支持了反应中心猝灭机制。使用泵浦探测光谱仪研究了光系统II中的电子转移动力学，该光谱仪同时测量了820纳米处的闪光诱导吸光度变化（氧化的光系统II反应中心色素P680+的形成）和可变荧光产率（还原的光系统II电子受体QA-的形成）。低pH值下P680+寿命的大幅增加与荧光猝灭相关。光系统II闪光激发后，荧光猝灭的消失与P680+的还原具有相同的动力学。与基于P680+和QA-之间电荷复合的基于反应中心的猝灭机制不同，QA-的氧化速率不受低pH值影响，并且在所有条件下都比P680+的还原速率慢。我们的数据将根据基于P680+直接猝灭的光系统II中低pH值依赖性非光化学猝灭模型进行讨论。
