Department of Biochemistry, University of Cambridge, Tennis Court Road, CB2 1QW, Cambridge, UK.
Photosynth Res. 1993 Aug;37(2):147-58. doi: 10.1007/BF02187473.
Photosystem II cyclic electron transport was investigated at low pH in spinach thylakoids and PS II preparations from the cyanobacteriumPhormidium laminosum. Variable fluorescence (Fv) quenching at a very low light intensity was examined as an indicator of cyclic electron flow. A progressive quenching of Fv was observed as the pH was lowered; however, this was shown to be mainly due to an inhibition of oxygen evolution. Cyclic electron flow in the uninhibited centres was estimated to occur at a rate comparable to or smaller than 1 μ mole O2 mg Chl(-1) h(-1) in the pH range 5.0 to 7.8.The quantum yeeld of oxygen production is known to decrease at low pH and has been taken to indicate cyclic electron flow (Crofts and Horton (1991) Biochim Biophys Acta 1058: 187-193). However, a direct all-or-none inhibition of oxygen production at low pH has also been reported (Meyer et al. (1989) Biochim Biophys Acta 974: 36-43). We have analysed the effects of light intensity on the rates of oxygen evolution in order to calculate ΦU, the quantum yield of open and uninhibited centres. ΦU was found to be constant over a broad pH range, and by using ferricyanide and phenyl-p-benzoquinone as electron acceptors the maximum possible rate of cyclic electron transport was equivalent to no more than 1 μmole O2 mg Chl(-1) h(-1). The rate was no greater when the acceptor was adjusted to provide the most favourable conditions for cyclic flow.
在菠菜类囊体和蓝藻 Phormidium laminosum 的 PS II 制剂中，研究了低 pH 值下的光系统 II 循环电子传递。作为循环电子流的指示剂，研究了非常低光强下可变荧光（Fv）猝灭。随着 pH 值的降低，观察到 Fv 逐渐猝灭；然而，这主要是由于氧释放的抑制。在未被抑制的中心，循环电子流的估计速率在 pH 值为 5.0 至 7.8 的范围内与或小于 1 μ mole O2 mg Chl(-1) h(-1)相当。已知氧产生的量子产率在低 pH 值下降低，并被认为表明存在循环电子流（Crofts 和 Horton（1991）生物化学与生物物理学学报 1058：187-193）。然而，也有报道称在低 pH 值下氧产生的直接全有或全无抑制（Meyer 等人，1989 年，生物化学与生物物理学学报 974：36-43）。我们分析了光强对氧释放速率的影响，以便计算ΦU，即开放和未被抑制中心的量子产率。发现ΦU 在很宽的 pH 范围内保持恒定，并且使用铁氰化物和苯-p-对苯醌作为电子受体，循环电子传递的最大可能速率相当于不超过 1 μmole O2 mg Chl(-1) h(-1)。当将受体调整为提供最有利于循环流动的条件时，速率并没有增加。
