ON THE RATES OF CYCLIC ELECTRON-TRANSPORT AROUND PHOTOSYSTEM-II IN THE PRESENCE OF DONOR SIDE LIMITATION

Photosystem II cyclic electron transport was investigated at low pH in spinach thylakoids and PS II preparations from the cyanobacterium Phormidium laminosum. Variable fluorescence (F(v)) quenching at a very low light intensity was examined as an indicator of cyclic electron flow. A progressive quenching of F(v) 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 mu mole O2 Mg Chl-1 h-1 in the pH range 5. 0 to 7.8. The quantum yield 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 PHI(U), the quantum yield of open and uninhibited centres. PHI(U) 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 mumole 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.