The Effect of Some Inhibitors of Photosynthetic Electron Transport on the Kinetics of Redox Changes of the Reaction Centre Chlorophyll of Photosystem I (P700) and of Cytochrome f at Sub-Zero Temperatures

1 The effect of five inhibitory treatments on the extent of photooxidation of the reaction centre chlorophyll of photosystem I (P700) and cytochrome f in saturating continuous light and on the kinetics of their subsequent dark reduction was studied in spinach chloroplasts suspended at–17°C in a fluid medium containing 50 % (v/v) ethanediol: water. Under these conditions the turnover time of the uninhibited electron transport system is of the order of seconds. 2 Treatment with KCN or polylysine inhibited the photooxidation of cytochrome f and the reduction of P700, but not the reduction of the cytochrome or P700 oxidation. These observations are those expected if inhibition is at the level of plastocyanin and this component mediates electron flow between cytochrome f and P700. 3 Inhibition with bathophenanthroline or 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone had no effect on the photooxidation of either cytochrome f or P700. In contrast the rate of dark reduction of both components was reduced in parallel by both inhibitors. This suggests a site of inhibition common to the reduction pathways of both cytochrome f and P700. 4 HgCl2 had the same inhibitory effects as polylysine and KCN but in addition the reduction of cytochrome f was inhibited. It is suggested that this inhibitor may act at more than one site. 5 In most cases cytochrome f reduction showed first-order kinetics and in no case was there any evidence for a lag in the reduction. Inhibitors preventing electron flow from plastocyanin to P700 caused no increase in the rate of cytochrome f reduction. These results do not fit the expected properties of a linear scheme of electron transfer from plastoquinol to P700 via cytochrome f and plastocyanin.