The effect of redox potential on the kinetics of fluorescence induction in pea chloroplasts. I. Removal of the slow phase.

The effect of alteration of redox potential on the kinetics of fluorescence induction in pea chloroplasts has been investigated. Potentiometric titration of the initial (Fi) level of fluorescence recorded upon shutter opening gave a two component curve, with Em(*7) at -20 mV and -275 mV, almost, identical to results obtained using continuous low intensity illumination (Horton, P. and Croze, E. (1979) Biochim. Biophys. Acta 545, 188--201). The slow or tail phase of induction observed in the presence of DCMU can be eliminated by poising the redox potential at approx. 0 to +50 mV. At this potential Fi was increased by less than 10% and the higher potential quencher described above was only marginally reduced. The disappearance of the slow phase titrated as an n = 1 component with an Em(7) of +120 mV. Therefore it seems unlikely that the slow phase of fluorescence induction is due to photoreduction of the -20 mV quencher. These results are discussed with reference to current ideas concerning heterogeneity on the acceptor side of Photosystem II.