Studies on the kinetics of the 515 nm absorbance changes in chloroplasts: Evidence for the induction of a fast and a slow P515 response upon saturating light flashes

Abstract

A great deal of attention has been given in recent years to the functional reIatiollship between the generation of an electrical potential, proton transfer, conformational changes and ATP synthesis in chloroplast membranes, associated with the primary photosynthetic light reactions [l-71. Conclusions about magnitude and time course of the transmembrane potential were obtained either by means of indirect potential measurements such as the light-induced M5r5 of the carotenoid chlorophyll b pigment complex (PS 15), or by direct measurements of the chloroplast (thylakoid) transmembrane potential with microcapillary glass electrodes [3,8-l 11. The rise and decay of the field-indicating MsIs upon single turnover saturating light flashes have been reported to occur with single first-order reaction kinetics in broken chloroplasts suspended in isotonic rnediul~l under nonphosphorylating conditions [ 1,12,13]. The rate constant of the dark decay appears to be determined by the passive ion permeability of the membrane. The dark decay has been shown to become biphasic in the presence of ADP and inorganic phosphates. This has been suggested to be due to potential-dependent changes in the proton conductance through the ATP synthesizing enzyme complex [14-l 81. Recently it has been reported that the flash-induced PS 15 AA in intact (class I) chloroplasts, like in intact algal cells [ 191, occurs with complex multi-phasic rise and decay kinetics [20]. However, rise and decay of the flashinduced potential changes in intact chloroplasts measured with micro-electrodes have been shown to occur with single first-order reaction kinetics [21-231. The