Electrical Events and P515 Response in Thylakoid Membranes

Abstract

Since its discovery (Duysens, 1954) the light-induced absorbance change in the 480–550 nm wavelength region, with its characteristic difference spectrum, has been studied in much detail. Ample evidence has been presented (e.g. Junge, 1977) that this so-called P515 absorbance change is due to an electrochromic band shift of a pigment moiety with a maximum around 515 nm. This has made it a useful intrinsic probe for studying the magnitude and kinetics of formation and decay of the transmembrane electric potential of the inner chloroplast (thylakoids) membranes. There has been found reasonable good agreement on the value of the initial potential generated by saturating single turnover light flashes, measured directly by micro-electrodes and calculated by calibrating the absorbance change caused by salt-induced diffusion potentials (e.g. Vredenberg, 1981). The kinetics of P515 responses in continuous light as well as in single turnover flashes are however at variance with the responses measured with microelectrodes. There is now general agreement, based on observations in intact cells as well as in intact and broken chloroplasts that the P515 absorbance change in single turnover flashes is a multiphasic phenomenon, reflected by a biphasic rise and a decay which consists of at least three different single exponentials.