SUPPRESSION OF FLASH-INDUCED PSII-DEPENDENT ELECTROGENESIS CAUSED BY PROTON-PUMPING IN CHLOROPLASTS

Pre-illumination of the thylakoid membrane of Peperomia metallica chlo roplasts leads to a reversible suppression of the flash-induced electr ical potential as measured either with the electrochromic bandshift (P 515), microelectrode impalement or patch-clamp technique. The energiza tion-dependent potential suppression was not observed in the presence of 1 mu M nigericin suggesting the involvement of proton and/or cation gradients. Energization in the presence of 3-(3,4-dichlorophenyl)-1,1 -dimethylurea (DCMU) and N,N,N',N'-tetramethylphenylenediamine (TMPD), i.e. cyclic electron flow around photosystem (PS) I, results in the a ccumulation of TMPD(+) in the thylakoid lumen. The reversible suppress ion of the flash-induced membrane potential was not observed in these conditions indicating that it is not a general cation-induced increase of membrane capacitance. Cyclic electron flow around PSI in the prese nce of DCMU and phenazine methosulfate (PMS) results in the accumulati on of PMS(+) and H+ in the thylakoid lumen. The absence of reversible suppression of the flash-induced membrane potential for this condition shows that accumulation of protons does not lead to (1) a reversible increase of membrane capacitance and (2) a reversible suppression of P SI-dependent electrogenesis. Reversible inactivation of PSII by a low pH in the thylakoid lumen is therefore proposed to be the cause for th e temporary suppression of the flash-induced electrical potential. The flash-induced PSII-dependent membrane potential, as measured after ma jor oxidation of P700 in far-red background light, was indeed found to be suppressed at low assay pH (pH 5) in isolated spinach (Spinacia ol eracea) chloroplasts.