Suppression of flash‐induced PSII‐dependent electrogenesis caused by proton pumping in chloroplasts

Abstract

Pre‐illumination of the thylakoid membrane of Peperomia metallica chloroplasts leads to a reversible suppression of the flash‐induced electrical potential as measured either with the electrochromic bandshift (P515), microelectrode impalement or patch‐clamp technique. The energization‐dependent potential suppression was not observed in the presence of 1 μM 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 accumulation of TMPD+ in the thylakoid lumen. The reversible suppression 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 presence of DCMU and phenazine methosulfate (PMS) results in the accumulation 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 PSI‐dependent electrogenesis. Reversible inactivation of PSII by a low pH in the thylakoid lumen is therefore proposed to be the cause for the temporary suppression of the flash‐induced electrical potential. The flash‐induced PSII‐dependent membrane potential, as measured after major oxidation of P700 in far‐red background light, was indeed found to be suppressed at low assay pH (pH 5) in isolated spinach (Spinacia oleracea) chloroplasts.