STIMULATION OF PHOTOCURRENT IN CHLOROPLASTS RELATED TO LIGHT-INDUCED SWELLING OF THYLAKOID SYSTEM

The photocurrent in patch-clamped chloroplasts of Peperomia metallica was strongly modified by full or partial replacement of 50 mM KCI with NH4Cl in the external medium. Two effects of ammonium were observed a t zero holding potential: (1) the sustained current in prolonged light pulses was substantially increased and (2) the peak current induced b y a single-turnover flash (8 mu s) was amplified, with a concomitant s hortening of its relaxation time. Under control conditions, in the pre sence of KCl, the photocurrent rose to an initial peak and declined in continuous light, whereas in the presence of NH4Cl, an initial rise o f inward current was followed by a further rise (within 0.5-1 s) to a high steady level. This effect of ammonium is partially explained by t he absence of pH gradient which makes electron turnovers more frequent . However, the current induced by an 8-mu s flash in the presence of N H4+ was also stimulated (by 50-150%) after 1-s preillumination of dark -adapted plastids. Hence, the apparent photoactivation of electrogenic activity was not only due to acceleration of electron turnovers. Eith er (i) latent reaction centers turn active during preillumination or ( ii) the patch pipette becomes a more effective sink for current, owing to NH4+-dependent swelling of lumen and the resulting reduction in th e longitudinal lumenal resistance. In support of the latter view, the flash-induced response in NH4+-treated plastids had a shorter relaxati on time in preilluminated than in dark-adapted plastids. Thus, the lig ht-induced structural changes of the thylakoid system, confirmed by el ectron microscopy, strongly affect the photocurrents probed by the pat ch-clamp technique. (C) 1998 Elsevier Science S.A. All rights reserved .