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 NH(4)Cl in the external medium. Two effects of ammonium were observed at zero holding potential: (1) the sustained current in prolonged light pulses was substantially increased and (2) the peak current induced by a single-turnover flash (8 mu s) was amplified, with a concomitant shortening of its relaxation time. Under control conditions, in the presence of KCl, the photocurrent rose to an initial peak and declined in continuous light, whereas in the presence of NH(4)Cl, an initial rise of 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 the absence of pH gradient which makes electron turnovers more frequent. However, the current induced by an 8-mu s flash in the presence of NH(4)(+) 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. Either (i) latent reaction centers turn active during preillumination or (ii) the patch pipette becomes a more effective sink for current, owing to NH(4)(+)-dependent swelling of lumen and the resulting reduction in the longitudinal lumenal resistance. In support of the latter view, the flash-induced response in NH(4)(+)-treated plastids had a shorter relaxation time in preilluminated than in dark-adapted plastids. Thus, the light-induced structural changes of the thylakoid system, confirmed by electron microscopy, strongly affect the photocurrents probed by the patch-clamp technique. (C) 1998 Elsevier Science S.A. All rights reserved.