Photoactivation of electrogenic activity in chloroplasts and its relation to photoinduced swelling of thylakoids

In patch-clamp experiments on isolated chloroplasts of Peperomia metallica Lind. et Rodig. (Piperaceae), the replacement of 50 mM KCl in a medium with 50 mM NH4Cl strongly influenced the parameters of photocurrent known to reflect the generation of electric potential in thylakoids. The addition of NH+ 4 to the medium modified the induction curves of the photocurrent as well as the currents induced by single-turnover flashes in preilluminated chloroplasts. Under the action of a prolonged light pulse (∼ l s), the steady-state current was much higher in the ammonium-containing medium than in the presence of K+. Preillumination of a dark-adapted chloroplast with a l-s light pulse suppressed the current induced by a single-turnover flash (6 μs) in the presence of K+ but caused an elevation (by 50-150%) of the flash-induced current and shortening of its relaxation time in the presence of NH+ 4. The origin of different induction kinetics for the photocurrent in K+ and NH+ 4 media is partly clear, because ammonium prevents generation of the pH gradient and, subsequently, eliminates the ΔpH-dependent suppression of the electron transport rate. However, this does not explain the origin of NH+ 4-dependent photostimulation of the current generated by single-turnover flashes. This phenomenon arises from the thylakoid swelling caused by the accumulation of NH+ 4 in the lumen and from the respective changes in the network resistances. The network element most sensitive to thylakoid swelling is the lateral resistance of the lumen: it decreases upon enlargement of the cross-section area. Stimulation of the flash-induced current by preillumination in the presence of NH+ 4 was accompanied by accelerated relaxation of the current, indicating that the phenomena observed are caused by the reduction of network resistance involved in the discharge of the membrane capacity. Thus, the light-induced structural changes in the thylakoid system have a marked effect on the currents measured with the patch-clamp technique.