TRANSMEMBRANE MOVEMENTS OF ARTIFICIAL REDOX MEDIATORS IN RELATION TO ELECTRON-TRANSPORT AND IONIC CURRENTS IN CHLOROPLASTS

Electrochemical data obtained with TMPD(+)-sensitive electrodes indica te that ammonium-uncoupled chloroplasts retain TMPD (N,N,N',N'-tetrame thyl-p-phenylenediamine) mainly in the reduced form during illuminatio n, whereas uncoupled DCMU-treated chloroplasts accumulate TMPD in the oxidized form (TMPD(+)). This observation indicates that the reduced p lastoquinol is the preferred electron donor for photosystem I (PSI) an d TMPD can only compete efficiently when plastoquinone reduction is bl ocked. After adding DCMU the formation of a transmembrane gradient for TMPD(+) is reflected by a slow-down of the electrogenic electron tran sport and by the emerging of the overshoot of the membrane current in the light-off response. A light-dependent increase in photoelectric cu rrent generated by chloroplasts in the presence of NH4Cl and TMPD is o bserved and considered to be caused by a reversible release of current limitation in the interfacial conductance barriers in the lumen.