Abstract
Electrochemical data obtained with TMPD+‐sensitive electrodes indicate that ammonium‐uncoupled chloroplasts retain TMPD (N,N,N′,N′‐tetramethyl‐p‐phenylenediamine) mainly in the reduced form during illumination, whereas uncoupled DCMU‐treated chloroplasts accumulate TMPD in the oxidized form (TMPD+). This observation indicates that the reduced plastoquinol is the preferred electron donor for photosystem I (PSI) and TMPD can only compete efficiently when plastoquinone reduction is blocked. After adding DCMU the formation of a transmembrane gradient for TMPD+ is reflected by a slow‐down of the electrogenic electron transport and by the emerging of the overshoot of the membrane current in the light‐off response. A light‐dependent increase in photoelectric current generated by chloroplasts in the presence of NH4Cl and TMPD is observed and considered to be caused by a reversible release of current limitation in the interfacial conductance barriers in the lumen.
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