Distribution of electric potential and ion transport in the hypocotyl of Vigna sesquipedalis VI. The dual structure of radial electrogenic activity

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The electrophysiological structure in bean hypocotyl was investigated by the intracellular electrode method in combination with surface electric potential (V_s) measurement and respiratory inhibition by anoxia, with special reference to the membrane transport of ions and the formation of an absorption centre in the elongating (E) zone. The radial potential difference (V_<sx> : electric potential difference between the organ surface and a xylem vessel), on which axial distribution of V_s was dependent, comprised two components; V_<sx>=V_<px>-V_<ps>. |V_<px>| (the potential difference between the inside of a parenchyma symplast and a xylem vessel) was at a maximum in the E-zone, while |V_<ps>| (the intracellular electric potential with respect to the organ surface) was largest in the G-zone (mature zone), resulting in the characteristic distribution pattern of V_s with a minimum in the E-zone. There were two independent electromotive forces which were both partially dependent on respiration; one corresponding to V_<ps> located at the surface of the parenchyma symplast (P) and the other to V_<px> located between P and the xylem (X). The electrogenic component of V_<px> was relatively small both in the hook (H) zone and the G-zone, but maximal in the E-zone of the hypocotyl. This result was consistent with the emergence of a maximum pH difference between P and X in the E-zone, where accumulation of K^+ and water were at a maximum, suggesting maximum activity of an H^+-pump extruding protons from P into X in exchange for K^+.
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Distribution of electric potential and ion transport in the hypocotyl of Vigna sesquipedalis VI. The dual structure of radial electrogenic activity

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