The role of membrane potential for the control of elongation growth of vigna hypocotyl: Response of a hollow cylinder to osmotic and ionic stress

We have devised an experimental system of perfusion through a hollow cylinder of a Vigna hypocotyl to examine the control mechanism of plant stem elongation. When the cylinder was subjected to osmotic stress, it began to shrink and then spontaneously resumed elongation. Not only the membrane potential difference between the parenchyma symplast and the central bore (V_px), but also that between the parenchyma symplast and the organ surface (V_ps), showed hyperpolarization a few minutes after the cylinder began to shrink. Removal of the stress caused an immediate increase in elongation rate followed by depolarization of both membrane potentials a few minutes later. When the cylinder was subjected to KCl stress, V_px showed transient depolarization and recovery, while V_ps showed only immediate hyperpolarization. Increasing the KCl concentration caused V_px to depolarize, and the cylinder simultaneously to cease to elongate for about 5min, even when the osmotic concentration of the perfusion solution was kept almost constant. An inverse reaction was observed when the KCl concentration was decreased.These two reversible responses suggest that control of V_px may regulate the elongation of hollow cylinders, and that the xylem pump plays an important role in the regulation of intact stem elongation.