Distribution of electric potential and ion transport in the hypocotyl ofVigna sesquipedalisI. Distribution of overall ion concentration and the role of hydrogen ion in generation of potential difference

摘要

Overall concentration of free inorganic ions distributes in the hypocotyl of a bean seedling {Vigna sesquipedalis) at a constant level (H+) or decreases monotonously from the cotyledonary node towards the base (K+, Na+, Ca++and Mg++, phosphate, NO3−). According to our theory, this is inconsistent with the distribution of electric potential having a definite minimum in the elongating region. The discrepancy can not be explained by regional variance in radial potential difference or histological differentiation in passive ionic permeability of the cell membrane. Short circuit current observed through a hypocotyl segment corresponded to a net flux in ions of 10–24 pEq/cm2.sec. It is questionable, however, whether this is due to active ion transport, which can be the source of electric potential difference, or is a passive flow due to histological heterogeneity in ion concentration.In order to investigate the latter possibility, pH of sap exuding from stumps made at various intervals along the hypocotyl axis was measured, since H+is the ion electro-osmotically most effective. pH Values of acropetal exudates distributed along the axis closely corresponding to the distribution of electric potential. This suggests that potential distribution is determined by a passive flow of H+through a specific channel in the vascular system. The fact that H+production and the uptake of ions and water are most active at the elongating zone of hypocotyl is discussed from a physiological point of v