Influence of the Counter-ion on the Absorption Isotherm for Chloride at Low Temperature

Abstract

The influence of the counter-ion on the Cl absorption isotherm for potato (Solanum tuberosum) slices was studied with the use of Cl36. Whereas at 30[degree] the Cl- absorption isotherm in KC1 for slices aged 24 hours is hyperbolic, with freshly cut slices at this temperature and with both fresh and aged slices at 0[degree], Cl36 absorption resembles an exponential function of external concentration. In the presence of CaCl2, however, the absorption isotherm is hyperbolic under all conditions. Maintaining K+ at constant concentration (40 mM) while varying Cl- from 1 mM to 20 mM has the effect of altering the Cl-absorption isotherm at 0[degree]C from a concave-upwards to a convex-upwards pattern. It is concluded that Cl-uptake at 0[degree]C is rate-determined by passive movement across the plasma membrane in accordance with the electrochemical potential gradient. These results are shown to be consistent with a straightforward electrochemical model (or theory) of the transport of ions across plant cell membranes. This model is discussed in detail and particular attention is given to the assumption inherent in the model and to the predictions arising from it. Two predictions, viz., the effect of Ca++ and the effect of constant K+ on the Cl- absorption isotherm are substantiated in this paper.