Permeability of periderm and cuticular membranes to alkali cations

The rates of penetration of alkali cations through isolated potato periderm and pear fruit cuticular membranes as a function of pH and the external solution concentration were determined. Steady-state rates were measured using a diaphragm diffusion cell, and permeabilities and the corresponding diffusion coefficients were calculated from the lag time. The observed penetration rates of the alkali cations in the periderm and cuticular membranes were in the order Cs+>K+>Na+>Li+. This is the order of increasing crystal radii and the order of decreasing exchange selectivity. The diffusion coefficients of all counterions increased as the external concentration was increased. © 1995 by Academic Press, Inc.