Ion Transport Across Leaf Cuticles: Concepts and Mechanisms

Abstract

The chemistry of leaf cuticles (e. g., Holloway, 1994) and the permeation of cuticles by nonelectrolytes (e. g., Kerstiens, 1994; Riederer et al, 1994) are much better understood than is the permeation of ionic species, e. g., salts and acids. Ions differ from nonelectrolytes in that they carry a net electric charge. Ions and nonelectrolytes both diffuse in response to concentration gradients in solution, but ions are unique in being moved by electric gradients in addition to concentration gradients. If the force associated with an electric gradient is larger in magnitude and opposite in direction to the concentration gradient, then ionic species can move from regions of low to high concentration. In general, when salts and acids diffuse, an electric gradient, called a diffusion potential is generated; this electric gradient will retard the rate of diffusion of some ions and speed up the rate of others. Failure to recognize the influence of electric gradients can lead to errors or vagueness in interpretation of results (see below).