Leaf cuticles behave as asymmetric membranes : evidence from the measurement of diffusion potentials

Cuticles were isolated enzymatically from the leaves of two maple species (Acer saccharum Marsh and A. platanoides L.) and from orange (Citrus aurantium L.). The cuticles were placed in a plastic cuvette and different concentrations of KCl were perfused over the physiological inner and outer surfaces while the electrical potential (E¹⁰) that developed across the cuticles and was caused by ion diffusion was measured. E¹⁰ was always positive, indicating that the permeability of K⁺ was always greater than that of Cl^-. Measured E¹⁰ in cuticles did not fit the Goldman equation, whereas, E¹⁰ measured during KCl diffusion across selected artificial membranes fit the equation. The magnitude of E¹⁰ in cuticles and artificial membranes also was dependent on ionic strength, decreasing as ionic strength increased. These observations are explained by combining classical transport equations with equations that describe the equilibrium ion distribution between ionic double layers in the cuticle or membranes and the bathing solution.