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Diffusion and Electric Mobility of Ions within Isolated Cuticles of Citrus aurantium: Steady-State and Equilibrium Values
Authors:Tyree M T  Wescott C R  Tabor C A
Institution:Northeastern Forest Experiment Station, P.O. Box 968, Burlington, Vermont 05402.
Abstract:We report a new method for measuring cation and anion permeability across cuticles of sour orange, Citrus aurantium, leaves. The method requires the measurement of two electrical parameters: the diffusion potential arising when the two sides of the cuticle are bathed in unequal concentrations of a Cl salt; and the electrical conductance of the cuticle measured at a salt concentration equal to the average of that used in the diffusion-potential measurement. The permeabilities of H+, Li+, Na+, K+, and Cs+ ranged from 2 × 10−8 to 0.6 × 10−8 meters per second when cuticles were bathed in 2 moles per cubic meter Cl salts. The permeability of Cl was 3 × 10−9 meters per second. The permeability of Li+, Na+, and K+ was about five times less when measured in 500 moles per cubic meter Cl salts. We also report an asymmetry in cuticle-conductance values depending on the magnitude and the direction of current flow. The asymmetry disappears at low current-pulse magnitude and increases linearly with the magnitude of the current pulse. This phenomenon is explained in terms of transport-number effects in a bilayer model of the cuticle. Conductance is not augmented by current carried by exchangeable cations in cuticles; conductance is rate limited by the outer waxy layer of the cuticle.
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