Naphthaleneacetic Acid Permeability of Citrus Leaf Cuticle

Abstract

Summary The fluxes of naphthaleneacetic acid (NAA) across astomatous isolated cuticular membranes (CM) and cutin matrices (MX) (cuticular membranes from which cuticular waxes has been extracted with lipid solvents) of Citrus aurantium L. leaves were analyzed on the basis of distribution ( λ ), permeability (P) and diffusion coefficients (D). NAA distribution coefficients ( λ )([NAA] in the solid in moles kg −1 /[NAA] in buffer solution in moles kg −1 ) were determined for cuticular membranes and the cutin matrices from leaves of Brassaia spec, Citrus aurantium L., Nerium oleander L., from tomato fruit (Lycopersicon esculentum L.) and for the cuticular waxes triacontan, hexacosanol and hexadecanoic acid. λ (CM) values (133 to 194) were smaller than λ (MX) values (167 to 212). At p H values above three (the isoelectric point of the cuticles) distribution coefficients were a function of the degree of dissociation of NAA. The anionic form of NAA could not enter the cutin matrix because of Donnan exclusion. Below p H 3 the NAA anion entered the cutin matrix by counter ion exchange. Sorption of the nondissociated form of NAA by the cutin matrix was not influenced by the nature or density of the charges fixed to the cutin matrix. Distribution coefficients for the three cuticular waxes amounted to 5 to 7. Permeability coefficients for diffusion of NAA across the cutin matrix were 170 to 300 times greater than the values measured for cuticular membranes. The data are explained by treating the lipid components of the cuticle (wax and cutin matrix) as two resistances acting in series. NAA permeability of Citrus leaf cuticular membranes is shown to be completely determined by cuticular waxes, because of the very low solubility of NAA in the waxes. All the evidence presented is consistent with the movement of NAA within the lipid components of the membranes and not in aqueous pores.