Plant Cuticles Are Polyelectrolytes with Isoelectric Points around Three

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Abstract
The isoelectric points of isolated cuticles from Citrus aurantium L. (3.15), Prunus armeniaca L. (3.45), and Pyrus communis L. (2.90) leaves were determined from membrane potentials. At pH values below the isoelectric point, cuticular membranes carry a net positive charge and are permselective to anions (determined using 82Br−). Above the isoelectric point, they carry a net negative charge and are permselective to cations (determined using 24Na+). There are no gradients of fixed charges across the cuticular membranes as indicated by the absence of asymmetry potentials. Positive charges in the membranes originate from residues of basic amino acids of proteins or polypeptides contained in a nonextractable form within the cuticle. The exchange capacity of basic fixed groups in the cuticles of six species (Lycopersicon esculentum Mill., Capsicum annuum L. fruit cuticles, and Brassaia spec. leaf cuticles in addition to the above species) varied between 0.010 and 0.025 meq g−1 cuticle. Fixed acidic groups were donated by residues of acidic amino acids, polygalacturonic acid, and nonesterified -COOH groups of the cutin polymer. At pH 8, total cation exchange capacity as determined using 45Ca2+ varied between 0.26 (Citrus) and 0.30 (apricot) meq g−1.