Permeability of Apricot Leaf Cuticle

Although cutinized tissue in some form is the first barrier to the entry of chemicals into plants, direct measurements of the permeability properties of cuticular membranes are limited and qualitative (1, 5). From direct measurements on isolated cuticle Skoss (9) has reported that English ivy leaf cuticle is relatively impermeable to water, and that lemon leaf cuticle is impermeable to the sodium salt of 2,4-dichlorophenoxyacetic acid, to the ammonium salt of dinitro-o-sec-butylphenol, and to silver and chloride ions. Similarly Orgell (7) has reported little, if any, penetration of isolated apricot leaf cuticle by "polar solutes in aqueous solution, e.g. acids, bases, salts, dyes, and miscellaneous other compounds." Adding to these his own observation that oils do not penetrate the upper cuticle of apricot leaves (11), van Overbeek (12) concluded that penetration of leaf cuticle by chemicals is unlikely. Mitchell, Smale, and Metcalf (5), however, accepting that cuticle in some form covers the entire aboveground surface of the plant, concluded that hydrated cuticle is permeable to a degree to water and to dissolved compounds associated with this water, and is highly permeable to nonpolar compounds. The objectives of this work were to quantitatively measure penetration through stomata-free isolated leaf cuticle and attempt to assess the importance of this entry pathway in intact plants; to measure the effect of substituent variation on penetration by the model compound a-chloroacetamide (ClCH,CONH2); and to seek a correlation between penetration and chloroform-water partitioning that might allow a prediction of leaf penetration rates of other compounds.