Studies on Foliar Penetration

Abstract

The effects of chlorine substitution on the movement of phenoxyacetic and benzoic acids through enzymatically-isolated cuticles of Lycopersicon fruits were determined by following the transfer of each acid containing ¹⁴C from a donor to a receiver solution. This cuticle is characterized by an isotropic cutin matrix, within which patches of birefringent cuticular waxes are found near the outer surface. The outer, morphological surface is relatively smooth while at the junction with the outer walls of the epidermal cells there is extensive cuticular development extending down between the anticlinal walls. The epicuticular wax appears as a soft sheet-like covering of which the surface is relatively featureless. Chlorination of phenoxyacetic acid results in an enhanced transfer across the isolated cuticle. The order was 2,4,5- and 2,4,6-trichlorophenoxyacetic > 2,4- and 3,5-dichlorophenoxyacetic > 2-chlorophenoxyacetic > phenoxyacetic acid. Removal of the epicuticular wax resulted in greater permeability for all compounds; transfer of the more polar acids was favoured. In contrast, chlorination of benzoic acid decreases passage through the cuticle; the rate is highest for benzoic acid followed in descending order by 2-chlorobenzoic, 2,4- and 2,5-dichlorobenzoic and 2,3,6-trichlorobenzoic acid. Chlorination also depresses the passage of both phenoxyacetic and benzoic acid through a dialysis membrane. The effects of chlorination on the lipid solubility of both series of compounds are discussed in relation to differences in transfer across the cuticle.