Review of sorption and diffusion of lipophilic molecules in cuticular waxes and the effects of accelerators on solute mobilities

Many agrochemicals are applied to the leaf surfaces of crop plants. Systemic chemicals have to penetrate through the cuticle, which forms an effective transport barrier. The barrier properties of cuticles are mainly due to the cuticular waxes deposited as partially crystalline aggregates on the outer surfaces of leaves. Substances increasing the mobilities of agrochemicals in cuticular waxes are called accelerators and it is shown that they act as plasticizers when absorbed by cuticular waxes. They decrease the barrier properties of the waxes and thus increase the mobilities of the agrochemicals through them. In order to analyse the efficiency of different accelerators, the sorption and mobility of both agrochemicals and accelerators within cuticular waxes was measured. Such information was used to establish correlations between the internal concentrations of accelerators and their mobility-enhancing effects on agrochemicals in the cuticle. This, in turn, allowed the determination and comparison of the intrinsic effects of different accelerators and to rationalize the effect of accelerators on the cuticular permeability of agrochemicals. Results describing the sorption (partition coefficients) and mobility (diffusion coefficients) of lipophilic organic molecules in reconstituted cuticular waxes from different plant species, and the effect of two different classes of accelerators (alcohol ethoxylates and n-alkyl esters), on the mobility of organic molecules are presented and discussed.