Penetration by Herbicides

The properties of the cuticle which determine herbicide absorption have been reviewed by Price and Kirkwood. Permeability of the cuticular membrane is determined largely by the wax component of the cuticle and is influenced by such factors as cuticle thickness, wax viscosity, molecular radius of the penetrating molecule, partition coefficient, effective area, formulation, and temperature. Pores and canals, which have been demonstrated to exist in the cuticular membrane of many species, appear to act 

The barrier to penetration of water and solutes across the cuticular membrane occurs at the air-cuticle interface. This solid-state barrier is composed of cutin and waxes, and the latter are presumed to be relatively inaccessible to polar compounds. The determining factor in cuticle penetration by herbicides appears to involve the HLB of the compound and its ability to partition from air into the nonpolar wax components and thence into the more polar regions within the basal regions of the cuticle. 

The characteristics of cuticle penetration by nonpolar (lipophilic) chemicals have been investigated in depth by Schonherr and his co-workers, with particular reference to accumulation in plant cuticles, permeation across plant cuticles, and desorption from plant cuticles.  

Plant cuticles can be regarded as potential sorption compartments for lipophilic compounds, and sorption of moderately lipophilic phenolic compounds can be predicted from the octanol/water partition coefficients (Xow). Partition coefficients in the cuticle/water system are important parameters for the transport of compounds across plant cuticles and for the uptake of such compounds.  

Kerler and Schonherr have studied the accumulation of lipophilic chemicals in plant cuticles. Using cuticles isolated enzymatically from mature leaves of Citrus aurantium L., they examined the sorption of a range of C-labeled compounds of widely differing lipid solubilities. They found that logarithms of cuticle/water (Kcm/w) and octanol/water (/(Cow) partition coefficients between 10 and 10, the logarithms were correlated by linear regression equations with coefficients of correlation from 0.980 to 0.988. 

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