Recent developments in our understanding of the plant cuticle as a barrier to the foliar uptake of pesticides†

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The plant cuticle is a highly complex membrane which forms the outer surface of the aerial portion of plants. The nature of the plant cuticle is reviewed with particular regard to its action as a potential barrier to the penetration of pesticide molecules; the role of the cuticular waxes is highlighted. The physicochemical properties of the cuticle influence the behaviour of spray droplets and, in turn, may affect the rate and efficiency of cuticle penetration. The permeation of active ingredients is influenced by their solubility characteristics as indicated by octanol/water (log K ow ) and cuticle/water (K cw ) partition coefficients. Penetration of hydrophilic compounds (low log K ow ) may be enhanced by hydration of the cuticle, while transcuticular transport of non-polar solutes (high log K ow ) is increased by factors which reduce wax viscosity. The use of in-vitro models involving isolated cuticle membranes, isolated cuticle waxes, or isolated leaves has helped to focus on the activities of the cuticle in the absence of other physiological factors. Using these systems, the role of the waxes as a transport-limiting barrier has been identified and the factors influencing sorption, permeance and desorption examined. The action of surfactants, in vitro and in vivo, has been briefly addressed in regard to their role in facilitating cuticle penetration; other steps involving surfactant/solute/cuticle are complex, and synergy appears to depend on a number of factors including test species, concentration of active ingredient, surfactant type and concentration. Adjuvants may greatly influence the surface properties of the droplet, predispose the cuticle to solute transport, and enhance pesticide activity. The nature of these complex inter-relationships is discussed.