Desorption of chemicals from plant cuticles: Evidence for asymmetry

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Abstract

Isolated cuticular and polymer matrix membranes from four plant species (Citrus, Ficus,Lycopersicon, Capsicum) were preloaded with14C-(2,4-dichlorophenoxy)acetic acid (2,4-D) by sorption from solutions and subsequently subjected to simultaneous bilateral desorption. With cuticular membranes, only 2 to 3% of the 2,4-D initially contained in the cuticles could be desorbed from the outer surface, while 86 to 92% were desorbed from the inner surfaces within 6 hr. Initial desorption rates where 50 to 80 times higher from the inner surfaces than from the outer surfaces. This asymmetrical desorption is mainly due to the presence of apolar and crystalline soluble lipids (waxes) on the surface and in the outer layers of the cuticles. These constituents drastically decrease the mobility of 2,4-D in the outer layers of of the leaf cuticles. In fruit cuticles, extensive cutinization of anticlinal and periclinal walls increases the inner surface area and thus contributes significantly to asymmetry. The fate of chemicals sorbed in cuticles depends on the presence or absence of a sink. If a sink exists (metabolization, translocation away from the epidermis) all cuticles will exhibit a pronounced inward permeability and accumulation in the cuticles is not likely to occur. Accumulation in the cuticles will occur only in absence of a sink. Ecotoxicological implications such as monitoring environmental pollution history by analyzing contents of sorbed chemicals in cuticles and detoxification by leaching are discussed.