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

Abstract

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 inýuence the behaviour of spray droplets and, in turn, may aþ ect the rate and efficiency of cuticle penetration. The permeation of active ingredients is inýuenced by their solubility characteristics as indicated by octanol/water and cuticle/ (log K ow ) water partition coefficients. Penetration of hydrophilic compounds (low may be (K cw ) log K ow ) enhanced by hydration of the cuticle, while transcuticular transport of non-polar solutes (high log is increased by factors which reduce wax viscosity. The use of in-vitro models involving isolated K ow ) 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 identiüed and the factors inýuencing sorption, permeance and desorption examined. The action of surfactants, in vitro and in vivo, has been brieýy 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 inýuence 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.

Biochemical analysis of elastic and rigid cuticles of Cirsium horridulum

“…One of the functions of the cuticle is as a barrier for the permeation of solutes (Kirkwood 1999) and water (Holloway 1994). Cuticular resistance to water loss depends on lipids embedded in the polymer matrix (Schreiber et al 1996).…”

Foliar Interactions and Uptake of Agrichemical Formulations–Present Limits and Future Potential

“…Though uptake is predominantly required by systemic bioactives, non-systemic actives can also benefit by absorption into the surface or epicuticular wax, or cuticle of the organism. Uptake into plant leaves can occur by two pathways, either stomatal infiltration or by diffusion through the leaf cuticle [27,28]. Insects or their larvae may ingest insecticides (intestinal absorption), absorb them directly through their cuticle, or their spiracles can be infiltrated by solutions.…”

Modeling Xenobiotic Uptake and Movement: A Review

“…The cuticle proper, which bears a superficial layer of soluble epicuticular waxes, constitutes the outer region of the cuticle and consists mainly of soluble and polymerized aliphatic lipids (including intracuticular waxes), whereas the cuticular layer represents the inner layer, which forms by impregnation of the cell wall and contains large amount of various cell-wall polysaccharides. Adjacent to the cuticular layer is a pectinaceous layer of pectin-rich cell-wall polysaccharides (35,36).…”

“…This physical structure plays an important role in the barrier property of plant cuticles (6,45). Although the epicuticular waxes play an important role in the wettability of leaf surfaces and affect spray deposition, distribution and retention (35,46,47), their impact on the rate of penetration of a foliar-applied xenobiotic into leaves is limited (36,48). The main barrier to the penetration of foliar-applied xenobiotic consists of the cutin matrix and the embebbed intracuticular waxes (49,50).…”

“…Water permeability of cuticles has been extensively studied (30,(57)(58)(59). The interest for improved efficacy of foliar-applied agrochemicals mainly pesticides has led to extensive research in the understanding of cuticular penetration of these xenobiotic compounds (5,35,(60)(61)(62)(63). Plant cuticles are permeable to water, non-polar lipophilic compounds, ionic and other polar compounds, and it has been suggested that movement in the cuticular membrane follows two parallel pathways for lipophilic and hydrophilic xenobiotics, respectively (64,65).…”