Estimation of path lengths for diffusion of organic compounds through leaf cuticles

Abstract

The effect of waxes on the mobilities of organic solutes in isolated leaf cuticular membranes (CM) of six species was investigated using compounds with molar volumes ranging from 99 cm 3 mol -1 (salicylic acid) to 349 cm 3 mol -1 (cholesterol). When log(solute mobility) was plotted versus molar volumes straight lines were obtained for all plant species showing that the mobilities decreased exponentially with increasing molar volumes of solutes. The slopes of these graphs represent the size selectivity of the CM. Extracting waxes from the CM using chloroform-methanol had no effect on size selectivity, but absolute values of mobilities increased by 28-fold (Citrus grandis L. and Camellia assamica L.) to 759-fold (Ilex paraguariensis St.-Hil). Since the removal of waxes did not affect size selectivity it is argued that extraction reduced the path lengths and these values are a measure for tortuosity, that is the ratio of path lengths in CM and MX. Mobilities in the polymer matrix of extracted cuticles did not differ much among species with the exception of C. assamica. Although the mobilities in CM of I. paraguariensis and C. assamica were similar, the mobilities in the polymer matrix of C. assamica were almost 40-fold lower than in I. paraguariensis. Neither the mobilities in CM and MX nor the tortuosity were correlated with thickness of CM or amounts of wax.Key-words: cuticular wax; diffusion; foliar uptake; penetration; permeability; size selectivity; tortuosity. INTRODUCTIONThe cuticle of plants is the outermost part of the epidermis that covers all primary organs exposed to air and it is also present in mature embryos within seeds (Martin & Juniper 1970). As the interface between aerial plant organs and the environment the cuticle has many functions. One important feature is its low permeability for water, inorganic ions and organic solutes. Cuticular transpiration determines the minimum water loss from a plant when all stomata are closed (Kerstiens 1996). Often, plant cuticles limit the rates of foliar penetration of agrochemicals into leaves and fruits and effectively prevent leaching of solutes from the apoplast (Schönherr & Baur 1994. Cuticles are heterogeneous membranes with high variability in structure among species (Holloway 1982;Jeffree 1996). Three layers can be distinguished which differ in composition, sorptive properties and solute mobilities. Depending on species the surface is covered by a more or less pronounced layer of epicuticular waxes which are deposited on the thin transport-limiting skin composed of intracuticular waxes and cutin, while the bulk of the cuticle between the limiting skin and the epidermal cell wall is made up of cutin and polysaccharides. The latter part below the limiting skin adds little to the barrier properties of the cuticle for organic compounds but it has the highest sorptive capacity (Schönherr & Baur 1994).Due to the heterogeneity in structure and transport properties and the ill-defined thickness of the limiting skin of the cuticular membranes (CM), diffu...

Cloning and Characterization of the WAX2 Gene of Arabidopsis Involved in Cuticle Membrane and Wax Production

“…wax2 has reduced visible density in the cuticle membrane, suggesting that the internal molecular packing of cuticle lipid molecules may be less dense. If this is so, the proportion of crystalline regions in the cuticle, which serve as exclusion zones for diffusion (Baur et al, 1999), may be reduced, leading to more direct pathways for water diffusion through wax2 cuticles. The effect of wax2 on cuticles that overlie the stomatal ridges, pores, or substomatal chambers (Osborn and Taylor, 1990;Zhao and Sack, 1999) or the inner epidermis adjacent to the mesophyll (Pesacreta and Hasenstein, 1999) is unknown.…”

Probability distribution, sampling unit, data transformations and sequential sampling of European vine moth, Lobesia botrana (Lepidoptera: Tortricidae) larval counts from Northern Greece vineyards

“…Maher & Thiéry (2004) showed that females of L. botrana can perceive chemical information from grape berries and that the intensity of their oviposition response is associated with the strength of the stimulus. The active compounds may originate from the interior tissues of berries and reach the surface via diffusion paths in cuticle layers (Baur et al, 1999). Hence, injury to grapes caused by an earlier infestation of larvae may increase the concentration and vagility of those compounds, resulting in an increase in stimulatory chemical information for ovipositing females.…”

Water and oxygen permeance of phellems isolated from trees: the role of waxes and lenticels

“…However, cuticles are not impermeable. They show measurable and characteristic permeances for water (Becker et al 1986), chemicals (Baur et al 1999), and gases (Lendzian and Kerstiens 1991;Kerstiens 1996). For lipophilic organics like agrochemicals the cuticle seems to serve as the sole pathway for entrance into the plant (Scho¨nherr and Riederer 1989;Buchholz et al 1998).…”