Finite dose diffusion studies: I. Characterizing cuticular penetration in a model system using NAA and isolated tomato fruit cuticles

A finite dose diffusion system was employed to study cuticular penetration of 2-(l-naphthyl) [1-C-14]acetic acid (NAA) from simulated spray droplets t hrough enzymatically isolated tomato fruit cuticles (Lycopersicon esculentu m Mill cv Pik Red). Isolated cuticles were mounted on diffusion half-cells with the cell wall surfaces facing a 20mM citric acid receiver solution (pH 3.2, volume 2.9ml, prepared with deionized water). A 5-mul donor droplet c ontaining NAA at 100 muM in 20mM citric acid buffer (pH 3.2) was applied to the outer surface. Penetration was monitored by repeated sampling of the r eceiver solution. NAA penetration was characterized by (1) an initial lag p hase of about 2.3h, (2) a phase of nearly constant maximum rate of penetrat ion averaging 6.3% of applied NAA h(-1) (equivalent to 0.032 nmol h(-1)) an d (3) a plateau phase approaching an asymptote at 81.2% of applied NAA (equ ivalent to 0.406 nmol) at 120h. Within 1h after application droplets appear ed dry on visual inspection. Immediately after droplet drying, 7.0% of the applied NAA was sorbed to the cuticle, but only 0.5% penetrated into the re ceiver solution, indicating that penetration occurred almost exclusively fr om the apparently dry deposit. At 120h, 5.2% of the NAA applied was associa ted with the deposit and 4.3% with the cuticle. The distribution of maximum rates of penetration was lognormal, but penetration at 120h followed a nor mal distribution. Cuticle thickness (estimated 5-25 mum) had no significant effect on NAA penetration. Maximum rates of penetration through pepper fru it and citrus and ficus leaf cuticles were 4.9-, 2.6- and 0.1-times that th rough tomato fruit cuticles. At 120h, penetration averaged 85.5, 79.5 and 3 4.7% fbr pepper, citrus and ficus cuticles, respectively. Extracting epicut icular and embedded waxes increased NAA penetration rates through tomato fr uit cuticle more than three-fold, but had little effect on penetration at 1 20h (71.0 vs 87.7% for cuticular vs dewaxed cuticular membranes). The maxim um penetration rate and total penetration were found to be useful parameter s in describing the penetration time-course. (C) 2000 Society of Chemical I ndustry.