Current agricultural crop management practices stand in need of more effective ways to enhance penetration by spray-applied systemic materials through rate-limiting, plant cuticular surfaces. A goal of this study was to develop a plant cuticular penetration model that may aid in identifying and quantifying factors in transcuticular transport that can be effectively managed to promote penetration. A diffusion model was developed that embodied a time-dependent diffusivity and an instantaneous plane source that simulated a finite-dose spray solution containing a systemic active ingredient, with and without an additive intended to increase penetration. The time-dependent, three-layer, apparent diffusivity model was intended to simulate a donor layer on the plant surface whose driving force changed with time owing to spray-droplet drying and alteration of active-ingredient properties by solution additives. The model was validated by comparing its predictions of cuticular- penetration with laboratory data for the anionic form of 1-naphthylacetic acid (NAA) in the presence and absence of ammonium nitrate (AMN), which increases NAA uptake. Data were obtained with a finite-dose diffusion cell under defined laboratory conditions. The model satisfactorily simulated the experimental observations over a time course of 120 h, other than a tendency to overestimate penetration during the first 10 h following application. Model results also support the possibility that AMN alters the anionic form of NAA to the more readily penetrating nondissociated form.
