The last few years have seen the publication of experimental cuticular mobilities for diverse chemicals in several plant species. The measurements are made in vitro using either cuticles isolated enzymatically, or using reconstituted wax extracts from the cuticles; it can be shown that these different methods are consistent with each other. Mobility parameters have been measured both in the presence and absence of uptake-enhancing adjuvants. After accounting for partitioning effects, cuticular diffusion coefficients can be calculated and these diffusivities are observed to have a simple dependence on the molar volume of the diffusing chemical.
This paper presents anecdotal evidence of several kinds to suggest that this dependence can be used to predict when an active ingredient will be sensitive to the use of mobility-enhancing adjuvants. Specifically, it is expected that foliar-active herbicides will be sensitive to such adjuvants if the logarithm of their cuticular mobility [m2/s] is between about -16 and -15. We show also that in the presence of an optimal adjuvant for the specific system, the movement of a chemical through the cuticle can conform to the familiar case of diffusion through a fluid, and is governed by the Stokes-Einstein relation when modified to account for the effect of hindered diffusion through the narrow transport paths within the cuticle. The observed dependence of diffusivity on molar volume then implies a very low value for the “free” volume (voidage) that is available for diffusion within the cuticle. In turn this low value implies the presence of structural features within the cuticle, and microscopic studies of the cuticle are consistent with this interpretation. Thus there appears to be a direct connection between in vitro measured cuticular mobilities, the need for adjuvants to promote uptake, and the structure of the plant cuticle.