Modelling Foliar Penetration: Its Role in Optimising Pesticide Delivery

Abstract

Rates of foliar uptake are often limited by slow cuticular penetration of active ingredients (ai). Rates of cuticular penetration have been modelled based on two processes, namely solute mobility in cuticles (k*) and partitioning of ai or adjuvants between the formulation residue, and cuticular waxes (Kwxfr). The model distinguishes between effects of ai and adjuvants on k* and on driving force of penetration, which is proportional to the product Kwxfr.Cfr, the latter variable being the concentration of the ai in the formulation residue. Solute mobilities in cuticles varied greatly among species but for neutral ai it depended only on molar volumes and not on lipophilicity. Solute mobility decreased exponentially with increasing size of ai but size selectivity (β′) was the same for cuticles from all plant species tested and also for extracted cuticles (polymer matrix). Differences in solute mobility among species were related to the term k*0, which represents the mobility of a hypothetical molecule having zero molar volume. While size selectivity did not change on extraction of cuticular waxes k*0 increased by orders of magnitude depending on plant species. Evidence is presented showing that differences in solute mobility among species and higher solute mobility in extracted cuticles are related to the tortuosity factor. Crystalline waxes increase the lengths of the diffusion paths and decrease mobility in cuticles proportional to the tortuosity factor. Solute mobilities increased with increasing temperatures and it is argued that at 5 to 10 ° solute mobilities can become so low that significant penetration of ai is prevented.