DETERMINATION OF MOBILITIES OF ORGANIC-COMPOUNDS IN PLANT CUTICLES AND CORRELATION WITH MOLAR VOLUMES

Mobilities of selected pesticides in plant cuticles have been measured using unilateral desorption from the outer surface (UDOS). The outer layer of the cuticle limits the rate of foliar uptake and solute mobilities in this layer have been determined. The primary parameter measured was the first-order rate constant of desorption. From these rate constants, permeances, half-times of penetration and diffusion coefficients could be calculated. Rate constants were lowest in bitter-orange Citrus aurantium L.) leaf cuticles and this cuticle turned out to be very size-selective, as rate constants decreased by a factor of 264 when molar volumes increased by only a factor of about three. Rate constants measured using green-pepper (Capsicum annuum L. cv. Bell Boy) fruit cuticles were larger by one to two orders of magnitude and this cuticle was considerably less size-selective. A good correlation between rate constants and molar volumes was observed for cuticles from both plant species, which means that rate constants can be estimated from molar volumes of pesticides. Rate constants of desorption are mobility parameters which do not depend on lipophilicity of solutes. Thus, it is now possible to separate the contributions of mobilities and solubilities in cuticles to rates of foliar uptake of pesticides.