Comparison of sorption and diffusion by pyridate and its polar metabolite in isolated cuticular wax of Chenopodium album and Hordeum vulgare.

Sorption and diffusion of the herbicide pyridate and its metabolite CL9673 were measured in reconstituted cuticular waxes isolated from Chenopodium album L. and Hordeum vulgare L. (cultivar Igri) leaves. The compounds have the same basic chemical structure, except that pyridate is characterized by a C8-alkyl chain bound via a thioester to the ionizable hydroxyl group of CL9673. Sorption of the weak acid CL9673 from aqueous solutions into cuticular waxes was pH-dependent, and the apparent wax/water partition coefficients decreased with increasing pH. Wax/water partition coefficients of pyridate were not dependent on pH, and they were about 4 orders of magnitude higher as compared to the nondissociated species of CL9673. Diffusion coefficients measured in reconstituted cuticular wax for CL9673 fitted established models predicting diffusion coefficients in relation to molar volumes. However, this was not the case with pyridate, which was characterized by a self-accelerating effect leading to diffusion coefficients, which were up to 2 orders of magnitude higher than predicted from the molar volume. This is a remarkable result since pyridate represents a compound combining the properties of an active ingredient and of a plasticizer.