Phloem mobility of xenobiotics: tabular review of physicochemical properties governing the output of the Kleier model

Abstract

The Kleier model of phloem-mobility of xenobiotics combines the intermediate permeability hypothesis with the acid trap mechanism for weak acids. The output of the model is dependent on the lipophilicity of a compound, for which octanol/water partition coefficients (log Kow) have been used as a measure. The membrane permeability of xenobiotics is predicted from these partition coefficients, and the nature of the sieve tube membranes has been modelled using regressions derived from Nitella or potato permeability data. A wide range of log Kow values for herbicides, fungicides, insecticides and experimental compounds (≈400) have been tabulated along with the model output for various membrane parameters. The application of the model is in broad agreement with literature and experimental observations on many of the known phloem mobile herbicides and predicts low phloem mobility for the fungicides and insecticides considered here, again in agreement with the literature. The behaviour of herbicides representative of the main chemical families and modes of action are reviewed, along with examples of the few phloem-mobile fungicides and insecticides identified.