Phloem Mobility of Xenobiotics: II. Bioassay Testing of the Unified Mathematical Model

Abstract

Two bioassays were used to test phloem mobility of selected xenobiotic compounds: (a) excised bean leaf assay; (b) rooted bean leaf assay. Compounds assayed were N-alkylpyridiniums with systematic variation in octanol-water partition coefficients (log K  _ow), substituted benzoic acids with about the same log K  _ow value but variable acidities. Results of the assays strongly conform, quantitatively, to the predictions of the unified mathematical model. Results also indicate that the membrane permeability value of a compound, which depends directly on log K  _ow value, is the overriding factor in determining phloem mobility. When the weak acid functionality of a compound confers increased phloem mobility, it does so principally by making the log K  _ow value, and consequently the membrane permeability of the compound more optimal.