Phloem mobility of xenobiotics. II. Bioassay testing of the unified mathematical model

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.