AbstractA simple and relatively rapid test has been developed that enables the phloem mobility of xenobiotics to be assessed. Compounds are applied by injection of an aqueous solution into the petioles of plants of castor bean (Ricinus communis var. Gibsonii), and phloem sap is collected from two incisions made in the stem below the treated leaves over the period of 2 to 4 h after application. Amounts of compounds in the sap are measured by liquid scintillation counting following thin‐layer chromatography for radio‐labelled samples or by high‐pressure liquid chromatography for non‐radio‐labelled compounds. A semi‐quantitative estimate of mobility in phloem can be obtained from these tests, including an estimate of the potential for long‐distance transport via phloem. Mobilities in phloem measured for 28 compounds, mostly weak acids, are placed within a general framework for understanding the phloem transport of compounds in terms of their physico‐chemical properties. This approach can be used to predict and interpret the behaviour of new compounds from a knowledge of their polarity an
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