A Simple Theory Regarding Ambimobility of Xenobiotics with Special Reference to the Nematicide, Oxamyl

A theory is presented to explain the phloem mobility of certain systemic xenobiotics that are not weak acids. It is shown that there is a theoretically optimum permeability that permits optimum circulation through the symplasm and apoplast (including the phloem and xylem) of Solanum tuberosum plants. The optimum permeability is large enough to permit substantial passive permeation into sieve cells in the source leaf and yet is small enough to permit phloem transport with some retention. The optimum permeability is a function of the velocity of sap flow in sieve tubes, the radius of the sieve tube, the over-all length of the plant, and the length of the carbohydrate and xenobiotic sources. It is argued that the nematicide, oxamyl, is near the optimum permeability under some experimental conditions. It is shown that depending on the strength of the carbohydrate sink in roots or growth points and depending on the permeability of the xenobiotic, there can be passive accumulation of xenobiotics in the sieve tubes in the carbohydrate sink regions.