Phloem Mobility of Xenobiotics. III. Sensitivity of Unified Model to Plant Parameters and Application to Patented Chemical Hybridizing Agents

Abstract

A refined model for the phloem translocation of xenobiotics in living plants is presented. The model accounts for the translocation of a xenobiotic that can exist in several ionic forms in terms of the permeabilities of these forms and the fractions of the forms that exist both within the sieve tubes and in the surrounding apoplast. Predictions of the model are quite sensitive to the description of the membranes used in the model, with optimum log Kow's and pKa's becoming more positive as the membranes become less permeable. The literature generally supports the predictions of the model. When applied to patented chemical hybridizing agents, the model predicts that most are phloem-mobile. This prediction strongly suggests that the chemical hybridizing agents exert their pollen-suppressing activity by direct effects on pollen and/or the surrounding maternal tissues. With low transpiration rates and the discontinuity of xylem in many reproductive tissues, phloem translocatability may be an essential requirement for the whole plant activity of many chemical hybridizing agents.