Phloem Mobility of Xenobiotics

Abstract

A passive diffusion model has been developed which simultaneously accounts for the dependence of phloem mobility on permeability and acid dissociation. The model is consistent with the observation that the addition of an acid moiety to an otherwise phloem immobile compound may en-hance that compound's ability to move in the phloem. However, acid trapping in the basic phloem is not the only enhancement factor. Acid functionalization also lowers the effective permeability usually towards its optimum value. The unified theory predicts that for a given acid disso-ciation constant there is an optimum permeability and conversely for a given permeability there is an optimum dissociation constant. According to one theory, the phloem mobility of a compound depends upon the presence of a weak acid functionality within that compound (2). Another theory deals with compounds which are nonelectrolytes at physiological pH and ascribes efficient phloem mobility to compounds whose membrane permeabilities fall within some optimum range (6, 8). The optimum range is determined by attributes such as plant length, leaf size, phloem sap velocity, etc. The latter theory has been developed into a mathematical model (8). Neither of these theories invokes a carrier mechanism and the two theories are not necessarily in opposition. This paper describes an extension of the mathemat-ical model (8) that unifies the two theories. The extended model explains in a straightforward fashion the enhanced phloem mo-bility of weakly acidic compounds without invoking a carrier mechanism. While the model provides for acid trapping within the sieve tubes, the mobility of weak acids is in large part due to their intermediate permeability.