Phloem mobility of xenobiotics. IV. Modelling of pesticide movement in plants

Abstract

AbstractA model is described that accounts for the effect of physical properties on the mobility of a xenobiotic within the phloem tissue of a plant. This model builds on the intermediate permeability hypothesis by incorporating the effect of acid dissociation. The relative importance of lipophilicity and acid‐base properties of compounds is discussed. The sensitivity of the model predictions to plant parameters such as the nature of the sieve tube membranes is also explored. Experimental support for the model is presented using examples from the literature. The systematic behaviour of mobility as a function of physical properties for both non‐ionized and acidic compounds is well accounted for by the model with only rare exceptions. Application of the model to commercial plant‐protection chemicals is generally consistent with experimental observations. Thus, many of the herbicides that are known to be phloem‐mobile are predicted to be so by the model. However, none of the selected fungicides and only a few of the insecticides are predicted to be phloem‐systemic, again in general agreement with observation.