Physicochemical Aspects of Phloem Translocation of Herbicides

Abstract

A physicochemical approach to understanding phloem transport of xenobiotics requires that similarities in transport processes in different plant species far outweigh any differences. There is now evidence that this is so, though additional factors such as rate of cuticular penetration, metabolism, and phytotoxicity may differ among plant species and thus may influence distribution patterns. Most herbicides that are translocated in phloem are weak acids, and their transport behavior can now be explained, at least in part, in terms of accumulation and retention in phloem. These processes can, in turn, be explained reasonably well by considering the polarity and acid strength of each compound. Specific carrier processes do not appear to be involved in the transport of most phloem-mobile herbicides. Phloem transport of herbicides has been assessed using the castor bean plant. For acids of pKa <4, intermediate lipophilicity is required for good phloem transport, while weaker acids of pKa >5 and nonionized compounds need to be more polar in order to move well.

New proton donors in electrochemical mechanistic studies in non-aqueous solution dimethylsulfoxide: Chlorinated hydroxybenzonitriles

“…One can conclude that the ability to provide protons increases in order p-hydroxybenzonitrile (pK a = 8.0) < ioxynil 3 (pK a = 4.0) < 3-chloro-4-hydroxybenzonitrile 2 (pK a not available) < chloroxynil 1 (pK a = 3.1). This is in agreement with the decreasing pK a of these compounds reported in literature [24,25].…”

Leaf Epidermal Cells: A Trap for Lipophilic Xenobiotics

“…Therefore, the effect of hydrophilic xenobiotics on plants should not be neglected. Indeed, such compounds tend to be phloem mobile (Bromilow et al 1990) and could be even more noxious on a whole-plant scale.…”

Modeling of Uptake of Xenobiotics in Plants

“…Among the contributions dealing with the phloem in an overall manner, Wardlaw (31), Fischer and Oparka (32), Hsu and Kleier (33)(34)(35)(36)(37)(38)(39)(40), MacRobbie (41), and Van Bell (42) and others (43)(44)(45)(46)(47)(48)(49) deserve special attention. The character of transport that takes place in the phloem and xylem was dealt with by Peterson (50), who distinguished between three xenobiotic transportation paths in a plant:…”