The herbicide glyphosate and related molecules: physicochemical and structural factors determining their mobility in phloem

The herbicide glyphosate shows excellent symplastic distribution to roots and new growth when applied to the foliage of plants. Though the phloem transport of non‐ionised and of monobasic weak acids can be understood in terms of the intermediate permeability theory, the multiple ionisable functionality of glyphosate, which has three acid groups and a strong amine base, precludes simple interpretation of its systemicity. Accordingly, the behaviour of an analogue of glyphosate based on a phenyl ring and carrying the same ionisable functionalities has been examined in castor bean ( Ricinus communis L) plants, together with that of related compounds in which the number of ionisable groups has been sequentially reduced. The close glyphosate analogue was symplastically transported almost as efficiently as glyphosate itself, whereas its monoethyl phosphonate ester, together with simpler analogues such as phenylphosphonic acids and phenylalanine, were less well translocated. It is thus the unique combination of ionisable functionality that confers good symplastic mobility, and loss of one or more of these ionisable functionalities leads to decreased movement in phloem. The reasons for such functional requirements for effective transport via phloem are not understood, though there is no evidence for recognition by specific carriers; the accumulation of glyphosate in phloem vessels is good but not exceptional, but once transported to roots and rhizomes it is exceptionally well retained. © 2000 Society of Chemical Industry