Mechanisms of Herbicide Action

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Abstract
This review is concerned primarily with the biochemical aspects of mechanisms of herbicide actio and is limited to major trends and developments of the past five years. The major trend in mechanism research appears to have been toward determining the fate of herbicides in plants. The accumulated evidence suggests that an individual herbicide may be applied in one molecular form, translocated in a metabolized form, and express its inhibition at a site of action as still another chemical structure, whereas another herbicide may be absorbed, translocated, and accumulated at the site of action as the original molecule. Most herbicides, however, are rapidly and extensively metabolized. In a few instances, metabolism converts a relatively nontoxic material to an herbicidally active molecule; examples include reduction of dipyridyl compounds and B-oxidation or ester hydrolysis of phenoxy compounds. In most instances, however, metabolism seems to reflect detoxication mechanisms mediated through biochemical degradation or through physical or chemical conjugation with cellular constituents. Selective toxicity of herbicides in many instances can be explained by differences in abilities of species to carry out the various activation or inactivation reactions. Species differences in susceptibility are usually effected by a combination of morphological, physiological, and biochemical factors which influence the concentration of toxic material reaching the site or sites of action at any one time. The site(s) of action in different species may also be differentially sensitive to a toxicant; but such an explanation for selective action is not yet documented by experimental evidence. The diversity of principles involved in herbicide action precludes unqualified generalizations on mechanisms. However, most of the compounds reviewed herein fall into 2 major groups. One group is composed of compounds which move only in the xylem with the transpiration stream and consequently accumulate in the leaves. Their herbicidal action is generally attributed to inhibition of photosynthesis. Compounds of the other group seem to move either with the transpiration stream in the xylem or with photosynthate in the assimilate stream of the phloem; consequently, they become distributed throughout the plant and generally accumulate in young expanding tissues. For most of these herbicides, multiple sites and mechanisms of action must be considered a probability along with the possibility that the most sensitive site differs among species. Many possible sites of action have been described, but their contribution to lethal action is uncertain. The most difficult aspect of mechanism research on herbicidal action is substantiating the physiological significance of in vitro results under field conditions. There is a bibliography of 298 references.