The success of the auxin-type herbicides is principally due to their highly selective action. However, since these herbicides behave differently in many species, it is thought that many factors interact and contribute to selectivity in various ways. Several examples of this variation are cited by Pillmoor and Gaunt (1981) in their extensive review of phe-noxyacetic acid herbicides. At an extreme level, a few micrograms may kill a susceptible species, but a tolerant crop may withstand milligram doses, although increasing the dose will eventually induce phytotoxicity in the crop. Furthermore, species sensitivity clearly varies with plant and tissue morphology and age. For example, young seedlings of cucumber (Cucumis sativus) and wild carrot (Daucus carota) are sensitive to 2, 4-D but develop tolerance as tissues mature.
Generally, herbicide selectivity is achieved either by differences in herbicide concentration reaching an active site or by differences in sensitivity at an active site. The former involves a consideration of herbicide uptake, movement and metabolism, since the amount of herbicide in a sensitive tissue is determined by its import and transport from the site of application and its metabolic fate in the largest tissue. A full balance sheet is therefore ideally needed for all these factors in both resistant and susceptible species to satisfactorily account for selectivity. However, such detailed information is invariably lacking with auxin-type herbicides. In general, differences in uptake and movement have been reported in many species, but no correlation between uptake, movement, and selectivity has been convincingly demonstrated. On the contrary, uptake and movement are sometimes faster in tolerant species! Studies with radiolabelled herbicides show that foliar uptake is typically rapid, and that active ingredients accumulate at major growth regions, especially the apical meristem, as a result of phloem transport (Figure 7.14). Slow but significant rates of root excretion of auxin-type herbicides has also been reported in some species, but how this is achieved, and to what extent it contributes to selectivity, remains unclear.
The pattern and extent of metabolism of auxin-type herbicides is also highly variable. Conjugation, hydroxylation, and side-chain cleavage are the principal routes for the metabolism of phenoxyalkanoic acids, the eventual products depending on the sequence of these processes.
Thus, the 2-methyl group of MCPA is highly susceptible to oxidation, and the hydroxyl group so formed is rapidly used in glycoside formation (Figure 7.15A). Direct conjugation of phenoxyacetic acids to form glucose and aspartate esters has also been widely reported, as has side-chain degradation to a corresponding phenol. This oxidation also yields glycolic acid, which is subsequently metabolised in photorespiration to carbon dioxide.
In addition, a reaction unique to the metabolism of 2,4-D, known as the NIH shift, is commonly observed in many species. Here, the migration of a chloride atom, usually from the 4- to the 5-carbon position, is probable evidence for an epoxide intermediate in ring hydroxylation, and the product is a substrate for further glucosylation (Figure 7.15B).
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