Although most authors accept the selective inhibition of ACCase as a primary target of the AOPPs and CHDs, it has also been proposed that these and other graminicides can also act as anti-auxins. Diclofop-methyl, for example, has no auxin activity alone, but can inhibit several auxin-mediated processes such as coleoptile elongation and proton-efflux (Figure 8.4), and alter cell membrane potentials, possibly by acting as a protonophore (Wright and Shimabukuro, 1987) . Disruption of memberane function in the form of rapid depolarisation of the plasma membrane electrogenic potential is reported after ACCase inhibitor treatment of susceptible species. This phenomenon is not observed when the same herbicides are applied to ACCase-resistant biotypes or species. However, this alternative hypothesis of graminicide action has been rejected by many workers as a secondary consequence of the inhibition of lipid biosynthesis which would, it is argued, alter membrane function. Furthermore, for this hypothesis to gain acceptance it would be necessary that this anti)auxin activity satisfy the following four criteria:
(a) shows at least equal sensitivity to the micromolar concentrations known to inhibit ACCase,
(b) is observed within minutes,
(c) shows stereospecificity, and
(d) is selective in grasses.
Figure 8.4 Inhibition of auxin-induced proton-efflux by diclofop-methyl (DM). O, Control; 50 |M DM; 10 |M MCPA; ■, 10 |M MCPA + 50 |M DM (from Cobb and Barnwell, 1989, with permission).
Several studies have provided supportive or circumstantial evidence to fulfil each of the above criteria, and more detailed and convincing data are slowly emerging. For example, micromolar concentrations of several AOPPs can inhibit auxin-induced proton-efflux from sensitive coleoptile segments within minutes (Figure 8.4, Cobb and Barnwell, 1989), and stereospecificity has been demonstrated (Andreev and Amrhein, 1976). Evidence is also accumulating for a selective anti-auxin activity being confined to grasses. Young, rapidly elongating monocotyledonous tissues are especially sensitive to graminicides, and a rapid retardation of grass internode elongation is commonly observed, with the result that sensitive grasses show significant stunting when compared to control plants and cereal crops. The need for a rapid rate of tissue extension has recently been identified as a prerequisite for optimal graminicide activity, since decreased extension rates associated with water stress, for example, severely reduce diclofop efficacy (Andrews et al., 1989) . Finally, it is surely pertinent that a foliar-applied graminicide must first come into contact with a cell membrane before its subsequent intracellular metabolism and translocation to the grass meristem. The opportunity therefore exists in vivo for a graminicide to demonstrate anti-auxin activity in addition to an inhibition of ACCase.
This additional activity of graminicides is thought to be of some commercial significance when herbicide mixtures are taken into account. Since an arable field will contain a mixture of both monocotyledonous and dicotyledonous weeds, it would be of obvious advantage to the farmer to mix a broadleaf weed, auxin-type herbicide with a graminicide. This single treatment could ensure a broad spectrum of weed control with additional savings in time, labour and fuel costs. However, it is well known that such tank mixtures commonly result in reduced efficacy and crop damage is often observed. Antagonism of ACCase inhibitor action by a number of herbicides including 2,4-D, dicamba, ALS inhibitors, bentazone and acifluorfen has been reported. This antagonism does not appear to be at the sites of action of the herbicide and may be due to the decreased uptake and movement of the ACCase inhibitors. Indeed, broadleaf weed herbicides such as bentazone are able to reduce the uptake of some graminicides, and so account for a reduction in gramini-cide efficacy.
On the other hand, if the broadleaf weedkiller is an auxin-type herbicide it may be argued that the anti- auxin activity of the graminicide is overcome by the addition of 'extra auxin'. Hence, less stunting is observed in treated grasses and plants may recover to set seed. The real significance of these observations remains to be established, although further understanding of this interaction may eventually lead to the design of more compatible graminicide/broadleaf weedkiller mixtures in the future.
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