Selectivity

Crop selectivity to the thiocarbamates is achieved through depth protection and metabolism. These herbicides are volatile and so need to be rapidly incorporated into the soil to be effective. Indeed EPTC, the most volatile example of this class, must be incorporated within 15min of application! Such instability ensures that EPTC can be used to clear a soil of germinating couch grass (Elytrigia repens) or wild oats (Avena spp.) and that it mostly disappears before a crop is planted in the following week or so. Triallate is incorporated into the top 2.5 cm of soil and also controls grasses at germination. Cereal seeds are then drilled at a minimum 4 cm depth and their seedlings grow through this zone, the sensitive meristem being protected by the coleoptile and primodial leaves. On the other hand, germinating wild oat seedlings have an elongating mesocotyl (first internode) that extends the unprotected wild oat meristem into the phytotoxic chemical barrier. However, wild oats germinating at the soil surface or at greater depths than 4 cm may not be controlled by this treatment, and so mixtures with more persistent soil-applied herbicides such as atrazine are often used in practice. In addition, thiocarbamates appear to undergo bio-activation in susceptible species by the process of sulphoxidation. The sulphoxides so formed create an electrophilic centre in the molecule which seems to be linked to an increase in phytotoxicity. Tolerant crops such as cereals, and especially maize, appear able to detoxify the sulphoxides by conjugation with glutathione.

The chloroacetamides, such as alachlor and metolachlor, are absorbed onto soil colloids and may remain active for two or three months without major soil leaching. Selectivity

Cl carboxylesterase

cooch3

diclofop-methyl (inactive)

Figure 8.6 Metabolism of diclofop-methyl in wheat and wild oat.

OH t glucose (inactive)

Figure 8.6 Metabolism of diclofop-methyl in wheat and wild oat.

Table 8.4 Effect of haloxyfop and tralkoxydim on plant growth (ED50) and inhibition of ACCase in vitro (I50) (from Secor et al., 1989, with permission).

Species

ED5C (|M)

i50 (|M)

Haloxyfop

Tralkoxydim

Haloxyfop

Tralkoxydim

Maize

19 (S)

1B (S)

C.5C

C.52

Wheat

Ba (S)

>760 (T)

1.22

C.91

Tall fescue

133 (S)

225 (S)

C.94

C.4C

Red fescue

125C (T)

>6CCC (T)

23.32

13.B3

Soybean

>6CCC (T)

>6CCC (T)

13B.5C

516.72

S, susceptible; T, tolerant.

S, susceptible; T, tolerant.

appears to be achieved by rapid metabolism to inactive glutathione conjugates in barley, sorghum, maize and sugarcane seedlings.

Ester hydrolysis appears central to the selectivity of the alaninopropionates in the control of wild oats in cereals. Thus, the inactive esters of benzoylprop and flamprop, for example, are rapidly hydrolysed in susceptible wild oats to their respective phytotoxic acids. In contrast, de-esterification is far slower in wheat, and any acid formed is rapidly inactivated by glycosylation. The carboxylesterase responsible for benzoylprop-ethyl hydrolysis in Avena fatua has been studied in some detail by Hill and colleagues (1978), and has also been implicated in the selective metabolism of the AOPPs. In this case Shimabukuro et al. (1979) found that diclofop-methyl was rapidly de-esterified in both wild oat and wheat. However, aryl hydroxylation rapidly inactivated the phytotoxic acid in the crop, and an ester glucoside was formed in the weed from which the toxic species could be easily and rapidly regenerated (Figure 8.6).

Metabolism also appears to form the basis of selectivity of the CHDs. Sulphoxidation, aryl hydroxylation, and molecular rearrangement have been observed with cycloxydim in tolerant species, and rapid conjugation of these groups leads to more polar and inactive by- products.

The striking selectivity of the ACCase inhibitors haloxyfop and tralkoxydim has been investigated in some detail by Secor et al. (1989). In this study, both susceptible and tolerant plants were sprayed with a range of herbicide concentrations to establish the dose at which growth was inhibited by 50% (ED50) and values compared to the herbicide concentrations needed to inhibit in vitro ACCase activity by 50% (I50). Their results, presented in Table 8.4, indicate that plant tolerance to these herbicides is clearly related to the insensitivity of ACCase. The identification of two forms of ACCase with very different sensitivities to ACCase inhibitors has confirmed that this is the major basis of selectivity between monocotyledonous and dicotyledonous crops (see section 8.3 and Table 8.4). Of the five species examined, soybean was the most tolerant at both the whole plant and the enzyme levels, although the opposite was the case in maize. Interestingly, wheat proved to be tolerant to tralkoxydim even though the isolated ACCase was sensitive to inhibition, which may imply metabolic inactivation in this crop. Further differences in grass sensitivity to the ACCase inhibitors have been characterised by Lichtenthaler and colleagues

(1989). It is therefore clear that grass tolerance to these graminicides is a consequence of both metabolism and lower sensitivity of ACCase, and that the apparent resistance of many dicotyledonous plants (e.g. soybean in Table 8.4) is due to insensitivity of the target enzyme itself.

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