Selectivity and m etabolism

The contact activity of the peroxidising herbicides may lead to poor selectivity. Cinidon-ethyl, however, selectively controls broadleaf weeds in cereals, with selectivity resulting from increased metabolism in wheat.

The basis of isoxaflutole selectivity in weeds and maize also appears to be differential rates of metabolism. The herbicide is rapidly taken up and translocated following both soil and leaf applications. In maize the herbicide is rapidly metabolised, so that 6 days after a root application 59% of recoverable activity was found in a benzoic acid derivative and 29% in the active diketonitrile. Conversely, in the susceptible weed Abutilon theophrasti- after the same period, 82% remained as the active diketonitrile, with only 12% of recoverable activity found in the benzoic acid metabolite (Pallet et al. - 1998- . The pathway of isoxaflutole metabolism is shown in Figure 6.11 .

Sulfentrazone shows selective pre-emergence activity in soybeans and peanuts at doses of 125-500 gai ha-1. Its selective action is explained by oxidative metabolism (Figure 6.12), specifically demethylation. Interestingly, the tolerant weed sicklepod (Cassia obtusifolia) also metabolises the herbicide in the same way (Dayan et al., 1996 ).

The diphenyl ether herbicides have shown commercially successful selectivity in soybean and rice. Soybean contains homoglutathione (y-glutamyl-cysteinyl-P-alanine) instead of glutathione and uses this alternative thiol in herbicide metabolism. As examples, acifluorfen and fomesafen are rapidly detoxified by homoglutathione conjugation in soybean. Interestingly, diphenyl ether herbicides can increase the expression of glutath-ione ^-transferases (GSTs) in soybean. The activity of the GST in question (GM GSTU1-1) was selectively enhanced by homoglutathione rather than by glutathione (Skipsey et al., 1997).

Tolerance by peas to the diphenyl ether fluorodifen is due to rapid conjugation with glutathione and the GST responsible has been further characterised (Edwards, 1996 ).

O SO2CH,

O SO2CH,

rapid

O O SO2CH3 CF, isoxaflutole (inactive)

diketonitrile (active)

relatively fast in maize, slow in weeds

O SO2CH,

O SO2CH,

Figure 6.11 Metabolism of isoxaflutole in crops and weeds.

benzoic acid derivative (inactive)

Figure 6.11 Metabolism of isoxaflutole in crops and weeds.

NHSO2 ch3

CF2H

CF2H

CH2OH

COOH

Figure 6.12 Metabolism of sulfentrazone in soybean.

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