A Lolium multiflorum Lam. biotype resistant to diclofop-methyl was found in a Triticum aestivum field in France (Normandy) that had been treated for several years with diclofop-methyl. Based on plant survival evaluated 21 d after herbicide application, the biotype exhibited a high level of resistance to diclofop-methyl and moderate resistance to CGA-184927-propargil and PP-604. The resistant biotype exhibited a small increase in tolerance to haloxyfop-methyl, quizalofop-ethyl, sethoxydim, and BAS-517-H, but was controlled by recommended field rates for these herbicides. The mechanism of resistance was investigated for diclofop-methyl. There was little or no difference in diclofop-methyl absorption by leaves of resistant and susceptible biotypes measured 48 h after treatment. For both biotypes, less than 1% of absorbed radiolabel was translocated during 48 h following foliar application of 14C-diclofop-methyl. Metabolism of diclofop-methyl was not significantly altered in the resistant biotype. In both biotypes, diclofop-methyl was rapidly metabolized to diclofop acid followed by a slow rate of formation of a polar conjugate. Two multifunctional acetyl coenzyme A carboxylase isoforms (ACCase I and ACCase II) were isolated from leaf tissue of resistant and susceptible biotypes. Both isoforms exhibited a subunit molecular mass of 203 kDa. For both resistant and susceptible biotypes, ACCase I constituted approximately 80% of total ACCase activity. Graminicide concentrations required to inhibit ACCase activity by 50% (I50 values) were determined for both ACCase isoforms from resistant and susceptible biotypes. The ACCase II isoform was highly resistant to graminicides in both biotypes. In contrast, the I50 value for diclofop inhibition of ACCase I was 19-fold greater for the enzyme isolated from the resistant biotype compared with the susceptible biotype. It is concluded that diclofop resistance in the L. multiflorum biotype from Normandy is caused by the presence of a resistant form of the ACCase I isoform.

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This work studied the tolerance mechanisms of chickpea (Cicer arietinum L.) to pyridate, a post-emergence herbicide used in other crops such as maize, peanut and wheat. Foliar absorption of pyridate in chickpea was limited and occurred mainly during the first 24 h after treatment. Over 83% of the herbicide absorbed remained in the treated leaf 72 h after treatment. Twelve hours after herbicide treatment, 50% of the total herbicide applied was converted to CL9673-N-Gly, an intermediate metabolite essential in the activation of the pyridazine ring for following conjugations. The N-glycoside was conjugated to form more-polar metabolites, C1 and C2: C1 was identified as CL9673-N-Gly- glutathione; and C2 was identified as CL9673-N-Gly-cysteine. C1 appeared 3 h after treatment and reached a maximum of around 40% of recovered radioactivity 72 h later, while C2 appeared 24 h after treatment and reached a maximum of 10% 72 h later. Chickpea glutathione transferases do not seem to be involved in the formation of C1, thus the final amount of this conjugate mainly depends on the amount of N-glycoside formation in vivo.