Abstract
Studies have been carried out to determine the basis for the unique postemergence selective action of the rice herbicide, 3′,4′-dichloropropionanilide (DPA), which controls a wide variety of weed species including barnyardgrasses.
Absorption studies have shown that both rice (Oryza sativa, L.) and barnyardgrass (Echinochloa crusgalli, L.) foliage absorb DPA readily and equally and, consequently, morphological differences cannot account for its unique selective action. Through the use of uniformly 14C ring labeled and carbonyl labeled DPA in short-term studies (1-3 days), it has been found that rice and barnyardgrass leaves hydrolyze DPA to 3,4-dichloroaniline (DCA). It is further shown that this is not a direct 1-step hydrolysis, but rather an oxidative metabolism of DPA to 3′,4′-dichlorolactanilide (DLA) followed by hydrolysis to DCA and lactic acid. In rice, DLA is a transient intermediate and not isolatable under normal conditions, whereas DLA accumulates in barnyardgrass and is readily isolated. The oxidative metabolism of DPA is rapid in rice but slow in barnyardgrass. Thus, DPA is detoxified rapidly in rice, but accumulates in barnyardgrass to lethal proportions and this difference is proposed as the primary basis for the selective action of DPA.
Additional evidence for this mechanism comes from studies on the interaction of certain insecticides with DPA. It is known that the tolerance of rice and other crops to DPA can be completely eliminated when an insecticide such as carbaryl is present with the herbicide in the crop. Under these conditions, it has been found that DLA accumulates in the rice as it does in DPA treated barnyardgrass.
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