Mechanisms of selective action of a protoporphyrinogen IX oxidase-inhibiting herbicide pyraflufen-ethyl between wheat (Triticum aestivum) and cleavers (Galium aparine)

To elucidate the mechanisms of selective herbicidal action of pyraflufen-ethyl [ethyl 2-chloro-5-(4-chloro-5-difluoromethoxy-1-methylpyrazol-3-yl)-4-fluorophenoxyacetate], the intrinsic activity on the target enzyme, foliar deposition and absorption, and metabolism of the compound were compared between wheat (Triticum aestivum L.) and cleavers (Gallium aparine L.). Although protoporphyrinogen IX oxidase activities from wheat and cleavers chloroplasts were almost equally inhibited by pyraflufen-ethyl, a foliar application of the compound caused much more accumulation of protoporphyrin IX in cleavers than in wheat. The foliar deposition and absorption of the compound were much greater in cleavers than in wheat. Metabolic detoxification in wheat was much greater than that in cleavers. These differences in foliar deposition and absorption, and in the rate of metabolic detoxification would play an important role in the accumulation of protoporphyrin IX and the resulting selectivity of pyraflufen-ethyl between wheat and cleavers.