Differential Susceptibilities of Wheat and Barley to Diphenyl Ether Herbicide Oxyfluorfen

Wheat is known to be relatively tolerant to diphenyl ether herbicides. Growth and physiological responses of wheat to diphenyl ether herbicide oxyfluorfen were examined in comparison with those of oxyfluorfensusceptible barley. Compared to barley, wheat was significantly less susceptible to the herbicide with preemergence and postemergence treatments. The differential susceptibilities of wheat and barley to the herbicide were more apparent with postemergence than with preemergence treatment. The effects of the herbicide on causing cellular leakage, chlorophyll loss, and lipid peroxidation were much lower in wheat than in barley leaves. Protoporphyrin IX accumulated in both wheat and barley leaves treated with oxyfluorfen, in a concentration-dependent manner. However, the magnitude of the protoporphyrin IX accumulation was much lower in wheat than in barley leaves treated with oxyfluorfen at a concentration of higher than 0.33 muM. Protoporphyrinogen oxidase from wheat leaves was found to be less susceptible to oxyfluorfen than that from barley leaves. The I50 concentrations of oxyfluorfen on protoporphyrinogen oxidase activity were approximately 0.2 and 0.06 muM for wheat and barley etioplasts, respectively. Whereas the inhibition of protoporphyrinogen oxidase in barley etioplasts increased with increasing concentration of oxyfluorfen, protoporphyrinogen oxidase in wheat etioplasts was not further inhibited beyond 0.33 muM oxyfluorfen concentrations. In addition, both plant species similarly responded to superoxide anion-generating paraquat and to singlet oxygen-generating rose bengal, implying that the relative tolerance of wheat to oxyfluorfen is not due to protective mechanism against active oxygen species. Our results suggest that the differential susceptibilities of wheat and barley to oxyfluorfen are at least partly due to the differential inhibition of protoporphyrinogen oxidase by the herbicide, and thereby wheat is relatively tolerant to the herbicide.