Differential susceptibilities of wheat and barley to diphenyl ether herbicide oxyfluorfen

Wheat is known to be relatively tolerant to diphenyl ether herbicides. Grow th and physiological responses of wheat to diphenyl ether herbicide oxyfluo rfen were examined in comparison with those of oxyfluorfensusceptible barle y. Compared to barley, wheat was significantly less susceptible to the herb icide with preemergmce and postemergence treatments. The differential susce ptibilities of wheat and barley to the herbicide were more apparent with po stemergence 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 accumulat ion was much lower in wheat than in barley leaves treated with oxyfluorfen at a concentration of higher than 0.33 mu M. Protoporphyrinogen oxidase fro m wheat leaves was found to be less susceptible to oxyfluorfen than that fr om barley leaves. The I-50 concentrations of oxyfluorfen on protoporphyrino gen oxidase activity were approximately 0.2 and 0.06 mu M for wheat and bar ley etioplasts, respectively. Whereas the inhibition of protoporphyrinogen oxidase in barley etioplasts increased with increasing concentration of oxy fluorfen, protoporphyrinogen oxidase in wheat etioplasts was not further in hibited beyond 0.33 mu M oxyfluorfen concentrations. In addition, both plan t species similarly responded to superoxide anion-generating paraquat and t o singlet oxygen-generating rose bengal, implying that the relative toleran ce of wheat to oxyfluorfen is not due to protective mechanism against activ e oxygen species. Our results suggest that the differential susceptibilitie s of wheat and barley to oxyfluorfen are at least partly due to the differe ntial inhibition of protoporphyrinogen oxidase by the herbicide, and thereb y wheat is relatively tolerant to the herbicide. (C) 1999 Academic Press.