Propanil activity, uptake and metabolism in resistant Echinochloa spp. biotypes

Field resistance of Echinochloa spp. to propanil has been previously report ed in Costa Rica, Colombia and Arkansas (USA). In this study, the mechanism of resistance was investigated in three resistant (R) and three susceptibl e (S) biotypes. The shoot fresh weight reduction in pot-grown plants from a post-emergence spray of propanil at 2.44 kg a.i. ha(-1) on biotypes R/S fr om Costa Rica, Colombia and Arkansas was 35/98%, 25/79% and 20/82% respecti vely. In vitro chlorophyll fluorescence data from leaf tissue incubated in propanil showed that photosynthesis was inhibited in all biotypes, indicati ng that the propanil-binding site and enzyme were not altered. After transf er to herbicide-free solution, photosynthesis recovered only in resistant b iotypes, indicating that the mechanism of resistance was caused by enhanced metabolism of the herbicide. Simultaneous treatment with fenitrothion, an aryl acylamidase inhibitor, prevented the recovery of photosynthesis in lea f tissue in two resistant biotypes. In contrast, the cytochrome P450 mono-o xygenase inhibitor, 1-aminobenzotriazole, did not prevent recovery from pro panil in leaf tissue. Application of C-14-propanil to the second leaf of in tact Echinochloa plants showed that c. 90% of the radioactivity remained in the treated leaf for up to 72 h after application. No major differences in translocation between R and S biotype plants were found. TLC analysis of t issue extracts from the treated leaves showed substantially less radioactiv ity associated with propanil, present after 72 h in rice or in the three R biotypes, compared with S biotypes.