Abstract
Verification of barnyardgrass [Echinochloa crus-galli (L.) Beauv.] resistance to propanil has now been confirmed and its distribution found to be widespread in rice (Oryza sativa L.)-producing areas of Arkansas. We conducted laboratory and greenhouse experiments on propanil-resistant (R-BYG) and propanil-susceptible (S-BYG) barnyardgrass, and rice (cv. “Newbonnet”) to determine the resistance mechanism(s). Propanil absorption and translocation were not different in the two barnyardgrass biotypes. In vivo chlorophyll fluorescence data from leaf disks exposed to propanil solutions at various concentrations indicated that the propanil binding site of photosynthetic electron transport in PS II was not altered. Propanil-R-BYG exhibited no cross-resistance to other PS II inhibitors such as atrazine, diuron, fluometuron, or linuron, since both biotypes had equal mortality and injury ratings when treated with these herbicides, and atrazine (50 µM) totally inhibited PS II as evidenced in chlorophyll fluorescence tests. Metabolic studies using 14C-propanil applied to rice, R-BYG, and S-BYG indicated that detoxification of propanil to dichloroaniline (DCA) was more rapid in the R-BYG than the S-BYG biotype. Two additional polar metabolites were also detected in R-BYG, with TLC Rf values in two solvent systems equal to those of propanil metabolites found in rice. The detoxification appears to be via aryl acylamidase, since DCA and metabolites corresponding to DCA-saccharide conjugates were found and because the insecticide carbaryl, an inhibitor of this enzyme, increased injury or caused mortality of R-BYG when propanil and carbaryl were applied simultaneously. Metabolism of propanil appears to be the operative resistance mechanism in this propanil-R-BYG found in Arkansas.
Published Version
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