Resistance levels of sulfonylurea‐resistant Schoenoplectus juncoides (Roxb.) Palla with various Pro197 mutations in acetolactate synthase to imazosulfuron, bensulfuron‐methyl, metsulfuron‐methyl and imazaquin‐ammonium

An investigation, using herbicidal pot tests in a greenhouse condition, was conducted to determine the whole‐plant dose–response relationships to several acetolactate synthase ( ALS )‐inhibiting herbicides of sulfonylurea ( SU )‐resistant Schoenoplectus juncoides with various Pro 197 mutations in ALS that was collected from Japanese rice paddy fields. All the tested SU ‐resistant accessions with a Pro 197 mutation were highly resistant to two commonly used SU herbicides (imazosulfuron and bensulfuron‐methyl), but were much less resistant to another SU herbicide, metsulfuron‐methyl, and were substantially not resistant to imazaquin‐ammonium. These cross‐resistance patterns have been known previously in fragments of S. juncoides and other weed species and were comprehensively confirmed in this study with a whole set of Pro 197 mutations. The analyses of resistance levels, based on ED 90 values, newly showed that different accessions with a common amino acid substitution in ALS 1 showed similar responses to these herbicides (confirmed with four amino acid substitutions), that the rankings of resistance levels that were conferred by various Pro 197 mutations in ALS 1 differed among the SU herbicides and that the resistance levels of the ALS 2‐mutated accessions were higher than, lower than or similar to those of the corresponding ALS 1‐mutated accessions, depending on the compared pair, but the deviation patterns were generally similar among the SU herbicides in each compared pair. The final finding might suggest that the abundance of ALS 2 is not as stable as that of ALS 1. In addition, as a result of these new findings, together with expected further research, a suggested possibility is that substituting amino acids at Pro 197 generally could be estimated by plotting each accession's ED 90 values of imazosulfuron and bensulfuron‐methyl in a two‐dimensional graph.