Tribenuron‐Methyl‐Resistant Sinapis arvensis Exhibits Reduced Acetolactate Synthase Expression and Activity but Retains Competitiveness Against Wheat Under Glasshouse Conditions

Abstract

Sinapis arvensis is a problematic weed species distributed across cropping regions worldwide. Tribenuron‐methyl has been one of the primary herbicides used to manage this species in Iran. However, resistance to tribenuron‐methyl has been increasingly reported in various regions of the country. Despite its prevalence, the underlying mechanism of resistance has not yet been characterized. Experiments were conducted to evaluate the level of resistance, mechanisms of resistance, and potential fitness costs associated with tribenuron‐methyl resistance in S. arvensis . Resistance indices based on the herbicide dose that reduces survival by 50% (LD 50 ) ranged from 20.7 to 27.7 in the herbicide‐resistant populations. Molecular analysis revealed that the resistance was associated with the Asp‐376‐Glu mutation in the ALS gene. Furthermore, ALS gene expression and enzymatic activity were approximately twice as high in the susceptible populations compared to the resistant ones. Assessments of physiological traits revealed no apparent fitness penalties in the resistant individuals. Additionally, the resistant populations produced more seeds than wheat under competitive conditions. These results suggest that, despite the presence of target‐site resistance conferred, and lower ALS gene expression and enzymatic activity, no measurable fitness cost on fecundity was observed. Future studies should assess multigenerational fitness and performance under varied environmental conditions to detect potential context‐dependent trade‐offs.