Coexistence of target-site and non-target-site mechanisms of glyphosate resistance in Amaranthus palmeri populations from Argentina

ABSTRACT. Amaranthus palmeri S. Waston is currently one of the most problematic weeds worldwide. Biotypes with resistance to herbicides such as glyphosate and ALS inhibitors are now present in almost all Argentinian cultivable areas. In this work, we studied glyphosate resistance in three different populations, some of them previously characterized as resistant to ALS inhibitors. Dose-response curves were conducted in order to assess the effect of glyphosate on the survival and dry biomass of the populations. Subsequently, the presence of target-site resistance (TSR) was studied. Results confirmed the glyphosate resistance in the three populations, showing different levels of resistance, being R2 and R3 significantly more resistant than r1 population. A high prevalence of the P106S substitution was detected in the three resistant populations, while none increase in the relative EPSPS copy number was noticed. Some surviving plants without any of the TSR mechanisms for glyphosate were detected in R3 population, suggesting the presence of non-target-site resistance (NTSR).