Distribution of target-site resistance mechanisms to nicosulfuron and glyphosate in Amaranthus palmeri accessions from two different regions of Turkiye

Palmer amaranth (Amaranthus palmeri S. Watson) is a highly competitive weed that has evolved resistance to several modes of action and has recently become a significant problem in Turkiye. This study was designed to determine the distribution of common target-site resistance (TSR) mechanisms to the acetolactate synthase (ALS)-inhibiting herbicide, nicosulfuron, and the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS)-inhibiting herbicide, glyphosate, in A. palmeri accessions collected from two agricultural regions of Turkiye; the Cukurova Region and the Gediz Basin. A total of 96 accessions were analyzed for ALS gene mutations and relative EPSPS gene copy number variation. Whole-plant responses were also evaluated based on relative dry weight following treatment with nicosulfuron and glyphosate applied at twice the field rate. The ALS mutations Pro197Ser, Trp574Leu, and Ser653Asn were far more prevalent in plants from the Cukurova Region than those from the Gediz Basin. Elevated relative EPSPS gene copy numbers were more common and higher in the Gediz Basin. Single rate herbicide screening experiments confirmed that accessions from the Cukurova Region exhibited higher dry weight response to nicosulfuron, whereas those from the Gediz Basin had higher to glyphosate Multivariable regression analyses revealed that nicosulfuron response was primarily associated with the Pro197Ser and Trp574Leu mutations, while glyphosate response was correlated with EPSPS gene copy number. These findings reveal distinct regional patterns of TSR mechanisms in A. palmeri in Turkiye that are consistent with differences in herbicide use and/or introduction history, but the roles of local selection, demographic processes, and gene flow remain unclear.