Distribution of glufosinate resistance and glutamine synthetase copy number variation among Palmer amaranth (<i>Amaranthus palmeri</i>) accessions in northeast Arkansas

Abstract The presence of glufosinate-resistant Palmer amaranth ( Amaranthus palmeri S. Watson) is of concern for Arkansas farmers. The objective of this study was to understand the distribution of glufosinate resistance among A. palmeri accessions collected in 2023 from locations surrounding MSR2 (a highly glufosinate-resistant accession) in 2020, focusing on the distance and direction patterns. Additionally, the cytosolic ( GS1 ) and chloroplastic ( GS2 ) glutamine synthetase copy number were quantified in glufosinate survivors. In 2023, a total of 66 A. palmeri samples were collected within a 15-km radius of MSR2. Amaranthus palmeri seedlings were treated with glufosinate at 590 g ai ha −1 . Plant tissues were collected, and gene copy number assays were conducted with survivors from accessions showing less than 96% mortality. Glufosinate provided ≥80% mortality in most of the accessions evaluated. Nonetheless, a few accessions showed low mortality rates, with values as low as 34%. Within and among accessions, there was no variation for GS1.1 and GS1. 2, while the GS2.1 and GS2.2 copy numbers varied greatly. There was no evidence that the geographic distance between samples and MSR2 impacted mortality or gene copy number. However, there was strong evidence that direction, relative to MSR2, affected both mortality and GS2.1 copies. Samples collected north from MSR2 showed lower average mortality rates (83%) with a higher number of GS2.1 copies (2.3). For comparison, average mortality ranged from 90% to 95% and GS2.1 copy number ranged from 1 to 1.2 in the other directions. The predominant summer and fall wind directions do not explain the movement of resistance in a specific direction. These findings indicate that there are multiple A. palmeri accessions capable of surviving a label recommended use rate of glufosinate in northeast Arkansas, and resistance distribution needs to be further investigated.