| Summary: | The mechanism and expression of resistance to glyphosate at different plant growing temperatures was investigated in an <i>Amaranthus palmeri</i> population (VM1) from a soybean field in Vicuña Mackenna, Cordoba, Argentina. Resistance was not due to reduced glyphosate translocation to the meristem or to <i>EPSPS</i> duplication, as reported for most US samples. In contrast, a proline 106 to serine target-site mutation acting additively with <i>EPSPS</i> over-expression (1.8-fold increase) was respectively a major and minor contributor to glyphosate resistance in VM1. Resistance indices based on LD<sub>50</sub> values generated using progenies from a cross between 52 PS106 VM1 individuals were estimated at 7.1 for homozygous SS106 and 4.3 for heterozygous PS106 compared with homozygous wild PP106 plants grown at a medium temperature of 24 °C day/18 °C night. A larger proportion of wild and mutant progenies survived a single commonly employed glyphosate rate when maintained at 30 °C day/26 °C night compared with 20 °C day/16 night in a subsequent experiment. Interestingly, the P106S mutation was not identified in any of the 920 plants analysed from 115 US populations, thereby potentially reflecting the difference in <i>A. palmeri</i> control practices in Argentina and USA.
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