Glyphosate resistance in common ragweed ( A mbrosia artemisiifolia L.) from Mississippi, USA

Glyphosate is one of the most commonly used broad-spectrum herbicides over the last 40years. Due to the widespread adoption of glyphosate-resistant (GR) crop technology, especially corn, cotton and soybean, several weed species have evolved resistance to this herbicide. Research was conducted to confirm and characterize the magnitude and mechanism of glyphosate resistance in two GR common ragweed (Ambrosia artemisiifoliaL.) biotypes from Mississippi, USA. A glyphosate-susceptible (GS) biotype was included for comparison. The effective glyphosate dose to reduce the growth of the treated plants by 50% for the GR1, GR2 and GS biotypes was 0.58, 0.46 and 0.11kgaeha(-1), respectively, indicating that the level of resistance was five and fourfold that of the GS biotype for GR1 and GR2, respectively. Studies using (14) C-glyphosate have not indicated any difference in its absorption between the biotypes, but the GR1 and GR2 biotypes translocated more (14) C-glyphosate, compared to the GS biotype. This difference in translocation within resistant biotypes is unique. There was no amino acid substitution at codon 106 that was detected by the 5-enolpyruvylshikimate-3-phosphate synthase gene sequence analysis of the resistant and susceptible biotypes. Therefore, the mechanism of resistance to glyphosate in common ragweed biotypes from Mississippi is not related to a target site mutation or reduced absorption and/or translocation of glyphosate.