Glyphosate efficacy in Avena sterilis and Lolium rigidum populations

Abstract

The physical, physiological and molecular basis of tolerance/resistance to glyphosate in Avena sterilis and Lolium rigidum, respectively were examined. Samples of these species were collected from fields with a long history of glyphosate usage (Exposed – “E”) and from areas that were not exposed to herbicides (UnExposed – “UE”). A. sterilis (“E” and “UE”) was found to have roughly 3 times greater natural resistance to glyphosate than L. rigidum (“UE”). “E” population of L. rigidum was found to be 5 times more resistant to glyphosate than the “UE” population. There was no difference in shikimic acid accumulation between the “E” and “UE” population for A. sterilis, but, the “UE” L. rigidum population accumulated 6 times more shikimic acid than the “E”. Both species absorbed 14C-glyphosate differentially, absorption decreased in the following order L. rigidum “UE” > A. sterilis “E” and “UE” > L. rigidum “E”. There were also marked differences between species regarding the amount of 14C-glyphosate translocated from the treated leaf to the rest of plant shoots and roots. Glyphosate translocation from treated leaf to roots was maximal in L. rigidum (“UE”) followed by A. sterilis (“E” and “UE”) and minimal in L. rigidum (“E”). Comparison of the EPSPS gene sequences between L. rigidum populations indicated that the “E” has a serine amino acid substitution at position 182 of the predicted protein instead of the proline amino acid present in the “UE” population.