Glyphosate resistance has been found in two populations of Italian ryegrass (Lolium multiflorum) following many years of glyphosate application in New Zealand vineyards. Dose-response experiments showed that both glyphosate-resistant Italian ryegrass populations had 10-fold levels of resistance to glyphosate compared to a susceptible population. Possible mechanisms of glyphosate resistance target site mutation at position Pro-106 of 5-enolpyruvyl-shikimate-3-phosphate synthase gene and alterations in glyphosate absorption/translocation in these populations were investigated. Genotyping assays demonstrated that there was no point mutation at Codon 106 of the 5-enolpyruvyl-shikimate-3-phosphate synthase gene in either of the resistant populations. Glyphosate-resistant and susceptible populations did not differ in ¹⁴C-glyphosate absorption. However, in both resistant populations, much more of the absorbed ¹⁴C-glyphosate was retained in the treated leaf than occurred in the susceptible population. Significantly more ¹⁴C-glyphosate was found in the pseudostem region of susceptible plants than resistant plants. Based on these results, it was suggested that alterations in glyphosate translocation patterns plays a major role in glyphosate resistance for Italian ryegrass populations from these New Zealand vineyards.

Keywords: EPSP synthase mutation, Glyphosate, Herbicide resistance, Mechanism of resistance, Weeds

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