Restricted Herbicide Translocation Was Found in Two Glyphosate-resistant Italian Ryegrass (Lolium multiflorum Lam.) Populations from New Zealand

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.