Non‐target‐site mechanism of glyphosate resistance in Italian ryegrass ( Lolium multiflorum )

In Shizuoka Prefecture, Japan, glyphosate-resistant Lolium multiflorum is a serious problem on the levees of rice paddies and in wheat fields. The mechanism of resistance of this biotype was analyzed. Based on LD50, the resistant population was 2.8–5.0 times more resistant to glyphosate than the susceptible population. The 5-enolpyruvyl-shikimate-3-phosphate synthase (EPSPS) gene sequence of the resistant biotype did not show a non-synonymous substitution at Pro106, and amplification of the gene was not observed in the resistant biotype. The metabolism and translocation of glyphosate were examined 4 days after application through the direct detection of glyphosate and its metabolite aminomethylphosphonic acid (AMPA) using liquid chromatograph-tandem mass spectrometer (LC-MS/MS). AMPA was not detected in either biotype in glyphosate-treated leaves or the other plant parts. The respective absorption rates of the susceptible and resistant biotypes were 37.90 3.63% and 41.09 3.36%, respectively, which were not significantly different. The resistant biotype retained more glyphosate in a glyphosate-treated leaf (91.36 1.56% of absorbed glyphosate) and less in the untreated parts of shoots (5.90 1.17%) and roots (2.76 0.44%) compared with the susceptible biotype, 79.58 3.73%, 15.77 3.06% and 4.65 0.89%, respectively. The results indicate that the resistance mechanism is neither the acquisition of a metabolic system nor limiting the absorption of glyphosate but limited translocation of the herbicide in the resistant biotype of L. multiflorum in Shizuoka Prefecture.