Greenhouse and laboratory experiments were conducted to investigate two Lolium spp. populations from a northern Italian vineyard (biotype “R-332”) and a southern Italian olive grove (biotype “R-336”) that were not controlled by the registered rates of glyphosate. The fraction of resistant (R) plants of each population was determined through screenings using two doses of glyphosate and three doses of ACCase inhibitors: cycloxydim and fluazifop. Shikimate accumulation at two doses was measured spectrophotometrically at 24, 48, 72, 96 and 216 hours after treatment. A quick germination dose-response test in petri dishes using glyphosate solutions at different concentrations was set up to select the R individuals within the population for further investigations. The R seedlings were discriminated on the basis of shoot and root development. The screening results confirmed resistance outlining the following profile for the northern R-332 biotyope: 84% (Standard Error 1.4) and 56% (3.2) of survivors and 104% (6.6) and 43% (3.7) of fresh weight at lower (360 g a.e. ha-1) and higher doses (1440 g a.e. ha-1) respectively, referred to the untreated check. Southern R-336 biotype: 98% (0.25) and 88% (3.1) of survivors and 110% (2.9) and 60% (3.5) of fresh weight. Both ACCase inhibitors controlled the glyphosate resistant biotypes.
The shikimate accumulation clearly discriminated the R populations from the susceptible (S-204) at the lower dose while at the maximum dose applied the northern population had an accumulation comparable to the S pop but R plants fully recovered in 21 days. The southern population did not accumulate shikimate as the S population neither at the higher level. The quick test outlined a concentration above 100 μM to discriminate S from R populations.