Submitted to: Weed Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/17/2005
Publication Date: 12/1/2005
Citation: Perez-Jones, A., Park, K.W., Colquhon, J., Mallory-Smith, C., Shaner, D.L. 2005. Identifcation of glyphosate-resistant italian ryegrass (lolium multiflorum) in oregon. Weed Science. 53:775-779 Interpretive Summary: The selection of glyphosate resistant weed biotypes is becoming an increasing concern. In order to manage glyphosate resistant weeds, it is important to understand the different mechanisms of resistance. In this paper we showed that the resistance mechanism in a glyphosate resistant ryegrass that was selected in Oregon is not due to an alteration of the target site for the herbicide but seems to be due to a non-target site mechanism. In addition, the results show that a simple leaf assay could be used to differentiate between susceptible and glyphosate resistant biotypes. These results increase our understanding of the different mechanisms of glyphosate resistance as well as a method for screening ryegrass populations in the field for resistance.
Technical Abstract: A suspected glyphosate-resistant Italian ryegrass biotype was collected from a filbert orchard near Portland, OR, where glyphosate was applied multiple times per year for about 15 yr. Greenhouse studies were conducted to determine if this biotype was glyphosate resistant. The plants were sprayed with glyphosate (0.01 to 3.37 kg ae ha'1) 14 d after planting and shoot biomass was determined 3 wk after herbicide treatment. Based on the dose–response experiments conducted in the greenhouse, the suspected Italian ryegrass biotype was approximately fivefold more resistant to glyphosate than the susceptible biotype. Plants from both susceptible and resistant biotypes were treated with glyphosate (0.42 and 0.84 kg ha'1) and shikimic acid was extracted 12, 24, 48, and 96 h after treatment. The susceptible biotype accumulated between three and five times more shikimic acid than did the resistant biotype. Leaf segments from both susceptible and resistant biotypes were incubated with different glyphosate concentrations (0.5 to 3000 'M) for 14 h under continuous light. Shikimic acid was extracted from each leaf segment and quantified. At a concentration up to 100 'M, leaf segments from the susceptible biotype accumulated more shikimic acid than leaf segments from the resistant biotype. The epsps gene was amplified and sequenced in both susceptible and resistant biotypes; however, no amino acid change was found in the resistant biotype. The level of resistance in this biotype is similar to that reported for a glyphosate-resistant Italian ryegrass biotype from Chile.