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One of the crucial factors in the evolution of herbicide resistance in weeds is the mode of inheritance. Experiments were conducted to determine the inheritance of glyphosate resistance in a population of perennial ryegrass (Lolium perenne) caused by restricted glyphosate translocation. First, the degree of dominance of the glyphosate resistance trait was evaluated by generating first filial generation families through pair-crossing resistant with susceptible plants. Dose-response experiments showed that these first generation families had an intermediate level of glyphosate resistance compared with that of the parent plants. The phenotypic segregation of the resistance trait was then investigated by backcrossing the first generation offspring with individuals from the original susceptible population. Results obtained from spraying the resulting backcrossed families with two rates of glyphosate gave segregation values that best fitted the one-gene model. Hence, glyphosate resistance in perennial ryegrass caused by the restricted herbicide translocation trait is controlled by a single nuclear gene with incomplete dominance. [ABSTRACT FROM AUTHOR]

Three quick tests were evaluated for detecting glyphosate-resistant biotypes of Italian ryegrass and perennial ryegrass. Biotypes of these two species were used that were 13.4 and 7.3 times more resistant to glyphosate, respectively, than susceptible biotypes when assessed using a sprayed pot study. One assay exposed germinating seeds of resistant and susceptible populations for 8 days to different concentrations of glyphosate. Measurements of reduction in seedling root growth gave an estimate of resistance magnitude of 10.8 and 8.9 for Italian ryegrass and perennial ryegrass, respectively, similar to that from the sprayed pot study. Two other assays were tested and were able to differentiate resistant and susceptible biotypes. One measured the relative levels of shikimic acid in leaves following exposure to glyphosate; four to seven times more shikimic acid accumulated in glyphosate-susceptible populations of Italian and perennial ryegrass than in resistant populations. The other assay involved growing tillers of the plants in glyphosate solutions and the results showed that at glyphosate concentrations of 10 and 40 mg/L, the plantlets of resistant plants showed negligible visual injury and growth reduction compared with susceptible ones. Results obtained from these assays were not affected by whether the glyphosate formulation used was an isopropylamine salt or potassium salt. [ABSTRACT FROM AUTHOR]