Mechanism of paraquat tolerance in perennial ryegrass

Abstract The mechanism of paraquat tolerance was investigated in lines of perennial ryegrass (Lolium perenne L.) which had been selected for resistance to the herbicide. Uptake, metabolism and translocation of paraquat were studied. Susceptible cultivars and a tolerant line were not found to differ in uptake of radioactive paraquat applied to the leaf surface or supplied to the cut ends of excised leaves. Distribution of herbicide within leaf tissue was similar in tolerant and susceptible plants and no metabolites of ¹⁴C-paraquat were detected in tolerant or susceptible plants treated with sub-lethal concentrations of the herbicide. Autoradiography and quantitative determinations showed much variation in translocation of ¹⁴C-paraquat out of treated leaves of intact plants, but the variation was not related to the degree of susceptibility to the herbicide. It is concluded that paraquat tolerance in perennial ryegrass is unlikely to depend upon reduced uptake, enhanced metabolism or altered translocation of the herbicide.