Herbicide resistance in Black-grass ( Alopecurus myosuroides Huds.)

Abstract

Herbicide resistant grass weeds are a growing problem throughout the UK with Alopecurus myosuroides Huds. (black-grass) considered a major problem in winter cereals. Blackgrass control is hindered by the presence of populations resistant to herbicides. Research indicates that resistance in black-grass is due in part to enhanced metabolism involving glutathione S-transferases (GSTs) and that increased activities of these enzymes may confer resistance in this species. The work described in this thesis has characterised resistance in black-grass and examined the role GSTs play in herbicide resistance with respect to herbicide application and timing. Characterisation of herbicide resistance in three black-grass populations tested against isoproturon, fenoxaprop-P-ethyl, clodinafop-propargyl, sethoxydim, flupyrsulfuron-methyl and AC210 in the glasshouse revealed that the commercially available population, Herbiseed, may be used as a standard susceptible reference population when testing unknown populations. Novel resistance ratings were applied to Herbiseed for future reference. A 2 year study was performed to investigate glutathione S-transferase activity in five UK black-grass populations from field sites situated in the East Midlands. Findings indicate there is a natural elevation of endogenous GST activity in response to black-grass growth and development and natural environmental changes from winter to spring. Clear correlations between GST activity, temperature, solar radiation and sunshine hours have been observed. It is proposed that increasing GST activity is required as part of an antioxidant defence system until tillering (GS30) has ceased. It is speculated that this endogenous change in enzyme activity with plant development in the field contributes to reduced efficacy of some graminicides applied in the spring. Further investigation in a controlled environment focused on the effect of temperature on plant growth and antioxidant status of resistant and susceptible black-grass. Results indicated that temperature has a developmental and metabolic effect on the growth of resistant black-grass plants, which may be critical in the response of plants to herbicide treatment. Increased temperature was accompanied by a natural elevation in endogenous GST activity in resistant plants and changing temperature increased the concentration of antioxidants. It is speculated that these endogenous responses are part of a natural mechanism of acclimation to environmental change in resistant plants of this species and provide protection against subsequent stress such as herbicide treatment. In conclusion, it is postulated that the antioxidant system of black-grass plants is vital for survival under normal plant growth and development and climatic conditions. It is speculated that these endogenous responses are part of a natural mechanism of acclimation to environmental change whilst supporting normal plant development, suggesting that GSTs have direct cytoprotective activity. These observations lend further weight to the suggestion that the development of resistance in black-grass is in part due to evolution and elevation of GST activity. It is speculated that in striving to achieve maximum herbicide efficacy in resistant black-grass populations, the period of environmental change from autumn to winter as temperature decreases, in combination with smaller growth stages of plants would be the best time for graminicide application for black-grass control.