Environmental effects on the evolution of herbicide resistance in the grass weed Alopecurus myosuroides

Understanding the adaptive responses of weeds to abiotic factors and xenobiotics and the way this interact is a major unresolved issue in agricultural science. Plant defence mechanisms that prevent or minimize the number of damaged or dead plants can be defined as resistance. Of particular interest in the context of agricultural weeds, is the idea that shared pathways (e.g. physiological pathways in plants) of resistance to abiotic factors and xeno-biotics might promote resistance to both drought and herbicides. The grass weed Alopecurus myosuroides (black-grass), has evolved resistance to several herbicides, making it an ideal species to investigate whether exposure to environmental factors promotes the evolution of resistance in grass weeds. By conducting four linked pot experiments in a greenhouse, evidence was obtained that drought stress accelerates the evolution of herbicide resistance in the subsequent generation of grass weed populations. Through the first two experiments, the exposure of grass weed populations with no previous history of herbicide applications to drought stress was found to confer herbicide resistance in the next generation in comparison to controlled plants. As the highest survivor plants were recorded for the plants under high droughted treatment and high dose of herbicide treatment. The second two experiments provided evidence that the mechanisms that may be underlying this evolution of herbicide resistance were epigenetic. Results show that exposure of maternal plants to high drought stress may confer heritable herbicide resistance through epigenetic inheritance in the first generation. In a final study, I investigated how several agronomically important characteristics including seed germination and emergence characteristics respond to environmental condi-tions. Seeds produced at high density were significantly higher in weight, size, viability and germination compared to the seeds that were produced at low density. Furthermore, a sig-nificant relationship was found between density and previous herbicide applications in parental plants. The majority of herbicides in interaction with high density caused an increase in seed germination, viability, seed weight and seed size across populations. The results also suggest an increase in seed germination, viability, seed weight and seed size in response to density in interaction with soil type. Overall, these results suggest that the environmental conditions in which the parental plants develop can be characterized as a stress-force shaping adaptation and evolution in the characteristics of weed seeds. This could occur through the phenotypic changes in the offspring that enable them to adapt in changing environments, specifically developing adaptation traits towards herbicide application. Generally, this research has shown that environmental factors can have an important role in the evolution of herbicide resistance in grass weeds. These findings provide strong evidence that the interaction between environmental factors in which the weedy plants grow in interaction with xenobiotics, can affect the abil-ity of grass weeds to adapt to changes in their environment.