Plant form is a compromise between resource gathering, reproduction and the tolerance to physical demands of the abiotic and biotic environment. In an agricultural field in addition to the natural factors causing stress, humans also introduce physical and mechanical stresses, and chemical pesticides into the environment. Many of these factors are hazardous, since they represent stresses to which plants are unable to develop defense mechanisms. However, weeds have persisted in the agricultural environment despite the efforts to eradicate them. They have adapted to environmental changes such as crop rotation and have developed tolerance to stressors like pesticides in very short periods of times (less than 10 years), much less time than normally expected for evolutionary responses to occur. Perhaps a key to why weeds persist in stress-dominated habitats is the way they compromise between yield and survival. The mechanisms that explain which process is relevant in the control of seed production or seedling growth relate to the ecophysiology of the individual plants. However, trade-offs between plants physiological functions will have implications at both population and community levels. Climate change, air pollution and water scarcity are examples of environmental stresses that particularly affect agriculture. Herbicides are a major technological tool for agriculture and are responsible, at least in part, for significant increases in crop production during the last quarter of the century. The research presented in this dissertation was developed to understand the extent that individual responses to multiple environmental stresses can be extrapolated to population-level responses in an annual weed species. The specific objectives were to assess (1) the impact of three anthropogenic stresses (herbicide, UVB light and ozone) and their interactions on individual Italian ryegrass ontogeny and reproduction and (2) the potential evolutionary effect of these stresses and combinations on changes in population size and structure over time. Plants were capable of growth and reproductive compensation under the studied stresses. Stress factors with similar biochemical mechanisms had different effects at the individual plant and population levels of organization. Compensation occurred at all levels of organization: as individuals modifying growth and allocation to different organs and as populations modifying birth, and death rates and density dependent responses. The ability to compensate sometimes decreased with the number of stress factors (e.g. herbicide and UVB). In other cases, compensation ability increased with the number of stress factors (e.g. ozone and herbicide). / Graduation date: 2004