The herbicide atrazine kills plants by disrupting the "light" reactions in the photosynthetic pathway. It works by blocking the action of a critical protein (QB) in the electron transport chain of photosystem II. The resistance takes the form of a modification of the QB protein of photosystem II which prevents atrazine binding to the QB protein. An additional effect of this modification of the QB protein is to reduce photosynthetic capacity. This gives us biotypes with high photosynthetic capacity (atrazine resistant) and low photosynthetic capacity (atrazine susceptible). This project examines the link between photosynthetic capacity and fitness, using atrazine resistant and susceptible biotypes of Abutilon theophrasti, a common invader of agricultural fields in Eastern North America. Atrazine resistance is confered by a single nuclear gene in this species. It has classically been assumed that a reduction of photosynthetic capacity will translate directly into a decrease in plant fitness; however, this assumption has rarely been tested. The fitness consequences of this reduction in photosynthetic capacity will be investigated in three experiments, all conducted in the absence of atrazine. The first will compare resistant and susceptible biotypes of A. theophrasti grown as well spaced individuals, the second will be an intra- specific competition experiment in which A. theophrasti will be competed with other commonly occurring weedy annual species. In all three experiment, demographic growth analysis, a non- destructive indirect method of growth analysis, will be performed in order to provide a mechanistic interpretation of the link between physiology and fitness.
