The quantitative genetics and plasticity of size and architectural traits in relation to nutrient availability were investigated in families of Amaranthus albus. This summer annual continually produces flowers and seeds clustered in leaf axils along growing stems. The importance of size and architectural traits (number of branches and total branch length) to reproductive fitness (seed output) was determined. Seeds from seven maternal genotypes collected from a field in southern Wisconsin were used to raise the first generation of plants in a greenhouse under constant conditions. Seed families from four first-generation plants per genotype were used to raise a second generation in the greenhouse over a 10 week period. For each family, nine of the plants were unfertilized and nine others were regularly fertilized. In both unfertilized and fertilized groups, stepwise regression revealed that size, estimated as volume of space occupied at 5 and 8 weeks, and total branch length were all significant determinants of fitness. Genotype by treatment interactions were apparent for size and architectural traits, indicating plasticity in relation to soil nutrients. Significant effects of genotype on size and architectural traits and fitness were detected, but only for the unfertilized group. Continuous lengthening of multiple branches increases seed output throughout the growing season; plasticity in size and branching in response to improved soil resources allows opportunistic increases in fitness in a heterogeneous habitat. However, selection is most likely to differentiate among genotypes in stressful, nutrient-poor environments.

Key words: Amaranthaceae, Amaranthus albus, annual weed, architecture, fitness, plasticity