CHEPLICK, GREGORY PAUL. Department of Biology, College of Staten Island-City University of New York, Staten Island, NY 10314. - Plasticity of size and architectural traits in relation to fitness in an annual weed.
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