Plasticity in Yield Components in Natural Populations of Three Species of Sesbania

To discover whether plasticity in resource allocation differs among related species, we examined variability in seed mass, seeds per fruit, ovules per flower, fruit survivorship, and fruits per unit plant size in Sesbania macrocarpa, S. drummondii, and S. vesicaria. The species differed in the amounts of variability of the different components of yield, and seed mass was not always the least variable component of yield. The components of yield differed significantly among years, sites, and plants. Differences among plants in size, density of surrounding plants, distance to the nearest neighbor, and density of surrounding fruits had little measurable effect on the components of yield. Differences in the position of fruits on single plants affected seed mass in S. drummondii and S. vesicaria and seeds per fruit in S. vesicaria. The components of yield showed a negative relationship among species, but were only weakly correlated within species, probably because environmental and genetic effects on the yield components acted in different directions. The differences in plasticity of yield components among species probably reflected differences among the species in response to stresses such as herbivory and drought, but may also have reflected their different life histories (annual vs. perennial). Given that each response pattern carries a set of consequences, natural selection may have acted to produce a different, least costly response to stress in each species.