Regulation of Seed Yield Within and Among Populations of Prunella Vulgaris

Abstract

Knowledge of how seed production is regulated by components of seed yield can provide insight into what determines fitness in natural populations. This study examines patterns and sources of variation and interaction among components of seed yield within and among 10 populations of a perennial weed. Much of the variation in total yield among individuals within populations could be explained by differences in plant size. In general, relationships between components of yield were weak, indicating that total yield was not tightly buffered by trade—offs among yield components. The pattern of relationships between yield components differed among populations, demonstrating that these relationships are not a constant characteristic of a species. The patterns of relationship between each component and an estimate of the number of successful seedlings produced also differed among populations, but seed number was generally more important than biomass per seed in determining plant fitness. Yield component means and total yield differed between populations in old—field and woodland habitats. Populations in woodland habitats tended to produce fewer flowers per inflorescence and larger seeds than populations in old—field habitats. Reciprocal transplant and common garden studies showed that these differences were due to phenotypic plasticity. In contrast to the pattern observed within populations, variation among populations in mean yield was buffered by a trade—off between the numbers of flowers per inflorescence and seed mass. The results of this study indicate that the environment plays an important role in determining patterns of seed production and therefore fitness in natural populations. The high degree of phenotypic plasticity in components of seed yield also suggests that response to selection on these traits would be slow.