The influence of achene type and plant density on growth and biomass allocation in the heterocarpic annual Catananche lutea (asteraceae)

Catananche lutea is a heterocarpic species with a complex fruit polymorphism in terms of shape, size, mass, dispersal, and germination. This species produces two types of subterranean achenes (amphic-1 and amphic-2) and three types of aerial achenes (peripheral, intermediate, and central). Both types of subterranean achenes are larger than aerial achenes and do not disperse; their germination produces plants in dense clusters, which results in competition among them. Aerial achenes are smaller, can disperse, and produce plants that, under normal conditions, do not compete. We examined the effect of plant density on growth and biomass allocation and studied competition at variable densities in C. lutea plants produced by amphic-1, peripheral aerial and central aerial achenes. Plants from the different types of achenes were grown at different densities (two, four, and eight plants/pot) in pure and mixed stand in a greenhouse. In pure stands, the effect of competition was similar for the plants produced by amphic-1 and peripheral aerial achenes at the densities studied. Density stress was found to affect vegetative biomass markedly, the number of subterranean capitula and, especially, that of aerial capitula. The plants produced by these two types of achenes are initially more vigorous and flower earlier than those produced by central aerial achenes, in which an increased plant density affects moreover other variables (achene biomass). These plants respond differently to density stress from those produced by amphic-1 and peripheral achenes. Competition has different effects on the production of subterranean and aerial achenes. The negative influence of density stress on the number of aerial capitula and achenes is much more marked relative to subterranean capitula and achenes. When the environment remains stable, plants from subterranean achenes gather in dense clusters, and competition among them favors the production of subterranean (nondispersing) fruits over aerial (dispersing) fruits. The tests conducted on mixed cultures revealed that the plants from subterranean achenes exhibited a higher relative competitive ability; by contrast, plants from central aerial achenes were the least competitive.