Résumé
Crepis sancta is an annual composite that produces heteromorphic seeds. Peripheral achenes, named from the position they occupy within the fruiting heads, are heavier and without a pappus whereas central achenes are lighter and have a pappus. The morphological differentiation is associated with a difference in embryos size: embryos from peripheral achenes are twice as heavy as those from central achenes. As seedling size has consequences for growth, we tested the hypothesis that the difference in weight between peripheral and central achenes leads to a differentiation in resistance to biotic and abiotic stress. We studied the reproductive biomass produced by plants from each seed morph, grown under different environmental conditions: at high and low nutrient levels, and at various densities of intraspecific and interspecific competition. In the absence of competition, there was no difference in reproductive biomass between the two morphs at high and low nutrient levels. In intraspecific competition, the plants derived from peripheral achenes dominated those derived from central achenes. In interspecific competition, the greater size of the embryo of peripheral achenes was advantageous at high density. The difference between morphs was more important in competition with an annual species of high RGR, Bromus madritensis, than with a perennial species of low RGR, Bromus erectus. The low weight of central achenes represents a disadvantage when competing with other annual species. Theory suggests that each seed morph must differ ecologically in order to maintain heteromorphism. The differentiation in competitive ability represents one of the ecological differences between the two types of achenes and may therefore contribute to the success of the mixed strategy in Crepis sancta.