SEED HETEROMORPHISM IN CREPIS-SANCTA (ASTERACEAE) - PERFORMANCE OF 2 MORPHS IN DIFFERENT ENVIRONMENTS

Crepis sancta is an annual composite that produces heteromorphic seeds . Peripheral achenes, named from the position they occupy within the f ruiting heads, are heavier and without a pappus whereas central achene s 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 di fference in weight between peripheral and central achenes leads to a d ifferentiation in resistance to biotic and abiotic stress. We studied the reproductive biomass produced by plants from each seed morph, grow n under different environmental conditions: al high and low nutrient l evels, and at various densities of intraspecific and interspecific com petition. In the absence of competition, there was no difference in re productive biomass between the two morphs at high and low nutrient lev els. 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 ma dritensis, than with a perennial species of low RGR, Bromus erectus. T he low weight of central achenes represents a disadvantage when compet ing with other annual species. Theory suggests that each seed morph mu st differ ecologically in order to maintain heteromorphism. The differ entiation in competitive ability represents one of the ecological diff erences between the two types of achenes and may therefore contribute to the success of the mixed strategy in Crepis sancta.