E. Imbert et al., SEED HETEROMORPHISM IN CREPIS-SANCTA (ASTERACEAE) - PERFORMANCE OF 2 MORPHS IN DIFFERENT ENVIRONMENTS, Oikos, 79(2), 1997, pp. 325-332
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.