Phenotypic plasticity for dispersal ability in the seed heteromorphic Crepis sancta (Asteraceae)

In several species of Asteraceae with seed heteromorphism, the proportion o f seeds equipped with wind-dispersal structure decreases plastically under stressful conditions. This contradicts the adaptive expectation that the di spersal rate should increase with stress. Alternatively. developmental cons traints related to the ontogeny of the capitulum may explain these observat ions. as they may explain the effect of senescence on seed morph proportion s. In the present paper, we report on an experiment investigating the effec t of stress and senescence on the seed heteromorphic Crepis sancta (Asterac eae). Plants were subjected to one of four different treatments: control, s imulated herbivory, nutrient depiction, and simulated herbivory plus nutrie nt depletion. Plants experiencing herbivory produced more fecund seed heads immediately after the application of stress, which was later compensated b y a decrease in fecundity per seed head. Environmental stress increased the proportion of seeds with dispersal structures. Though senescence had large effects on fecundity, it had no effect on seed morph proportions in nonstr essed plants. These findings agree with adaptive expectations about the eff ect of stress on dispersal ability, and contrast with results obtained in o ther seed heteromorphic Asteraceae. Our results cast doubts on the existenc e of strong developmental constraints acting on the seed morph proportions in Crepis sancta.