Abstract
In several species of Asteraceae with seed heteromorphism, the proportion of seeds equipped with wind-dispersal structure decreases plastically under stressful conditions. This contradicts the adaptive expectation that the dispersal rate should increase with stress. Alternatively, developmental constraints related to the ontogeny of the capitulum may explain these observations, as they may explain the effect of senescence on seed morph proportions. In the present paper, we report on an experiment investigating the effect of stress and senescence on the seed heteromorphic Crepis sancta (Asteraceae). Plants were subjected to one of four different treatments: control, simulated herbivory, nutrient depletion, and simulated herbivory plus nutrient depletion. Plants experiencing herbivory produced more fecund seed heads immediately after the application of stress, which was later compensated by 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 nonstressed plants. These findings agree with adaptive expectations about the effect of stress on dispersal ability, and contrast with results obtained in other seed heteromorphic Asteraceae. Our results cast doubts on the existence of strong developmental constraints acting on the seed morph proportions in Crepis sancta.