Echinacea angustifolia grows in fragmented prairie.  Many generalist pollinating insects visit Echinacea, which may deposit inter-specific pollen onto styles.  Foreign pollen may clog stigmas or make them unreceptive to conspecific pollen, resulting in reduced seed set. Style shriveling in Echinacea indicates receipt of compatible E. angustifolia pollen, whereas style persistence indicates no receipt of compatible pollen. I hypothesized that pollen from co-flowering species deposited on the styles of E. angustifolia would make styles unreceptive and prevent fertilization. I predicted that florets receiving foreign pollen only would have lower style persistence than those receiving no pollen and that florets receiving foreign and E. angustifolia pollen would have greater style persistence compared to those receiving only E. angustifolia pollen.

We used 20 randomly chosen plants growing in a common garden experiment.  On each plant, groups of 6 florets were randomly assigned to one of twelve pollen combinations.  The foreign pollen treatment had four levels: one for each co-flowering Asteraceae species pollen: Heliopsis helianthoides, Coreopsis palmata, and Carduus acanthoides, and no pollen. The conspecific pollen treatment had three levels: application at the time of foreign pollen, application four hours later, or no pollen. We observed style shriveling 24 and 48 hours after pollination.  
Results/Conclusions

H. helianthoides pollen shriveled 70% of styles when applied alone compared to < 20% in the other two foreign pollen treatments applied alone.  Control treatments of no pollen showed low shriveling (<10%), while any application of E. angustifolia pollen showed high proportions of shriveling   (>80%).  C. acanthoides applied with E. angustifolia in the morning showed somewhat lower shriveling rates than E. angustifolia morning pollination alone (82 and 95%).  C. palmata pollen alone showed somewhat higher shriveling rates (20%) than no pollen control (<10%). These results reveal strong effects of foreign pollen on floret receptivity that vary with the identity of the foreign pollen.

Future results of seed set may indicate pollen interference post deposition.  For example, H. helianthoides pollinated florets that shriveled may not produce embryos, or combinations of E. angustifolia and foreign pollen with high proportions of shriveling may not have equivalently high seed set. Pollen interference due to generalist pollinators carrying mixed loads of pollen may partly explain the low reproduction we have observed in isolated plants with many pollinator visits.  Closely related Asteraceae species may be more likely to interfere, as H. helianthoides shares the same tribe as Echinacea and had the greatest effect.