Secondary metabolites have been found in the floral nectar of at least 21 angiosperm families, but patterns of distribution across families or species have not been examined. Comparing the identity and concentration of nectar secondary metabolites can provide critical information about the origin and function of so-called ‘toxic’ nectar. The genus Asclepias has a rich literature describing its chemical defenses along with a published phylogeny, making it a good candidate for comparative nectar analyses. Focusing on the Asclepias series Incarnatae, which exhibit substantial variation in leaf cardenolide concentrations, we examined the relationship between leaf and nectar cardenolide concentrations within species, as well as comparing nectar cardenolides across species. We collected nectar and leaves from twelve species that were part of a large greenhouse collection and used HPLC to determine gross cardenolide concentration. Using the nectar data, we developed pollinator preference assays to assess the ecological consequences of nectar cardenolides on bumble bee behaviour.
Results/Conclusions
Nectar cardenolides varied substantially within and between Asclepias species, with concentrations ranging from 0 to 175 ng/ μL of nectar. Half of the species in this study had no nectar cardenolides despite the presence of cardenolides in their leaves. Quantification of leaf cardenolide concentrations is currently underway. Correlations between cardenolide concentrations in nectar and leaves would support the hypothesis that 'toxic' nectar is a pleiotropic consequence of defense. Pollinators did not discriminate between artificial nectar comprised of sucrose alone and artificial nectar enriched with cardenolides, suggesting that nectar cardenolides do not reduce pollination services for a plant. This result is in stark contrast to the effect of nectar alkaloids, which can significantly reduce the number of flower visits made by a pollinator. Nectar alkaloids are also reported to correlate with alkaloid concentrations in leaves. Our results therefore suggest that the adaptive function and ecological costs of nectar secondary metabolites are compound-specific. This study is the first to identify and quantify cardenolides in Asclepias nectar and to explore their effects on pollinators.
