Pollen and vegetative secondary chemistry of three pollen-rewarding lupines

Authors: HEILING, JACOB - North Carolina State University; Cook, Daniel; Lee, Stephen; IRWIN, REBECCA - North Carolina State University

Submitted to: American Journal of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/19/2019
Publication Date: 5/2/2019
Citation: Heiling, J.M., Cook, D., Lee, S.T., Irwin, R.E. 2019. Pollen and vegetative secondary chemistry of three pollen-rewarding lupines. American Journal of Botany. 106(5):643–655. https://doi.org/10.1002/ajb2.1283.
DOI: https://doi.org/10.1002/ajb2.1283

Interpretive Summary: Optimal defense theory predicts that selection should drive plants to disproportionally allocate resources for herbivore defense to tissues with high fitness values. Pollen’s primary role is the transport of gametes thus plants may be expected to defend it from herbivory. However, for many animal-pollinated plants, pollen serves a secondary role as a pollinator reward. These dual roles may present a conflict between selection to defend pollen from herbivores and selection to reward pollinators. Here, we investigate whether pollen secondary chemistry in three pollen-rewarding Lupinus species better reflects the need to defend pollen or reward pollinators. Lupinus (Fabaceae) species are nectarless, pollen-rewarding, and produce defensive quinolizidine and/or piperidine alkaloids throughout their tissues. Gas chromatography was used to identify and quantitate the alkaloids in four aboveground tissues (pollen, flower, leaf, stem) of three western North American lupines, L. argenteus, L. bakeri, and L. sulphureus. Alkaloid concentrations and composition were compared among tissues within individuals. Pollen alkaloid concentrations in L. argenteus and L. sulphureus were 11–35% of those found in other tissues. No alkaloids were detected in L. bakeri pollen, though they were present in other tissues. Alkaloid concentrations were not strongly correlated among tissues within individuals. Fewer alkaloids were detected in pollen compared to other tissues, and pollen contained no unique alkaloids. These results are consistent with the hypothesis that, in these pollen rewarding species, pollen secondary chemistry may reflect the need to attract and reward pollinators more than the need to defend pollen from herbivory.

Technical Abstract: Optimal defense theory predicts that selection should drive plants to disproportionally allocate resources for herbivore defense to tissues with high fitness values. Pollen’s primary role is the transport of gametes thus plants may be expected to defend it from herbivory. However, for many animal-pollinated plants, pollen serves a secondary role as a pollinator reward. These dual roles may present a conflict between selection to defend pollen from herbivores and selection to reward pollinators. Here, we investigate whether pollen secondary chemistry in three pollen-rewarding Lupinus species better reflects the need to defend pollen or reward pollinators. Lupinus (Fabaceae) species are nectarless, pollen-rewarding, and produce defensive quinolizidine and/or piperidine alkaloids throughout their tissues. Gas chromatography was used to identify and quantitate the alkaloids in four aboveground tissues (pollen, flower, leaf, stem) of three western North American lupines, L. argenteus, L. bakeri, and L. sulphureus. Alkaloid concentrations and composition were compared among tissues within individuals. Pollen alkaloid concentrations in L. argenteus and L. sulphureus were 11–35% of those found in other tissues. No alkaloids were detected in L. bakeri pollen, though they were present in other tissues. Alkaloid concentrations were not strongly correlated among tissues within individuals. Fewer alkaloids were detected in pollen compared to other tissues, and pollen contained no unique alkaloids. These results are consistent with the hypothesis that, in these pollen rewarding species, pollen secondary chemistry may reflect the need to attract and reward pollinators more than the need to defend pollen from herbivory.