Norditerpene alkaloid concentrations in tissues and floral rewards of larkspurs and impacts on pollinators

Abstract

Plant secondary compounds mediate interactions with insects and other animals. The norditerpene alkaloids are significant secondary compounds in Delphinium (larkspur) species which are divided into two classes: the 7, 8-methylenedioxylycoctonine (MDL-type) and N-(methylsuccinimido) anthranoyllycoctonine (MSAL-type), and are known to be toxic to herbivorous insects and livestock. Alkaloid concentrations were measured in a whole plant context in vegetative and floral tissues as well as rewards (pollen and nectar) in Delphinium barbeyi and Delphinium nuttallianum. Alkaloid concentrations differed between vegetative tissues, floral tissues and floral rewards. Alkaloid concentrations in floral parts were consistent with optimal defense theory, with tissues more closely tied to plant fitness, such as fruits, being more heavily defended than foliage. However, alkaloid concentrations were significantly lower in nectar compared to other tissues. The norditerpene alkaloids influenced the activity of bumble bees, the dominant pollinator of larkspur, but the effects were concentration dependent. Alkaloids in nectar are found at concentrations that have no effect on bee activity; however, if alkaloid concentrations in nectar were similar to those in foliage bee activity would be reduced significantly. These results suggest that nectar with low alkaloid concentrations may be beneficial to plant fitness by limiting adverse effects on pollinator activity.