The nectar produced by plants and consumed by pollinators is usually a sweet brew, containing ingredients such as sugar, amino acids and vitamins. But in addition to these components, nectar can also contain secondary metabolites such as nicotine and other toxic compounds. Why would plants risk poisoning the insects and birds that provide vital pollination services? Max Planck researchers, Danny Kessler and Ian Baldwin, decided to investigate what possible advantage such nasty ingredients could provide to the plant.

They examined the nectar of a wild tobacco species, Nicotiana attenuata, and discovered that it contained 35 secondary compounds. The researchers then tested 16 of these with three groups of native visitors - hawkmoths, hummingbirds (both pollinators) and ants (nectar thieves that don't provide any pollination services). While some compounds were attractive and others weren't, certain nectar blends seem to increase a flower's chances of being visited by useful pollinators while discouraging nectar thieves.

The most abundant repellent found, nicotine, was found to affect both pollinators and nectar thieves in the same way. Both types of visitor removed less nectar per visit when nicotine was present.

To investigate further, the researchers genetically transformed N. attenuata plants to create nicotine-free plants, which were then planted into a natural population. They found that the native floral visitors removed much more nectar from the plants that had no nicotine than from the normal nicotine-containing plants. But the question remained, why would a plant produce nectar that repels pollinators?

Kessler and Baldwin hypothesize that when nectar contains nicotine, the amount of nectar consumed per visit decreases but the number of visitations increases. They posit that increasing the number of visitors might increase the genetic diversity of the offspring produced. The researchers are planning to test this hypothesis in the upcoming field season.

Source: The Plant Journal