Aconitum offers bees more nectar to carry its toxic pollen

Male flowers of Aconitum protect their pollen by adding alkaloids. Without pollen as a reward, botanists asked how the flower compensated pollinators.

The idea that insects pollinate plants is well understood. It’s no simple task for the plants to produce the pollen and nectar. In addition, any pollen the insects take for food is pollen that cannot fertilise another flower. Aconitum is a dichogamous plant, its flowers have a male and female phase. For the male phase, the task is to attract pollinators and protect the pollen to maximise delivery to female phase flowers. Aconitum is also a toxic plant, if you eat the wrong part. It protects its pollen with a chemical cocktail that will harm insects that eat a lot if it. So if insects don’t want to be near the pollen, how can it attract pollinators? It’s a puzzle that has interested A.-L. Jacquemart and colleagues. “Studies that examine the balance between pollinator attraction (signals) and rewards as a function of dichogamous sexual phase are rare, as are studies that focus on male reproductive success in toxic plant species,” wrote the authors in Scientific Reports.

Image: Canva.

“We examined reproductive fitness in male- and female-phase flowers in the protandrous hermaphrodite Aconitum napellus spp. lusitanicum Rouy (Common monkshood, Ranunculaceae). In contrast to previous studies that examined only one aspect of plant–pollinator interactions (toxicity, robbing, signals, etc.), we combined observations in natural populations (pollinator behaviour) with experimental approaches (detection of toxic compounds) and fine chemical analyses of signals and rewards.”

The Aconitum flowers open with a male phase to provide pollen. This lasts five or six days, before the organs wither and then the female organs are accessible to pollen. Jacquemart and colleagues examined how the flowers varied over this period.

They found that there was higher production of scents and nectar during the male phase. There was over four times as much nectar produced in the male phase compared to the female phase. The sugar concentrations, however, seemed be comparable.

While the nectar was attractive, Jacquemart and colleagues found the pollen definitely was not. The botanists concluded that the alkaloids in the pollen were potentially lethal to bees. And this combination of nectar and pollen is what makes Aconitum a successful plant, the scientists said. “Pollen toxicity may be considered a chemical defence that helps plants decrease herbivory and excessive harvesting of pollen. As alkaloids at high concentrations could deter, harm, or kill visitors, they may deter non-pollinating insects; this may prevent pollen from being wasted and thus improve pollen transfer among plants. ”

“The deterrence effect of pollen toxicity and high production of sugar-rich nectar work together to ensure the reproductive success of this specialised protandrous plant species, mainly by high male fitness.