Flowers in a Fix: How to Get Pollinated When You Produce Toxic Pollen

Abstract

To promote pollination, flowers offer nutrients in pollen and nectar. Toxic plant metabolites that defend against herbivores also occur in nectar and pollen, but little is known about the effects of these compounds on pollinators. Here, we examined the toxic, bee-pollinated plant Aconitum napellus ssp. lusitanicum. In this protandrous species, pollinators visit female-phase flowers, then move acropetally to male-phase flowers. Male flower fertility usually increases with increasing pollinator visits, while female fertility requires only a few visits to reach saturation. We hypothesized that male flower reproductive success is pollinator-limited, which should favour showy floral displays and higher volatile and nectar levels than female-phase flowers. Further, we expected insects to forage only for nectar, due to pollen toxicity. Comparison of floral signals detected no differences in flower size, shape, and colour between male and female phases. The volatile molecules benzene ethanol and trans-β-ocimene were detected at greater levels in male-phase than female-phase flowers. Alkaloids were more diverse and more concentrated in pollen compared to nectar. Nectar volumes were higher during the male phase than the female phase. Bee visitors differed in their behaviour, fidelity, and detection of aconitine and, despite alkaloid contents in pollen and little active pollen collection, insects visited flowers during the male phase. Nectar had low toxicity and attracted insect robbers; this likely improved male-phase fitness, as robbers exported pollen. Therefore, our results suggest that both pollen and nectar properties have important roles in promoting A. napellus reproductive success.