Abstract
Many plants require animal pollinators for successful reproduction; these plants provide pollinator resources in pollen and nectar (rewards) and attract pollinators by specific cues (signals). In a seeming contradiction, some plants produce toxins such as alkaloids in their pollen and nectar, protecting their resources from ineffective pollinators. We investigated signals and rewards in the toxic, protandrous bee-pollinated plant Aconitum napellus, hypothesizing that male-phase flower reproductive success is pollinator-limited, which should favour higher levels of signals (odours) and rewards (nectar and pollen) compared with female-phase flowers. Furthermore, we expected insect visitors to forage only for nectar, due to the toxicity of pollen. We demonstrated that male-phase flowers emitted more volatile molecules and produced higher volumes of nectar than female-phase flowers. Alkaloids in pollen functioned as chemical defences, and were more diverse and more concentrated compared to the alkaloids in nectar. Visitors actively collected little pollen for larval food but consumed more of the less-toxic nectar. Toxic pollen remaining on the bee bodies promoted pollen transfer efficiency, facilitating pollination.
Highlights
Many plants require animal pollinators for successful reproduction; these plants provide pollinator resources in pollen and nectar and attract pollinators by specific cues
Our results revealed a clear sexual difference in floral scent production, with one alcohol, benzene ethanol, and one monoterpene, trans-β-ocimene, produced exclusively or at higher rates in male- than in female-phase flowers
It is the first time than odour detection was tried on Aconitum species
Summary
Many plants require animal pollinators for successful reproduction; these plants provide pollinator resources in pollen and nectar (rewards) and attract pollinators by specific cues (signals). Some visitors directly access nectar without touching reproductive organs of the flower by piercing the corolla (primary robbing), using holes already present (secondary robbing) or sliding between the petals (base working)[9] Such nectar robbing is usually observed in complex specialised flower structures, often bearing long spurs and hidden nectaries[10]. In response to this specialization, visitors with short mouthparts must make holes in floral structures or have no contact with reproductive organs, depriving plants of pollen transfer and some specialist pollinators of nectar[9,10,11,12]. Sexually dimorphic male- and female-phase flowers can direct pollinator movements from male to female flowers, making more opportunities for fertilization[20]
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