High toxin concentration in pollen may deter collection by bees in butterfly-pollinated Rhododendron molle

The hypothesis that plants evolve features that protect accessible pollen from consumption by flower visitors remains poorly understood.

To explore potential chemical defence against pollen consumption, we examined the pollinator assemblage, foraging behaviour, visitation frequency and pollen transfer efficiency in Rhododendron molle, a highly toxic shrub containing rhodojaponin III. Nutrient (protein and lipid) and toxic components in pollen and other tissues were measured.

Overall in the five populations studied, floral visits by butterflies and bumblebees were relatively more frequent than visits by honeybees. All foraged for nectar but not pollen. Butterflies did not differ from bumblebees in the amount of pollen removed per visit, but deposited more pollen per visit. Pollination experiments indicated that R. molle was self-compatible, but both fruit and seed production were pollen-limited. Our analysis indicated that the pollen was not protein-poor and had a higher concentration of the toxic compound rhodojaponin III than petals and leaves, this compound was undetectable in nectar.

Pollen toxicity in Rhododendron flowers may discourage pollen robbers (bees) from taking the freely accessible pollen grains, while the toxin-free nectar rewards effective pollinators, promoting pollen transfer. This preliminary study supports the hypothesis that chemical defence in pollen would be likely to evolve in species without physical protection from pollinivores.