Reproductive Biology and Nectar Secretion Dynamics of (Plantaginaceae): a Perennial Herb with a Mixed Pollination System?

Background: In many plant species, pollination syndromes predict the most effective pollinator. However, other floral visitors may also offer effective pollination services and promote mixed pollination systems. Several species of the species-rich (Plantaginaceae) exhibit a suite of floral traits that suggest adaptation for pollination by both hymenopterans and hummingbirds. Transitions from the ancestral hymenopteran pollination syndrome to more derived hummingbird pollination syndrome may be promoted if the quantity or quality of visits by hummingbirds is increased and if the ancestral pollinator group performs less efficiently. The quantification of such shifts in pollination systems in the group is still limited. We aimed to investigate floral traits linked to this pollination syndrome in with flowers visited by bumblebees and hummingbirds.

Methods: We investigated the floral biology, pollinator assemblages, breeding system and nectar production patterns of inhabiting a temperate montane forest in central Mexico. Pollination experiments were also conducted to assess the pollinator effectiveness of bumblebees and hummingbirds.

Results: flowers are protandrous, with 8-d male phase (staminate) flowers, followed by the ∼1-7 d female phase (pistillate phase). Flowers display traits associated with hymenopteran pollination, including purple flowers abruptly ampliate-ventricose to a broad throat with anthers and stigmas included, and long lifespans. However, the nectar available in the morning hours was abundant and dilute, traits linked to flowers with a hummingbird pollination syndrome. Two hummingbird species made most of the visits to flowers, (30.3% of all visits), followed by (11.3%). Bumblebees (, and ) accounted for 51.8% of all recorded visits, but their foraging activity was restricted to the warmer hours. Hummingbirds made more foraging bouts and visited more flowers than hymenopteran species. Flowers experimentally pollinated by produced significantly more fruits than those pollinated by . However, there was no statistical difference in the number of seeds produced per fruit when a bumblebee or a hummingbird was the pollinator.

Conclusions: We have shown that bumblebees and hummingbirds visit and pollinate flowers. Despite floral traits resembling the hymenoptera pollination syndrome, flowers of offer characteristic nectar rewards to flowers with a hummingbird pollination syndrome. Although pollination efficiency is higher among flowers visited by hymenoptera, the noteworthy percentage of fruit production and number of seeds per fruit derived from hummingbird pollination highlights the importance of hummingbirds as a functional group of pollinators that might have potential evolutionary consequences to the plants.