Anther arrangement influences pollen deposition and removal in hermaphrodite flowers

1. Effects of anther arrangement on pollen deposition and removal in hermaphrodite flowers were experimentally quantified to assess the ecological significance of stamen structure. The anther arrangement of Brassica rapa was modified into six types with respect to anther number and the spatial arrangement of long and short stamens. Each anther treatment was combined with a nectar-intact and nectar-depleted treatment. After single visits by bumblebees in a glasshouse, the visit duration to each flower and the number of pollen grains deposited on stigmas and pollen grains remaining in long and short anthers were measured. 2. Visits were significantly shorter in nectar-depleted flowers. Short anthers also extended visit duration, but modification of the long anther arrangement had no effect. Pollen deposition on stigmas depended on the number of long anthers and on visit duration. Although an increase in long anther number accelerated self-pollination, removal of short anthers did not influence pollen deposition. 3. Pollen removal from long anthers depended on anther treatment and visit duration, but not on nectar availability. Any modification of anther arrangement tended to increase pollen removal during single visits. Long anthers contributed to the decrease in pollen removal per visit, while pollen removal from short anthers depended only on visit duration. Short anthers appeared to have a different function from the restriction of pollen removal during single visits by bumblebees. 4. Thus, long and short anthers made different contributions to pollination efficiency of nectar-feeding bumblebees. The maintenance of different types of stamens in Brassicaceae plants (tetradynamous stamens) may reflect functional advantages for a diverse pollination syndrome if the pollination efficiency of long and short anthers varies depending on the type of pollinator.