A modified petal and stamen dimorphism interact to enhance pollen placement by a buzz-pollinated flower.

Background: Floral adaptations supposedly favour pollen grains to cross the numerous barriers faced during their journey to stigmas. Stamen dimorphism and specialized petals, like the cucculus in the Cassieae tribe (Fabaceae), are commonly observed in flowers that offer only pollen as a resource for bee pollinators. Here, we experimentally investigated whether the stamen dimorphism and cucculus enhance pollen placement on the bee's body.

Methods: We used 3D-printed bee models to apply artificial vibrations on the flowers of Chamaechrista latistipula with their cucculus deflected or maintained in its original position and their anther pores manipulated. After each simulated flower visit, we captured photographs of the artificial bee from four distinct angles. Employing digital imaging techniques, we documented the presence and location of pollen and stigma on the bee's body.

Key results: Our findings reveal that the cucculus redistributes pollen grains on the bee's body. There is a remarkable increase in pollen density (approximately tenfold) on the lateral side adjacent to the cucculus, precisely where the stigma contacts the bee when the cucculus is unmanipulated. Furthermore, the cucculus also enhances pollen placement on the ventral region of the bee, indicating its additional function. The cucculus also increases the accuracy of pollen grains on the adjacent lateral region of the bee's body, irrespective of the pollen grains released by small or large anthers.

Conclusions: Floral specialized traits, such as modified petals and stamen dimorphism, can modify the fate of pollen grains and ultimately contribute to the male reproductive performance in pollen flowers with poricidal anthers. The cucculus exhibits a dual role by promoting pollen placement in optimal regions for pollination and probably supporting pollen grains for bee feeding. These findings provide valuable insights into the adaptive significance of floral traits and their impact on the reproductive success of pollen flowers.