Breeding system and bumblebee drone pollination of an explosively pollen-releasing plant, Meliosma tenuis (Sabiaceae)

Touch-sensitive stamens enhance pollen dispersal by scaring away visitors

Animal-pollinated plants have to get pollen to a conspecific stigma while protecting it from getting eaten. Touch-sensitive stamens, which are found in hundreds of flowering plants, are thought to function in enhancing pollen export and reducing its loss, but experimental tests are scarce. Stamens of Berberis and Mahonia are inserted between paired nectar glands and when touched by an insect's tongue rapidly snap forward so that their valvate anthers press pollen on the insect's tongue or face. We immobilized the stamens in otherwise unmodified flowers and studied pollen transfer in the field and under enclosed conditions. On flowers with immobilized stamens, the most common bee visitor stayed up to 3.6× longer, yet removed 1.3× fewer pollen grains and deposited 2.1× fewer grains on stigmas per visit. Self-pollen from a single stamen hitting the stigma amounted to 6% of the grains received from single bee visits. Bees discarded pollen passively placed on their bodies, likely because of its berberine content; nectar has no berberine. Syrphid flies fed on both nectar and pollen, taking more when stamens were immobilized. Pollen-tracking experiments in two Berberis species showed that mobile-stamen-flowers donate pollen to many more recipients. These results demonstrate another mechanism by which plants simultaneously meter out their pollen and reduce pollen theft.

Ingenious floral structure drives explosive pollination in Hydrilla verticillata (Hydrocharitaceae)

In explosive pollination, many structures and mechanisms have evolved to achieve high-speed stamen movement. The male flower of the submerged plant Hydrilla verticillata is reported to be able to release pollen explosively some time after leaving the mother plant time, but the mechanism of stamen movement and the related functional structure in this species are unclear. In this study, we observed the male flower structure and pollen dispersal process of H. verticillata. We analysed the stamen movements during the pollen dispersal process and conducted several controlled experiments to study the process of storage and release of elastic potential energy in explosive pollination. When the male flower of H. verticillata is bound to the united bracts, the sepals accumulate elastic potential energy through the expansion of basal extensor cells. After the male flower is liberated from the mother plant, the stamens unfold rapidly with the sepals under adhesion and transfer the elastic potential energy to the filament in seconds. Once stamens unfold to a critical angle, at which the elasticity of the filament just exceeds the adhesion between sepals and anthers, the stamens automatically rebound and release pollen in milliseconds. These results reveal that Catapult-like stamens, spoon-shaped sepals and enclosed united bracts in the spathe together constitute the functional structure in rapid stamen movement of H. verticillata. They ensure that the pollen can be released on the water surface, and thus adapt successfully to the pollen-epihydrophilous pollination.