Gradual pollen release in a buzz pollinated plant: Investigating pollen presentation theory under bee visitation

The rate at which flowers dispense pollen across successive pollinator visits is a key functional trait influencing male reproductive success. Pollen presentation theory (PPT) predicts that when pollinators are abundant and actively groom their bodies to collect pollen, plants should evolve gradual pollen release. Despite its intuitive appeal and generality, PPT has rarely been tested empirically using real bee visits, especially in buzz pollinated plants where pollen dispensing is controlled mechanically. Buzz pollinated plants are ideal to study PPT because they are often nectarless and rely on pollen-grooming bees, which must apply vibrations to specialised tube-like or poricidal anthers to release pollen. The vibrations generated by some bees directly determine pollen release and thus yield a direct mapping between bee behaviour and floral function. Here, we provide the first quantitative assessment of pollen dispensing in a buzz pollinated plant, Solanum rostratum, using real bumblebee (Bombus terrestris) visits. We conducted 80 trials with individual bees and measured over 2,000 pollination buzzes to estimate the relationship between total buzzing time and pollen dosing. For each trial we measured the total time spent buzzing and the amount of pollen remaining in the flower after visitation. We also characterised the properties (frequency and duration) of individual buzzes. Individual pollination buzzes had a fundamental frequency of 340 ± 26 Hz (mean ± SD) and were relatively short in duration (0.748 ± 0.604 s). We found that S. rostratum shows a strong pattern of gradual pollen release with flowers dispensing on average 2-4% of the available pollen grains for each buzzing second. Our study links an ecologically relevant pollinator behaviour to a key floral functional trait that affects male fitness, offering empirical support for PPT in a buzz pollinated system.