Thank you for sending the link.
Let me share it with colleagues and students. I see you found the section in Materials and Methods describing what the authors had to do to hand-pollinate each flower.
Prior to pollen deposition the stigmatic
surface was rubbed with a brush in order to release stig-
Here is the perfect reason why entomologists and botanists always need to work together on field studies in pollination ecology. Thanks to the research of Yolande Heslop-Harrison, and her colleagues, botanists have known about the thin but strong pellicle that covers the pistil tip of most cross-pollinated, papilionoid legumes (peas, beans, clovers. alfalfa, etc.) for at least 35 years. Now, every bee-keeper knows that honeybees pollinate crop and garden legumes but "tripping" the flower but few know what the weight and pressure of the insect does to the protective coat (pellicle) on the pistil tip. The weight of the "right" insect ruptures the pellicle making it receptive to incoming pollen.
Remember, almost all papilionoid legumes are bisexual and anthers open and release pollen in the flower bud so the pellicle is nature's chastity belt (and probably more efficient than the human hymen). If an insect of the wrong size and weight lands on the keel petal containing the pistil then nothing happens. Cross-pollination can occur only when the appropriate pollinator contacts the keel petal and that petal rubs against the tight pellicle.
As luck would have it, Steve Callen (Allison Miller Lab, St. Louis U.) is completing a competitive, research poster for this April on a local legume. Part of the poster will show how the pistil tip resembles an angry, red pimple (pellicle intact) and becomes a pollen accepting "funnel" (pellicle ruptured).