The explosive pollination mechanism in Papilionoideae (Leguminosae): an analysis with three Desmodium species

In the papilionaceous flowers with explosive pollination mechanism, the androecium and the gynoecium are subjected to great pressure in the keel. It has been suggested that at the first visit nearly all pollen grains are released and that the floral parts do not return to their original positions; this would be a cue for pollinators to avoid activated flowers. The dependence on the pollinators suggests that explosive flowers will not produce fruits if the mechanism is not activated. In this paper, we have tested these ideas studying the reproductive biology of three Desmodium species, D. incanum, D. subsericium and D. uncinatum. Specifically, we studied the floral traits, reproductive system, pollination mechanism and floral visitors, for the three species. The results indicated that the keel and the wings play a key role in the activation of the explosive mechanism. The three species have a secondary pollen presentation, diurnal flowers, and a short floral cycle. According to the results, all the species studied were self-compatible, with two species producing fruits by autonomous self-pollination. Desmodium flowers were visited and activated by native and exotic bee species. During the first visit nearly all the pollen was released; after that, new visits were low or null. Our results indicated that the activation of the mechanism is not essential for the production of fruits, that explosive flowers are visited only once and that the changes in the positions of the floral parts are the cues that keep pollinators away from the flowers with activated mechanism.