Pollination biology and breeding system of Desmodium grahamii (Fabaceae, Papilionoideae): functional aspects of flowers and bees

Abstract

This study aims to understand the role of floral traits in determining the pollination and reproduction of Desmodium grahamii (Fabaceae, Papilionoideae) with explosive floral mechanism in populations occurring in a natural reserve and botanical garden in southern Mexico City. We assessed the breeding system by quantifying floral and pollinator activity, compatibility, pollination, and reproductive success, assisted by field and laboratory analyses. Results showed that cross-pollination and self-pollination coexist, but bees are required for fruit and seed set. Flower colour is the primary attractant. Floral scent is also likely important because the petals were covered with papillae, although no scent was perceptible. Morphological and functional observations of D. grahamii exhibit simultaneous pollen release and stigma receptivity when the flower is activated by a pollinator and there is no secondary pollen presentation; this contrasts with what has been reported on other species of Desmodium. Pollen is the pollinators’ only reward, and its deposition and collection must be done synchronically, occurring when a bee lands and introduces its proboscis in the flower. In this study, we identified seven floral visitors: five bee pollinators and two syrphid flies as pollen thieves. On some occasions, Apis mellifera also behaves as a pollen thief. Fruit and seed set of flowers that were isolated from visitors may indicate a delayed pollination mechanism. Pollination results also suggest that Apis bees and syrphid flies may contribute to fruit and seed production when they forage on flowers at post-anthesis.