Bumblebee visitation and pollen dynamics in Palicourea coriacea (Rubiaceae): does coflowering with congeneric species matter?

Abstract

The flowering phenology of many closely related species in the Brazilian Cerrado coincides with the onset of the rainy season, where sequential flowering often occurs with some overlap. Transitioning from solitary flowering to coflowering with congeneric species may alter the pollination environment, affecting pollen delivery and deposition patterns. Coflowering among conspecifics concurrently requires pollination niche differentiation to minimize reproductive costs. This study tested these concepts in the distylous Palicourea coriacea during two flowering periods: early in the season when it flowered alone, and later with conspecific P. officinalis, also distylous. Pollination syndromes were assessed by measuring corolla length, nectar volume and sugar concentration, and reproductive organ height. Palicourea coriacea shows yellow and shorter corollas with higher sugar concentration in the nectar, while P. officinalis presents yellow to orange longer corollas with more diluted nectar, aligning with bee and hummingbird pollination syndromes, respectively, as reported in the literature. However, the species exhibited significant overlap in stigma and anthers height. The main floral visitor in the two species during the study was Bombus pauloensis. Visitation increased through the season, particularly in conspecific patches of P. coriacea, resulting in higher pollen delivery. In contrast, pollen deposition was similar or higher in congeneric patches with P. officinalis during the coflowering period. Visits to P. coriacea were higher than in P. officinalis, suggesting a bumblebee preference for the former. The study highlights the complex interplay between flowering phenology, floral traits, and pollinator behaviour in shaping reproductive outcomes and potential niche differentiation. While differences in flowering and flower morphology may prevent potential costs of pollinator sharing, the risk of reproductive interference remains significant. Future research should focus on comprehensive pollination dynamics throughout the entire flowering season, measuring pollinator behaviour, pollen dynamics and plant fitness, to further elucidate the mechanisms driving floral evolution and niche differentiation in sympatric species.