Sweet Rain from Bat-Pollinated Flowers: How Does Sugar Concentration Modulate Nectar Retention?

Premise of research. Bat-pollinated flowers have wide morphologies and produce copious and diluted nectar that commonly spills from flowers. Considering that sugar concentration modulates rheological properties in aqueous solutions, we estimated its role in regulating nectar retention. Methodology. We took three bat-pollinated species with open floral morphologies (Calliandra aeschynomenoides; Hymenaea cangaceira, Fabaceae; and Tarenaya longicarpa, Cleomaceae) as models. We evaluated how sugar concentration affects their nectar retention through experiments with artificial nectar in different sugar percentages. We also reported the volume and concentration of sugar in the nectar drops of these flowers under natural conditions. Pivotal results. Depending on the plant species, higher sugar concentration can improve nectar retention, reduce it, or even present intermediate values that enable the retention of larger nectar volumes. Nevertheless, sugar concentrations from flowers in the field do not coincide with those that allow a greater accumulation of nectar, and flowers retain smaller volumes than recorded in experiments. Conclusions. Peculiarities of each flower (e.g., size, thickness, and quantities of structures available for nectar adhesion) ensure distinct effects from sugar concentration on nectar retention. Even so, it seems that there is no strategy from plants to prevent spillage of nectar by regulating the sugar concentration. Shifts in the sugar concentration of nectar may influence other aspects of the pollination biology of the species. Spilling part of the produced nectar, despite seeming to be a paradoxical waste of resources, can be a less impactful factor.

Automated summary

The study found that different plant species respond differently to sugar concentration, and the sugar concentration in flowers in the field does not correspond to the ability to accumulate large amounts of nectar, indicating that plants may not have a strategy to prevent nectar spillage by regulating sugar concentration.