Functional aspects of floral nectar secretion of Ananas ananassoides, an ornithophilous bromeliad from the Brazilian savanna

Several members of Bromeliaceae show adaptations for hummingbird pollination in the Neotropics; however, the relationships between floral structure, nectar production, pollination and pollinators are poorly understood. The main goal of this study was to analyse the functional aspects of nectar secretion related to interaction with pollinators by evaluating floral biology, cellular and sub-cellular anatomy of the septal nectary and nectar composition of Ananas ananassoides, including an experimental approach to nectar dynamics. Observations on floral anthesis and visitors were conducted in a population of A. ananassoides in the Brazilian savanna. Nectary samples were processed using standard methods for light and transmission electron microscopy. The main metabolites in nectary tissue were detected via histochemistry. Sugar composition was analysed by high-performance liquid chromatography (HPLC). The accumulated nectar was determined from bagged flowers (‘unvisited’), and floral response to repeated nectar removal was evaluated in an experimental design simulating multiple visits by pollinators to the same flowers (‘visited’) over the course of anthesis. The hummingbirds Hylocharis chrysura and Thalurania glaucopis were the most frequent pollinators. The interlocular septal nectary, composed of three lenticular canals, extends from the ovary base to the style base. It consists of a secretory epithelium and nectary parenchyma rich in starch grains, which are hydrolysed during nectar secretion. The median volume of nectar in recently opened ‘unvisited’ flowers was 27.0 mL, with a mean (sucrose-dominated) sugar concentration of 30.5 %. Anthesis lasts approx. 11 h, and nectar secretion begins before sunrise. In ‘visited’ flowers (experimentally emptied every hour) the nectar total production per flower was significantly higher than in the ‘unvisited’ flowers (control) in terms of volume (t 1⁄4 4.94, P 1⁄4 0.0001) and mass of sugar (t 1⁄4 2.95, P 1⁄4 0.007), and the concentration was significantly lower (t 1⁄4 8.04, P 1⁄4 0.0001). The data suggest that the total production of floral nectar in A. ananassoides is linked to the pollinators’ activity and that the rapid renewal of nectar is related to the nectary morphological features.