Floral nectaries in Spathicarpa hastifolia (Araceae: Spathicarpeae): insights from ultrastructure and biology of the secretory process

The Neotropical tribe Spathicarpeae (Araceae) exhibits unique floral traits, among which staminodes seem to be crucial for reproductive biology in most of its genera. Given the existing gap in the literature on pollination ecology, along with the uncertainties regarding nectar secretion, we studied Spathicarpa hastifolia to describe staminode ultrastructure, investigate secretory processes, and characterize the composition of exudates sought by floral visitors. Flower buds and flowers in different anthesis phases were collected, fixed, and processed for light and electron microscopy. Flowers of S. hastifolia lack a perianth and comprise a gynoecium surrounded by staminodes. Staminodes exude a hyaline fluid during the pistillate and staminate phases of anthesis, attracting floral visitors like cockroaches, ants, and sap beetles. The staminodes are slightly globose, vascularized, and covered by a uniseriate epidermis with widespread stomata that are mostly open pores. The ground tissue is a starch-rich parenchyma whose starch content varies throughout the day due to synthesis and hydrolysis cycles. Chemical analyses showed that staminode exudates constitute sucrose-rich nectar. On the other hand, the fused stamens or synandria exude tiny drops of liquid from their tops, sometimes containing sugars, but in too low concentration to be qualified as nectar. The structure of the secretory cells of staminodes, both from the epidermis and parenchyma, was shown to be compatible with nectar synthesis. Mitochondria, plastids, rough endoplasmic reticulum, and vacuoles prevail in these cells. Altogether, our results show that the staminodes of S. hastifolia are floral nectaries whose secretory activity extends throughout the entire period of anthesis and mediates interactions with potential pollinators.