Anatomy of the floral nectary of ornithophilous Elleanthus brasiliensis (Lindl.) Rchb.f. (Orchidaceae: Sobralieae)

Abstract

Orchidaceae show enormous floral diversity. However, anatomical studies of nectary tissues relative to nectar composition and pollinators are scarce. This work aims to present a detailed anatomical study of the labellar nectary of Elleanthus brasiliensis, analyse the chemical composition of its nectar and relate these findings to pollination biology. Basally, the labellum bears a pair of fleshy, whitish, ovoid calli on its adaxial surface. Nectariferous callus tissue consists of a papillate epidermis and enlarged subepidermal parenchyma cells with thin walls, large nuclei and dense cytoplasm which stained positively for hydrophilic substances, interpreted as pre-nectar. The paired calli lack vascular tissues, but at the point of callus insertion, the diameters of vascular bundles supplying the lip are larger. Nectar is secreted as droplets on the adaxial callus surface. It is produced in small quantities, c. 4 μL per flower. Callus cell contents tested negative for polysaccharides, lipids and phenolic compounds. The nectar is sucrose-dominant, as in other hummingbird-pollinated species. It is suggested that other ornithophilous species of Sobralieae have anatomically similar nectaries.