The structure of the spur nectary in Dendrobium finisterrae Schltr. (Dendrobiinae, Orchidaceae)

Magdalena Kamińska,Małgorzata Stpiczyńska

doi:10.5586/aa.2011.003

Magdalena Kamińska, Małgorzata Stpiczyńska

Open Access

https://doi.org/10.5586/aa.2011.003

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Abstract

To date, the structure of the nectary spur of <i>Dendrobium finisterrae</i> has not been studied in detail, and the present paper compares the structural organization of the floral nectary in this species with the spurs of other taxa. The nectary spur of <i>D. finisterrae</i> was examined by means of light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It is composed of a single layer of secretory epidermis and several layers of small and compactly arranged subepidermal secretory cells. The secretory cells have thick cellulosic cell walls with primary pits. The secretory tissue is supplied by vascular bundles that run beneath in ground parenchyma and are additionally surrounded by strands of sclerenchymatous fibers. The flowers of the investigated species displayed morphological features characteristic of bee-pollinated taxa, as they are zygomorphic, creamy-green coloured with evident nectar guides. They also emit a weak but nice scent. However, they possess some characters attributed to bird-pollinated flowers such as a short, massive nectary spur and collenchymatous secretory tissue that closely resembles the one found in the nectaries of certain species that are thought to be bird-pollinated. This similarity in anatomical organization of the nectary, regardless of geographical distribution and phylogeny, strongly indicates convergence and appears to be related to pollinator-driven selection.

Highlights

Within the Orchidaceae family, the great diversity in the forms of flowers frequently reflects adaptation to pollination (Dressler, 1968; Johnson and Steiner, 2000; Fenster et al 2004; Cozzolino and Widmer, 2005)
The spurs were collected on the day of flower opening, and the nectaries were examined using light microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM)
Small pieces of nectary tissue were fixed in 2.5% glutaraldehyde / 4% formaldehyde in phosphate buffer for 2h at ambient temperature, and afterwards they were carefully washed three times in phosphate buffer, and post-fixed in 1.5% osmium tetroxide for 1.5h and washed in distilled water two times for 5 min

Summary

Introduction

Within the Orchidaceae family (the largest within angiosperms), the great diversity in the forms of flowers frequently reflects adaptation to pollination (Dressler , 1968; Johnson and Steiner , 2000; Fenster et al 2004; Cozzolino and Widmer , 2005). The flower spur has a great adaptive significance, and it occurs in diverse taxonomic groups of orchids indicating multiple and independent evolution (Johnson and Steiner , 1997, 2000). In Spirantheae, the spur is composed of the proximal parts of the lateral sepals, the lip and column (Singer and Sazima , 1999), whereas in Hexisea imbricata, it is formed by fusion of the column and margins of the labellum (Stpiczyńska et al 2005). It is unilocular, but the spur divided into two locules occurs in Stereochilus dalatensis (Stpiczyńska et al 2011). In rewardless orchids the presence of a spur can be sufficient to attract pollinators (B e l l et al 2009)

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