Diverse labellar secretions in AfricanBulbophyllum(Orchidaceae: Bulbophyllinae) sectionsPtiloglossum,OreonastesandMegaclinium

Abstract

Floral food‐rewards of Bulbophyllum range from nectar to protein‐rich mucilage and lipid‐rich labellar secretions. For the first time, the structure of the labellum and the secretory process are investigated for four African Bulbophyllum species. The most specialized type of labellar organization occurred in B. schinzianum, the deep, narrow, median longitudinal groove consisting of palisade‐like secretory cells flanked by trichomes containing lipid droplets, and the copious secretion containing sugar. This groove was absent or poorly defined, shallow and wide, in the remaining taxa, the scant secretion containing lipid. All taxa possessed a striate cuticle lacking cracks and pores, and micro‐channels were present, cuticular blisters occurring only in B. schinzianum. The labellum contained storage parenchyma (B. lupulinum) or mesophyll‐like parenchyma (B. schinzianum), but in section Megaclinium (B. falcatum and B. maximum), these were replaced by aerenchyma. In B. schinzianum, the form of the labellar groove, sweet fragrance and sugary secretion suggest pollination by Hymenoptera, the food‐reward and fragrance indicating that pseudocopulation is unlikely. Conversely, the form of the labellum of taxa having smaller flowers, and the lipid‐rich secretion, suggests pollination by small flies. The labellar aerenchyma may facilitate this process or even aid wind‐assisted pollination. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 179, 266–287.

Evidence for the Dual Role of Floral Secretory Cells in Bulbophyllum

“…Racemosae, unlike those of many other Bulbophyllum species examined to date, including Neotropical and African species (e.g., de Pádua Teixeira et al, 2004;Stpiczyńska et al, 2015), consisted mainly of protein and tested negatively for lipids and the present paper). Indeed, Contrary to previously investigated species of sect.…”

“…Racemosae that form the subject of the present study (Bulbophyllum dissitiflorum, B. lilacinum and B. tricorne) and that of previously investigated members of the section (Bulbophyllum careyanum, B. morphologorum, B. orientale and B. wangkaense - is very similar, the labellum being linguiform in each case, with an adaxial median longitudinal sulcus containing verrucae, and the internal anatomy being more or less identical. Similar labellar morphology also occurs in Neotropical and certain African members of the genus (de Pádua Teixeira et al, 2004;Nunes et al, 2014Nunes et al, , 2015Stpiczyńska et al, 2015). Based on the little information we currently have for Bulbophyllum, the labellum, as viewed in transverse section, generally comprises an epidermis, a subepidermis (or hypodermis) of small, isodiametric, starch-laden parenchymatous cells, and central ground parenchyma (referred to as mesophyll by Nunes et al, 2014Nunes et al, , 2015 containing collateral vascular bundles and idioblasts with bundles of raphides.…”

“…Thus, in many respects, the labellar morphology and anatomy of species of Asian Bulbophyllum sect. Racemosae studied to date resemble those of certain Neotropical and African members of the genus (de Pádua Teixeira et al, 2004;Nunes et al, 2014Nunes et al, , 2015Stpiczyńska et al, 2015). Indeed, de Pádua Teixeira et al (2004) have commented on the highly conservative character of the labellum in Bulbophyllum.…”

“…To date, very few anatomical studies of the labellum have been conducted on Bulbophyllum (de Pádua Teixeira et al, 2004;Nunes et al, 2014Nunes et al, , 2015, and there have been even fewer investigations of its ultrastructure and secretions Kowalkowska et al, 2015;Stpiczyńska et al, 2015). The labellar morphology of the representative members of sect.…”

“…Many of its members are pollinated by dipterans (van der Pijl and Dodson, 1969;van der Cingel, 2001) attracted by foodrewards and fragrance. However, to date, these food-rewards have been investigated for relatively few species, mostly Neotropical and African taxa, and have generally been found to be lipid-rich and produced by palisade-like epidermal cells of the labellum (de Pádua Teixeira et al, 2004;Nunes et al, 2014Nunes et al, , 2015Stpiczyńska et al, 2015). Other food-rewards are heterogeneous and contain both lipids and sugars (e.g., Pohl, 1935;Stpiczyńska et al, 2015).…”