What do floral anatomy and micromorphology tell us about NeotropicalBulbophyllumsectionDidactyle(Orchidaceae: Bulbophyllinae)?

Abstract

Bulbophyllum section Didactyle comprises seven species, but distinction between these is often problematic. These species are pollinated by milichiid flies and air currents move the hinged labellum and press the pollinator against the gynostemium. The labellum structure is considered to be homogeneous and conservative for the genus. Therefore, the floral anatomy and micromorphology of B. section Didactyle were studied in order to identify characters useful for distinguishing the species. All species have sunken glandular trichomes on the abaxial surface of the sepals (possible osmophores) and a trilobed labellum, clothed with trichomes, with a secretory cavity in the callus that is bound by scale-like papillae. Of the c. 100 characters assessed, 25 varied between species, and each pair of species differed by at least four character states, mainly occurring on the labellum. The data presented allow for a distinction to be made between species and corroborates their grouping in B. section Didactyle, as proposed previously. The presence of osmophores and a nectary on the labellum is confirmed, although their structure is more diverse than anticipated. Moreover, structural differences between B. weddellii and the core of the section might be the result of the odour-mediated attraction of pollinators rather than flower morphology and thus phylogeny.

Evidence for the Dual Role of Floral Secretory Cells in Bulbophyllum

“…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.…”

“…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. The median, longitudinal groove is lined with palisade-like epidermal secretory cells.…”

“…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).…”