Comparative floral micromorphology and anatomy of species ofBulbophyllumsectionNapelli(Orchidaceae), a Neotropical section widely distributed in forest habitats

Abstract

Bulbophyllum section Napelli, as recently circumscribed based on molecular analyses, comprises 12 species. These occur as epiphytes in the Brazilian Atlantic Rain Forest and in gallery forests in the cerrado vegetation, and thus differ from other Neotropical sections of Bulbophyllum, which are more variable in habit and habitat. To identify diagnostic characteristics that are representative of this section and to confirm whether there are characteristics that are related to their habit and habitat, their floral micromorphology and anatomy were studied using conventional techniques. Bulbophyllum section Napelli is characterized by sepals with sunken glandular trichomes and stomata on the abaxial surface, reduced petals with one vascular bundle (sometimes lacking), an entire labellum with a secretory sulcus on the adaxial surface and a keel with stomata or glandular trichomes on the abaxial surface, two pollinia, and crystalliferous idioblasts in all floral parts. Our data enable us to distinguish between species and show congruence with the present circumscription of B. section Napelli. The labellar epidermal surface of three closely-related species was unusual in its possession of unicellular trichomes and these appear to function as osmophores. A relationship between striate surfaces, iridescence and pollinator attraction related to the epiphytic habit and forest habitat is proposed.

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).…”