The Orchidaceae family, with its huge number of species that evolved different pollination systems, is known for the variety and complexity of its floral scents, which according to Kaiser (1993), could potentially cover all the spectrum of fragrances occurring in nature
[1]. Floral scent, which derives from the composition of volatile organic compounds emitted by the flowers’ tissues (floral VOCs), is fundamental for the defense against pathogens/herbivores and pollinator responses
[2]. This trait, together with other characteristics of flowers, such as the color, the presence of nectar, and other peculiarities of the reproductive portions, contributes indeed to defining pollination syndromes
[3]. The genus
Dendrobium Sw., 1799 (Epidendroideae; Dendrobiinae), which accounts for about 1100 species distributed in Pacific Islands, Asia, and Australia, is one of the largest of the family
[4]. As potted and cut flowers,
Dendrobium species and hybrids are of great economic interest, being at the top ten among the most commercially traded orchid taxa
[5]; several species are also grown and sold for medicinal purposes
[6][7]. A large number of taxa, the great morphological diversity, and the wide distribution range have contributed to taxonomic ambiguities that are currently under debate
[4][8][9]. In the phylogenetic revision of the genus, Takamiya et al. (2014) considered the presence of papillae on the flower’s lip in entities belonging to different clades. They demonstrated that this character evolved as an adaptation to bee pollination by
Dendrobium species
[4]. As stated in previous studies, bee-pollinated orchid flowers exhibit papillose carpets, identified as osmophores, structures of accumulation of substances responsible for floral fragrances
[10][11]. Takamiya et al. (2014) recorded odor-producing cells in all species of Section
Densiflora and the majority of the Section
Dendrobium, thus hypothesizing that this character has probably been acquired after the divergence between the Asian and the Australasian Superclades
[4]. Despite the great number of studies aimed at optimizing in vitro propagation protocols (i.e., Marting and Madassery, 2006; Teixera da Silva et al., 2015; Calevo et al. 2020; and references therein)
[12][13][14], and at characterizing anatomical and chemical traits (Carlsward et al., 1997; Xu et al., 2013; Devadas et al., 2016 and references therein)
[15][16][17], the genus
Dendrobium has been little investigated from the point of view of the reproductive biology, and even less is known about floral volatilome
[18]. To the best of our knowledge, only a few authors had carried out characterizations of floral volatiles from
Dendrobium species. Flath and Ohinata (1982) investigated the VOCs of
D. superbum Rchb. f. (syn.
D. anosmum Lindl.), which is pollinated by the melon fly (
Dacus cucurbitae), finding a significant amount of 4-phenylbutan-2-one, whose structure is closely related to another known fly attractant
[19]. Brodmann et al. (2009) worked on
D. sinense Tang and F.T.Wang and reported that this species emits (
Z)-11-eicosen-1-ol (a molecule present in the alarm pheromone of honeybees) to attract hornets for pollination
[20]. Silva et al. (2015) recognized terpenes as the most abundant class of compounds in the floral volatiles of
D. nobile Lindl.
[21]. Julsrigival et al. (2013) found a prevalence of 2-pentadecanone in
D. parishii Rchb.f.
[22]. Robustelli della Cuna et al. (2017), instead, compared the essential oil of different portions of
D. moschatum (Buch.-Ham.) Sw., including the inflorescence: They observed differences among the volatile compositions, and then hypothesized that compounds like ketones or long-chain methyl and ethyl esters play a role as pollinator attractants
[23]. The few reports dedicated to reproductive biology have stated that there are various ways for which
Dendrobium species attract pollinators: There are cases of shelter mimicry
[24][25][26][27][28], nectar rewarding
[18], chemical and visual attraction
[29], rest and mating place offering, or generalized food deception strategies like a simulation of other co-flowering species occurring in the same habitat
[30]. In this work, we aimed to characterize and compare the floral volatiles of five
Dendrobiums belonging to sections
Dendrobium and
Densiflora of the Asian Superclade
[4][9]. In particular, we characterized the volatile fractions of the inflorescences of
D. chrysanthum Wall. ex Lindl. (
Figure 1A),
D. harveyanum Rchb. f. (
Figure 1B) and
D. wardianum R.Warner (
Figure 1C) from section
Dendrobium, Core subclade of Clade A, and
D. chrysotoxum Lindl. (
Figure 1D) and
D. amabile (Lour.) O’Brien (
Figure 1E) from Clade A and C, respectively, of section
Densiflora (according to Takamiya et al. 2014)
[4].