## 1 Introduction  
_Dendrobium Ioddigesii_ Rolfe, a perennial epiphytic herb in the Orchidaceae family [1], is a precious medicinal material in traditional Chinese medicine because of its secondary metabolites production [2, 3]. Furthermore, _D. Ioddigesii_ has a high economic value in ornamental horticulture due to its pleasing morphology and fragrance, and it is often used as the source material for essence and fragrance extraction in the cosmetic industry [4]. Floral scent is one of the most significant traits that constitute the ornamental value of flowering plants [5, 6]. However, due to the complexities in the composition, structure, and biosynthesis process of floral scent, research findings are insufficient compared to other important ornamental characteristics such as flower shape and color [7, 8]. Meanwhile, floral scent is not a breeding criteria during _Dendrobium_ hybridization for the long term, which leads to the lack of fragrance in many commodity dendrobiums [9]. The selection of aromatic varieties has become one of the most urgent needs, and important trends in the breeding of _Dendrobium_, in which the role of _D. Ioddigesii_ cannot be ignored.  
Floral scent is composed of various low-molecular-weight volatile chemicals mostly belonging to three major groups: terpenoids, benzenoids/phenylpropanoids, and fatty acid derivatives [10]. The floral scents of common ornamental plants such as rose and peony have been well studied [11; 12], some research studies about aromatic compounds of _Dendrobium_ also have been reported. In total, 33 volatile organic compounds (VOCs) were identified in _D. chrysotoxum_, including 15 terpen, 7 esters, 6 alcohols, 2 aromatic compounds, 2 ketones, and 1 aldehydes [9]. Meanwhile, 41 VOCs were distinguished at the flowering stage of _D. moniliforme_, and the alkenes were the most important compounds, which affected its floral scent [13]. A recent study has identified esters as the main components of the aroma in _D. lod digessi_ sampled from Yunnan province, China [14]. However, another research suggests that the main ingredients of floral scent in _D. lod digessi_ are terpenes [15]. These findings suggested the complexity of volatile components in flowers of _Dendrobium_ species.  
Due to the invisibility and highly variable characters of floral scent and lack of efficient methods to identify genetic variations, research on _Dendrobium_ has focused on the stage of volatiles identification for a long time. The genetic and molecular mechanisms responsible for the floral scent in _Dendrobium_ species are poorly revealed. Thanks to the rapid developments of molecular biology and high-throughput sequencing technologies, recent research studies on floral scent have elucidated several biosynthetic pathways of fragrance compounds and some structural genes encoding key enzymes in _D. official_[16; 17]. Nonetheless, there are few studies on the molecular mechanism of synthesis of _D. lod digessi_'s aromatic ingredients.  
To investigate the characteristic and molecular mechanisms of floral scent in _D. loddigesi_, the flowers of high-scented _D. loddigesi_ (TH) originated from Guizhou province and light-scented _D. loddigesi_ (TL) from Guangdong province of China were used as materials to screen differential metabolites (DMs). There were obvious differences in olfactory sense between the two varieties: flowers of TH had a strong floral smell, while TL has a grassy smell. Furthermore, we conducted the first de novo assembly of the transcriptome for the species by high-throughput RNA sequencing. The differential expressed genes (DEGs) and enriched pathways between two varieties were identified. Our findings provide important gene resources for further study of the biosynthesis pathway and regulation mechanism responsible for floral volatiles in _D. loddigesi_. This work holds merits for exploring the molecular mechanism of aroma biosynthesis of _D. loddigesi_ and providing preliminary preparations for breeding new aromatic _Dendrobium_ varieties and perfume blends.