## 4 Discussion  
There were significant differences in the volatile components and relative contents of different _Dendrobium_ species, and the main volatile components played a decisive role in their fragrance. For instance, esters are the aroma volatile components of _D. lolohense_, while in _D. densiflorum_ are mainly alkanes. The volatile content of linoleic acid and linoleic acid accounted for more than 60% of the total content in _D. thyrsiflorum_[48]. Geraniol and linalool are major floral scent compounds in _D. hancockii_[49]. The main volatile terpenids in _D. chrysanthum_ flowers were \(\alpha\)-phellandrene, \(\alpha\)-pinene and \(\alpha\)-thujene [50]. In _D. officulae_, different cultivars with different smells had different contents of volatile compounds such as \(\alpha\)-thujene, linalool, and \(\alpha\)-terpineol [29; 51]. In this experiment, SPME and GC-MS  
Figure 7: Relative expressions of 15 genes at the transcriptome and mRNA levels. were used to extract and detect the volatile components of flowers. Although there were certain components and content differences in different florescences and parts, 60 and 52 volatile compounds were collected from _D. moniliforme_ and _D._ \"H1\", respectively. These compounds, according to their main chemical group, were classified as terpen, aromatics, fatty acids, nitrogenous compounds, ketone, alcohols, and alkanes. The results showed that terpenoids were the main components of aromatic compounds in the two species, and their relative contents were 76.69% (_D. moniliforme_) and 79.09% (_D._ \"H1\") at full bloom stage, respectively. In petals, the contents of terpenoids in _D. moniliforme_ and _D._ \"H1\" accounted for 77.25% and 50.38%, respectively. Among terpenoids, \(\alpha\)-Pinene is the most abundant compound in the four florescences and different parts of the two species flowers. At the same time, we found that the contents of volatile terpenoids in two _Dendrobium_ species were significantly different. For example, linalool was detected in four flowering stages in _D. moniliforme_, with the content ranging from 10.21% to 27.84% and reaching 30.86% in the petals. In contrast, _D._ \"H1\" can only be detected in three flowering stages, with the content ranging from 0.44% to 1.37% and only 2.73% in the petals. We hypothesized that LIS content is one of the reasons for the difference in scent between the two species. Plant volatile terpen play critical roles in the formation of orchid floral scents, which include monoterpenes, sesquiterpenes, and diterpenes [52]. In the current study, up-regulated KEGG enrichment pathway analysis showed that the terpenoids backbone biosynthesis pathway was highly enriched in _D. moniliforme_. The findings reinforce that terpenoids biosynthesis increased in high-scent _Dendrobium_. This study suggests a consistent relationship between the _Dendrobium_ of floral scents and terpenoids biosynthetic pathways. It is urgent to study the differential gene functions of these two species. In addition to terpenoids, the floral scents of _Dendrobium_ contain a series of secondary metabolites such as benzenes/phenylpropanes, fatty acid derivatives, sulfur, or nitrogen compounds released by flowers [53]. Thus, a total of 95 genes related to these pathways in KEGG were selected for PPI network analysis to clarify the functional network that controls Aroma formation. The results showed that there were 15 closely connected genes, among which 4 were LIS genes, and their expressions were up-regulated. Real time PCR analysis of the 15 closely related genes showed that their expressions were completely consistent with the transcriptome data, indicating that the transcriptome data used in this study had strong credibility. As an important member of the monoterpene family, linalool has a unique fragrance and various biological properties, which show important application value in attracting predators and repelling pests [54; 55]. For example, Arabidopsis plants release linalool by transferring the TPS gene to repel aphids [56].