## 3 Results  
Analysis the Characteristics of Volatile Constituents in Different Floescences of Two Dendobium Species  
According to the SPME and GC-MS analysis of volatile compounds from \(D\). _moniliforme_ and \(D\). \"H1\", a total of seven main chemical groups, including terpen, aromatics, fatty acids, nitrogenous compounds, ketone, alcohols, and alkanes, were collected. However, there were significant differences in the compositions and content of volatile compounds in the two _Dendrobium_ species. As shown in Figure 2, terpenids are the main components of the aromatic compounds in the flowers at the four flowering stages. The contents of terpenids in \(D\). _moniliforme_ were 71.78% at the bud stage, 89.83% at the first flowering stage, 76.69% at the full bloom stage, and 64.33% at the declining stage. Otherwise, the terpenids contents of \(D\). \"H1\" in the four stages are 53.27%, 89.74%, 79.09%, and 35.78%, respectively. Among the 60 volatile compounds identified in \(D\). _moniliforme_, 1R-\(\alpha\)-pinene and Linalool were found in all four flowering stages. The relative contents of 1R-\(\alpha\)-pinene and Linalool accounted for more than 40% of all compounds, which was the highest among all compounds in the four flowering stages (Supplementary Table S1). On the other hand, we found 52 volatile compounds in the flowers of \(D\). \"H1\", and the relative contents of 1R-\(\alpha\)-pinene were more than 21.11%, which was detected in four flowering stages. Except for the bud stage, the content of cis-\(\beta\)-ocimene was also relatively high (10.36-54%) (Supplementary Table S2). These results indicated the composition of volatile compounds becomes more complex as the flowers open, with the most complicated in the full bloom stage, which has 41 (\(D\). _moniliforme_) and 33 (\(D\). \"H1\") volatile compounds, respectively. In particular, the emissions of Linalool (one of the terpenids) were higher in \(D\). _moniliform_ throughout all the flowering stages, which accounted for 10.21-27.84% of the total volatiles in \(D\). _moniliform_ but for only 0-1.37% in \(D\). \"H1\". The difference in linalool content may be one of the main volatile components that distinguish floral fragrance release between these two _Dendrobium_ species.  
Analysis of the Characteristics of Volatile Constituents in Different Organss of Two Dendrobium Species  
By GC/MS analysis, the components and relative contents of the same compounds in different flower parts of the two _Dendrobium_ species at full bloom stage were measured (Table 1). The highest relative content in the petals was still the terpenoids contents, and there were 15 kinds of terpenoids identified in _D. moniliforme_ with a relative total content of 77.25%. _D._ 'H1' contained 13 kinds of terpenoids with a relative total content of 50.38%. Although there were only 7 kinds of terpenoids in the gynandrium of the two species, the relative total contents reached 81.75% (_D. moniliforme_) and 63.31% (_D._ 'H1'), respectively.  
Figure 2: Volatile constituents in different florescences of two _Dendrobium_ species.  
The results in Tables S1 and S2 showed that at the full flowering stage, the volatile compounds of _D. moniliforme_ and _D._ \"H1\" in petals were 26 and 36, respectively, and 17 and 19 volatile compounds in gynandrium, respectively. Thus, the volatile compounds of the petal are more complex than those of the gynandrium. These results suggested that the petals may be the most important part affecting the release of volatiles.