## 3 Results  
### Dynamic Changes of Floral Aroma at Different Stages  
A total of 70 volatile compounds were detected in the three species of _Tilia_ at different flowering stages, 43 aroma compounds were identified, 27 components remained uncertainly labelled as NI (Table 2). At different stages, there were 28, 28 and 26 components from _T. cordata_; 17, 20 and 17 from _T. tomentosa_; 22, 24 and 26 from _T. miapeliana_, respectively. Figure 2 demonstrated that the three species had a wide range of floral aroma compound quantities, and the relative content of terpenids were dominating in the identified aroma components. The primary aroma components released from _T. cordata_ flowers are \(\beta\)-Ocimene, Germacrene D and Linalool. With the gradual blossoming, the relative content of \(\beta\)-Ocimene increased from 29.47% to 34.44% and lastly to 36.90% at different flowering stages; Linalool was 14.74%, 18.73% and 21.30%. Both \(\beta\)-Ocimene and Linalool showed an increasing trend. The relative content of Germacrene D, on the other hand, kept on diminishing from 22.30% to 18.67% and 16.83%. \(\beta\)-Ocimene, D-Limonene, and Cyclohexene, 1-methyl-4-(1-methylylidene)- were predominant fragrant substances of _T. tomentosa_. However, \(\beta\)-Ocimene and Cyclohexene,1-methyl-4-(1-methylylidene)- did not vary significantly at the three flowering stages. The relative content of D-Limonene increased marginally, containing 19.88% and 19.04% at the bud and half-open stages, respectively, and going up to 22.55% during the blooming period. The dominating aroma substance of _T. miqueliana_ was \(\alpha\)-Farnesene, with an inconspicuous increase in relative content at the three flowering stages, containing 58.63%, 59.35% and 60.37%, respectively. The amount of fragrance compounds present in the three species at various flowering phases varied greatly, as shown by Table 2 and Figure 2.  
At different flowering phases, _T. cordata_ had 25 aroma substances in common at the three flowering stages; the bud stage shared 2 exclusive substance with the half-open stages, and only 1 particular component in the half-open stage and the blooming stage. _T. tomentosa_ contained 14 aroma substances in common across the three flowering stages, the half-open stage shared 2 unique components with the bud stage and the blooming stage, respectively. _T. miqueliana_ possessed 20 fragrant compounds that appeared in all three flowering stages, the bud stage shared 1 particular component with the half-open stage and the blooming stage, respectively. Some aroma components were only available during special periods, for example, there was only 1 exclusive component in the bud stage of _T. cordata_; For _T. tomentosa_, the bud stage and the blooming stage had 1 exclusive component, respectively, and 2 exclusive components appeared in the half-open stage; For _T. miqueliana_, 3 exclusive components in the halogen stage, 5 exclusive components in the blooming stage. Moreover, Tridecane was detected at the half-open stage from _T. miqueliana_, which has been proven to be one of the ingredients of plant waxes that covers not only leaves but also other plant organs like flowers and fruits [26]. Thus, it is possible to assume that wax layers are present on the floral surface of _T. miqueliana_. Additionally, _T. cordata_ shared 2 aroma components with _T. tomentosa_: \(\beta\)-Ocimene and Germacrene D, while only sharing Lilac aldehyde (isomer II) with _T. miqueliana_. 4 aroma compounds including D-Limonene, \(\alpha\)-farnesene, Phenylethyl Alcohol, and Acetic acid, 2-phenylethyl ester were discovered in both _T. miqueliana_ and _T. tomentosa_. However, only 2 components in commom among the 3 species. Considering \(\beta\)-Ocimene, Germacrene D, D-Limonene and \(\alpha\)-farnesene diversely all exhibited high relative percentages diversely among the three species, it may be inferred that they are the typical scent chemicals in flowers of _Tilia_. Figure 2: UpSet diagrams based on the aroma components at different flowering stages of the three species.