## Discussion  
Differences in Aroma Composition of Lingtou Dancong Oolong Tea among Plants Grown at Different Altitudes  
In the evaluation of oolong tea, sensory evaluation is widely used to score the aroma quality. However, this method is subjective. The combination of GC-MS with an organoleptic analysis can evaluate the aroma of oolong tea more objectively. The GC-O/MS system is useful for identifying the contribution of aroma-active compounds in a complex matrix. We found that the categories of odorants in Lingtou Dancong oolong tea products from different altitudes were the same, but the content and proportion of each category differed among materials from different altitudes. The contents of esters (mainly _cis_-jasmine lactone) and nitrogen heterocyclic compounds (mainly indole) were lower in medium-altitude teas than in high- or low-altitude teas (Figure 2 and Table 1), resulting in the lowest total aroma compound content in medium-altitude tea. As a result of _cis_-jasmine lactone's (honey) low odor threshold, the low content of _cis_-jasmine lactone in the medium-altitude tea also directly led to the deficiency of a honey fragrance (Tables 1 and 2, Figure 4). Comparing tea materials from all altitudes, the total proportions of alcohols, aldehydes, terpen, and nitrogen heterocyclic compounds tended to first increase, and then decrease with elevation (Figure 2). Another study found that aldehydes accounted for the largest proportion of odor compounds in most oolong teas [5]. However, we found that esters accounted for the largest proportion in Lingtou Dancong oolong tea. For example, the contents of \(\gamma\)-nonanolactone and \(\gamma\)-octanolactone, which confer the coconut fragrance of green tea, were significantly higher in medium- and high-altitude teas than in the low-altitude tea [12; 13]. Among the alcohols, linalool and geraniol share the same precursor, GPP, but they have different odors [14]. Linalool has a floral-orange fragrance, while geraniol has an obvious rose fragrance [15]. Among the tea products, those from high and medium altitudes had higher linalool and geraniol contents in the fresh leaves, and higher contents of their shared precursor GPP (Table 1 and Figure 5). Aldehydes in tea can be produced through lipid oxidation and Strecker degradation under thermal conditions [16]. For example, hexanal (OAV > 1) and (_E_, _E_)-2_A_-nonadien (OAV > 1) are generated by lipid oxidation, and butanal, 2-methyl- (OAV > 1) is produced by Strecker degradation. We found that the contents of all three of these compounds were higher in medium- and high-altitude teas than in low-altitude tea, and they were identified as the main contributors of the green grass odor in tea. Among the ketones, \(\alpha\)-ionone (OAV > 1), which has a woody fragrance, was present at higher levels in high-altitude tea than in medium- and low-altitude teas. (_E_)-\(\beta\)-damascenone (OAV > 1) and _trans_-\(\beta\)-ionone (OAV > 1) are two representative odor compounds generated by carotenoid degradation, and both are essential contributors to aroma [5; 17]. As reported in previous studies, indole is another important aroma substance in oolong tea. The accumulation of indole is caused by turning-over during oolong tea processing, and this contributes to the floral fragrance [18]. We used OAV and FD analyses to evaluate whether a single odor compound makes a major contribution to fragrance. We detected eight odor compounds with FD \(\geq\) 128 and OAV \(>\) 1 as the highest contributors, and they also had high sensory intensity (Figure 4B, Tables S3-S5). Among the eight fragrances, the contents of hotrienol (honey),linal (floral), geraniol (floral), and \(\alpha\)-ionone (woody, herbal) were positively correlated with altitude, while the _trans_-\(\beta\)-ionone (floral) content was highest in medium-altitude tea (Table 2). The _cis_-citral (honey) content was significantly higher in medium-and high-altitude teas than in low-altitude tea.