## 3 Results and Discussion
### Comparative Analysis of HS-GC-IMS and HS-SPME-GC-MS in Identifying VOCs
#### 3.4.4 VOC Change of Different Tea Cultivars Following Drought Stress  
To gain deeper insights into the effects of field drought on the VOCs profiles of fresh tea leaves from different cultivars, a within-group comparative analysis was conducted between control and drought-treated samples of the FD and WNZ cultivars, respectively. Specifically, differential VOCs were screened using T-test analysis (\(p\)-value \(<\) 0.05) and fold change (FC \(\geq\) 2 or \(\leq\) 0.5) as the selection criteria. As illustrated in Figure 7, the volcano plots clearly depict the upregulation and downregulation of VOCs between drought-stressed and control samples. A total of 17 and 18 differential VOCs were identified between the FD-D vs. FD-CK group and the WNZ-D vs. WNZ-CK group using HS-GC-IMS data (Figure 7A). In the FD-D vs. FD-CK comparison, six VOCs were significantly increased, including 2-hexanone, methyl-5-hepten-2-one, 3-octanone, cis-linalool oxide, trans-linalool oxide, and (E)-4-methyl-2-(pent-1-enyl)-1,3-dioxolane. Meanwhile, 11 VOCs were significantly decreased, namely octanal, delta-carene, ethylpyrazine, hexanal, (E)-2-hexen-1-ol, 2-pentylfuran, linalool-D, but-(E)-2-enal, \(\beta\)-ocimene, 2-ethylfuran, and (E,E)-2,4-hexadien. In contrast, the WNZ cultivar exhibited significant alterations in VOCs under drought stress, with six VOCs showing marked upregulation. These included methyl-5-hepten-2-one, cis-linalool oxide, 2-hexanone, 3-octanone, trans-linalool oxide, and (E)-2-hexenyl acetate-M. Meanwhile, 12 VOCs were significantly decreased, such as styrene-D, (E,E)-2,4-hexadien, ethylpyrazine, (E)-2-hexen-1-ol, hexanal, octanal, methyl salicylate, but-(E)-2-enal, 2-ethylfuran, linalool-M, an unidentified volatile compound, and formic acid hexyl ester-M. From the above analysis, it is evident that the increased volatiles in both tea cultivars under drought stress exhibited a high degree of similarity, while the decreased VOCs showed some variations. Overall, the significantly increased volatiles predominantly displayed  
Figure 7: Volcanic map of differential VOCs detected by HS-GC-IMS (**A**) and HS-SPME-GC-MS (**B**). (a) FD-D vs. FD-CK; (**b**) WNZ-D vs. WNZ-CK. sweet, floral, earthy, fruity, and citrus aroma characteristics, whereas the decreased VOCs were mainly associated with fresh green, grassy, and leafy aromas. Figure 7B provides the volcano plots for VOCs in the FD-D vs. FD-CK and WNZ-D vs. WNZ-CK groups as measured by HS-SPME-GC-MS, with increased and decreased VOCs highlighted in red and blue, respectively. A total of 55 VOCs were significantly elevated, and 47 VOCs were reduced in the drought-treated groups compared to the control groups for the FD and WNZ cultivars. Detailed information regarding these VOCs is available in Tables 56 and 57. For the FD-D vs. FD-CK comparison, 30 VOCs were significantly elevated, including dodecanamide, dihydro-\(\beta\)-ionol, benzaldehyde, benzeneacetaldehyde, 2-hexenal, alkenes, medium- and long-chain fatty alcohols, and unsaturated fatty alcohols. In contrast, 25 VOCs were significantly reduced, primarily consisting of linalool oxide IV, linalool oxide III, coumarin, sesquiterpen, and several green-odor-type alcohols and esters. In the WNZ-D vs. WNZ-CK comparison, 23 VOCs were significantly elevated and 24 VOCs were significantly reduced. The elevated VOCs mainly exhibited sweet, floral, and fruity odors, including volatile terpenids (VTs), volatile phenylpropanoids/benzenoids (VPBs), and medium- to long-chain volatile fatty acid derivatives (VFADs, including fatty acid aldehydes, fatty acid alcohols, and fatty acid esters). Examples include benzeneacetaldehyde, 2,4,6-trimethyl-benzaldehyde, linalool oxide I, linalool oxide II, linalool oxide III, (3Z)-\(\beta\)-ocimene, \(\alpha\)-farnesene, \(\gamma\)-muurolene, \(\alpha\)-guaiene, methyl salicate, (Z)-jasmone, and sulcatol.