## 3 Results and Discussion  
### Qualitative and Quantitative Analysis of the VOCs by HS-GC-IMS  
Aroma volatiles of fresh tea leaves were identified and analyzed through HS-GC-IMS, employing retention indexes (RIs), retention times, and drift times referenced from the NIST library. A total of 50 peaks were detected across four groups, with 44 peaks successfully identified as specific chemical compounds, while 6 peaks remained unidentified and were classified as unknown substances, requiring further validation. The detailed analysis results are provided in Table S3. The detected VOCs were categorized into several groups, including aldehydes, ketones, esters, terpenes, furrans, alcohols, aromatics, and pyrazines (Figure 1A). Among these, aldehydes, ketones, and terpenes were the three most dominant volatile groups, collectively constituting over 60% of the total volatiles (Figure 1B), with 12, 7, and 6 aroma compounds, respectively (Table S3). The VOC content exhibited variation between different tea cultivars. Overall, the FD cultivar demonstrated higher VOC levels compared to the WNZ cultivar. Drought stress led to an increase in VOC levels in both cultivars. Notably, in the FD cultivar, ketones, terpenes, and aromatics showed a substantial increase under drought conditions, while aldehydes, esters, furans, alcohols, and pyrazines decreased. In comparison, in the WNZ cultivar, aldehydes, ketones, and terpenes exhibited a marked increase under drought stress, while the levels of esters, furans, alcohols, aromatics, and pyrazines decreased. The results indicate that the differential accumulation of aroma compounds under drought stress is related to the varying drought adaptability of different tea cultivars [40]. In summary, across both cultivars, drought stress induced a notable increase in ketones and terpenes, contributing to enhanced citrus, floral, sweet, earthy, and herbal aroma characteristics in fresh tea leaves. These results suggest that ketones and terpenes play a key role not only in the drought resilience of tea plants but also in the modulation of tea flavor under environmental stress. Interestingly, tea plantation smallholders serving as informants from tea production communities in Yunnan Province, China, also observed that the aroma of spring tea becomes more intense during drought periods, which they believed positively influences the market price of the tea [22]. This observation aligns with the findings of our study, where the content of volatile compounds increased in both FD and WNZ tea cultivars following drought stress (shown in Figure 1A). Figure 1: Comparison of VOCs in 4 groups detected by HS -GC-IMS. (**A**) Stacked column graph of VOC content; (**B**) 100% stacked column graph of VOC relative content. Drought stress influences the differential accumulation of VOCs in the fresh leaves of tea plants. Compared to the control group, drought-stressed plants exhibited higher levels of ketone VOCs, including methyl-5-hepten-2-one, 3-octanone, mesily oxide, 2-pentanone, 2-butanone, and acetone. Additionally, methylated VOCs ((E)-4-methyl-2-(pent-1-enyl)-1,3-dioxolane, methyl acetate, 5-methylfurfural, 2-furanmethanol, 5-methyl-, and 3-methyl-3-buten-1-ol), linalool oxides (trans-linalool oxide, cis-linalool oxide), (E)-2-hexenyl acetate-D, oct-1-en-3-ol, p-xylene, and benzaldehyde were also elevated (Figure 2A-C). These VOCs contribute predominantly to sweet, caramel-like, floral, earthy, and fruity aroma characteristics (shown in Table S3). It is important to note that certain VOCs exhibited distinct accumulation patterns across different tea cultivars. For example, VOCs such as (E)-hept-2-renal-D, butanal, (E)-2-hexenyl acetate-M, ethyl acetate, and delta-carene were present in significantly higher concentrations in the WNZ cultivar under drought stress (Figure 2B), whereas their levels decreased in the FD cultivar under the same conditions (Figure 2A).