Untargeted Metabolomics and GC–IMS Reveal Quality-Related Differences in Qilan Fresh Tea Leaves Grown in Wuyi and Baisha

In this study, fresh leaves of the same Camellia sinensis (L.) O. Kuntze cv. Qilan cultivar grown at similar elevations but contrasting latitudes in Wuyishan, Fujian (the 2L-FQ cultivation site), and Baisha, Hainan (the 2L-HQ cultivation site), were systematically compared using environmental characterization, enzyme activity assays, gas chromatography–ion mobility spectrometry (GC–IMS), and untargeted metabolomics. Black teas produced from the two groups of fresh leaves under identical standardized processing conditions were further evaluated using electronic nose and electronic tongue analyses. Correspondingly, 2L-HQ leaves were longer and wider than 2L-FQ leaves, with higher moisture content and water activity. The lower L*, b*, and chroma values of 2L-HQ, together with a ΔE*ab of 6.54, indicated a darker leaf appearance, which was accompanied by higher chlorophyll and carotenoid concentrations. Compared with 2L-FQ, 2L-HQ contained 11.8% more tea polyphenols, 26.4% more soluble polysaccharides, 11.4% more caffeine, and 10.6% more free amino acids. The non-gallated catechins catechin, epicatechin, epigallocatechin, and gallocatechin were significantly enriched in 2L-HQ. DPPH, ABTS, and FRAP antioxidant capacities were also 6.1%, 4.9%, and 2.2% higher in 2L-HQ, respectively. Lipoxygenase, polyphenol oxidase, and peroxidase activities increased by 137.0%, 98.0%, and 93.8%, respectively, whereas β-glucosidase activity remained unchanged. GC–IMS resolved 61 volatile compounds, of which 46 differed significantly between the two origins. 2L-FQ was enriched in several aromatic aldehydes, β-phenyl ethanol, and fruity esters, whereas 2L-HQ contained higher levels of selected alcohols, unsaturated aldehydes, furanic compounds, and lipid-derived esters. Untargeted metabolomics retained 4359 metabolic features, of which 4356 were shared by both groups. After multiple-testing correction, 793 differential metabolic features were identified using VIP > 1, FDR-adjusted p < 0.05, and |log2FC| > 1, including 420 features with higher abundance and 373 features with lower abundance in 2L-HQ relative to 2L-FQ. These differential features were mainly associated with carbohydrate, amino acid, flavonoid-related, lipid, and cofactor metabolism. Overall, regional cultivation was associated with coordinated differences in leaf morphology, hydration, pigmentation, taste-related composition, antioxidant capacity, oxidative-enzyme potential, volatile fingerprints, and metabolic allocation, providing a data foundation for region-adapted processing of Qilan tea leaves.