Metabolomics-based mechanism of colour change in Cuibi No.1 flue-cured tobacco leaves during early aging

Early aging is critical for colour formation in tobacco leaves, yet the underlying metabolic mechanisms remain elusive. This study integrated image processing with untargeted and targeted LC-MS to construct a metabolic network characterising colour evolution in flue-cured tobacco (cv. Cuibi-1, CB-1 ) during early aging (0–120 days). Results indicated that leaf colour significantly brightened during the first 30 days, followed by deepening and reddening. Specifically, L* and b* values showed an initial increase followed by a decrease, whereas the a* value exhibited a sustained upward trend. The constructed metabolic network was anchored in saccharide metabolism, connecting downstream phenolic, flavonoid, sphingolipid, and terpenoid pathways. Sixteen differentially accumulated metabolites (DAMs) were identified across categories including alcohols, flavonoids, lignans, nucleotides, phenolic acids, saccharides, and xanthophylls. These DAMs exhibited distinct change patterns and significant correlations with colour parameters (L* and a*, r ranging from −0.88 to −0.58 and 0.56–0.74, respectively; P  < 0.05). For the first time, this study elucidated the metabolic mechanisms governing colour changes in CB-1 tobacco leaves during early aging for the first time, providing novel insights for assessing aging progression and maintaining high-value raw material quality.