Effects of moisture-preserving yellowing process optimization on cell physiology and post-curing chemical composition

BACKGROUND: Yellowing is a critical post-harvest process that determines the flavor, quality, and economic value of crops, particularly tobacco (Nicotiana tabacum L.). Although the optimization of temperature and humidity has been widely studied, the physiological mechanisms underlying tobacco responses to varying humidity conditions during yellowing remain insufficiently understood. RESULTS: This study investigated the effects of a moisture-preserving yellowing curing process (average relative humidity 82.7%) on the antioxidant systems, ultrastructure, untargeted metabolites, and post-curing chemical composition in model plant-tobacco cultivar Yunyan 87. Cellular ultrastructure showed this humidity condition maintained the structural stability of cell membranes and walls and reduced nucleolar disintegration during the yellowing stage. Metabolomics analysis indicated that it regulated lignin metabolism by downregulating (-)-secoisolariciresinol, which slowed dehydration, and modulated linoleic acid metabolism by downregulating 16-hydroxyhexadecanoic acid and 13 S-hydroxyoctadecadienoic acid, thereby mitigating membrane lipid peroxidation. Moreover, it enhanced antioxidant defense by increasing catalase (CAT) and superoxide dismutase (SOD) activities and maintaining higher carotenoids levels as non-enzymatic antioxidants. In addition, the moisture-preserving yellowing process promoted chlorophyll degradation and nutrient cycling, resulting in improved coordination of chemical components, enhanced aroma characteristics, and a more uniform, bright yellow color. CONCLUSION: Optimizing the moisture-preserving yellowing process significantly alleviates oxidative stress and preserves cellular integrity in tobacco leaves. By improving the balance between enzymatic and non-enzymatic antioxidants and promoting metabolic homeostasis, this process enhances the chemical composition, aroma quality, and visual appearance of flue-cured tobacco. These findings provide theoretical guidance for establishing appropriate humidity conditions in curing, fermentation, preservation, and storage of tobacco and other postharvest crops.