Variations of flavonoid and carotenoid metabolites in cured tobacco and their relationships with color fading

To investigate the characteristics of metabolites in cured tobacco during color fading and their relationship, the easily fading flue-cured tobacco variety Yunyan 87 was treated under 200 μmol/(m²·s) light intensity for 0, 15, and 30 d to obtain tobacco leaves with different degrees of fading. The color differences and the characteristic flavonoid and carotenoid metabolites before and after color fading were observed, and their relationships with the degree of fading were analyzed. The results showed that: 1) The L^* value first decreased and then increased, and the a^* and b^* values decreased continuously during the fading process of cured tobacco. 2) A total of 4 987 metabolites belonging to 25 categories were detected in the cured tobacco after different fading treatments. The more severe the fading was, the higher the amounts of differential metabolites and down-regulated differential metabolites. 3) The results of metabolic pathway analysis showed that the transformation metabolic pathways of flavonoid and carotenoid metabolites were significantly enriched during the fading process, indicating that these two pathways were closely related to the development of tobacco color. Quantitative analysis revealed that there were strong correlations between the flavonoid and carotenoid metabolite contents (mass fractions) and the color indexes of the cured tobacco leaves. The degradation and transformation of flavonoid and carotenoid metabolites were closely related to the decrease of redness and yellowness values of the cured tobacco, which might be the main internal cause of cured tobacco fading.