Differential chlorophyll and carotenoid degradation underpin poor curing colour of low-quality flue-cured tobacco leaves

This study investigates the physiological basis for the inferior curing quality of low-quality flue-cured tobacco leaves. Using 'Yunyan 300' tobacco, we compared normal (CK), multi-fertilizer (LH), and reviving (FQ) leaves. Post-curing, LH and FQ leaves exhibited uneven, discoloured appearances versus the uniform golden-yellow CK. Physiologically, LH showed delayed water loss and electrolyte leakage, while FQ exhibited premature stress responses. Chemically, low-quality leaves maintained significantly higher post-curing nitrogenous compounds (total nitrogen, alkaloids, protein) and aberrant starch metabolism. Transmission electron microscopy revealed delayed chloroplast disassembly in LH and FQ, with persistent starch granules and tightly packed thylakoids. Pigment analysis showed significantly impaired degradation of chlorophylls and carotenoids in LH and FQ, leading to 30-40% greater accumulation of intermediate catabolites like pheophytin a and pheophorbide a (p < 0.05). Concurrently, the generation of final aroma-active degradation products (e.g., neophytadiene) was reduced. We conclude that the elevated initial levels of chlorophylls and carotenoids, coupled with metabolic dysregulation in low-quality tobacco leaves, collectively impair the complete enzymatic degradation of these pigments. This results in the accumulation of intermediate catabolites and an insufficient generation of key aroma precursors, thereby ultimately leading to the deterioration of both visual appearance and flavor quality.