Integrated analysis of physiological and metabolic data uncovers essential dynamic mechanisms involved in the maturation of cigar tobacco leaves

The quality of cigar tobacco leaves is profoundly affected by the timing of their harvest, with both early and late collections resulting in inferior characteristics. While the relationship between maturity and physiological metabolic processes is acknowledged, a comprehensive understanding of the physiological behavior of cigar leaves harvested at different stages remains elusive. This research investigated the physiological and metabolomic profiles of the cigar tobacco variety CX-014, grown in Danjiangkou City, Hubei Province, with leaves sampled at 35 (T1), 42 (T2), 49 (T3), and 56 (T4) days post-inflorescence removal. Leaf color transitioned from green to yellow, accompanied by the appearance of white mature spots. Notable increases in photosynthetic pigments and gas exchange parameters occurred between T1 and T2, followed by decline at T3 and T4. The optimal sugar-to-nicotine and potassium-to-chlorine ratios, critical determinants of smoking quality and tobacco combustibility, were observed at T3, indicating a superior chemical balance in leaves harvested at this stage. Metabolomic analysis revealed 2153 distinct metabolites, with the most significant changes occurring between T2 and T3, highlighting critical physiological transformations during this interval. Pathway enrichment analysis via KEGG pinpointed notable shifts in amino acid synthesis pathways, particularly those involving tryptophan, alanine, and aspartate. Tryptophan metabolism and zeatin biosynthesis were substantially altered, with compounds like indolepyruvic acid, N-formylpurine nucleotide, isopentenyladenine nucleotide, and dihydrozeatin showing marked reductions at T3. This study also explored how the timing of lower leaf harvest influences the physiological processes of middle leaves, finding that a plethora of metabolites associated with the breakdown of arachidonic acid-a primitive metazoan signaler implicated in plant stress and defense networks-were abundant in T3 leaves when lower leaves were harvested 43-38 days prior. Overall, these findings elucidates the complex physiological dynamics of cigar leaves during maturation, highlighting critical metabolites involved in essential metabolic pathways.