Abstract
The timing of maturity significantly impacts the quality of cigar tobacco leaves, with both premature and delayed maturation leading to quality degradation. Despite the known association between maturity and physiological metabolic activities, there is a paucity of concrete evidence detailing the physiological behavior of cigar leaves harvested at varying times. This research involved a comprehensive physiological and metabolomic examination of the cigar tobacco variety CX-014, cultivated in Danjiangkou City, Hubei Province. The study focused on leaves picked at 35 (T1), 42 (T2), 49 (T3), and 56 (T4) days following the removal of the apical inflorescence. As the harvest period progressed, the leaves’ hue transitioned from green to yellow, displaying white mature spots. Between T1 and T2, there was an uptick in pigment indices (like chlorophyll a and b) and photosynthetic traits (such as stomatal conductance), which then diminished in the T3 and T4 samples. Optimal levels of sugar-to-nicotine and potassium-to-chlorine ratios—key indicators of smoking quality and tobacco combustibility—were observed at T3, suggesting a more balanced chemical composition in the leaves harvested at this stage. Metabolomic analysis revealed 2,153 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 to 38 days prior. These findings suggest that the harvest timing of lower leaves may sway the maturation physiology and environmental adaptability of middle leaves. Overall, this investigation sheds light on the intricate physiological dynamics of cigar leaves throughout maturation and pinpoints crucial metabolites that signify pivotal metabolic pathways.