Metabolome of flue-cured tobacco is significantly affected by the presence of leaf stem

Abstract

Background: Leaves of tobacco (Nicotiana tabacum L.) are flue-cured to use as a key industrial supply in various parts of the world. The quality of tobacco leaves is dependent on chemical components and their proportions. Generally, the stem attached to tobacco leaf is detached before curing. However, the leaf stem remains green for an extended period of time (as compared to leaf) during flue-curing. Hence, it is expected to affect the quality of tobacco's final product.

Results: To understand the impact of the green stem of leaf on the metabolome of flue-cured tobacco, we employed a broad targeted metabolomics approach. We selected two tobacco cultivars (Yun87 and K326) and cultivated them in five geographic locations in China. For flue-curing, leaves were harvested without a stem (L) or with an attached stem (SPL). After metabolome analysis, a total of 1027 metabolites were annotated in these samples. A variable number of metabolites were differentially accumulated between both types of leaves (depending on geographic location or cultivar) representing an influence of environment or genotype. Interestingly, only 68 metabolites were differentially accumulated between L and SPL samples irrespective of the cultivar or geographic location. These differentially accumulated metabolites belonged to major groups of primary and secondary metabolites. We have discussed the importance of identified metabolites in terms of carbon, nitrogen, and polyphenolic metabolism.

Conclusion: The present research is the first comprehensive description of several metabolites in tobacco leaves related to the contribution of leaf stem. The current study opens novel prospects for investigating the potential of such metabolites in improving the quality of flue-cured tobacco.

Keywords: Leaf; Metabolites; Postharvest treatment; Quality; Tobacco.

Fig. 1

General representation of tobacco leaves used for metabolome analysis. The left panel represents the leaves normally used for flue-curing and the right panel represents the leaves along with green stems. For the analysis, only the six leaves present at the top of tobacco plant were used

Fig. 2

A general description of identified metabolites. A Classification as well as the composition of the 1027 metabolites of tobacco leaves. B Heatmap regarding hierarchical clustering analysis (HCA) of metabolites of all samples. Location of cultivation (L1, L2, L3, L4, L5), Genotype (V1, V2), only leaf (L), stem plus leaf (SPL)

Fig. 3

Analysis of differential metabolite in all samples. A Principal component analysis to visualize the sample distributions, B OPLS-DA generated score plots of the differential metabolites; C The volcano plot representing differential metabolites among different samples; D the Venn diagram showing differential metabolites in different samples, E Total number of differentially accumulated metabolites in different samples. Location of cultivation (L1, L2, L3, L4, L5), Genotype (V1, V2), only leaf (L), stem plus leaf (SPL). OPLS-DA, orthogonal partial least squares discriminant analysis

Fig. 4

Analysis of metabolic enrichment pathway in two comparative groups. A KEGG classification, B Enriched compounds with the highest fold change

Fig. 5

Physiological and biochemical properties. A Chemical indices and Polyphenol substances composition of tobacco leaves, B Amino acid composition of tobacco leaves. Red represents higher content and green represents lower content. L1, L2, L3, L4, and L5 represent five geographic locations. V1 and V2 represent two varieties of tobacco. L and SPL represent Leaf only and Stem plus leaf, respectively. Three-letter codes represent amino acids