Free amino acids content characteristics and their association with leaf quality of Sichuan flue-cured tobacco

【Objective】The purpose of this study was to explore the correlation between free amino acids and sensory and appearance quality of flue-cured tobacco, and to identify the key amino acid indexes affecting the quality of tobacco, so as to provide theoretical references for the directional regulation and quality enhancement of high-quality tobacco in Sichuan.【Method】A total of 188 tobacco samples were collected from five main flue-cured tobacco producing areas in Sichuan(Liangshan, Panzhihua, Yibin, Luzhou and Guangyuan), and the appearance quality and sensory quality were evaluated, as well as the content of free amino acids. Pearson correlation analysis and Mantel test were used to analyze the relationship between the content of free amino acids, the proportion of various amino acids and the sensory and appearance quality indexes, identify the key amino acid indexes affecting the quality of Sichuan tobacco and analyze the distribution characteristics of the key amino acid indexes in the five production areas.【Result】Atotal of 17 free amino acids were detected in Sichuan flue-cured tobacco, with an average total content of 14.2 mg/g. Among them, the content of proline was the highest, accounting for 83.4% of the total amino acid, and the content of isoleucine was the lowest, with an average of only 7.6 μg/g. The contents of the 17 free amino acids were showed as upper leaves > the middle leaves, and the differences between the stalk positions reached the extremely significant level(P<0.01, the same below). Correlation analysis showed that there were mainly positive correlations among 17 free amino acids. Isoleucine, phenylalanine, valine, threonine, histidine, glutamine, leucine, the proportion of alkaline amino acids, the proportion of aromatic amino acids, the proportion of hydroxyl amino acids, and the proportion of amide amino acids were negatively correlated with tobacco quality, while the proportion of imino acids was positively correlated with tobacco quality. Mantel test further revealed that the proportion of alkaline amino acids, histidine, phenylalanine and valine were the key amino acid indexes affecting the quality of Sichuan flue-cured tobacco. In general, the contents of key amino acid indexes in tobacco leaves of Liangshan among the five flue-cured tobacco producing areas of Sichuan were mostly at a medium or low level, and the contents of most key amino acid indexes in tobacco leaves of Guangyuan were relatively high. 【Conclusion】There are moderate or high correlations among the free amino acid of Sichuan primary flue-cured tobacco, and most of the free amino acid are negatively correlated with the quality of tobacco. In production, appropriate control of free amino acid content and improvement of imino acid ratio are beneficial to the quality improvement of Sichuan flue-cured tobacco.