Effects of Preharvest Shading on Dynamic Changes in Metabolites, Gene Expression, and Enzyme Activity of Three Tea Types during Processing

Preharvest shading significantly influences tea flavor. However, little attention has been given to the mechanism of shading on metabolites, genes, and enzymes in the processing of different tea types. Our study identified 1028 nonvolatile metabolites covering 10 subclasses using a widely targeted metabolome. The results show that shading had a greater effect on the compositions of amino acids, flavonoids, and theaflavins in tea leaves. The combined transcriptomics and enzyme activity analysis results indicate that the upregulated expression of asparagine, aspartate, and tryptophan synthesis genes and proteolytic enzymes promoted the accumulation of amino acids. The downregulated enzyme genes resulted in the reduction of nongalloylated catechins and flavonoid glycosides. Simultaneously, the accumulation of TFs in shaded tea was due to the enhanced enzymatic activities of polyphenol oxidase and peroxidase during processing. Theaflavin-3-3′-di-O-gallate was also significantly positively correlated with the antioxidant and hypoglycemic activities of shaded tea. The results contribute to a better understanding of how preharvest treatments influence summer tea quality.采前遮荫对茶叶风味具有显著影响。然而目前针对不同茶类加工过程中，遮荫对代谢物、基因及酶类的调控机制相关研究仍较为匮乏。本研究借助广泛靶向代谢组（widely targeted metabolome）技术，共鉴定出1028种非挥发性代谢物，涵盖10个亚类。研究结果显示，遮荫对茶叶中氨基酸、黄酮类及茶黄素（theaflavins）的组成具有更为显著的调控作用。转录组与酶活性联合分析结果表明，天冬酰胺、天冬氨酸及色氨酸合成基因与蛋白水解酶的表达上调，可促进氨基酸的积累；而酶基因的下调表达则会导致非没食子酰基儿茶素与黄酮苷类物质的含量降低。与此同时，遮荫处理茶叶中茶黄素的积累，源于加工过程中多酚氧化酶（polyphenol oxidase）与过氧化物酶（peroxidase）的酶活性得以提升。茶黄素-3,3'-双没食子酸酯与遮荫茶叶的抗氧化及降血糖活性亦呈显著正相关。本研究结果有助于更深入地理解采前处理对夏茶品质的调控机制。