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.