Shading effects revisited: Comparisons of spring and autumn shading treatments reveal a seasonal-dependent regulation on amino acids in tea leaves

Shading is an effective way to improve tea quality by modulating the accumulation of the flavor-related secondary metabolites of tea leaves. However, shading effects on amino acids have been conflicting for years as both shading-induced increases and decreases were detected. This study aims to investigate the effects of shading levels on catechins and amino acids, in particular, a comparison was made in April and August to better understand the involvements of seasonal environmental parameters. Shading reduced major catechins (catechin, gallocatechin, epicatechin, epigallocatechin, epicatechin gallate, and epigallocatechin gallate) in both seasons. Heavy shading (80%-90%) increased total and individual amino acids (Threonine, Glutamic acid, and Alanine) in April but decreased those in August. In both seasons, the o-Phosphoethanolamine increased under heavy shading, but α-aminoadipic acid, Ornithine and Glutamine showed decreases. Theanine showed no shading response in April but decreased under heavy shading in August. These results indicate that shading-induced reduction of leaf catechins is mainly determined by shading level, but the shading effects on amino acids are seasonal-dependent. As the first report focuses on the synergistic effects of seasons, this study improves our understanding of shading regulation on secondary metabolites of tea leaves and guides shading treatments to improve tea quality.