Exogenous melatonin improves tea quality under moderate high temperatures by increasing epigallocatechin-3-gallate and theanine biosynthesis in Camellia sinensis L.

Global warming has multifarious effects on crop growth and productivity. Nonetheless, the effects of moderate-high temperatures and melatonin on tea yield and quality remain unclear. In this study, we found that melatonin, a universal growth stimulatory molecule, not only promotes photosynthesis and biomass accumulation in tea plants (Camellia sinensis L.) but also improves tea quality under sub high temperature (SHT). SHT increased the dry biomass and photosynthesis by 40.8% and 28.1%, respectively, and exogenous melatonin caused a further improvement. Moreover, SHT increased the total polyphenol concentrations and decreased the free amino acid concentrations, leading to a significant increase (68.2%) in polyphenol to free amino acid ratio. However, melatonin decreased the polyphenol to free amino acid ratio by delicately improving the concentrations of polyphenols and amino acids. Consistent with the total polyphenol, melatonin increased the concentrations of (-)-catechin, (-)-gallocatechin (GC), and (-)-epigallocatechin-3-gallate (EGCG) in tea leaves. The qRT-PCR analysis revealed that melatonin increased the transcript levels of catechins biosynthesis genes, such as CsCHS, CsCH1, CsF3H, CsDFR, CsANS, CsLAR, and CsANR under SHT. Meanwhile, the theanine concentration was decreased by SHT, which was attributed to the attenuated expression of CsGS, CsGOGAT, CsGDH, and CsTS1. Nonetheless, melatonin significantly increased those transcripts and the content of theanine under SHT. Melatonin also increased the caffeine content by inducing the expression of CsTIDH, CssAMS, and CsTCS1. These results suggest that melatonin could positively alter tea growth and quality by modulating the photosynthesis and biosynthesis of polyphenols, amino acids, and caffeine in tea leaves under SHT.