**5. Conclusions**

In this study, a LC-MS-based metabolomics approach was used to evaluate the difference in metabolites during the change of leaf color of *C. sinense* 'Red Sun'. This is the first metabolomics study on *C. sinense*. A total of 196 flavonoid-related metabolites were detected. 42 metabolites were identified as differential metabolites during the process of leaf color change. In anthocyanin biosynthetic pathway, 15 metabolites were identified and the contents of them all showed decrease. Especially the contents of Naringenin chalcone, an important intermediate in anthocyanin synthesis, and two important anthocyanins metabolites, Pelargonidin O-acetylhexoside and Anthocyanin 3-O-beta-D-glucoside, decreased to zero in the green leaf stage. The enzyme activity of 10 enzymes related to anthocyanin synthesis showed different change patterns, while the expression of corresponding encoding genes was all down-regulated in the process of leaf color change. Overall, this study substantially contributes to the knowledge flavonoid-related metabolites composition in *C. sinense* and provides important reference values for breeders to improve the leaf color of *C. sinense.*

**Supplementary Materials:** Supplementary materials can be found at http://www.mdpi.com/1422-0067/21/5/1869/ s1.

**Author Contributions:** Conceptualization, F.Y. and G.Z.; methodology, J.G. and R.R.; software, Y.W.; validation, J.J. and J.W.; formal analysis, J.G.; resources, C.L. and C.Z.; writing—original draft preparation, J.G.; writing—review and editing, S.A. and R.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by grants from National Key Technologies R & D Program 2018YFD1000401, the National Natural Science Foundation of China (31902065), the Natural Science Foundation of Guangdong province (2017A030312004), the Orchid Industry Technology Innovation Alliance (2019KJ121), R & D plan in key areas of Guangdong Province (2018B020202001), Foundation of President of the GuangDong Academy of Agricultural Sciences (2019B030316033), Guangzhou Key projects of scientific research plan (201904020026) and team construction project of Guangdong Academy of Agricultural Sciences (201612TD), Discipline team building projects of Guangdong Academy of Agricultural Sciences in the 13th Five-Year Period (201612TD) and Special fund for scientific innovation strategy-construction of high level Academy of Agriculture Science(R2016PY-QF015 and R2018QD-103).

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
