Abstract: In order to understand the major metabolites of walnut endocarp and compare the difference in the metabolite compositions of walnut endocarps of different colors, broadly targeted metabolomics based on ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) was used to determine and compare the metabolite compositions of light yellow and purple walnut endocarps from the same habitat. The obtained data were analyzed by cluster analysis, correlation analysis and orthogonal partial least squares discriminant analysis (OPLS-DA). The results showed that a total of 680 metabolites belonging to 23 classes were detected in the endocarps, of which 658 metabolites were shared by both, while 670 metabolites were present in the yellowish one and 668 metabolites in the purple one. Totally 244 differential metabolites were identified between the two endocarps, accounting for 37.08% of the total metabolites common to them. Furthermore, 93 (38.11%) of the 244 differential metabolites had higher content in the purple endocarp than that in the yellowish one, whereas 151 metabolites (61.89%) showed the opposite result. The significantly differentially expressed 244 metabolites were mainly distributed in 20 metabolic pathways, and the first five dominant ones were the secondary metabolite biosynthesis pathway, flavonoid biosynthesis pathway, flavone and flavonol biosynthesis pathway, environmental microorganism-related metabolic pathway and phenylpropanoid biosynthesis pathway, in which 90 (36.89%), 48 (19.67%), 37 (15.16%), 30 (12.30%) and 23 (9.43%) metabolites were involved, respectively. The types and contents of differential anthocyanin metabolites in the purple endocarp were higher than in the yellowish one, which may be one of the major causes the different colors of walnut endocarp.

Key words: walnut endocarp; Juglans sigillata; metabolomics; polyphenol; anthocyanin