Objective  This paper aims to investigate the distribution of phenolic substances in the cell structure of sapwood and heartwood of Eucalyptus cloeziana and the main phenolic components of heartwood accumulation.

  Method  The distribution of phenolic substances in sapwood and heartwood was characterized by laser confocal Raman spectroscopy, and then the changes of phenolic components in xylem and their contents were identified by ultra performance liquid chromatography-mass spectrometry.

  Result  The Raman spectroscopy absorption intensity of heartwood was significantly higher than that of sapwood, and the phenolic content at the cell corner was significantly higher than that of the intercellular level and secondary wall. Further determination of the phenolic content of the sapwood and heartwood of E. cloeziana showed that the total polyphenol content in the heartwood was 22.378 mg/g higher than that of the sapwood, indicating that a large amount of phenolic substances was deposited at the cell corners during the formation of the heartwood. A total of 21 phenolic components were identified for the first time by UPLC-MS/MS from methanolic extracts of E. cloeziana wood, mainly containing phenolic acids and derivatives, flavonoids, ellagic acid derivatives, galloyglucose derivatives and hydrolysable tannins, with relatively high contents of ellagic acid, ellagic acid-rhamnoside, gallic acid, ellagic acid-pentoside and methylphloroglucinol-O-galloyl-glucose. The relative contents of ellagic acid and gallic acid were significantly higher in the heartwood than in the sapwood, and the quantitative analysis showed that ellagic acid was 0.560 and 9.283 mg/g in the sapwood and heartwood, respectively, and gallic acid was 0.019 and 0.043 mg/g in the sapwood and heartwood, respectively.

  Conclusion  It is clear that the phenolic composition is concentrated in the cell corners during the formation of the heartwood of E. cloeziana, and that ellagic acid is the main component accumulated in the heartwood.