Objective The heartwood of Acacia auriculiformis features beautiful wood grain and unique patterns. As a fast-growing tree species, understanding the formation mechanism of its heartwood can enhance its utilization value. However, the key physiological and chemical mechanisms of heartwood formation in A. auriculiformis remain unclear. This study aims to explore the physiological and chemical basis of heartwood formation in A. auriculiformis to gain a deeper understanding of this process.

Method To achieve the above objectives, this study used histological staining to observe changes in morphology and quantity of nuclei, starch grains, and reducing sugars in parenchyma cells in different parts of xylem of A. auriculiformis. Additionally, UPLC-MS/MS was employed to identify the components of extracts from heartwood, transition wood, and sapwood, and to analyze their chemical characteristics. These methods were used to systematically investigate the physiological and chemical basis of heartwood formation and reveal key changes during the process.

Result During the transition from sapwood to heartwood, the number of nuclei gradually decreased, their length-to-width ratio decreased, and their shape changed from elongated to round, completely disappearing in the third growth ring. Starch grains and reducing sugars were mainly distributed in axial parenchyma cells surrounding ray parenchyma cells, with their quantities gradually decreasing and disappearing in the third growth ring. In the fourth growth ring, nuclei changed from oval to round, and starch grains almost disappeared, suggesting this area was the inner side of transition zone. These changes reflected the gradual adjustment of physiological activities within cells during heartwood formation. Chemically, the total phenol (45.80 mg/g) and total flavonoid (16.13 mg/g) contents in A. auriculiformis heartwood were significantly higher than those in sapwood (total phenol 3.25 mg/g, total flavonoid 0.50 mg/g). Using UPLC-MS/MS, 21 phenolic compounds were identified in heartwood and sapwood extracts of A. auriculiformis, including flavonoids, phenolic acids, tannins, stilbenes, and coumarins. Among these, compounds such as dihydroquercetin, eriodictyol, umbelliferone, and 3-methoxy luteolin had relatively high contents.

Conclusion This study reveals the changing patterns in physiological activity and metabolites in A. auriculiformis, clarifying the key parts of its xylem chemical composition. These findings provide important theoretical foundations for heartwood formation and color changes in A. auriculiformis, and offer deeper scientific evidence for its efficient utilization.