Physiological and metabolic changes during gibberellin- and 6-benzyladenine-induced heartwood formation in Dalbergia odorifera

Heartwood formation is a complex developmental process involving the transformation of living sapwood into a metabolically specialized tissue enriched in secondary metabolites. In Dalbergia odorifera , the prolonged period required for natural heartwood formation represents a major constraint for commercial production, yet the physiological and metabolic responses associated with hormone-induced heartwood development remain poorly understood. All gibberellin (GA) and 6-benzyladenine (6-BA) treatments induced visible heartwood formation within six months, whereas no heartwood developed in untreated controls. Among the tested treatments, 2% 6-BA produced the largest heartwood area and the highest total flavonoid content. Across the sapwood-to-heartwood gradient, non-structural carbohydrates and antioxidant enzyme activities generally declined, whereas malondialdehyde increased, indicating substantial physiological changes during heartwood development. PAL activity showed a strong positive association with flavonoid accumulation ( r = 0.95), and the PAL/PPO ratio was also positively associated with flavonoid content ( r = 0.86). GA treatments were associated with higher NSC and starch concentrations in sapwood tissues, whereas 6-BA treatments, particularly 2% 6-BA, were associated with higher PAL activity and total flavonoid content in developing heartwood. GC–MS analysis further revealed treatment-dependent differences in volatile compound profiles, with 6-BA associated with higher levels of phenolic antioxidants and GA associated with higher proportions of several sesquiterpene compounds. Hormone-induced heartwood formation in D. odorifera is accompanied by coordinated changes in carbohydrate status, oxidative responses, and secondary metabolism. GA and 6-BA were associated with distinct physiological and metabolic signatures, indicating distinct treatment-associated patterns of carbohydrate status and secondary-metabolite accumulation during induced heartwood development. These findings provide new insights into the physiological basis of hormone-induced heartwood formation and support the development of artificial heartwood production strategies in D. odorifera .