Annual Dynamics of Endogenous Hormones Reveal the Mechanism of Off-Season Flowering in Macadamia

Off-season flowering of Macadamia integrifolia has been observed in certain high-altitude regions; however, the endogenous hormonal mechanisms underlying this phenomenon remain unclear. In this study, the annual dynamics of four key endogenous phytohormones (ZT, GA3, IAA, and ABA) were quantified in the leaves and branches of trees from both normal and off-season flowering sites using high-performance liquid chromatography (HPLC). Hormonal ratios and correlation network analyses were further performed to investigate their roles in flowering regulation. Floral bud differentiation at the off-season site occurred approximately 1 to 2 months earlier than at the normal site. This advancement was associated with sustained low levels of GA3 (below 100 μg·g−1 FW), while ZT and ABA levels peaked in September at 108.66 μg·g−1 FW and 24.25 μg·g−1 FW, respectively. The ratios of ABA to GA3, ABA to IAA, and ZT to GA3 increased significantly between July and September, indicating the early establishment of a hormonal environment favorable for floral induction. Correlation analysis revealed that IAA, GA3, and ZT formed a synergistic module promoting flowering, whereas ABA functioned as an antagonistic regulator. These findings provide insight into the hormonal regulation of off-season flowering in macadamia and offer a theoretical basis for precision flowering control in high-altitude cultivation systems.