Physiological regulation of flower bud differentiation in Bougainvillea under different drought stress intensities

Flowering in Bougainvillea is closely associated with plant water status; however, the optimal drought level for floral induction and the underlying physiological regulatory mechanisms remain unclear. In this study, Bougainvillea glabra ‘Mrs. Eva White’ was used to evaluate the effects of different drought intensities on floral bud differentiation, with particular emphasis on the relationships among mineral nutrient dynamics, carbon metabolism, endogenous hormones, and flowering responses. The results showed that moderate drought (MD, 40% FC) was the most effective treatment for promoting flowering, as indicated by earlier floral initiation, higher flower bud differentiation index (FBDI) and flower bud differentiation rate (FDR), and more flowers per plant than in the other treatments. Compared with the control, MD promoted active remobilization of nitrogen (N), phosphorus (P), and potassium (K) and reduced the phosphorus-to-nitrogen ratio (P/N ratio) and potassium-to-nitrogen ratio (K/N ratios), while also increasing abscisic acid (ABA) content and decreasing the indole-3-acetic acid to abscisic acid ratio (IAA/ABA ratio), indicating an ABA-dominated hormonal rebalancing associated with the transition from vegetative to reproductive growth. In contrast, severe drought did not further enhance flowering despite stronger increases in ABA and soluble sugars. Correlation analysis revealed that mineral nutrient status and hormonal balance were more closely associated with flower bud differentiation than carbon metabolism traits, suggesting that carbon metabolism played a supportive rather than dominant role. The results demonstrated that MD significantly promoted floral bud differentiation in Bougainvillea by establishing a coordinated physiological state involving nutrient reallocation, ABA-dominated hormonal rebalancing, and supportive carbon metabolism, thereby facilitating the shift from vegetative to reproductive growth.