Phosphorus-Driven Stem-Biased Allocation: NPK Synergy Optimizes Growth and Physiology in Dalbergia odorifera T. C. Chen Seedlings

Valued for furniture, crafts, and medicine, <i>Dalbergia odorifera</i> T. C. Chen confronts critically depleted wild populations and slow cultivation growth, necessitating precision nutrient formulation to overcome physiological constraints. Using a '3414' regression design with four levels of N, P, and K, this study identified phosphorus (P) as the most influential nutrient in regulating growth (P > N > K). Maximal growth enhancement occurred under T7 (N2P3K2), with height and basal diameter increments increasing by 239% and 128% versus controls (<i>p</i> < 0.05). Both traits exhibited progressive gains with rising P but unimodal responses to N and K, initially increasing then declining. T7 boosted total biomass by 50% (<i>p</i> < 0.05) with stem-biased partitioning (stem > root > leaf; 52%, 26%, 22%). Photosynthetic capacity increased significantly under T7 (<i>p</i> < 0.05), driven by P-mediated chlorophyll gains (Chla + 70%; Chlb + 75%) and an 82% higher net photosynthetic rate. Metabolic shifts revealed peak soluble sugar in T7 (+139%) and soluble protein in T9 (+226%) (<i>p</i> < 0.05), associated primarily with P and K availability, respectively. Correlation networks revealed significant associations among structural growth, photosynthesis, and metabolism. Principal component analysis established T7 as optimal, defining a "medium-N, high-P medium-K" precision fertilization protocol. These findings elucidate a phosphorus-centered regulatory mechanism governing growth in <i>D. odorifera</i>, providing a scientific foundation for efficient cultivation.