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

Valued for furniture, crafts, and medicine, Dalbergia odorifera 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 (p < 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% (p < 0.05) with stem-biased partitioning (stem > root > leaf; 52%, 26%, 22%). Photosynthetic capacity increased significantly under T7 (p < 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%) (p < 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 D. odorifera, providing a scientific foundation for efficient cultivation.