Exogenous hormones regulate the physiological responses and adaptation of Pinus yunnanensis seedlings to top-pruning by altering nutrient allocation and stoichiometric ratios

Pinus yunnanensis is a key afforestation tree species in southwest China, but its seedlings often suffer from slow early growth and the "squat seedling" phenomenon. To address this issue, a field experiment was conducted to explore the regulatory effects of exogenous indole-3-acetic acid (IAA) and 6-benzyladenine (6-BA) on nutrient allocation and stoichiometric characteristics of P. yunnanensis seedlings. A two-factor three-level (3 ×3) completely randomized block design was adopted, and the contents and allocation patterns of nitrogen (N), phosphorus (P), and potassium (K) in various seedling organs (primary root, lateral root, stump main stem, stump branches, stump needles, sprouted shoots, and sprouted needles) were determined under different treatments. The results showed that hormone treatments significantly altered the priority of nutrient allocation, with the order of N > K > P. Among all treatments, T4 (IAA: 150 mg/L; 6-BA: 0 mg/L) and T7 (IAA: 300 mg/L; 6-BA: 0 mg/L) resulted in the highest N allocation ratios in sprouted needles, reaching 55.6 % and 57.2 %, respectively. T3 (IAA: 0 mg/L; 6-BA: 200 mg/L) regulated the underground N allocation, while T9 (IAA: 300 mg/L; 6-BA: 200 mg/L) significantly affected K allocation. Compared with the control group, the K contents under T2 and T3 treatments decreased by 30.6 % and 29.6 %, respectively. IAA and 6-BA promoted root and bud growth, induced differential accumulation and consumption of N, P, and K, and thereby altered their stoichiometric ratios. High concentrations of 6-BA and IAA enhanced N accumulation but reduced P content, while IAA exerted a significant regulatory effect on underground K content. This study provides a theoretical basis and technical reference for improving the quality of P. yunnanensis seedlings and alleviating the "squat seedling" problem through hormone regulation. • Reveals complex nutrient reallocation networks between "source" and "sink" organs post-hormone treatment. • Establishes "hormone-nutrient-stoichiometry-growth" research chain, revealing multidimensional regulatory effects. • Expands ecological stoichiometry application to conifer cultivation studies.