Nitrogen addition regulates the growth of Pinus tabuliformis by changing distribution patterns of endogenous hormones in different organs

A controlled experiment with Pinus tabuliformis Carr Seedlings was conducted to investigate the effect of nitrogen (N) deposition on the growth of plant organs by influencing the distribution of different hormones in plant organs and provide an important reference for understanding forestland governance and forest regeneration under global atmospheric N deposition. Seedlings of P. tabuliformis were subjected to 0, 3, 6, 12 g of N m(-2) year(-1), then growth parameters and hormone concentration (trans-zeatin riboside (ZR), gibberellin (GA(1+3)), indole acetic acid (IAA), abscisic acid (ABA)) in leaf, stem, total root, coarse root (diameters > 2 mm), fine root (1-2 mm) and finest root (0-1 mm) were measured. The results showed that N addition increased the growth of all the plant organs, decreased the ratio of underground (the whole root) to aboveground (leaf and stem) growth, which is in line with the functional balance hypothesis. Additionally, N addition increased the ZR concentration in stems, while decreasing the ABA concentration in leaves and whole roots and the GA(1+3) concentration in whole roots, but IAA concentration was not significantly influenced by N addition. Furthermore, the stem growth was mainly affected by the synergistic effect of ZR and GA(1+3), while the leaf and total root growth was mainly impacted synergistically by ZR and IAA. Additionally, the root growth at each diameter class was also regulated by different hormones. In conclusion, N addition can change the distribution pattern of endogenous hormones in different plant organs and roots of different diameter classes, thereby regulating growth of Pinus seedlings, and the changes in growth are the result of the coordinated regulation of various hormones.