Abstract: Phosphorus is one of the essential macronutrient for plant growth and development. Root growth of different plants responds differently to phosphorus levels. Ormosia henryi Hemsl. & E. H. Wilson is a rare and endangered plant species with important economic and ecological values. In this study, we explored the responses of two-year-old O. henryi seedlings to different phosphorus levels. Five phosphorus levels of 0 (P0), 0.5 (P1), 1.0 (P2), 2.0 (P3) and 4.0 (P4) mmol·L^-1 were set up with the modified Hoagland nutrient solution. We analyzed the effects of different phosphorus levels on root morphology and phosphorus uptake of O. henryi seedlings. The results showed that: (1) Total root length, total root surface area, total root volume, average root diameter, and root vigor were highest under the P2 treatment and lowest under the P4 treatment. (2) Acid phosphatase activity from root base and middle regions was higher under the P1 treatment, and that from seedling root middle and tip regions was higher under the P2 treatment, which was significantly higher than that of the P0 and high-phosphorus treatments (P3 and P4). (3) The types and total amount of organic acids secreted by the roots were the largest under the P2 treatment, while the types and total amount of organic acids under the P0 and P1 treatments were higher than those under the P3 and P4 treatments. (4) Compared with the P0 treatment, phosphorus application increased the phosphorus uptake of shoots and roots, with the low phosphorus treatment (P1) performed best. In summary, the indicators of root morphology, root activity, acid phosphatase secreted from the middle and tip region of roots, and the type and content of organic acids secreted by roots were all maximized at the phosphorus level of 1.0 mmol·L^-1, which was the optimal phosphorus application level for the growth of O. henryi seedlings in this study. Under low P treatment (0.5 mmol·L^-1), the uptake of P was promoted by increasing root morphological indices, root activity, acid phosphatase activity secreted from the base and middle region roots, and the types and content of organic acids secreted. However, high phosphorus levels (2.0 and 4.0 mmol·L^-1) were not favorable to root growth of Ormosia henryi seedlings. This study provides a theoretical basis for scientific fertilization and efficient utilization of phosphate fertilizer in the seedling stage of O. henryi.

Key words: phosphorus level, Ormosia henryi, root morphology, phosphorus uptake