Responses of root morphological and physiological characteristics of different alfalfa genotypes to phosphorus levels

To clarify the response of the root morphology and physiological characteristics of different varieties of alfalfa (Medicago sativa) to phosphorus levels, the test method of nutrient solution hydroponics was used to analyze a total of six alfalfa plants under the phosphorus levels 0 (P₀), 50 µmol·L⁻¹ (P₁), 100 µmol·L⁻¹ (P₂), 500 µmol·L⁻¹ (P₃), 1 000 µmol·L⁻¹ (P₄), and 1 500 µmol·L⁻¹ (P₅). The response characteristics of root length, surface area, volume, phosphorus absorption efficiency, and acid phosphatase activity to phosphorus level was ascertained. It was found that there were differences in total root length, root surface area, root phosphorus absorption efficiency, and root acid phosphatase activity among varieties at the same phosphorus level. The total root length and root surface area reached a maximum at low phosphorus levels. The total root length and root surface area of the four varieties of Xinmu No.1, Magnum Salt, Zhaodong, and Juneng Ⅱ were higher than those of Longdong and Longmu 801 at low P₂ levels. The root volume of each variety was different only in P₀, P₁ treatment, and the root volume of Longdong was smaller than that of other varieties in the P₁, P₂, P₃, and P₅ treatments. The phosphorus absorption efficiency of roots of Xinmu No. 1 and Longmu 801 was higher than that of the other four varieties under low phosphorus conditions. The phosphorus absorption efficiency of the roots of Zhaodong was significantly higher (P < 0.05) than that of the other varieties under P₂, P₃, and P₄ treatments. The root acid phosphatase activity reached a maximum at P₁, and there was a significant difference (P < 0.05) among the varieties at the same phosphorus level. Under low phosphorus levels, Magnum Salt and Juneng Ⅱ had significantly higher root acid phosphatase activity than the other four varieties. The comprehensive low-phosphorus tolerance indicators show that Xinmu No. 1 and Magnum Salt have lower phosphorus tolerance than the other varieties are phosphorus efficient varieties, followed by Zhaodong and Juneng Ⅱ, whereas Longdong and Longmu 801 can be classified as phosphorus inefficient varieties.