Effects of root anatomical characteristics on phosphorus uptake efficiency under phosphorus deficiency in alfalfa

This study aimed to determine the response characteristics of root anatomical traits under phosphorus deficiency and explore the effects of root anatomical changes on phosphorus uptake efficiency in alfalfa (Medicago sativa). In this study, 12 alfalfa cultivars were selected as materials, and their root anatomical characteristics, such as stele area, cortex area, and cortical cell file, were assessed at the seedling stage under two phosphorus supply levels (control: 0.5 mmol·L⁻¹ NH₄H₂PO₄; and phosphorus deficiency: 0.05 mmol·L⁻¹ NH₄H₂PO₄). Phosphorus deficiency inhibited the growth and phosphorus uptake efficiency of all alfalfa cultivars. Significant differences were detected in the response of these traits to phosphorus deficiency among different alfalfa cultivars. All alfalfa cultivars were classified into four types based on their relative biomass and relative phosphorus uptake efficiency as follows: high yield and non-sensitive, high yield and sensitive, low yield and non-sensitive, and low yield and sensitive. All four types showed a decreased trend in cortex area, stele area, and metaxylem area under phosphorus deficiency. Compared to the other three types, alfalfa cultivars of the high yield and non-sensitive type not only exhibited a relatively greater decline in the cortical cell file but also a higher increase in the percentage of cortex in the aerenchyma and metaxylem areas. These performances enhanced the root horizontal phosphorus uptake efficiency and axial transport capacity and enabled this type of alfalfa cultivar with more stable and higher phosphorus uptake efficiency under phosphorus deficiency. Furthermore, stepwise regression analysis revealed that the cortical cell file, percentage of cortex that is aerenchyma (P < 0.05), and metaxylem area (P < 0.01) had more significant correlations with phosphorus uptake efficiency than the other root anatomical traits did. The metaxylem area, cortical cell file, and percentage of cortex that is aerenchyma were the main root anatomical characteristics that responded to phosphorus deficiency, and the metaxylem area was the most critical anatomical trait affecting the phosphorus uptake efficiency in alfalfa.