External phosphorus enhances the efficient acquisition of phosphorus by alfalfa (Medicago sativa L.) through its interaction with root morphological traits and rhizosphere carboxylates

Abstract

Root morphological characteristics and rhizosphere carboxylates are the critical factors for phosphorus (P) absorption in forage crops such as alfalfa (Medicago sativa L.) include root morphological characteristics and rhizosphere carboxylates. This study explored alfalfa P uptake across various P application levels and evaluated the respective contributions of root morphological traits and rhizosphere carboxylates to this process. We analyzed root morphological characteristics and composition and amounts of rhizosphere carboxylates and their contribution in P uptake after cultivating alfalfa in pots with seven distinct P application levels for 9 weeks. The results revealed that supplying P enhanced the growth of aboveground and underground plant parts. At a P application level of 12 mg kg⁻¹, alfalfa stems exhibited the highest concentration and uptake of P. Concurrently, root parameters such as root length (RL), root surface area (RSA), root volume, root tips and the amount of rhizosphere carboxylates were the highest. Although some P was allocated to aboveground parts, root exposure to higher P supply levels (48 and 96 mg kg⁻¹) can lead to P toxicity and reduce the P absorption. Structural equation modeling (SEM) and regression analysis demonstrated that RSA, root tips and rhizosphere carboxylate content significantly affected alfalfa P absorption. Among them, malate was crucial for promoting P uptake in alfalfa under high P concentrations. In summary, this study elucidated the dominant factors affecting plant P absorption and identified the optimal concentration (12 mg kg⁻¹) for alfalfa growth by examining the response of alfalfa to varying P concentrations. Additionally, this study also provides a foundation for the development of low-P agriculture in artificial alfalfa plantations.