Unveilling the impact of phosphorus availability on growth, root morphological and physiological traits of alfalfa (Medicago sativa L.) with contrasting aluminum tolerance in acidic soils

Background and aims The cultivation of alfalfa ( Medicago sativa L.) in acidic soils in southern China poses significant challenges, primarily due to two key factors: aluminum (Al) toxicity and phosphorus (P) deficiency. This study aimed to assess the morphological and physiological responses of alfalfa roots under conditions of Al stress and P limitation. Methods A pot experiment was conducted to compare the growth and root characteristics of an Al stress-sensitive cultivar (Longzhong) and an Al stress-tolerant cultivar (Trifecta) of alfalfa in acidic soil. This was achieved by supplying six P levels (0, 10, 20, 40, 80, and 120 mg P kg ^-1 soil) and utilizing two soil types: limestone (pH=6.01) and yellow soil (pH=5.46), both with high soil exchangeable Al concentration (Limestone soil 397 mg/kg, yellow soil 832 mg/kg). Results Phosphorus supply alleviated Al toxicity and enhanced plant growth, but aboveground growth did not further increase when the P supply exceeded 40 mg P kg ^-1 soil. Trifecta exhibited higher P uptake and P utilisation efficiency, along with increased total root length, compared to Longzhong. Both alfalfa cultivars allocated more biomass to the roots to maximize P extraction from the soil. Conclusions The superior root morphological traits observed in Al-tolerant alfalfa are responsible for enhanced P uptake and biomass production in acidic soil conditions. However, excessive P application leads to a significant decline in soil pH, increased levels of exchangeable Al content in the soil, and ultimately inhibits plant growth.