Effect of phosphorus deficiency on biomass and root system architecture in diverse Medicago accessions

Introduction Phosphorus (P) is essential for legume growth and symbiotic nitrogen fixation, yet its low availability in many soils frequently constrains plant productivity. Methods To evaluate phenotypic and genotypic variation in P efficiency, 200 genetically diverse accessions of the Medicago sativa complex were assessed under high and low P conditions in a greenhouse experiment, followed by detailed root system architecture (RSA) analysis in a subset of 20 accessions using a rhizotron system. Results Significant intraspecific variation was observed for shoot and root dry matter, root-to-shoot ratio, shoot P content, and P utilisation efficiency (PUE). Across the full panel of 200 accessions, shoot and root dry matter decreased by 24% and 23% respectively under low P, and in the subset of 20 accessions PUE increased by 38% under low P. Genome-wide association studies identified candidate genes associated with plant height and biomass related traits under high and low P conditions including genes involved in cell-wall modification, hormonal regulation, and growth-related stress responses. Rhizotron analyses revealed RSA plasticity, with increased specific root length and specific convex hull area under low P, reflecting morphological adjustments that enhance soil exploration. Discussion Accessions that combined stable biomass production, high PUE, and adaptive RSA features under low P represent valuable genetic resources for breeding nutrient-efficient alfalfa cultivars suitable for low-input agricultural systems.