Root morphology, exudate patterns, and mycorrhizal symbiosis are determinants to improve phosphorus acquisition in alfalfa

Differences in phosphorus (P) utilization efficiency (PUtE) and/or yield are closely linked to differences in root functional traits under low soil P availability. However, our understanding of how soil P availability mediates the intraspecific variation in root functional traits for breeding high-P efficiency genotypes to increase PUtE and yield remains limited. We investigated plant growth parameters and pivotal root functional traits associated with P acquisition in 20 alfalfa genotypes with contrasting P efficiencies and supplied with low, medium, or high levels of P. We observed that tradeoffs occurred in root functional traits among alfalfa genotypes under low-P stress. High P efficiency genotypes displayed higher shoot biomass and PUtE by relying on thicker and more robust roots, elevated concentrations of carboxylate exudates, and enhanced colonization by arbuscular mycorrhizal fungi. In contrast, low-P efficiency genotypes exhibited a relatively high root-to-shoot ratio and primarily depended on higher tissue P concentrations but relatively slender roots along with comparatively high rhizosphere pH. Consequently, high PUtE and productivity under low-P conditions among alfalfa can be identified by screening for a phenotype with thick roots, increased exudate concentrations, and mycorrhizal colonization, opening up the potential for breeding for P-efficient lines.