Rhizosphere Acidification Determines Phosphorus Availability in Calcareous Soil and Influences Faba Bean (Vicia faba) Tolerance to P Deficiency

Calcareous soils are known for their alkaline pH-promoting insoluble forms of certain nutrients, including phosphorus (P). Rhizosphere acidification is one of the main physiological mechanisms of phosphorus mobilization by plants. However, specific and genotypic differences in response to P deficiency are often observed, giving some genotypes particular tolerance abilities. This genetic potential gives us a new opportunity to colonize unused lands, improve yield in problematic soils, and install sustainable agrosystems. To this end, a potted experiment was conducted on three faba bean genotypes (Seville, SEV; Aguadulce, AGUA; and Tunisian, TUN) cultivated on calcareous soil (CS), as compared to fertile soil (FS). Measurements are made on plant growth, the SPAD index, photosynthesis, P distribution, rhizosphere acidification, and related interrelationships. Calcareous soil induced specific symptoms of P deficiency, reduced P concentration and decreased SPAD index, net photosynthesis, and plant growth. Rhizosphere acidification was significantly stimulated in CS. This activity determines the genotypic differences in response to P deficiency in faba bean. The genotype TUN was more adapted to calcareous-induced P deficiency than AGUA and SEV by increasing acidification activity, decreasing pH by 0.6 units in the rhizosphere, and having higher biomass production, photosynthesis, P remobilization, and P accumulation. The key functional traits (plant growth, chlorophyll biosynthesis, and photosynthesis) are strictly dependent on P availability, which remains in close relationship with the acidification capacity (AC). The tolerant genotype (TUN) expressed a lower stress index (SI) but higher P use efficiency (PUE), H-ATPase activity, and P uptake and translocation to shoots (PT), allowing it to maintain better metabolic functioning. AC, PT, PUE, and SI are among the main traits of P management in calcareous soils that promote resilient crops.