Selected autochthonous rhizobia, applied in combination with AM fungi, improve seed quality of common bean cultivated in reduced fertilization condition

Phaseolus vulgaris cv. Billò is an important crop, widely cultivated in Western Piedmont (Italy). It can form symbioses both with arbuscular mycorrhizal fungi (AMF) and bacteria belonging to the Rhizobium genus: AMF can improve plant nutrition and fruit/seed quality, whereas R. leguminosarum is a Gram-negative bacterium able to fix atmospheric nitrogen. In this study, 15 Rhizobium strains, isolated either from root nodules or soil, were characterized by molecular and biochemical point of view (P-solubilization, siderophore and IAA production, intrinsic antibiotic resistance) and one of the selected strains was used for field trial. The aims were: i) to select highly efficient symbiotic bacteria adapted to local conditions and ii) to assess bean seed yield and quality, in response to AMF/rhizobia inoculation at reduced fertilization level. At harvest, yield (pod and seed number and weight) and seed quality (starch, protein, fiber and mineral concentrations), root nodulation and mycorrhizal colonization were measured. Inoculation did not influence yield and fiber concentration. On the contrary, the total seed protein amount was higher in the seeds of rhizobium-inoculated plants in combination or without AMF than in uninoculated plants. The starch content in seeds was increased by rhizobia inoculation. Mg, K and Zn seed accumulation was positively affected by AMF, while the Mn concentration was higher in the presence of rhizobia. In conclusion, plant inoculation strongly affected seed characteristics, improving bean quality also in conditions of reduced fertilization.