Synergistic Interaction between Symbiotic N(2) Fixing Bacteria and Bacillus strains to Improve Growth, Physiological Parameters, Antioxidant Enzymes and Ni Accumulation in Faba Bean Plants (Vicia faba) under Nickel Stress

Several activities in the agriculture sector lead to the accumulation of Nickel (Ni) in soil. Therefore, effective and economical ways to reduce soil bioavailability of Ni must be identified. Five isolates of Rhizobium leguminosarum biovar Viceae (ICARDA 441, ICARDA 36, ICARDA 39, TAL–1148, and ARC–207) and three bacterial strains (Bacillus subtilis, B. circulance, and B. coagulans) were evaluated for tolerance and biosorption of different levels of Ni (0, 20, 40, 60, and 80 mg L(−1)). Pot experiments were conducted during the 2019/2020 and 2020/2021 seasons using four inoculation treatments (inoculation with the most tolerant Rhizobium (TAL–1148), inoculation with the most tolerant Rhizobium (TAL–1148) + B. subtilis, inoculation with the most tolerant Rhizobium (TAL–1148) + B. circulance, and inoculation with the most tolerant Rhizobium (TAL–1148) + B. coagulans) under different levels of Ni (0, 200, 400, and 600 mg kg(−1)), and their effects on growth, physiological characteristics, antioxidant enzymes, and Ni accumulation in faba bean plants (Vicia faba C.V. Nobaria 1) were determined. The results showed that Rhizobium (TAL–1148) and B. subtilis were the most tolerant of Ni. In pot trials, inoculation with the most tolerant Rhizobium TAL–1148 + B. subtilis treatment was shown to be more effective in terms of growth parameters (dry weight of plant, plant height, number of nodules, and N(2) content), and this was reflected in physiological characteristics and antioxidant enzymes under 600 mg kg(−1) Ni compared to the other treatments in the 2019/2020 season. In the second season, 2020/2021, a similar pattern was observed. Additionally, lower concentrations of Ni were found in faba bean plants (roots and shoots). Therefore, a combination of the most tolerant Rhizobium (TAL–1148) + B. subtilis treatment might be used to reduce Ni toxicity.