Bradyrhizobia and arbuscular mycorrhizal fungi modulate manganese, iron, phosphorus, and polyphenols in soybean (Glycine max (L.) Merr.) under excess zinc

The effect of Bradyrhizobia (R) and Arbuscular mycorrhiza (AM) on manganese, iron, phosphorus, and polyphenols was investigated in soybean cultivated in Zn treated soils. In a completely randomized 3 × 4 factorial, the experimental treatments - Zn addition (0, 200, and 400 mg Zn kg−1 of soil), and Inoculation (uninoculated control, R, AM, and R + AM dual inoculation) were set up in the greenhouse. Zinc treatment increased Zn in roots and shoots, Fe in roots, frequency of root mycorrhization, P in root, and Mn translocation; but decreased Fe translocation, Mn in root, and total polyphenols. Symbionts effected micronutrient balancing in the plants. Mycorrhizal inoculants increased Zn in shoots, and modulated Mn and Fe in shoots and roots; within the whole-plant crosstalk in Zn, Mn, and Fe status. In dual inoculation, synergy for much enhanced Mn translocation was indicated in 400 mg Zn kg−1 treatment. Modulations of Zn-Mn-Fe nutrition in dual inoculation was supported with a reduction in cumulative number of fallen leaves, and increases in leaf greenness, shoot P, and root polyphenols, to give higher shoot biomass. Our results suggest the possibility of optimizing the crop production and bio-inoculation outcomes by targeting Mn and Fe status (in roots, shoots, translocation, and ratios with Zn) during cultivation under excess Zn.