Genotypic Responses of Soybean to Bacterial Inoculation and Co-Inoculation: Implications for Grain Quality and Composition

This study evaluated the effects of different doses of Bradyrhizobium japonicum and Azospirillum brasilense, applied individually or in co-inoculation, on the physicochemical and physiological quality of soybean grains of the soybean genotypes BMX Zeus and BMX Valente. Grain quality was assessed through proximate composition, physiological parameters, and bioactive compounds. Significant effects of genotype and inoculation dose were observed (p < 0.05). Genotypic differences were observed independently of inoculation response. Overall, BMX Valente exhibited higher crude protein (41.45%), thousand-grain weight (191.01 g), and test weight (600.08 kg·m–3), together with lower electrical conductivity and free acidity. In contrast, BMX Zeus showed greater responsiveness to inoculation, with soluble protein reaching 80.87% and total carotenoids 14.30 µg β-carotene·g–1 under specific treatments. Excessive doses of B. japonicum alone negatively affected grain stability. Multivariate analysis identified the combination of A. brasilense (4.0 g·kg–1 of seeds) and B. japonicum (18.0 g·kg–1 of seeds) as the most favorable treatment, providing the best balance between physiological performance and grain quality.