Previous Incubation of Bradyrhizobium japonicum E109 and Azospirillum argentinense Az39 (formerly A. brasilense Az39) Improves the Bradyrhizobium -Soybean Symbiosis

The aim of this work was to evaluate under diverse plant growth conditions the previous incubation between B. japonicum E109 ( Bj E109) and A. argentinense Az39 ( Aa Az39) and elucidate their impact on the Bradyrhizobium -soybean symbiosis and plant growth. Five treatments were performed: (i) uninoculated seeds; (ii) seeds inoculated with Bj E109; (iii) seeds inoculated with Aa Az39; (iv) seeds co-inoculated at the seeds sowing with equal volume (1:1) of Bj E109 and Aa Az39; and (v) seeds inoculated with equal volume (1:1) of Bj E109 and Aa Az39 24 h before seed sowing. Each treatment was assessed through a seed recovery assay, glasshouse assays, and field assays. The single plant level differences were achieved under greenhouse conditions while differences at population level (crop) were achieved by a field assay. The previous incubation between Bj E109 and Aa Az39 improved the ability of Bj E109 to survive on soybean seeds with 25% and 10% of cell recovery at 4 h and 6 days post-inoculation respectively. As a result of the greater bacterial survival, the symbiosis parameters like nodule number, size, and biomass and nodulation percentage also significantly increased. In agreement with these observations, the grain yield under field conditions showed 13.3 and 17.3% greater than immediate combination or single Bj E109 inoculation respectively. The previous incubation between Bj E109 and Aa Az39 24 h before their inoculation improves the Bradyrhizobium -soybean symbiosis and increases both plant growth under culture controlled and crop productivity under field conditions, in comparison with the single inoculation with Bj E109 or the immediate inoculation using both strains.