Acid and high-temperature tolerant Bradyrhizobium spp. strains from Brazilian soils are able to promote Acacia mangium and Stizolobium aterrimum growth

In selection of rhizobia, it is important to consider tolerance of the strain to adverse conditions, as well as its symbiotic efficiency and capacity to perform other plant growth-promoting processes. In this study, we evaluated Bradyrhizobium spp. strains from Brazilian soils regarding tolerance to different pH and temperature conditions, inorganic calcium phosphate solubilization capacity, and symbiotic efficiency when inoculated on acacia (Acacia mangium) and velvet bean (Stizolobium aterrimum) in axenic conditions. In each experiment of symbiotic efficiency, 18 strains (including representatives of Bradyrhizobium sp., B. viridifuturi, B. brasilense, B. uaiense, B. stylosanthis, and B. forestalis) were compared to two uninoculated controls, one with low (5.25 mg L−1) and another with high (52.5 mg L−1) concentrations of mineral nitrogen (N), and one inoculant strain [BR 3617 (B. japonicum) in the acacia experiment and BR 2811(B. elkanii) in the velvet bean experiment]. All strains grew at the different pH values tested (4 to 10) and tolerated a wide temperature range (15 to 37 °C) and most of them solubilized inorganic calcium phosphate. Most strains did not nodulate acacia, but were able to promote growth of this species. The UFLA03–268 (Bradyrhizobium sp.) strain was the most efficient in symbiosis with acacia. In the velvet bean experiment, the UFLA03–144 (B. viridifuturi), INPA104A (Bradyrhizobium sp.), and UFLA03-321T (B. brasilense) strains outperformed the other treatments in shoot dry matter production of velvet bean, showing high potential for use as inoculants in this species. Strains representing the five Bradyrhizobium species and Bradyrhizobium sp. were similar or more efficient in shoot nitrogen accumulation (SNA) than the control with mineral N. They also led to 84 (UFLA 04–0212) to 246 (INPA 104A) % increases in SNA compared to inoculation with BR 2811. This study showed an efficient symbiotic relationship between B. viridifuturi, B. brasilense, B. uaiense, B. stylosanthis and B. forestalis and velvet bean.