How do plant growth-promoting bacteria and auxin influence the nutrition and morphophysiology of soybean?

Co-inoculation of plant growth-promoting bacteria and the application of hormones can be practical for soybeans. We aimed to evaluate the effect of co-inoculation methods of Azospirillum brasilense + Pseudomonas fluorescens and auxin application on the nutrition and morphophysiology of soybeans. The co-inoculation methods were: control; inoculation with Bradyrhizobium spp. only; and co-inoculation with Bradyrhizobium spp. and a commercial source containing A. brasilense + P. fluorescens applied via seed, via foliar at stage V3, or via seed + foliar, associated or not with the application of auxin at stage V4. The application of auxin had little influence on photochemical yield. The best values of photochemical activity in the reaction centers of photosystem II occurred in plants co-inoculated via seed + foliar. Photosynthesis was higher in plants co-inoculated via seed and via seed + foliar, as were intrinsic carboxylation and water use efficiency. The highest nitrogen contents were observed in the control treatment and in the seed + foliar co-inoculation, regardless of auxin application. The control plants showed higher potassium, magnesium, and sulfur contents, while auxin application increased calcium content in plants. The application of auxin contributed to an increase in leaf area and biomass of leaves, roots, and nodules. Co-inoculation via foliar and seed + foliar routes anticipated flowering and increased the number of pods per plant. Co-inoculation of A. brasilense + P. fluorescens via seed + foliar routes promoted improvements in soybean morphophysiology, while auxin application contributed positively to soybean morphophysiology and nutrition.