Co-Inoculation of Azospirillum brasilense and Bacillus sp. Enhances Biomass and Photosynthetic Efficiency in Urochloa brizantha

The synergism between plant growth-promoting bacteria species (PGPB) was evaluated regarding the effect of inoculation on productivity and the physiological aspects of Urochloa brizantha. The study included seven experimental groups arranged in a 3 × 2 + 1 factorial design consisting of three inoculants (Azospirillum brasilense, Bacillus sp. isolate EB-40, and Bacillus sp. isolate EB-40 + A. brasilense mixture), two application methods (seed and foliar spray), and controls. The MIX conjugate inoculation significantly increased plant height in all three harvests, with gains of 57%. At 60 and 90 days, MIX increased the number of tillers by 47% and the number of leaves by 61% compared to other treatments in all harvests. MIX also increased shoot dry mass in the second and third harvests, with improvements of 57–60% compared to the control. MIX improved the quantum efficiency of photosystem II and the ratio between variable and maximum chlorophyll fluorescence. Maximum fluorescence (Fm) was 11% higher in MIX-treated plants compared to the control, indicating increased potential photosynthesis. Variable fluorescence (Fv) efficiency improved by 22% for inoculation with A. brasilense and Bacillus sp. Our study reveals that A. brasilense plus the Bacillus sp. isolate EB-40 (MIX) has the potential to improve the resilience and productivity of U. brizantha.