Drought frequently results in significant losses in agricultural systems, including the soybean yield. Brassinosteroids exhibit multiple actions on essential processes, including chlorophyll fluorescence and gas exchange. Considering that the electron transport rate (ETR) into photosystems can exercise interference on net photosynthetic rate (PN), this research aims to determine whether 24-epibrassinolide (EBR) affects electron transport and find out if there is any repercussion on photosynthesis in soybean plants affected by the water deficit. The experiment was performed using a randomized factorial design, with two water conditions (control and water deficit) and three EBR concentrations (0, 50, and 100 nM EBR). The water deficit reduced effective quantum yield of PSII photochemistry, ETR, PN, and water-use efficiency. However, the exogenous application of 100 nM EBR mitigated these negative effects, increasing these variables. EBR reduced the oxidant compounds (superoxide and hydrogen peroxide) and membrane damages (malondialdehyde and electrolyte leakage) in stressed plants. Our study proved that EBR increased ETR and PN in control and stressed plants, revealing that ETR had a strong relationship with PN. These results suggest that soybean plants with higher values of ETR are more efficient in relation to PN.Additional key words: chlorophyll fluorescence; drought; gas exchange; Glycine max; 24-epibrassinolide. transport rate; ETR/PN -ratio between the apparent electron-transport rate and net photosynthetic rate; EXC -relative energy excess at the PSII level; F0 -minimal fluorescence yield of the dark-adapted state; Fm -maximal fluorescence yield of the dark-adapted state; Fv -variable fluorescence; Fv/Fm -maximal quantum yield of PSII photochemistry; gs -stomatal conductance to water vapor; LDM -leaf dry matter; MDA -malondialdehyde; NPQ -nonphotochemical quenching; PEG -polyethylene glycol; PN -net photosynthetic rate; PN/Ci -instantaneous carboxylation efficiency; qP -photochemical quenching; RDM -root dry matter; ROS -reactive oxygen species; STM -stem dry matter; TDM -total dry matter; WUE -water-use efficiency; ΦPSII -effective quantum yield of PSII photochemistry. Acknowledgements: This research had financial support from Fundação Amazônia de Amparo a Estudos e Pesquisas (FAPESPA/Brazil), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq/Brazil) and Universidade Federal Rural da Amazônia (UFRA/ Brazil) to AKS Lobato. While YC Pereira and WS Rodrigues were supported with scholarships from Programa de Educação Tutorial (PET/Brazil).

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Microorganisms in the soil and rhizosphere can release part of the total phosphorus in the soil through solubilization, mineralization, and an increase of the root absorption surface. The ability of phosphate solubilizing bacteria and mycorrhizal fungi to promote higher yield and profitability in co-inoculated soybean was investigated. For this purpose, field and greenhouse experiments were conducted in the years 2020 and 2021 in Brazil. In the field, the first factor was composed of microorganism application on soybean (simple inoculation with Bradyrhizobium; co-inoculation with Bacillus strains; co-inoculation with arbuscular mycorrhiza), and the second factor consisted of the application or not of phosphate fertilizer (0 and 100 kg ha -1 of P 2 O 5 ). In the greenhouse, treatments of the first factor were maintained with 50 % of the phosphate fertilization and one treatment added (standard inoculation with 100 % of the fertilization). Plant growth, roots, nodules, leaf nutrition, yield, and profitability were evaluated. In 2020, co-inoculation increased plant height, the number of pods, grains, and profitability index. The co-inoculation with Bacillus strains and arbuscular mycorrhiza promoted yield increase only associated with phosphate fertilization, by 813 and 761 kg ha -1 compared to standard inoculation, respectively. In 2021, there were increases for pods, grains, yield, gross profit, net income, and profitability index. Co-inoculation with Bacillus strains and arbuscular mycorrhiza promoted increased soybean yield and profitability, confirming itself as a sustainable technology for Brazilian soybean fields.

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