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).

Silicon (Si) frequently accumulates in plants tissues, mainly in roots of dicotyledons, such as cowpea. By contrast, Cadmium (Cd) is a metal that is extremely toxic to plant metabolism. This research aims to investigate if the deposition of Si in root can reduce Cd contents and minimize its negative effects on leaves, measuring gas exchange, chlorophyll fluorescence, antioxidant metabolism, photosynthetic pigments and growth, which may explain the possible role of Si in the attenuation of Cd toxicity in cowpea. This study had a factorial design, with all factors completely randomized and two Cd concentrations (0 and 500 µM Cd, termed as - Cd and + Cd, respectively) and three Si concentrations (0, 1.25 and 2.50 mM Si). Si reduced Cd contents in the roots and in other plant organs, such as stems and leaves. The Si contents were highest in roots, followed by stems and leaves, which was explained by the passive absorption of Si. The application of Si promoted increase in both the macro- and micronutrient contents in all tissues, suggesting that Si mitigates the effect of Cd on nutrient uptake. Si attenuated Cd-mediated effects on light absorption of photosystem II (PSII), increasing the effective quantum yield of PSII photochemistry and the electron transport rate. Additionally, toxic effects induced by Cd on gas exchange were mitigated by the action of Si. Plants treated with Cd + Si showed increase in the activities of antioxidant enzymes and reductions in oxidant compounds; these modifications were promoted by Si via detoxification mechanisms. Increases in the photosynthetic pigments and growth of plants treated with Si and exposed to Cd stress were detected and were due to the reduced deterioration of cell membranes and maintenance of chloroplasts, which had positive repercussions on growth and development. This study validated the hypothesis that the accumulation of Si in roots induces benefits on metabolism and alleviates the toxic effects caused by Cd in leaves of cowpea.