Drought is the primary global limiting factor for agricultural production, and soybean plants are sensitive to water deficits. Management strategies have been studied to improve the yield in water-district areas. Applying biochar (BC) improves substrates’ physical and chemical properties, increasing water retention and nutrient availability. This study aimed to investigate whether the application of BC derived from acai (Euterpe oleracea) agroindustry waste could mitigate the deleterious effects of water deficit in soybean plants. The experiment was randomized using five proportions of BC [0.0, 2.5, 5.0, 7.5, and 10.0% (w/w)] obtained from the pyrolysis of E. oleracea seeds at 700 degrees C. In general, BC provided the best results at a concentration of 10% (w/w). BC positively modulated the stomatal mechanisms of plants subjected to water deficit, with increased stomatal density and functionality detected on both faces. This soil conditioner benefited physiological performance, promoting significant increases in the net photosynthetic rate (121%), water use efficiency (88%), and carboxylation instantaneous efficiency (180%), and minimizing the deleterious effects of water deficit on the gas exchange due to the benefits of stomatal regulation. The use of BC induced significant increases in biomass, including leaf dry matter (327%), root dry matter (40%), and total dry matter (84%). This study proved that BC plays an essential role in leaf water content, favors leaf development, and improves the performance of the photosynthetic machinery in soybean plants subjected to water deficit.
