Biological strategies to mitigate the effect of water deficit on morphophysiology of soybean

Water stress is a major constraint to soybean productivity in tropical environments, impairing physiological processes and plant growth. Growth-promoting microorganisms have been proposed as a sustainable strategy to enhance plant tolerance to drought. This study evaluated the effects of inoculation with Bacillus subtilis and Trichoderma harzianum , alone or combined with the microalga Parachlorella sp., on soybean responses to water deficit. The experiment was conducted in a randomized block design with a 2 × 4 factorial arrangement, considering the presence or absence of the microalga and four cultivation conditions: irrigated control, water deficit, water deficit +  B. subtilis , and water deficit +  T. harzianum . Gas exchange, chlorophyll fluorescence, and growth parameters were assessed 15 and 30 days after application. Water deficit significantly reduced photosynthetic rate, stomatal conductance, chlorophyll index, maximum quantum efficiency of photosystem II, and electron transport rate, while increasing the F₀/Fm ratio, indicating damage to the photosynthetic apparatus. Microbial inoculation partially alleviated these effects. The combination of B. subtilis and Parachlorella sp. enhanced photosynthetic performance and Rubisco carboxylation under stress. The microalga also increased chlorophyll content, contributed to photosystem II stability, and promoted biomass accumulation, particularly in roots. These results indicate that B. subtilis and Parachlorella sp. can improve soybean tolerance to water deficit by maintaining photosynthetic efficiency and supporting adaptive physiological responses.