Combined Application of Bacillus aryabhattai and Silicon Enhances Membrane Stability, Biochemical Attributes, and Soil Biological Quality in Yellow Passion Fruit Under Water Deficit

Yellow passion fruit production is frequently limited by water scarcity, necessitating biotechnological strategies to ensure seedling quality. This study investigated the synergistic effects of Bacillus aryabhattai (Auras®) and silicon (Si) as mitigators of water deficit in Passiflora edulis seedlings. The experiment was conducted in a greenhouse in Catolé do Rocha, PB, Brazil, using 4 dm3 plastic bags. A randomized block design was used with a 4 × 3 + 2 factorial scheme, testing four available water contents (AWC: 50, 60, 70, and 80%) combined with three mitigation strategies (Auras, Si, and Auras + Si), plus two additional controls (50% and 100% AWC). Water deficit severely compromised growth and soil biological activity; however, mitigation treatments significantly improved physiological and biochemical responses. When applied separately, B. aryabhattai inoculation enhanced the accumulation of photoprotective pigments (carotenoids) and secondary metabolites (flavonoids and anthocyanins) under severe drought, while individual Si application provided homeostatic stability to plant biomass, maintaining dry matter production at levels comparable to moderate irrigation. The Auras + Si combination was the most effective, promoting the highest membrane stability, pigment maintenance, and vigorous growth even under 50% AWC. Furthermore, this interaction optimized soil microbial biomass and reduced the metabolic quotient by 56.7% compared to the stress control. These findings demonstrate that the combined application of B. aryabhattai and Si effectively mitigates the negative impacts of water scarcity on the initial development of passion fruit seedlings and soil microbial activity.