Biostimulation Effects of Plant Growth-Promoting Rhizobacteria and Spent Mushroom Compost on Maize Drought Tolerance

Maize (Zea mays L.) is one of the most important cereal crops worldwide but is highly vulnerable to drought stress, which impairs growth and yield through physiological and biochemical disruptions. Plant growth-promoting rhizobacteria (PGPR) and organic amendments have been proposed as sustainable strategies to mitigate water stress. This study aimed to evaluate the effects of PGPR isolates and spent mushroom compost (SMC) on maize growth under controlled drought conditions. A total of 23 bacterial isolates were initially screened for osmotic tolerance, biofilm formation, and exopolysaccharide (EPS) production. Several isolates demonstrated strong tolerance at 20% PEG-6000 and produced significant levels of EPS, supporting their potential role in enhancing stress resilience. Germination assays revealed that inoculated seeds exhibited higher germination rates and seedling vigor indices compared with uninoculated controls. In seedling assays, selected isolates significantly promoted root elongation, shoot length, and seminal root formation, indicating improved early growth performance. Two promising isolates, identified as Bacillus velezensis strains, were further tested in greenhouse conditions in combination with SMC at 30 t/ha (SMC30). Application of SMC alone enhanced plant height, root biomass, and chlorophyll content compared with untreated controls. Notably, the combined treatment of PGPR and SMC showed additive benefits, including greater root surface area, increased dry weight, and improved recovery following drought exposure. These improvements were accompanied by higher antioxidant enzyme activities and reduced stress markers, suggesting enhanced physiological adaptation to water deficit. Overall, the results demonstrate that both PGPR and SMC independently improve maize performance under drought, and their combined use may offer a synergistic strategy for sustainable maize production in water-limited environments.