Microbial Biostimulants Improve Early Seedling Resilience to Water Stress

Drought poses a major challenge for global agriculture, demanding strategies that improve crop resilience while safeguarding water and nutrient resources. Plant growth-promoting rhizobacteria (PGPR)-based biostimulants offer a sustainable approach to enhance resource-use efficiency under water-limited conditions. This study evaluated two commercial PGPR biostimulants applied to maize ( Zea mays L.) and tomato ( Solanum lycopersicum L.) seedlings grown under well-watered (80% field capacity) and water-stressed (40% field capacity) conditions. Both products improved plant growth and physiological performance, although responses were crop-specific. Inoculated tomato seedlings accumulated up to 35% more shoot biomass under optimal watering (1.6 g in non-inoculated seedlings compared with 2.5 g in inoculated seedlings), whereas maize maintained biomass production under drought, consistent with its higher intrinsic water-use efficiency, showing increases of approximately 50% (well-watered: 0.5 g versus 0.8 g; water-stressed: 0.3 g versus 0.7 g in non-inoculated and inoculated seedlings, respectively). Biostimulant application enhanced the acquisition and internal utilization of essential mineral resources, increasing leaf concentrations of (i) the macronutrients P (up to 300%), K (up to 70%), Mg (up to 220%), and Ca (up to 85%), and (ii) the micronutrients B (up to 400%), Fe (up to 260%), Mn (up to 240%), and Zn (up to 180%). Maximum nutrient increases were consistently observed in water-stressed maize seedlings inoculated with biostimulant 2. Antioxidant activities, particularly ascorbate peroxidase and catalase, increased by 20–40%, indicating more effective mitigation of oxidative stress. Principal component analysis revealed coordinated adjustments among growth, nutrient-use efficiency, and physiological traits in inoculated plants. Overall, PGPR-based biostimulants improved early drought tolerance and resource-use efficiency, supporting their potential as sustainable tools for climate-resilient agriculture. Field-scale studies remain necessary to confirm long-term agronomic benefits.