Bacillus subtilis and phosphorus improve wheat drought tolerance through water status and pigment stability

Abstract Background Drought stress is a major constraint to wheat ( Triticum aestivum L.) productivity, creating a need for integrated and sustainable strategies to improve crop resilience. Although plant growth-promoting rhizobacteria (PGPR) and mineral nutrients are known to influence drought responses, limited evidence is available on whether Bacillus subtilis performs more effectively with phosphorus (P) or potassium (K) across contrasting growth stages and drought systems. This study compared the effects of B. subtilis , P, and K, alone and in combination, on drought tolerance in three wheat cultivars under seedling-stage hydroponic osmotic stress and grain-filling pot drought. Results The combined B. subtilis + P treatment under drought produced the most consistent improvement compared with drought-only treatment and was generally more effective than B. subtilis + K. In hydroponics, B. subtilis + P improved relative water content, pigment stability, root–shoot growth, and biomass under PEG-induced drought. Faisalabad 2008 showed the strongest seedling-stage water-status response, while Dharab showed greater osmolyte accumulation through soluble sugars and proline. In the pot experiment, B. subtilis + P improved yield-contributing traits, particularly in Dharab and Faisalabad 2008, although cultivar-specific differences were evident. Conclusion B. subtilis combined with phosphorus was the most effective drought-mitigation treatment across the measured physiological, biochemical, growth, and yield-related traits. These findings suggest that PGPR–phosphorus integration may be a useful strategy for improving wheat performance under drought, especially where water limitation occurs during early growth or grain filling. Field validation and direct measurements of bacterial colonization, nutrient uptake, and oxidative stress markers are needed to confirm the mechanisms underlying these responses.