Optimizing a Hybrid Biofertilizer-Foliar Nutrition Strategy for Mexican Lime under Water Deficit: Insights into Water Status, Osmolytes, and Membrane Stability

Drought stress markedly limits citrus growth, highlighting the need for practical strategies to strengthen seedling resilience. Here, we assessed whether root inoculation with the plant growth-promoting bacterium Bacillus subtilis CS1, alone or combined with foliar potassium nitrate (0 and 2%), can mitigate water-deficit impacts in Mexican lime (Citrus aurantiifolia). Seedlings were exposed to three irrigation regimes representing 40, 70, and 100% evapotranspiration potential (ETP). Drought significantly depressed biomass accumulation, photosynthetic pigments, leaf area, and relative water content. Under severe stress (40% ETP), CS1 inoculation markedly enhanced plant water status, increasing relative water content by 48.2% and leaf area by 240.1% relative to untreated controls. Inoculated plants also maintained carbohydrate metabolism more effectively, exhibiting higher soluble sugars and reduced starch depletion during drought. Potassium nitrate spraying improved membrane stability, lowering electrolyte leakage, and promoted shoot and root fresh and dry weights under stress. Notably, the combined CS1+2% KNO3 treatment under drought increased leaf area by 7.98% and proline by 249.4%, while reducing malondialdehyde by 49.3% compared with the control, indicating alleviated oxidative damage. Overall, CS1 outperformed KNO3 alone and offers a sustainable, low-risk approach for improving drought tolerance in citrus cultivation.