| Summary: | We investigated the effect of pre-sowing seed treatment with endophytic <i>Bacillus subtilis</i> 10-4 (<i>B. subtilis</i>) on spring and winter wheat (<i>Triticum aestivum</i> L.; cultivars Ekada-70 (Ek) and Scepter (Sc), respectively) growth and tolerance under 1–24 h of drought stress, modulated by 12% polyethylene glycol 6000 (PEG). The results showed that drought decreased transpiration intensity (TI), root relative water content (RWC), osmotic potential (Ψ<sub>π</sub>) of cell sap, and induced proline accumulation and electrolyte leakage (EL) in both wheat cultivars. It was revealed that Sc was more responsive to PEG and <i>B. subtilis</i> treatments than Ek. Under drought, Ek did not significantly change root length, shoot height, or dry biomass. The pretreatment of wheat plants with <i>B. subtilis</i> performed significantly better under drought conditions through the enhanced TI, RWC, and Ψ<sub>π</sub> of the cell sap in comparison with the plants treated with 12% PEG alone. <i>B. subtilis</i> also reduced stress-caused EL, especially in the Sc cultivar. Under water deficit wheat seedlings, pretreated with <i>B. subtilis</i>, have a higher proline accumulation in comparison to untreated stressed plants. Taken together, our results demonstrate the crucial role of endophytic <i>B. subtilis</i> in ameliorating the adverse effects of water stress on the water balance of both winter and spring wheat cultivars.
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