Growth-Promoting Potential of Bacillus subtilis GMG1288 in Wheat Under Abiotic Stress: Effects of Application Method and Timing

Bacillus subtilis is one of the most widely used plant growth-promoting bacteria (PGPB), but the optimal method and timing of its application under abiotic stress have not been systematically compared across different stress types within a single experimental framework. Here, we evaluated the effect of B. subtilis strain GMG1288 on spring wheat (Triticum aestivum L.) under three abiotic stresses—soil salinity, PEG-induced osmotic stress, and heavy metal contamination—comparing 13 application variants (seed inoculation, and single or multiple watering with the bacterial suspension) applied at different time points in a 6-week pot experiment, alongside in vitro characterization of strain GMG1288. Two-way ANOVA showed that stress type was the dominant source of variability in wheat growth parameters, while application method and timing were significant but weaker, stress-dependent modifiers. Under salinity, seed inoculation was the key determinant of root mass gain; under osmotic stress, application timing was decisive, with efficacy confined to a narrow window at stress onset (days 14–21); under heavy metal stress, only repeated watering produced a consistent effect, while single applications and inoculation showed no reliable benefit. These results indicate that the effective application regimen for B. subtilis GMG1288 is stress-specific rather than universal, and suggest that application strategies for PGPB-based biological products should be guided by strain-specific phenotypic activity profiles rather than by generic method/timing schemes.