Biofertilizer and biostimulant potentials of phosphate-solubilizing Bacillus subtilis subsp. subtilis M1 strain and silicon in improving low phosphorus availability tolerance in rosemary.

The present study aimed to evaluate the single and combined effects of Si exogenous treatment and Bacillus subtilis subsp. subtilis M1 strain inoculation on rosemary tolerance to low phosphorus (P) availability. Hence, rosemary plants were fertilized with 250 µmol Ca3HPO4 (stressed plants) or 250 µmol KH2PO4 (control plants) under Si treatment and B. subtilis M1 strain inoculation. P starvation negatively affected rosemary growth and its P nutrition. However, exogenous Si supply or B. subtilis M1 strain inoculation significantly (P < 0.001) alleviated the deficiency-induced effects and significantly improved rhizogenesis, acid phosphatase activity, P uptake, and eventually dry weight of shoot and root. Moreover, Si-treatment and/or B. subtilis M1 strain inoculation significantly (P < 0.001) reduced the oxidative damage, in terms of malondialdehyde and hydrogen peroxide accumulation. This was found positively correlated with the higher superoxide dismutase activity, and the elevated non-enzymatic antioxidant molecules accumulation, including total polyphenols in Si-treated and inoculated P-deficient plants. Taken together, Si supplementation and/or B. subtilis M1 strain inoculation could be a good strategy to sustain rosemary plant growth under P starvation conditions.