Synergistic application of Bacillus subtilis IAGS174 and thiamine to mitigate salinity and lead stress in Helianthus annuus.

The increasing soil contamination with salinity and heavy metals poses serious threats to the cultivation of economically and ornamentally important plants such as Helianthus annuus (Sunflower). Bacillus subtilis IAGS174 and thiamine are well known for their role in increasing plant stress tolerance levels by multiple mechanisms. The present research aimed to assess the effect of B. subtilis IAGS174 and thiamine on H. annuus under salinity and lead (Pb) stress by analyzing the growth parameters, physiological markers, and biochemical assays. In a pot experiment, B. subtilis IAGS174 and thiamine were supplied to H. annuus plants grown in soil subjected to 500 mg/kg of salt and 150 mg/kg of Pb. The growth attributes and photosynthetic machinery of H. annuus plants were significantly reduced under single and combined stress of Pb and salinity. The combined stress of Pb and salinity declined the root length, shoot length, root fresh weight, shoot fresh weight, chlorophyll a, and chlorophyll b of H. annuus by 49%, 61%, 48%, 39%, 53%, and 55%, respectively, as compared to the control. Moreover, under stress, H. annuus plants exhibited higher levels of antioxidative enzymes, phenol, flavonoid and proline content. Nevertheless, the combined effect of B. subtilis IAGS174 and thiamine improved the fresh weight of shoots and roots, chlorophyll a, chlorophyll b and carotenoids by 34%, 38%, 15%, 18% and 16%, respectively, under the combined stress of salt and Pb to their respective controlled conditions. Supplementation of B. subtilis IAGS174 and thiamine significantly increased the antioxidative enzymes (superoxide dismutase, catalase, and peroxidase) and non-enzymatic antioxidants (phenol, flavonoids and proline) in sunflowers under combined and individual stress of sodium and Pb. Nevertheless, inoculation of B. subtilis IAGS174 accelerated the translocation of Pb and Na, while thiamine application reduced the uptake of these metals. Conclusively, single and combined application of plant growth-promoting rhizobacteria (PGPR) and thiamine proved a sustainable and effective option to improve plant tolerance against salt and Pb stress and offer new avenues for suitable agricultural practices in heavy metal and salt-contaminated soil.