Influence of simultaneous treatment of seeds with ZnONPs and Bacillus subtilis on the biological quality parameters of red cabbage seedlings

Minimizing the negative environmental impact of agrochemicals necessitates new strategies to achieve sustainable food production. Consequently, zinc oxide nanoparticles (ZnONPs) and plant growth-stimulating bacteria (Bacillus subtilis) have been proposed as a method to enhance the growth parameters. The effect of ZnONPs at a concentration range of 0.2–1.4 mg cm–3 on the planktonic growth of B. subtilis bacteria, production of the hormone indole-3-acetic acid, biofilm formation, the ability to biodegrade complex compounds such as Evans Blue, and an increase in oxidative stress was assessed. Concentrations of 0.2 and 0.4 mg cm−3 ZnONPs were used to further test the simultaneous effects of ZnONPs and B. subtilis on red cabbage growth. Moreover, the influence of the simultaneous use of ZnONPs and B. subtilis on seed germination, physiological characteristics, and the content of minerals in red cabbage seedlings grown in the soil was examined. The simultaneous use of ZnONPs and B. subtilis bacteria improves the number of germinated seeds, the length of red cabbage seedlings, and the content of photosynthetic pigments and antioxidants compared with the control or single treatment of seeds with only B. subtilis or ZnONPs. The simultaneous use of B. subtilis and zinc oxide nanoparticles resulted in a higher content of zinc and sodium in red cabbage seedlings, while the content of macronutrients such as Mg and K, and micronutrients such as Fe, Mn, and Co was lower or close to the control value. The combination of B. subtilis + 0.2 mg ZnONPs turned out to be better than B. subtilis + 0.4 mg ZnONPs, as it produced the highest number of germinated seeds, greater plant and root length, and a higher content of chlorophylls, phenolic compounds, and antioxidants. The results indicate that ZnONPs enhance the role of B. subtilis as plant growth-promoting bacteria.