Enhanced wheat growth and stress resilience under copper toxicity through ZnO─NPs and Azospirillum brasilense

Copper (Cu) pollution in agricultural soils is considered a serious health risk due to its accumulation in plants, but fewer studies have been conducted on its effects and alleviation strategies by using nanoparticles (NPs) and plant growth-promoting rhizobacteria (PGPR). Taking into consideration the positive effects of zinc oxide nanoparticles (ZnO─NPs) and Azospirillum brasilense in reducing Cu toxicity in plants, the present study was conducted. A pot experiment was conducted to determine the effects of the application of different levels of 25 and 50 ppm of ZnO─NPs and A. brasilense i.e., 10 and 20 ppm, on Cu accumulation, morpho-physiological and antioxidative defense attributes of wheat (Triticum aestivum L.) exposed to severe Cu stress (0, 100, and 200 mg kg⁻¹). Results from the present study showed that the increasing levels of Cu in the soil significantly (P < 0.05) decreased plant growth and biomass, photosynthetic pigments, gas exchange attributes, sugars, and nutritional contents from the roots and shoots of the plants. In contrast, increasing levels of Cu in the soil significantly (P < 0.05) increased oxidative stress indicators in terms of malondialdehyde, hydrogen peroxide, and electrolyte leakage, and also increased organic acid exudation patterns in the roots of T. aestivum. Although the activities of enzymatic antioxidants and the response of their gene expressions in the roots and shoots of the plants and non-enzymatic ones, such as phenolic, flavonoid, ascorbic acid, and anthocyanin contents, were increased by increasing the Cu concentration in the soil. The negative impacts of Cu injury were reduced by the application of ZnO─NPs and A. brasilense, which increased plant growth and biomass, improved photosynthetic apparatus, antioxidant enzymes, and mineral uptake, as well as diminished the exudation of organic acids and oxidative stress indicators in roots of T. aestivum by decreasing Cr toxicity. Research findings, therefore, suggest that the application of ZnO─NPs and A. brasilense can ameliorate Cu toxicity in T. aestivum, resulting in improved plant growth and composition under metal stress, as depicted by balanced exudation of organic acids.