Integrative Effects of Zinc Nanoparticle and PGRs to Mitigate Salt Stress in Maize

Salinity is one of the most critical problems for agricultural development and threatens future food safety. Therefore, we aimed to investigate root application of zinc oxide nanoparticles (ZnO-NPs; 0, 50, 100 mg/L), 24-epibrassinolide (EBL; 0, 0.02, 0.04 µM), and their combinations on the growth and performance of maize (<i>Zea mays</i> L.) as a model plant grown under salt stress (i.e., 0, 5 and 10 dS m⁻¹) in a hydroponic system. The results showed that the highest salt stress negatively affected growth, physiological, and biochemical traits of maize. However, the application of EBL, ZnO-NPs, and their combinations significantly mitigated salt stress and improved the growth and performance of the physiological system in maize plants. In particular, the combination treatment of 100 mg/L ZnO-NPs + 0.02 µM EBL surpassed all other root treatments and resulted in the highest root and shoot growth, leaf area, relative leaf water content, net photosynthesis, total chlorophyll content, and uptake of zinc (Zn) and potassium (K). Furthermore, it minimized salt stress by reducing Na uptake, Na/K ratio, and proline in stressed maize plants. For example, the combination treatment of 100 mg/L ZnO-NPs + 0.02 µM EBL improved root length by +175%, shoot length by +39%, leaf area by +181%, RWC by +12%, net photosynthesis by +275, total chlorophyll content by +33%, and total phenolic content by +38%, in comparison to those obtained from the control, respectively. Furthermore, it enhanced the roots and leaves uptake of Zn under high salt stress treatment (i.e., 10 dS m⁻¹) by +125% and +94%, and K⁺ by +39% and +51%, as compared to those grown without any of NPs or EBL treatments, respectively. Thus, the root application of 100 mg/L ZnO-NPs + 0.02 µM EBL can be a potential option to mitigate salt stress and improve the physiological, biochemical, and performance of strategy crops such maize.