Green-synthesized ZnO and MgO nanoparticles modulate physiology and antioxidant defense in maize under alkaline stress

Alkaline stress severely impairs the growth and yield of Zea mays L. by disrupting physiological and biochemical functions. This study evaluated green-synthesized ZnO and MgO nanoparticles (NPs), prepared using neem and licorice extracts, for mitigating alkaline stress. NPs were nanosized, crystalline, and functionalized by phytochemicals, confirmed by scanning electron microscopy, FT-IR spectroscopy, UV-vis spectroscopy, and energy dispersive X-ray spectroscopy. A pot experiment using NPs (25–200 ppm) under control and alkaline stress assessed morphological, physiological, biochemical, and ionic responses. Alkaline stress reduced root fresh and dry weight to 2.60 and 0.66 g (−59.6%, −31.0%), shoot fresh and dry weight to 2.60 and 0.38 g (−59.6%, −70.0%), and chlorophyll a, b, and carotenoids to 1.31, 0.67, and 2.40 mg g−1 (−62.4%, −54.7%, −62.8%), whereas it increased malondialdehyde (MDA) (244.6%), H₂O₂ (457.7%), and relative membrane permeability (RMP) (55.9%). The combined ZnO (50 ppm) and MgO (50 ppm) treatment improved chlorophyll a, b, and carotenoids to 3.48, 1.48, and 6.45 mg g−1 (165.4%, 120.3%, 168.5%), and total soluble protein (392.8%), total protein (301.0%), proline (105.5%), glutathione (35.6%), and ascorbic acid (44.2%). Antioxidant enzymes increased, with superoxide dismutase at 29.52 U mg−1 (452.8%), peroxidase at 24.44 U mg−1 (862%), and ascorbate peroxidase at 51.62 U mg−1 (560%), whereas MDA, H2O2, and RMP (−78.1%) were reduced. High NP concentrations (ZnO 100 ppm + MgO 100 ppm) were toxic. Moderate ZnO and MgO NP doses enhanced resilience, yield stability, and sustainable agriculture.