Sargassum-synthesized ZnO nanoparticles induce salt tolerance in maize plants through enhanced physiological and biochemical mechanisms

Salinity is one of the biggest limitations of agriculture in semi-arid regions of the world. It negatively impacts the growth and yield of Zea mays L. Especially the seedling stage is extremely sensitive to salt stress. Sargassum tenerrimum J. Agardh. is a macroalgae which produces a plethora of important secondary metabolites, micro and macro-nutrients, polyamines and natural phytohormones etc. These compounds have been reported to improve plant growth and alleviate the harmful effects of salt stress. The current study explores the effectiveness of algal based Zinc Oxide nanoparticles (ZnO_St-NPs) in alleviating NaCl stress in Zea mays. The study involved two levels of NaCl (0 = control and 200 mmol kg^{- 1}) and four levels (0, 30, 60 and 80 ppm) of ZnO_St-NPs applied through foliar spray. A notable enhancement was observed for morphological parameters Shoot length: 18.92%; root length: 18.56%; shoot fresh weight: 38.94%; dry weight: 23.74% and root fresh weight: 21.43% dry weight (27.97%) photosynthetic pigments (chl a: 40.98%; chl b: 93.55%; total chlorophyll: 55.38% and carotenoid 43.5%) and chlorophyll fluorescence parameters also exhibited a remarkable enhancement at 80 ppm ZNO_St-NPs under salt stress. Decrease in the levels of antioxidant enzymes (POD: 25.67%; SOD: 25.67%; CAT: 59.85% and APX: 54.21%) as well as non-enzymatic anti-oxidants (GSH: 34.67%; GR: 52.80%; AsA: 10.47% and lycopene: 34.40%) were also observed at 80 ppm ZnO_St-NPs under salt stress. The foliar application of ZnO_St-NPs significantly decreased MDA and H₂O₂ levels 38.40% and 32.80% and uptake of Na⁺ (34.25%) and Cl^- (38.65%) respectively. while an increase in the total soluble proteins (87.50%), K⁺ uptake (77.27%), water content (74.63%) and salt tolerance index (17.12%) was observed with application of 80 ppm ZnO_St-NPs. This study provides important insight into the potential of algal based ZnO-NPs as a low cost and ecofriendly method for managing salinized soil.