Nano ZnO Alleviates Salt Stress in Rapeseed Seedlings via Ionic Homeostasis and Antioxidant Enhancement

The seedling stage is one of the stages during which rapeseed is most sensitive to saline-alkali stress. Enhancing the tolerance of rapeseed seedlings is crucial for achieving high biomass and yield when cultivating rapeseed in saline-alkaline soils. This study utilized the salt-sensitive rapeseed variety Yangyou 9 as experimental material to investigate the physiological and molecular mechanisms by which foliar application of zinc oxide nanoparticles (ZnO NPs) improves salinity tolerance under salt stress during the seedling stage. The results indicated that the 150 mM NaCl stress significantly inhibited the growth of rapeseed seedlings. However, foliar application of ZnO NPs at the concentration of 100 mg L^-1 resulted in significant increases in biomass, plant height, leaf width, and leaf area of the above-ground parts of the plants. Furthermore, the contents of soluble sugars and soluble proteins increased by 57.03% and 33.43%, respectively. Under salt stress conditions, the application of ZnO NPs significantly enhanced the activities of POD, SOD, and CAT compared to the untreated control, reduced the levels of reactive oxygen species (ROS), and decreased electrolyte leakage by 27.7% as well as malondialdehyde (MDA) content by 30.7%. These findings indicated that ZnO NPs treatment could significantly alleviate oxidative stress and damage to cell membranes. Non-destructive micro-measurement techniques showed that after ZnO NPs treatment, the rates of K⁺ efflux and Na⁺ influx in the root tips and leaf mesophyll tissues of rapeseed seedlings were significantly reduced, thus maintaining the sodium-potassium ion balance and enhancing the salt tolerance of rapeseed during the seedling stage.