Copper and Nitrogen Codoped CDs for Alleviating the Damage of Reactive Oxygen Species for Cucumber Seedlings under Salt Stress

Dramatic changes in climate and soil environments have made growing conditions for crops more challenging. These crops are subject to a range of abiotic stresses in different environments, which can lead to significant yield losses, resulting in economic and environmental damages. Herein, we report a straightforward one-pot hydrothermal method for creating carbon dots codoped with copper and nitrogen (Cu,N-CDs). Under salt stress conditions, Cu,N-CDs demonstrate the ability to alleviate oxidative damage in cucumber seedlings by modulating antioxidant defense mechanisms and scavenging reactive oxygen species (ROS). Cucumber seedling biomass accumulation is greatly enhanced by Cu,N-CDs treatment in the presence of a ROS burst, leading to a notable rise in the dry weight, plant height, and fresh weight. Cu,N-CDs mitigate oxidative damage in cucumber seedlings by activating antioxidant defense systems, specifically enhancing the activities of superoxide dismutase (+34.08%), catalase (+28.11%), peroxidase (+17.54%), and ascorbate peroxidase (+31.54%) to scavenge ROS. Furthermore, Cu,N-CDs can enhance the levels of nonenzymatic elements within the antioxidant system, such as polyphenols (+23.60%), flavonoids (+15.43%), and carotenoid content (+51.73%), which strengthen the scavenging ability of cucumber seedlings against ROS. Meanwhile, Cu,N-CDs can induce a significant increase of soluble sugar and soluble protein content by 27.27 and 32.58%, respectively, which improves the osmotic pressure as well as stress tolerance of plants. Additionally, the accumulation of biomass was aided by the increase in the photosynthetic pigment content that Cu,N-CDs treatment can produce.