Chitosan‐Carbon Dot Composite Materials Form a Leaf Surface Barrier to Mitigate the Enrichment and Invasion of Nanoplastics: From Leaf Interface to Systemic Response

To address the potential risk posed by the atmospheric deposition of micro-nano plastics (M/NPs) to agricultural product safety, this study develops a chitosan-carbon dot (CS-CDs) composite suitable for foliar spraying. Its mechanism for alleviating polystyrene nanoplastic (PS) stress is systematically investigated at two growth stages (day 35 and 45) of Brassica rapa. Results demonstrate that CS-CDs form a homogeneous protective film on the leaf surface, which persistently and effectively blocks PS attachment and stomatal intrusion. Concurrently, the CDs component is absorbed by the plant and undergoes systemic translocation, synergistically enhancing plant resistance both externally and internally. By day 45, the fresh and dry weights of Brassica rapa increase by 26.17% and 30.77%, respectively. Oxidative damage is effectively mitigated. Photosynthesis has also been improved. Furthermore, the foliar microbial community exhibits a 103.93% increase in the Shannon index and shows enhanced richness of Pseudomonadota, Actinomycetota, and Chloroflexota. Defense pathways including keratin, cork, and wax biosynthesis and phenylpropane metabolism are also activated. This study elucidates the integrated mechanism by which CS-CDs alleviate PS stress across physiological, biochemical, and microbial levels, providing a novel strategy for using nanomaterials to reduce M/NPs accumulation and toxicity in plants.