Plant–Nanoparticle Interactions with g-C ₃ N ₄ : Regulator Development, Metabolic Responses, and Physiological Mechanisms

Graphitic carbon nitride (g-C₃N₄) has received increasing attention for environmental and agricultural applications. Here, homogeneous (H-CN) and heterogeneous (He-CN) g-C₃N₄ were synthesized and systematically compared with respect to structure, photocatalysis, and biological effects. H-CN exhibited an ordered lamellar structure and efficient charge separation, leading to a 92% methylene blue degradation within 3 h, compared with 76% for He-CN. In hydroponic lettuce, H-CN at 200 mg/L significantly enhanced plant growth, increasing height by 30%, stem diameter by 25%, and leaf area by 40% (<i>n</i> = 3, <i>P</i> < 0.05). Photosynthetic rate and chlorophyll content increased by 39% and 31%, respectively, while nitrate levels decreased by 42% (<i>n</i> = 3, <i>P</i> < 0.05). Antioxidant enzyme activities (SOD, POD, CAT) rose by 39%, 36%, and 54%, respectively, accompanied by a 36% reduction in ROS (<i>n</i> = 6, <i>P</i> < 0.05). Metabolomics revealed enhanced nitrogen assimilation and enrichment of flavonoids and amino acids. Cytotoxicity assays indicated lower toxicity of H-CN (80% HepG2 cell viability at 300 mg/L) compared with He-CN (20%) (<i>n</i> = 3, <i>P</i> < 0.05). These results establish a structure-performance-biological effect relationship, highlight the agricultural potential of H-CN, and underscore the importance of dosage control for safe and effective application.