The increasing application of carbon dots (CDs) inevitably leads to environmental leakage. To be environmentally friendly, degradable CDs will become a mainstream agricultural application in the future. We prepared three CDs with different functional groups. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis showed that –CC–H and CO groups on the CD surfaces are major sites for oxidative degradation of the CDs. The pyridine N is responsible for photoproduction of ˙OH and ˙O₂⁻, which will self-induce degradation. By liquid chromatography-mass spectrometry (LC-MS) method, the routes for N-doped CDs degrading into carboxylic acid compounds, CO₂, NH₄⁺, and H₂O are clearly illustrated. After spraying fluorescent CDs on lettuce leaves, only the group of lettuce that combined the light conversion function of N3-CDs and the final degradation products of N3-CDs exhibited a significant enhancement of the net photosynthesis rate by 1.13 times, acceleration of the ETR by 1.19 times, and improved expression of genes related to CO₂ fixation in the C₄ cycle and carbon assimilation in the C₃ cycle.