As the most important biological reaction on earth, photosynthesis directly or indirectly contributes to over 90% of the global agricultural yield. Improving photosynthetic performance is an effective approach to promoting the yield and fruit quality of horticultural plants. Nanomaterials have shown great potential in improving photosynthetic efficiency, but the mechanism underlying these effects is poorly understood. In this study, we focus on the enhancement of photosynthetic efficiency and fruit quality induced by nitrogen-doped carbon dots (N-CDs), and reveal the regulatory mechanism. The application of N-CDs can effectively increase the photosynthetic efficiency, which improves the quality of apple fruits in the orchard. In this process, N-CDs act as an electron donor and provide additional electrons to plastoquinone (PQ) in the photosynthetic electron transport chain through electron transfer, which improves photosynthesis by accelerating the redox cycle of PQ and thereby promoting fruit quality. This study uncovers a mechanism by which nanomaterials enhance plant photosynthesis and provide some insights that will be useful in the design of efficient nanomaterials for agricultural/horticultural applications.