The role of LED supplementary lighting in promoting graft necrotic layer formation in pumpkin-cucumber grafts

•LED supplementary lighting significantly accelerates the formation of graft necrotic layers in pumpkin-cucumber grafts.•Anatomical observations revealed complete necrotic layer formation by the second day post-grafting under LED lighting.•Enzyme activity assays showed a substantial increase in POD and PPO activities, enhancing the graft union's antioxidant capabilities.•LED lighting elevated soluble sugar and ATP levels, optimizing the environment for necrotic layer development.•Transcriptome analysis highlighted enhanced sugar metabolism and upregulation of genes involved in sugar synthesis and energy provision.•Hormone signal pathways, particularly abscisic acid, were influenced by LED lighting, expediting necrotic layer formation in the scion.•Findings offer valuable insights into graft healing mechanisms and endorse LED technology for advanced seedling cultivation. The rapid formation of the graft necrotic layer is crucial for preventing organic substance leakage and warding off external infections, significantly enhancing both the speed and quality of graft healing. This study investigates the impact of LED supplementary lighting on pumpkin-cucumber grafts, unveiling the molecular processes underlying necrotic layer formation. Specifically, three replications were conducted, where post-grafting cucumber seedlings were placed separately in healing chambers with LED light intensities of 0 μmol/(m2·s) and 50 ± 1 μmol/(m2·s) for three days. Anatomical observations, enzyme activity assays, energy product content detection, and a more comprehensive transcriptome sequencing analysis on the graft union of the rootstock and scion were conducted. Anatomical observations revealed that LED supplementary lighting accelerates the formation of the pumpkin-cucumber graft necrotic layer, establishing a complete layer by the second day post-grafting, one day earlier than the control. Additionally, at day 3 post-grafting, the activities of POD and PPO enzymes at the rootstock-scion junction significantly surpassed the control, exhibiting a remarkable increase of 74.12 % and 48.96 %, respectively (p < 0.01). Moreover, soluble sugar and ATP levels were notably higher at day 3 post-grafting in comparison to the control, showing significant differences (p < 0.05). Transcriptomic analysis unveiled that LED supplementary lighting not only enhances scion sugar metabolism but also stimulates the upregulation of sugar synthesis genes (SPP1, RPV1) and starch synthesis genes (AGP