Improving Tomato Graft Healing Efficiency Through Regulation of Red/Blue Light Ratios and Supplemental Green Light

As both an energy source and a signaling cue, light quality regulates graft healing by modulating endogenous phytohormone homeostasis, callus formation, and vascular reconnection. To elucidate the regulatory roles of red/blue (R/B) light ratios and green light supplementation on healing and seedling quality of grafted tomato (Solanum lycopersicum L.), a controlled-environment experiment was conducted in a plant factory using ‘Zhongza 105’ as the scion and ‘Zhezhen No. 1’ as the rootstock. LED lighting treatments were established with different R/B ratios (1.0, 2.5, 4.0, 5.5 and 7.0) with or without supplemental green light. The results show that moderate R/B ratios (4.0–5.5) significantly increased scion elongation, the stem diameter of both scion and rootstock, the mechanical strength of the graft union, and sap flow, while also enhancing leaf chlorophyll content, photosynthetic rate, and root activity. Under optimal R/B conditions, indole-3-acetic acid (IAA) and gibberellin (GA) levels were elevated, whereas abscisic acid (ABA) was reduced, favoring callus proliferation and vascular reconnection. Green light supplementation under moderate R/B further promoted stem thickening, leaf area expansion, water transport across the graft union, and total biomass accumulation. Overall, an R/B ratio of 4.0–5.5 combined with appropriate green light supplementation optimized the morphology, structure, and physiological performance of grafted tomato seedlings during the healing stage. The results aim to provide a scientific basis for optimizing light environments in a controlled environment, thus enhancing the stability and quality of grafted tomato seedlings.