Influence of Light Quality on the Growth of Machine-Compatible Tomato Seedlings Before and After Grafting

Tomato (Solanum lycopersicum L.) is an economically important horticultural crop. The application of mechanical grafting technology enables the efficient, large-scale production of grafted tomato seedlings, which is of great significance for overcoming continuous cropping obstacles and boosting tomato yield. In this study, tomato cultivar ‘Juxiang 1809’ as the scion and ‘T17-2’ as the rootstock were used to systematically investigate the effects of red-blue light quality pretreatments on tomato grafted seedlings. The rootstock and scion seedlings were cultivated under white (W), pure red (R), pure blue (B), and five mixed red-blue lights (R7B1, R3B1, R1B1, R1B3, R1B7). Our results demonstrated that R3B1 (Red: Blue = 3:1) yielded the highest scion comprehensive score (2.06), promoting balanced growth with robust stem diameter (2.75 mm) and high aboveground dry weight (0.36 g). For rootstocks, R3B1 also excelled, driving optimal root development with maximum root area (26.32 cm2) and dry weight (0.046 g). Post-grafting, R3B1-pre-treated seedlings maintained vigorous growth with enhanced photosynthetic capacity (37.10) and biomass accumulation. These findings demonstrate that R3B1 light quality is highly effective. It optimizes both scion vigor and rootstock root architecture. This offers a practical light-regulation strategy. It is applicable to the production of high-quality, machine-compatible tomato grafted seedlings in controlled environments.