A Study on the Directional Cultivation of Mechanization-Adapted Watermelon Scion Seedlings in a Plant Factory

Achieving high morphological uniformity and mechanical strength is critical for the automation of watermelon grafting; yet, specific light protocols targeting these traits are lacking. This study employed LED lighting to regulate the morphological development of watermelon scion seedlings in a controlled plant factory environment. Using the watermelon cultivar ‘Heimeiling’ as the experimental material, three sequential experiments were conducted: (1) Under conditions of 95 μmol·m−2·s−1 light intensity and a 12 h photoperiod, seven red/blue light ratios and a white light control were tested to identify the appropriate light quality. (2) Under the R3B1 light quality, gradients of the daily light integral (DLI) ranging from 2.88 to 17.28 mol·m−2·d−1 were established by adjusting the light intensity and photoperiod to determine the optimal DLI. (3) Based on the above results, an orthogonal experiment was designed, with factors including the light quality (R7B1, R3B1, R1B1; where R7B1 represents 87.5% red light and 12.5% blue light), light intensity (120, 160, 200 μmol·m−2·s−1), and photoperiod (16 h, 20 h, 24 h) to identify the optimal light environment combination for mechanical grafting. Results indicated that while monochromatic red light induced excessive elongation and suppressed metabolism, the R3B1 spectrum significantly enhanced the stem diameter, mechanical strength, and carbon–nitrogen accumulation while maintaining hormonal balance. Regarding the daily light integral (DLI), seedlings exhibited an optimal performance at 11.52 mol·m−2·d−1. Lower DLI levels led to etiolation, whereas higher levels caused photoinhibition and PSII damage. Furthermore, orthogonal analysis revealed that light intensity was the dominant factor driving stem thickening and biomass accumulation, while light quality primarily regulated plant height. Consequently, a combination of R3B1 light quality, 200 μmol·m−2·s−1 intensity, and a 20 h photoperiod was identified as the optimal strategy to satisfy the stringent morphological requirements for mechanical grafting.