This study aimed to determine the optimal conditions of UV-B exposure for regulating the growth of tomato grafted seedling in Plant Factory with Artificial Lighting by investigating growth characteristics, rhizosphere development, and chlorophyll fluorescence of seedlings. Tomato scion and rootstock were used in the experiment. UV-B treatment timing was divided into three stages from sowing to grafting. UV-B intensities were set at 1.44, 2.88, and 5.76 kJ·m−2·d−1. Results showed that morphology of plants did not exhibit significant differences up to 2.88 kJ·m−2·d−1 for tomato scion and rootstock. However, side effects such as leaf wilting were observed at 5.76 kJ·m−2·d−1. The length of hypocotyl, which is closely related to scion and rootstock suppression of stem elongation, was shortest when treated with 5.76 kJ·m−2·d−1 during the mid-growth stage across all treatment. Interestingly, rhizosphere characteristics such as root volume, surface area, and average root diameter showed improvement trends regardless of treatment timing when exposed to UV-B at 1.44-2.88 kJ·m−2·d−1. Quality indicators of seedlings showed best when 2.88 kJ·m−2·d−1 during the late-growth stage. Comparisons of chlorophyll fluorescence parameters revealed no significant effects of UV-B treatment on cucumber seedlings and grafting except for PIABS. However, tomato seedlings and grafting exhibited significantly reduced FV/FM and DIO/RC at late-growth stage when exposed to 5.76 kJ·m−2·d−1. Therefore, it is concluded that utilizing UV-B in the range of 1.44-2.88 kJ·m−2·d−1 during cucumber and tomato scion and rootstock production in Plant Factory with Artificial Lighting could minimize plant damage while expecting to suppress grafting effects.
