Acclimation and hardness enhancement through ultraviolet-B irradiation of fruit vegetable seedlings grown in a plant factory with artificial light

Abstract

This study determined the optimal ultraviolet (UV)-B dose of major vegetable seedlings in a plant factory with artificial light for labor saving. Four vegetable seedlings, cucumber, watermelon, tomato, and pepper, were used as test crops. UV-B irradiation was conducted at four different UV-B doses of 1.08, 2.16, 3.24, and 4.32 kJ·m⁻² d⁻¹ for 3 h a day for 3 days after graft-take and temporary planting of fruit vegetable seedlings. After UV-B irradiation, seedlings were moved to a greenhouse and exposed to ultraviolet rays of sunlight; growth was observed; and chlorophyll fluorescence was measured. When subjected to UV-B irradiation, minimal changes in external traits of fruit vegetable seedlings were observed at doses below 2.16 kJ·m⁻²·d⁻¹, while at higher intensities, issues such as chlorosis emerged. When UV-B was irradiated at 4.32 kJ·m⁻²·d⁻¹, cucumber exhibited a decrease in hypocotyl length by 6.4 cm, and watermelon by 1.7 cm compared with the control. Additionally, cucumber and watermelon both showed a decrease in leaf area by 75.6 cm² and 49.8 cm², respectively, indicating a tendency of growth inhibition. For tomatoes and peppers, when UV-B was irradiated at 1.08 kJ·m⁻²·d⁻¹, they exhibited an improvement in compactness by 5 and 2.5 mg·cm⁻¹, respectively, compared with the control. Chlorophyll fluorescence parameters indicated a positive correlation between ABS/RC and DI_O/RC with increasing UV-B doses, while F_V/F_M and PI_ABS showed a negative correlation. Using UV-B during the seedling acclimation period, seedlings can adapt to external UV radiation, thereby suppressing excessive growth and promoting the production of robust seedlings. Moreover, the utilization of UV-B within a plant factory allows for conducting the acclimation process within the facility, reducing the need for seedlings to be transported to conventional nurseries and offering potential benefits in terms of operational efficiency.