Tomato Cultivation in a Plant Factory with Artificial Light: Effect of UV-A Irradiation During the Growing Period on Yield and Quality of Ripening Fruit

Dwarf cherry tomatoes ‘Red Robin’ and ‘Tiny Tim Red’ were cultivated in two sections. The two sections were set as (1) section irradiated only by fluorescent lamps (main wavelengths 433, 543, and 610 nm; 230 μmol·m−2·s−1 of photosynthetic photon flux density) and non-treated with UV-A irradiation (hereafter, non-UVA), and (2) section irradiated with fluorescent lamps and treated with UV-A irradiation (maximum peak emission: 352 nm; 15.1 W·m−2 ultraviolet radiation intensity; hereafter, t-UVA). The fruit weight per plant and the number of fruits per plant were not significantly affected by cultivar or section. Regarding fruit quality, the fruit cracking rate was lower in the t-UVA than in non-UVA section. There were no anatomical or histochemical differences in fruit structure or distribution of pigments such as lycopene or β-carotene, but the shape of the pigment in the epidermal cells was needle-shaped in the non-UVA and unclearly-shaped in the t-UVA, so the pigment shape was different between the two sections. In addition, the number of layers of the hypodermis in the exocarp that accumulated the pigment was smaller in the t-UVA than in non-UVA section. Fruit components in t-UVA showed increased total soluble solids (TSS), titratable acidity (TA), and ascorbic acid content, but a decreased carotenoid content. Therefore, it was shown that UV-A irradiation had a positive effect on the TSS, TA and ascorbic acid content and had a negative effect on the size of the fruit per fruit and the carotenoid content. UV irradiation did not affect the yield per plant. Comparing ‘Red Robin’ and ‘Tiny Tim Red’, there was no significant difference in yield, but the fruit size of ‘Red Robin’ was larger, while and the fruit component of ‘Tiny Tim Red’ was higher than that of ‘Red Robin’. In conclusion, when cultivating tomato plants in a plant factory, further improvements in fruit size and composition are expected by appropriately adjusting the time and intensity of UV irradiation for each cultivar.