Difference between Nighttime and Daytime UV-B Irradiation with Respect to the Extent of Damage to Perilla Leaves

Although ultraviolet-B (UV-B) light (280–315 nm) irradiation effectively controls spider mites in horticultural crop production, it also causes plant damage, leading to growth suppression, changes in morphology, and leaf scorching. However, sensitivity to UV-B varies among plant species. Here, we assessed the effect of UV-B on growing perilla (Perilla frutescens var. crispa) plants. Three experiments were conducted. In the first experiment, plants of the variety ‘Akachirimenshiso’ were grown in a plastic house with natural light conditions and subjected to three treatments: natural UV (control), −UV, and natural UV plus artificial UV-B treatment (50 mW·m−2) from 0:00 to 3:00. The length of leaf blades that received additional UV-B treatment was reduced by 16.5% and 32.3% in two trials compared with that of the respective controls. In this experiment, additional UV-B irradiation turned the leaf color greener. During UV-B treatment, the values of leaf a*, an indicator of leaf redness, were significantly lower than their respective controls in both trials: 21.5 vs. 31.6 in trial I and 20.2 vs. 30.7 in trial II. For most of the parameters measured in this experiment, no differences were observed between the control and −UV treatment groups. In the second experiment, plants were irradiated with UV-B for 3 weeks at nighttime (0:00–3:00) or daytime (12:00–15:00). In the cultivar ‘Houkouakashiso’, the length of leaf blades significantly decreased by 15.9% and 20.6% under nighttime UVB irradiation at 80 and 120 mW·m−2, respectively, compared with that of the non-irradiated controls. Irradiation at 80 mW·m−2 also decreased the width of the leaf blades by 13.1% and that at 120 mW·m−2 further decreased it by 25.0%. These results showed that UV-B irradiation at night decreased the size of perilla leaves. In addition, the value of a* became lower under UV-B irradiation in the nighttime. Thus, UV-B irradiation appeared to turn purple perilla leaves green. When plants were irradiated with UV-B in the daytime, there was no significant difference between irradiated and non-irradiated plants in the length or width of leaf blades, a*, or fresh weight of aerial parts and number of nodes on the main stem. In the third experiment, visible rays (VIS) emitted by fluorescent lamps were applied at 0:00–3:00 and 6:00–22:00. Plants were irradiated with 120 mW·m−2 of UV-B at 0:00–3:00. UV-B + VIS treatment of ‘Akachirimenshiso’ with VIS from fluorescent lamps did not significantly affect the parameters measured in this experiment compared to −UV treatment. The results of this study suggest that UV-B damage to perilla leaves can be avoided by combined irradiation with visible light.