Effects of intensity and duration of UV-B irradiation on growth and intumescence development in two tomato cultivars at the seedling stage

Intumescence is a physiological disorder in plants characterized by hypertrophy and, in some cases, hyperplasia. Ultraviolet B (UV-B) irradiation inhibits intumescence development; however, the degree of intumescence inhibition by UV-B irradiation may differ depending on the cultivar. In this study, we determined the effective range and duration of UV-B irradiation for intumescence inhibition in two cultivars of tomato seedlings in a closed plant production system. Seedlings of two cultivars were grown in a 128-cell tray until 21 days after sowing (DAS). Plants were exposed to either white LEDs with three levels UV-B irradiation (0, 0.5 and 1.0 W m^‑2) using UV-B fluorescent lamps or white fluorescent lamps during the light period. In both cultivars, the intumescence area under LEDs alone treatment was the same, but those in 'Gohobi' were quite inhibited in all treatments with UV-B. In 'Rinka 409', the intumescence area was alleviated depending on the intensity of UV-B and was the smallest at 1.0 W m^‑2 among the treatments. In 'Rinka 409', total leaf area and dry weight of leaves in the LED alone treatment were the lowest among the treatments due to severe intumescence development, but no damage was observed on the leaves in all treatments in 'Gohobi'. To determine the effective duration of UV-B irradiation, half of the seedlings grown hydroponically under LEDs alone and half of those grown under UV-B (0.5 and 1.0 W m^‑2) were exchanged between treatments on 14 DAS and grown for the next seven days under new conditions, whereas the remaining half of the seedlings were grown under the same light conditions until 21 DAS. Results showed that intumescence development was inhibited only during the UV-B irradiation period in both cultivars, and intumescences occurred on all leaves, regardless of leaf age, under light conditions without UV-B.