Continuous UV-B Irradiation Induces Endoreduplication and Trichome Formation in Cotyledons, and Reduces Epidermal Cell Division and Expansion in the First Leaves of Pumpkin Seedlings (Cucurbita maxima Duch.×C. moschata Duch.)

We examined the effects of continuous ultraviolet-B (UV-B) irradiation (0.58 W m−2) for 15 d on pumpkin seedlings. Continuous irradiation for 15 d slightly reduced the expansion of epidermal cell area and leaf area, whereas total epidermal cell number did not change in cotyledons. Only normal trichomes consisting of three cells were observed on the surface of control cotyledons. On the UV-B-irradiated cotyledon surface from 3–7 d, smaller trichomes consisting of single cells were observed along with normal trichomes. The total number of trichomes increased after UV-B irradiation for 5–9 d compared with that of control cotyledons. Expansion of true leaves was disrupted by UV-B irradiation for 15 d. Expansion of epidermal cell area and leaf area was reduced substantially, and the total number of epidermal cells was reduced considerably after UV-B irradiation for 15 d. Thus, continuous UV-B irradiation of pumpkin seedlings induces endoreduplication and trichome formation in tissues lacking active cell division (cotyledons), and reduces cell division in tissues that are actively growing (shoot apical meristem including first leaves).