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Ultraviolet-B (UV-B; 280 320 nm) irradiation has pleiotropic effects on development, morphology, and physiology of higher plants (Frohnmeyer and Staiger, 2003). Exposure to UV-B causes DNA and membrane damage, reduced photosynthetic activity, inhibition of hypocotyl elongation, stunted growth, reduced leaf area, bronzing, and necrosis (Teramura, 1983; Ziska et al., 1992; Krizek et al., 1993; Teramura and Sullivan, 1994). Thus, UV-B irradiation inhibits plant growth. Plant growth is driven by cell division coupled with subsequent elongation and differentiation of the daughter cells (Beemster et al., 2003; Jakoby and Schnittger, 2004). Cell division plays a role in developmental processes that create plant architecture and in modulating plant growth rate in response to environmental conditions (Cockcroft et al., 2000; Beemster et al., 2002). In higher plants, the shoot apical meristem (SAM) and root apical meristem (RAM) are the tissues with active cell division. In other tissues that lack active cell division, cells can sometimes undergo endoreduplication, an alternative cell cycle process in which DNA replication continues without mitosis or cytokinesis (Nagl, 1976; Barlow, 1978). Endoreduplication in higher plants often occurs during cell elongation and differentiation, and some have suggested that it could play an important role in cell-size regulation (Traas et al., 1998). The DNA content of endoreduplicated nuclei is much greater than 2 C (C is the haploid DNA content). For example, trichomes are generated by endoreduplication of epidermal cells. Arabidopsis trichomes consist of single cells that emerge from the epidermis of leaves and stems. The single trichome nucleus continues to replicate its genomic DNA during differentiation, reaching levels of 20 32 C (Melaragno et al., 1993; Hulskamp et al., 1994). The surface of cucumber (Cucumis sativus L.) cotyledons contains trichomes composed of three cells with nuclear DNA levels of 6.7 8.2 C, which is greater than that in epidermal cells (Yamasaki et al., 2010). Cell division and endoreduplication are important processes for plant growth. Aerial parts of pumpkin (Cucurbita maxima Duch. C. moschata Duch.) seedlings contain the cotyledons, hypocotyl, and SAM. Although cell division is active in the SAM, it does not occur in open cotyledons. Therefore, pumpkin seedlings represent an excellent system to study the effects of UV-B irradiation on tissues with and without active cell division. To clarify the effects of UV-B irradiation on higher plants over short-term periods, an experimental system designed to provide continuous UV-B irradiation at intensities that do not cause severe damage to the plants is useful. We previously showed that continuous UV-B irradiation (0.57 W m 2 ) of cucumber (Cucumis sativus L.) seedlings induced endoreduplication, caused rapid expansion of the epidermal cells surrounding trichomes in cotyledons (Yamasaki et al., 2007; 2010), and reduced epidermal cell division and expansion in true leaves (Yamasaki et al., 2014). The effects of continuous

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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.^|^times;C. moschata Duch.)