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Chrysanthemum (Chrysanthemum morifolium Ramat.) is a major commercial cut flower produced year round worldwide (USDA, 2009; Flora Holland, 2014). In Japan in 2013, the annual shipping volume of cut chrysanthemum flowers was 1.6 billion, making up approximately 39% of the total shipping volume of cut flowers (MAFF of Japan, 2014). As chrysanthemum is a short-day plant, growers have been using techniques such as shading in summer or night-break (NB) treatment from autumn to spring in order to regulate its flowering for year-round production (NARO Institute of Floricultural Science, 2012). Incandescent lamps are used for NB treatment because of their good inhibitory effect (Hakuzan and Kooriyama, 2013). Although inexpensive, incandescent lamps require substantial amounts of electricity. The Japanese government recently decided to restrict the manufacture and sales of incandescent lamps to save energy, reduce emissions, and thus mitigate climate change (Ministry of Economy, Trade and Industry of Japan, 2008). Therefore, a new light source for NB treatment is required. Light-emitting diode (LED) lamps have the advantages of low electricity consumption, high efficiency, and long lifespan. However, as LED lamps emit single-wavelength light in contrast to the wide wavelengths of sunlight and incandescent lamps, their effects on plant growth are not entirely clear. Phytochrome is a plant photoreceptor that plays an important role in photomorphogenesis (Quail, 2002; Mockler et al., 2003; Nagatani, 2004; Takano et al., 2009). Phytochromes have 2 forms: Pfr and Pr, which respond to red light (maximally at 660 nm) and far-red light (maximally at 725 735 nm), respectively (Briggs and Rice, 1972; Kelly and Lagarias, 1985; Lagarias et al., 1987). Pfr, the biologically active form of phytochrome, has an important role in the flowering inhibition of short-day plants as a result of NB treatment. Thus, NB treatments with red light exert a similar inhibitory effect on chrysanthemum flowering as incandescent lamps (Ishikura et al., 2009; Ohishi et al., 2010). Meanwhile, NB treatments with blue and far-red light do not inhibit flowering in chrysanthemum (Higuchi et al., 2012; Sumitomo et al., 2012). There are 4 flowering types of chrysanthemum: summer flowering, summer-autumn flowering, autumn flowering, and winter flowering (Kawada and Funakoshi, 1988). Because each flowering type exhibits different critical day length and genetic background, different flowering types may have different responses to NB treatment with LED lamps emitting different wavelengths. Accordingly, the response to NB treatment with LED lamps emitting various wavelengths on floral bud formation differs between the summer-autumn flowering-type chrysanthemum “Iwa no hakusen” and autumn flowering-type “Jimba” (Hakuzan and Nagayoshi, 2013). However, it remains unclear whether NB treatment with LED lamps affects the initiation of flower bud differentiation or growth rate of flower buds. Therefore, this study investigated the differences in the effects of NB treatment with LED lamps emitting various wavelength of light on the initiation of flower bud differen-

Varietal Differences in Flowering and Plant Growth Under Night-Break Treatment with LEDs in 12 Chrysanthemum Cultivars