The chrysanthemum(Chrysanthemum morifolium Ramat.) cultivar Reagan' used in this study. Stock plants were grown in a greenhouse, which was maintained at an air temperature above 18°C, and ventilated when the temperature increased above 25 oc. A natural photoperiod with a 4-h night break(NB) (from 23:00 h to 03:00 h) was delivered by means of incandescent lamps(K- RD100V60W: Matsushita Electric Industrial Co. Ltd., Osaka, Japan). Rooted cuttings from the stock plants were planted into 7.5-cm plastic pots containing a commercial horticultural soil(Kureha Engei-Baido: Kureha Chemical Industry Co. Ltd., Tochigi, Japan), and grown in the greenhouse. The plants were transferred to a growth chamber maintained at 20°C with a 16-h photoperiod(long day(LD) conditions). Light was supplied by means of fluorescent tubes(FLA0SW: Mitsubishi Co. Ltd., Tokyo, Japan), at a photosynthetic photon flux density(PPFD: 400-700 nm) of 200 pmol m-2 s-1. After 7 d of growth under these conditions, the plants were trans- ferred to a growth chamber maintained 20°C, with a range of