Abstract
To clarify the temperatures responsible for delayed flowering and the production of proliferate inflorescences (i.e. capitula with involucral bracts borne among the ray and disc florets) in summer-to-autumn-flowering Chrysanthemum morifolium Ramat. ‘Seiun’, the effects of a range of day and night temperature regimes, and exposure to high temperatures at different stages of floral development, on flowering and inflorescence characteristics were investigated. Rooted cuttings of ‘Seiun’ were planted on early-June and grown with the night-break treatment to maintain vegetative growth in a greenhouse. Subsequently, the plants were transferred to phytotron rooms on June 19 and were grown at six constant day (6:00-18:00)/night (18:00-6:00) temperature regimes in short days. In addition, plants were subjected to high-temperature treatment of 35/25°C for two weeks at four temporal stages: from the end of night-break treatment (ENBT), from two to four weeks after the ENBT, from four to six weeks after the ENBT, and from six to eight weeks after the ENBT. The plants bearing proliferate capitula were counted when the number of involucral bracts in the capitulum exceeded 25. Flowering of ‘Seiun’ was delayed in plants grown in short days at a day temperature of 30°C or more, or at a night temperature of 20°C or more. Proliferate capitula were formed in plants exposed to a high day or night temperature exceeding 35°C or 25°C, respectively, during the period from two to four weeks after ENBT (at the late stage of involucre formation or early stage of floret formation). As a practical application, involucral inflorescences of plants in summer-to-autumn-flowering ‘Seiun’ could be reduced using cheesecloth to achieve a 50% cut of daylight in the greenhouse for 3 weeks from ENBT.
