On the Rosetting of Chrysanthemum Plants

Factors decreasing growth activity, or causing the physiological rosetting in chrysanthemum were studied. Mother plants chilled and non-chilled in winter were grown under various conditions, high temperature in spring and early summer, low light intensity, cool temperature in summer, and so on. Rooted cuttings propagated from those mother plants were planted and pinched, then they were grown under 9-hour daylength at cool temperatures. The chrysanthemum plants which had not experienced the temperatures lower than 15°C for several years showed the low growth activities continuously in all seasons. Whenever they were transfered to the short day condition at cool temperature (15°C), they always formed the rosetting shoots. The normal plants, chilled in winter, showed the high growth activities until autumn, and the activities decreased in October or November. Even if the plants had the low growth activities, they elongated well at the temperatures higher than 20°C, and formed flower buds normally at 25°C. The plants, chilled in winter and grown at 15°C in summer and at the temperatures higher than 10°C in the following winter, maintained the high growth activities until the following summer. Their growth activities, however, decreased earlier than those of the normal plants. High temperature treatments of the normal mother plants in the spring and early summer did not decrease the growth activities. When the mother plants were treated with the high temperature and low light intensity, their growth activities were lowered. It seems that, when a chrysanthemum plant grows under the normal conditions for a long period after chilling, and then it receives high temperature for several months, growth activity of the plant becomes lower.