On rossetting of suckers and measures for breaking the rossetting in chrysanthemums

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For the production of chrysanthemum flowers in early spring to early summer, rosette suckers are usually planted in winter. They do not elongate well' even under high temperature thereafter unless they are subjected to low temperature for certain period. Therefore, the suckers to be planted early in winter are obtained from the high altitude areas where temperature drops earlier. This study was made to elucidate the developmental process of suckers and factors responsible for rosette formation, and also to find the measures effective to break the rosetting of suckers. Suckers develop from the definite buds on the stem under or a little above the soil surface. Under the natural conditions, suckers start to develop after the end of August, simultaneously with the time of flower bud differentiation. When all the buds were pinched off, or were checked their growth by MH spraying, suckers started to develop before the flower bud differentiation. Therefore, it seemed that the absence of the growing buds on the stem stimulated the growth of the latent buds into suckers. Suckers developed in September or later form rosette, while those developed in July or August do not form rosette. Experiments were carried out to know which was responsible to the induction of rosetting of suckers, low temperature or short day length. The results revealed that suckers of ‘Shin-Toa’ and ‘Okayama-Heiwa’ were induced to rosetting by low temperature of about 15°C under long day condition, while the suckers of ‘Pink-Toa’ were induced to rosetting under short day condition and not under long day condition. The factors responsible for the rosette induction seemed to vary depending on the varieties. Some authors reported that the rosetting could be broken by low temperature. An experiment was made to know how soon the rosetting was broken in winter under the natural conditions in Tokyo. It was found that the suckers of ‘Shin-Toa’ and ‘Okayama-Heiwa’ grown in the open till Dec. 20, then moved into the greenhouse elongated well. So the low temperature period to Dec. 20 seemed to be long enough to break the rosetting of the suckers. An additional experiments was made to clarify the effects of ethylene-chlorhydrin, thiourea, and gibberellin on breaking the rosetting of suckers. Ethylene-chlorhydrin showed some effects, and thiourea not at all. Gibberellin (50 and 100 ppm) showed no effect when it was applied in early stage and also near the end stage of rosetting, while it showed a significant effect when applied on Jan. 10, apparently at the middle stage of rosetting.