Factors Controlling Flowering of the Chrysanthemum

Abstract

Experiments are described which indicate that a temporary exposure to low temperature (vernalization) hastens inflorescence bud initiation in the Chrysanthemum, as measured by the time to the macroscopic appearance of the bud and also by the number of leaves produced. This effect is found in both long-and short-day conditions. In the absence of vernalization plants kept in short day assume a diageotropic growth habit and remain vegetative for long periods, frequently for much more than one year. Unvernalized long-day plants also remain vegetative but have a normal geotropic reaction. While the day-length effect is less important for inflorescence bud initiation, the opening and further development of buds formed in long day depend normally on subsequent day-length treatment. A vernalization period of only three weeks appears to be fully adequate. The low-temperature treatment may be given discontinuously, and evidence to hand appears to indicate that it is more effective if given during the dark phase than during the light phase. Hence de-vernalization by temperatures of about 20–25° C. does not appear to take place. There is evidence that little or none of the stimulus is carried over from one year to another. The results are discussed in relation to the auxin metabolism of the plant and also with regard to the absence in the literature of previous mention of the cold requirement. The author is indebted to Professor F. G. Gregory and to Mr. F. J. Richards for their stimulating interest and helpful suggestions in the course of this work, and to Dr. M. Holdsworth for providing him with the results of earlier unpublished work. Messrs. H. Woolman Ltd. kindly supplied some of the plant material used.