Studies on flowering control of dahlia. IV.

Effect of day-length at the early stage of shoot growth upon the flowering time and the quality of cut-flowers

Abstract

This study deals with the flowering behavior of dahlia plants under various day-lengths using the variety“Futarishizuka”from cuttings. Particularly, the effects of day-length at the early stage of shoot growth on the flowering time and the quality of cut-flowers were examined. After decapitation, the plants were divided into two groups. One of them was grown under the photoperiod of 12 hours (SD), and the other was grown under 14-hour days (LD) during the early period of lateral shoot growth. At intervals of three or five days, the programmed lots of plants were moved from SD to LD, and from LD to SD respectively. The results obtained are as follows:
1. Flower primordia initiated simultaneously under, both photoperiods.
2. The longer the period of SD, the earlier the flowering time was. LD treatment for less than 20 days did not affect the flowering time, but under LD treatment for more than 20 days after cutting back, flowering time was delayed.
3. SD treatment given at the early stage of shoot growth decreased the shoot length, shoot weight, total number of florets and number of ray florets. On the contrary, the number of disk florets was increased by SD treatment.
4. The proportion in number of ray florets to total florets and that of double flowers to total flowers were clearly decreased by SD treatment given for only five days. As the duration of SD increased, these proportions decreased further. LD treatment for less than 17 days had no effect on these proportions. However, LD treatment for 20 days or more increased the percentage of both ray florets and double flowers.
5. When dahlia plants are exposed to SD at the early stage of growth, even for a few days, the physiological changes occur in the plants producing more disk florets. These physiological changes are durable under LD conditions which followed SD treatment.