Abstract
Some experiments have been carried out for the purpose of establishing the day-length treatment for cut flower production of dahlia plants during winter season. This report deals with the optimum day length and minimum intensity of supplementary light for flowering of dahlia.
Two varieties (Akane and Futarishizuka) bred in Japan were selected for the materials. Cuttings were made using the lateral shoots of plants grown under outdoor conditions in autumn or spring. The top of cutting was cut off at the lowest node to induce the new lateral shoots after rooting. Then, plants were shifted for day-length treatment. The results were as follows:
1. The day-length after the middle of September was not preferable for growth and flower formation of dahlia.
2. The growth of shoot was inhibited under the day-length shorter than 12 hours, while it was enhanced under that longer than 13 hours.
3. Blind flowers increased under the day-length shorter than 12 hours, while normal flowers were yielded under that longer than 13 hours. However, flowering delayed as the day-length became longer.
4. The total number of florets or the number of ray flowers increased with increasing day-length, while the number of disc flowers decreased to the contrary.
5. The optimum day-length was 13 hours for the variety Akane which hardly exposed the disc, and 14 hours for the variety Futarishizuka which readily exposed the disc.
6. Minimum light intensity for artificial irradiation in the long day treatment was 20 to 36lux.
7. Dahlia seems to be an indefinite short day plant according to the results of this study.
8. However, the day-length longer than a certain level is necessary for growth and flower formation. The limiting day length may be laid down about 12 hours, and growth or flower formation is suppressed under this limit.
