Studies on a Forage Crop, Silphium perfoliatum L. : IV. The effects of the day length on the growth and flowering of Silphium perfoliatum L.

With an aim to clarify the relationships of morphological and physiological characters of Silphium to day length, the experiments were conducted at the Akita Agricultural College during three years from 1974 to 1976. The results obtained were summarised as follows: 1. In the middle of August, the plant hight, leaf area, top-weight and root length under natural day length showed no significant difference from those under long day length with a few exceptions. These exceptions were (1) the values of the leaf area and the top-weight in the seeding year under natural day length were higher than those of plants grown under long day length, and (2) the values of the weight of underground parts in the third year after seeding under long day length were higher than that of plants grown under natural day length. Under short day treatment, the values of these characters were significantly small. However, in the middle of November, these characters showed higher values under long day treatment than under the natural day length. 2. As to the numbers of leaves and stems and the thickness of stem in the middle of August, there was no difference among the treatments of day length, but in the middle of November, the numbers of leaves and stems under long day length exceeded them under natural day length. 3. Elongation of the basal internode of stems did not occur in the year of establishment, which was almost same with the internodes of plant grown under eleven hour day length in the second and third year. The elongation of internode of plants grown under the long day length and the natural day length was found in the second and third year. And, it is assumed that the elongation of basal internode will necessitate fundamentally at least fifteen or sixteen leaves. 4. The long day length over natural day length appeared not to be effective to the bud formation, and the short day length of eleven hours was found to suppress bud formation almost completely. Also, the flowering of bud did not occur under the short day length in general. Plants grown under the long day length and natural day length treatments reached to flower at the same period. From these, Silphium was regarded as a quantitative long day plant. 5 Any difference of T/R ratio could not be found between natural day length and long day length and long day length treatments in the middle of August, but the ratio in the short day treatments was clearly lower than that of the former two. As the seedling stage was progressed, T/R ratio became lower. On the other hand, in the middle of November, T/R ratio in long day length was higher than in natural day length. In consequence, it was supposed that such vigorous growth of underground parts under long day length will result in the desirable top growth in the following spring. 6. The long day treatment gave no special effect on the seed production. On the contrary, short day treatment was found to suppress greatly its production.