Abstract
Flowering of morning glory (Pharbitis nil Choisy) occurs during the summer season in Japan. This species is well-known as a typical short-day plant. It is, however, not clear why morning glory flowers in summer season under long-day conditions. To answer this question, we analyzed the seasonal changes in the spectral characteristics of solar radiation and found that the ratio of blue component to photosynthetically active radiation increased in summer and decreased in winter, whereas the reverse held for green, and that of red did not undergo much changes throughout the year. In the present study, we investigated the effect of light quality on flower-bud formation in morning glory (cv. Murasaki), using monochromatic lights. Flowerbud formation occurred much earlier under blue light than either green or red light. It was accelerated by increasing the irradiance of blue light and the ratio of blue light to green light. We suggest that the seasonal changes in spectral composition of solar radiation, particularly the rate of blue component, can be one of the factors in controlling the flower-bud formation of morning glory.
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