Interaction of Photoperiod & Vernalization in Flowering of Sweet Clover (Melilotus)

Melilotus albus (alba) Desr. and M. officinalis (L.) Lam. are normally biennials flowering only in the 2nd year of growth. Under natural conditions the seeds germinate in the spring and the plants grow slowly during the first season, reaching a height of 1 to 2 feet. An enlarged storage root with a number of adventitious buds is formed in the fall and lives through the winter. In the second spring the plant grows rapidly to a height of 3 to 6 feet, flowers, produces seed, and dies (8). Since M. albus var. annuus reproduces as an annual plant, it becomes of interest to determine why the biennial forms require two seasons to mature. The late June or early July flowering of the biennials indicates a long-day response. The failure of these plants to flower the 1st year could be either an age or a size factor-the plants are only a few inches high in June of the 1st year-or it could be due to a lack of vernalization (12, 4). Smith (11) obtained flowering of biennial sweetclovers grown with 17-hour photoperiods within three months after seeding. Wiggans (14) found that the annual form of M. albus flowered on photoperiods of 12 to 20 hours, but the biennial form required 17 to 18 hours to flower as an annual. Since the biennial varieties flower in the second year in the field with a maximum daylength of 15 to 16 hours, a photoperiod-size or a photoperiod-temperature interaction is indicated. The present study was planned to clarify these responses.