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Role of Low Temperature Exposure in Floral Development of Red Clover (Trifolium pratense L.) Ecotypes¹

Floral stem development of ‘Pennscott’ and ‘Tanmaisto’ red clover, Trifolium pratense L, was compared when plants were exposed to different combinations of photoperiods and low temperature exposures. Over 90% of the Pennscott plants developed floral stems within 5 to 6 weeks after seeding when grown under 30C and a 16-hour photoperiod. Lengthening the photoperiod to 20 hours reduced this time to 4 weeks. Flowering of 90% of the plants of Tammisto was not observed until 9 weeks after seeding, and lengthening the photoperiod from 16 to 20 hours did not materially alter the cumulative percentage of floral plants. Exposure for 2 weeks at 9 or 10C had a stimulatory effect on the floral development of both varieties. The most pronounced stimulation was observed in the Tammisto plants grown out at a 20-hour photoperiod. Under these conditions the 2C exposure was somewhat more effective in stimulating floral stem development in the Tammisto plants than the 10C exposure,

Key Words: Photoperiodism • Low temperature equirement

² Research Agronomist, Crops Research Division, ARS, USDA, Lafayette, Ind. 47907.

Received for publication August 2, 1968.