Recently, evidence has accunmulated to indicate that the photoperiodic control-of floral induction is mediated by an endogenous rhythm with periods of approximately 24 hours (9, 10, 11, 12, 13, 18, 19, 20, 21, 22). It has long been known that the floral response of Biloxi soybean varies with the length of the photoperiod (4, 5, 6, 7, 18). The optimum response with 24-hour treatment cycles is obtained with photoperiods between 6 and 10 hours and normally no flowering is produced with photoperiods longer than 14 or perhaps 14½2 hours (1, 28). Hamner (18), however, has shown that if a dark period of 16 hours is given in each cycle after photoperiods of different lengths, flowering is obtained with photoperiods as long as 16 and 18 hours. In those experiments, however, the cycle duration was varied with the different lengths of photoperiods given. Since Nanda and Hamner (23) have demonstrated a periodic variation in floral response as a function of cycle durations, it was difficult to predict what results might be obtained with various photoperiods given in long cycles of standard duration. In the following experiments a 72-hour, or tridiurnal, cycle was used to further assess the effects of light breaks and differential photoperiod durations on photoperiodic and rhythmic flowering responses. Materials and Methods Biloxi soybean (Glycine miiax L. Merr.) seed, obtained from Dr. H. A. Borthwick (United States Department of Agriculture, Beltsville, Md.), were planted in 4 inch unglazed, earthen pots in the Plant Physiology greenhouse at the University of California, Los Angeles (UCLA). All experiments except experiment II were performed between February and August 1962 at UCLA. Experiment II was conducted between June and August of 1963 under '
