Studies on the nature of the endogenous rhythm affecting photoperiodic response of Biloxi soybean

Abstract

1. Biloxi soybean (Glycine max [L.] Merr.) plants were subjected to seven photoinductive cycles which varied in length from 24 to 96 hours. In experiment 1 the length of both the photoperiod and the dark period were varied. Plants were exposed to photoperiods of 8, 16, or 32 hours' duration. While the 8-hour photoperiod was given at both 27° and 12°C., the latter two were supplied at the lower temperature only. In experiment 2 the plants received a photoperiod of constant duration (8 hours at 27°C.), but the dark periods varied in length from 8 to 64 hours. In additional groups each dark period was interrupted by 30 minutes of high-intensity light, either at its mid-point or 16 hours after its beginning. 2. Flower-bud initiation showed a rhythmical pattern in the photoperiodic reaction. With an 8-hour photoperiod at high temperature (27°C.) the maxima were shown with cycle lengths 24, 48, and 72 hours long, and the minima with cycle lengths of 36 and 60 hours. 3. Low temperature during the 8-hour photoperiod depressed flowering to a very great extent and also appeared to cause a slight shift in the flowering rhythm related to cycle length. This depressing effect was ascribed to the low production of flowering stimulus C as a result of insufficient production of the high-intensity-light product A during the cold photoperiod. 4. A 16-hour photoperiod at low temperature also resulted in a rhythmical pattern in flowering related to total cycle length. The rhythm in this case, however, appears to have shifted by 12 hours, so that the maxima in flowering were shown with cycle lengths of 36 and 60 hours instead of at 24, 48, and 72 hours as in the plants exposed to a warm 8-hour photoperiod. 5. Flower initiation occurred even in plants exposed to 32-hour photoperiods when the photoperiod temperature was low (12°C.). The flowering rhythm related to cycle length appeared to correspond with that of the groups exposed to warm 8-hour photoperiods. A maximum at 48-hour cycle length was, however, not shown. 6. An interruption of the dark period by high-intensity light for 30 minutes affected the flowering response considerably. It appears that the flowering response in such cases is mainly determined by the second of the two subcycles into which the cycle length is divided by the light interruption. These results support those obtained by Lockhart and Hamner with Xanthium in which a second dark period following a brief exposure to light partially nullified the effect of an inductive dark period.