Studies of the Involvement of an Endogenous Rhythm in the Photoperiodic Response of Hyoscyamus niger.

An attempt was made to determine the involvement of an endogenous circadian rhythm in the flowering response of the long-day plant Hyoscyamus niger L. grown in a modified White's medium. Both variable-cycle-length and light interruption experiments were employed in this attempt. In the variable-cycle experiments, plants were subjected to light periods of 6, 12, or 18 hours followed by varying lengths of darkness. The total lengths of the cycles varied from 12 to 72 hours. In experiments utilizing a 6-hour photoperiod, a high level of flowering occurred in cycle lengths of 12, 36, and 60 hours. Flowering was suppressed in the 24-, 48-, and 72-hour cycles. When a 12-hour photoperiod was used the flowering response was low between 24 and 36 hours and flowering did not indicate a rhythmic response. When an 18-hour photoperiod was used, the flowering response was suppressed in the 36- and 60-hour cycles.Light-break experiments were conducted to study further the flowering response in Hyoscyamus. These experiments consisted of a 6-hour main photoperiod followed by varying lengths of darkness to make cycles of 24, 48, and 72 hours. At given intervals the dark period was interrupted by 2-hour light breaks. In a 24-hour cycle, flowering was promoted when a light break was given at either the twelfth or eighteenth hour of the cycle. In a 48-hour cycle, flowering was strongly promoted by light breaks given near the beginning or at the end of the dark period. In a 72-hour cycle, light breaks given at the eighteenth, forty-second, and sixty-sixth hour of the cycle stimulated flowering as compared with light breaks given at the thirtieth and fifty-fourth hour. These results are indicative of the involvement of an endogenous rhythm in the flowering response of Hyoscyamus niger.