Role of Phytochrome in Control of Flowering of Chrysanthemum

1. Flowering of chrysanthemum, Chrysanthemum morifolium Ramat., is prevented by dark-period interruptions of 4 hours, which convert phytochrome to the flower-inhibiting far-red-absorbing form (Pfr) and maintain it in this form throughout the light period. 2. Pfr is also maintained at an effective level by intermittent lighting provided the dark periods between successive periods of light are so short that dark reversion of Pfr to an ineffective level does not occur. If the light is from incandescent-filament lamps, the dark periods between successive light breaks must not greatly exceed 30 minutes. Cycles of light and dark from 30 minutes to 1 minute are equally effective. 3. Effective cycle length depends on the ratio of red to far-red energies in the light. As the relative amount of red increases, the amount of Pfr formed and the time required for its reversion in darkness to a level ineffective for flower inhibition also increase. For these reasons light from ruby-red, incandescent-filament, and fluore...