Role of Phytochrome in Control of Flowering of Chrysanthemum

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 fluorescent lamps is about equally effective in 15-minute cycles, but the effectiveness of ruby-red light fails in 30-minute cycles and of incandescent-filament in 60-minute cycles, whereas that of fluorescent light remains high in longer cycles. 4. In chrysanthemum the time required for the flower-inhibiting reactions catalyzed by Pfr is greater than the dark reversion time of Pfr to an inactive form. This fact accounts for the need of prolonged periods each night of continuous or intermittent light, a condition also encountered in control of flowering in certain long-day plants, regulation of growth in woody species, and positioning of the chloroplast in the alga Mesotaenium.