Day-extension Blue Light Inhibits Flowering of Chrysanthemum When the Short Main Photoperiod Includes Far-red Light

Chrysanthemum (Chrysanthemum ×morifolium) is a common ornamental crop with a qualitative short-day flowering response. Extending a short day with moderate blue [B (400–500 nm)] light inhibits flowering in greenhouse conditions with sunlight but does not indoors (without sunlight) under B + red [R (600–700 nm)] light or white light. We postulated that the contrasting responses to B light as a day extension depended on far-red [FR (700–800 nm)] light during the day, which is plentiful under sunlight but lacking indoors under B+R or white light-emitting diodes. To study this response in three chrysanthemum cultivars, we delivered indoor lighting treatments at two locations with an 11-hour main photoperiod of B, green [G (500–600 nm)], R, and FR light, where subscript values indicate the photon flux density (in µmol·m−2·s−1) of each waveband: B60R120, B60G60R60, and B60R60FR60. After each short main photoperiod, plants received 0 or 4 hours of day-extension lighting of 60 µmol·m−2·s−1 of B light (B60). Under all treatments except B60R60FR60 with day-extension B60, it took ‘Chelsey Pink’, ‘Gigi Gold’, and ‘Gigi Yellow’ 13 to 17 days to reach the first visible inflorescence and 42 to 51 days to the first open flower. In contrast, plants grown under B60R60FR60 with day-extension B60 took 41 to 67 days to reach the first visible inflorescence with few plants developing open flowers. Plants were tallest at the first open flower and after 9 weeks of treatments when grown under B60R60FR60 with day-extension B60. These results indicate that the inclusion of FR light, but not G light, in the main photoperiod is necessary for day-extension B light to inhibit flowering in chrysanthemum. On the basis of these results and those of other studies, we postulate that the spectral dependence of flowering in chrysanthemum depends on whether and how the phytochrome photoequilibrium changes during the day. In particular, a sufficiently high daytime phytochrome photoequilibrium (e.g., under B+R and B+G+R light) could establish a predominant mode of floral signaling that prevents perception of subsequent B light as a long day.