SPECTRAL SENSITIVITY OF FLOWERING AND FT-LIKE GENE EXPRESSION IN RESPONSE TO A NIGHT BREAK TREATMENT IN THE CHRYSANTHEMUM CULTIVAR 'REAGAN'

A night break, a short exposure to light near the middle of the night period, inhibits flowering in chrysanthemum (Chrysanthemum morifolium Ramat.), a short-day plant. We studied the effect of light quality (wavelength) of night break on flowering and on expression of the FLOWERING LOCUS T (FT)-like gene CmFTL3. Night break treatment with wavelengths from yellow to red showed strong inhibitory effects on flowering. Further study using monochromatic light from LED panels showed that the maximum effectiveness for the inhibition of flowering was near 596 nm, not 660 nm, and that levels of CmFTL3 mRNA were reduced in a pattern consistent with the observed inhibition of flowering. Wavelength regions from ultraviolet-A to blue and far-red light had no inhibitory effect on flowering. Our results also showed that the inhibitory effect of red light on flowering responses could be reversed by far-red light and confirmed that red/far-red photoreversibility occurs in chrysanthemum. These results suggest that phytochromes are involved in night break responses, and the absorption spectrum of leaf extracts suggests that the effect of wavelength is distorted by the screening effect of other plant pigments found in green leaves.