Effects of Twilight Irradiance on Photoperiodic Floral Induction in Autumn-Flowering Chrysanthemum

Abstract

Using an artificial light source, the effect of natural twilight irradiance intensity on the photoperiodic responses of autumn-flowering chrysanthemum was examined. When exposed to light from a red fluorescent lamp, four cultivars of chrysanthemum exhibited a higher inhibitory effect on flowering in the morning than in the evening under the same irradiance. When exposed to light from a red (R) light-emitting diode (LED) lamp, the threshold irradiance of the chrysanthemum cultivar ‘Jimba’ was less than 10 mW・m^–2 in the morning and approximately 70 mW・m^–2 in the evening. The effects of R and R + far red (FR) light were also investigated in ‘Jimba’. There was a difference in the morning treatment; however, there was no difference in the evening treatment. It was considered that the FR light did not influence the threshold of R light. In ‘Jimba’, the threshold irradiance for the photoperiodic flowering response increased with the age of the plant. The natural twilight irradiance was measured for 102 d in Ibusuki City. The results revealed that the sunrise and sunset light intensities changed for about 20 min each morning and evening owing to the influence of the terrain and weather. Using the threshold irradiance for the photoperiodic response with R light, we estimated the illuminance in the morning and evening in the dark period of ‘Jimba’. The results revealed that less than 8 lx in the morning and 54 lx in the evening are the light and dark thresholds for photoperiodic flowering. However, this estimate may vary depending on the weather factors and test environment conditions (place, cropping type, and growing environment).