Study on efficient lighting culture techniques for cut chrysanthemum production based on photoperiodism

Chrysanthemum is one of the most important floricultural crops in Japan. Chrysanthemum is a typical short-day plant. Year-around cut chrysanthemum production has been established by use of lighting (night break lighting) technique with two groups of chrysanthemum genotypes having different flowering characteristics in Japan. Flowering control by lighting is a key technique in year-around cut chrysanthemum production. Recently, lighting by using LED as a new light resource have been developed. Further development of efficient lighting culture techniques based on photoperiodism is required to achieve stable and efficient cut chrysanthemum flower production by use of new light source. The aims of this study are to reveal, 1. effect of light quality on floral inhibition of chrysanthemum, 2. relationship between lighting period and light irradiance intensity on floral inhibition of chrysanthemum, 3. effect of lighting treatment at different times on floral inhibition of chrysanthemum, 4. relationship between critical day length and effective lighting time 5. effect of lighting interruption on floral inhibition of chrysanthemum. 1. Effect of light quality on floral inhibition of chrysanthemum Effects of the wavelength of night-break light on the inhibition of flower bud differentiation in chrysanthemum were investigated using the cultivars 'Arajin2' and 'Iwanohakusen'. Wavelengths of 516, 593, 630, 663, 684, and 705 nm of light were produced using LED lamps. Effective wavelengths for floral inhibition in both 'Arajin2' and 'Iwanohakusen' were 593, 630, and 663 nm, and the most effective wavelength was 630 nm in 'Arajin2' and 593 and 630 nm in 'Iwanohakusen', respectively. The effectiveness of red light or a mixture of red and far-red light as a night-break light was compared using 'Arajin2' and 'Iwanohakusen'. No difference in the inhibition of flower bud differentiation was observed in 'Arajin2', whereas the mixed light of red light and far-red light was more effective than red light to inhibit flower bud differentiation of 'Iwanohakusen'. 2. Relationship between lighting period and light irradiance intensity on floral inhibition of chrysanthemum The relation between the night-break-lighting period and light irradiance intensity to inhibit flower initiation in chrysanthemum was investigated using 'Iwanohakusen' and 'Jinba', major summer-to-autumn and autumn flowering cultivars in Japan, respectively. Plants were subjected to night-break with different light irradiance intensities for 20 to 60 or 70 days from planting. The required light irradiance intensity to inhibit flower initiation was evaluated by the number of nodes on the main stem at flowering. The required light irradiance intensity increased as the night-break-lighting period became longer in both cultivars. In 'Iwanohakusen', flowering could not be prevented by night-break when the lighting period reached 60 days. On the other hand, flower initiation in 'Jinba' was inhibited successfully by night-break even though the lighting period reached 70 days. These results indicate that the light irradiance intensity of night-break should be higher when the lighting period is longer than a threshold, depending on cultivars. 3. Effect of lighting treatment at different times on floral inhibition of chrysanthemum The effects of night-break treatment at different times on flower bud initiation in chrysanthemum were investigated using 'Jinba' and 'Iwanohakusen'. These two genotypes are major autumn and summer-to-autumn flowering cultivars in Japan, with different critical day lengths. The most sensitive timing of NB treatment was not the middle of the night, but the late time zone of the night in 'Iwanohakusen'. When the dark period started earlier on using shade materials, the timing of the maximum sensitivity to NB treatment became earlier. The relationship between the day-length (12, 10, and 8 h) and timing of NB treatment in the flower initiation response of the autumn chrysanthemum cultivar 'Jinba' was investigated. The day length was longer as the timing of maximum sensitivity to NB treatment shifted to the late time zone of the night. The results suggest that the timing of light sensitivity in the NB response relates to the beginning of the dark period. In 'Jinba', the most sensitive timing of NB appeared about 9-10 h after the beginning of the dark period. In comparison with autumn flowering and summer-to-autumn flowering chrysanthemums, summer-to-autumn flowering genotypes showed a tendency whereby the most sensitive timing of NB treatment appeared at an earlier time from the beginning of the dark period. The results suggest the relationship between the critical day length and most sensitive time zone of the dark period. 4. Relationship between critical day length and effective lighting time The relationship between the critical day length and the most effective NB time on the depression of flower bud initiation in chrysanthemum was investigated using five cultivars of summer-autumn (SA) flowering types : 'Floral Yuka', 'Iwanohakusen', 'Southern Grape', 'Southern Chelsea', and 'Southern Pegasus', and five of autumn (A) flowering types : 'Jinba', 'Sanyo Ougon', 'Yukihime', . Hakusui', and 'Shuhouno-chikara', in Japan. The critical day length (night length) of the SA flowering types was found to be 15 (9)-16 (8) h during the August flowering and cropping period. The duration from dusk to NBmax, which is the most effective NB time, was 6.5-8.5 h when one hour of NB was included. On the other hand, the critical day length (night length) of the A cultivars was 13 (11) h during the December flowering and cropping period. The duration from dusk to NBmax was 9.0-10.5 h when 40 min of NB was included. Similarly, the SA cultivars with a short critical dark length had a shorter dusk-NBmax period compared to the A cultivars with a similar critical dark length and dusk-NBmax period. The results suggest that the dusk-NBmax duration is associated with the critical night length. The present study demonstrates that the critical day length (night length) is closely associated with the dusk-NBmax duration, thereby suggesting that estimation of the most effective NB time for each cultivar is possible if the critical day length is ascertained. 5. Effect of lighting interruption on floral inhibition of chrysanthemum Unexpected interruption of NB causes serious deterioration of the quality of cut chrysanthemum flowers. The effects of the timing and duration of NB interruption on flower bud formation were investigated using autumn and summer-autumn flowering chrysanthemum genotypes in Japan. In the autumn flowering genotype of 'Arajin2' flowering in December, more than three and four days of interruption of NB influenced flower bud formation when the interruption occurred 22 and 36 days after planting, respectively. On the other hand, no influence was observed of one to five days of interruption in those flowering in March. In the summer-autumn genotype of 'Iwanohakusen', flower bud formation was influenced by more than four days of interruption in July flowering, three days of interruption in August flowering, and more than two days of interruption in September flowering. To reveal genotype differences in the effects of the duration of NB interruption and the plant growth stages subjected to the interruption, three autumn flowering genotypes and two summer-autumn flowering genotypes were compared. Sensitivity to the NB interruption was genotype-dependent : the sensitive genotype showed a reduced leaf number flowering only a day of interruption, but the insensitive genotype showed no effect of three days of interruption. The sensitivity increased with plant growth. The present study demonstrated that the effect of NB interruption was changed by environmental factors such as the temperature, growing stage and juvenility of plants, and the cropping type.