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Hyeon-Hye Kim, Changhoo Chun, Toyoki Kozai and Junya Fuse

Spinach (Spinacia oleracea L.) was chosen to demonstrate that value-added transplant can be relatively easily produced under artificial light in a closed system. Transplant production under artificial light was divided into three periods, and the photoperiod during each period was varied. It was found that the rate of floral development could be controlled by photoperiod treatments, although floral initiation itself could not be manipulated. Short photoperiod treatments retarded floral development and stem elongation. This occurred even when the transplants were transferred for transplanting to natural conditions with long days and high temperatures. In conclusion, by providing the short photoperiod during the transplant production process, marketable plants with negligible bolting can be produced under natural long-day conditions. Moreover, the production cost per transplant in summer could be reduced by using a combination of natural and artificial lighting during the transplant production process. These results open the possibility to produce value-added transplants of different species under artificial lighting conditions and control their floral development and/or stem elongation for a timely and profitable harvest.

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Hyeon-Hye Kim, Changhoo Chun, Toyoki Kozai and Junya Fuse

Spinach (Spinacia oleracea L.) was chosen to demonstrate that the respective vegetative or reproductive conditions of transplants can be controlled in their early stages of development under artificial light in a closed system. Transplant production under artificial light was divided into three growth phases and the photoperiod during each of these phases was varied. The rate of floral development was controlled by photoperiod, but floral initiation itself was not affected. Short photoperiod treatments (8 or 12 hours/day) retarded floral development and stem elongation (bolting). This delay continued even after the transplants were transferred to natural long-day (15.5 hours/day on average) conditions with high temperatures (17 and 37 °C minimum and maximum). We concluded that by using short photoperiods during transplant production, marketable plants with reduced bolting could be produced under natural long-day conditions. In Japan, spinach with this rosetting capacity would be of greater value. Further, this concept opens the possibility of producing better quality transplants of several species under artificial lighting conditions of appropriate length, and thereby controlling their floral development and/or bolting.

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Changhoo Chun, Ayumi Watanabe, Toyoki Kozai, Hyeon-Hye Kim and Junya Fuse

Spinach (Spinacia oleracea L. cv. Dimple) was chosen to determine whether bolting (i.e., elongation of flower stalks) could be controlled by manipulating the photoperiod during transplant production in a closed system using artificial light. Plants grown under various photoperiods during transplant production were transferred and cultured under natural short photoperiods and artificial long photoperiods. Vegetative growth at transplanting tended to be greater with the longer photoperiod because of the increased integrated photosynthetic photon flux. Bolting initiation reacted qualitatively to a long photoperiod, and the critical photoperiod for bolting initiation was longer than 13 h and shorter than 15 h. The plants grown under a longer photoperiod during transplant production had longer flower stalks at harvest. The long photoperiod and/or high temperature after transplanting therefore promoted flower stalk elongation. Growing plants under a photoperiod that was shorter than the critical photoperiod during transplant production reduced elongation of the flower stalks, thus there was no loss of market value even though the plants were cultured under a long photoperiod and high temperature for 2 weeks after transplanting.