A simplified temperature control system for the unit-type multistage transplant culture system was developed and applied to tomato transplant culture using a small-sized refrigeration chamber. The transplant culture unit was installed into the chamber maintained at approximately 15°C. Air temperature inside of the culture unit rose to a higher temperature than the outside during the photoperiod. When the temperature inside of the culture unit reached a set point of 25°C, ventilation fans located above the transplants were turned on with a temperature controller, thereby decreasing the temperature inside the culture unit. The fans were turned off when that ventilation reduced the temperature inside of the culture unit below a predetermined temperature. In this manner, the culture unit temperature was controlled between 24.5°C and 26.5°C during the photoperiod even though the outside temperature fluctuated between 13.5°C and 17.5°C. After culture of 16 d, height of the tomato transplants in winter season culture were significantly more developed than in summer season culture, despite use of the semi-closed chamber of the equal temperature setting and artificial lights for both cultures. We infer that such growth resulted from the higher relative humidity in the winter culture than in the summer culture by seasonal variation of cooling load and refrigerator ability. The relative humidities inside of the culture unit in the summer and winter season cultures were 40% and 60%, respectively.
