Effect of Heat Insulation Management on Growth and Quality of Borecole under Unheated Plastic Greenhouse Cultivation during Winter in Hokkaido, a Subarctic Region of Japan

Hokkaido is located in a subarctic area of Japan, where attempts have been made to produce leafy vegetables with high Brix values in winter under cold-stress conditions. Kale has recently become popular due to its high nutritional value. ‘Winterbor’, a cold-tolerant cultivar of borecole, a type of kale, grows without any cold stress damage in unheated plastic greenhouses at temperatures as low as −7°C. In this study, it was found that the growth rate of ‘Winterbor’ in winter was less than that of the collard kale cultivar ‘Hi-Crop’ and had a higher dry matter ratio and Brix value. The air temperature within the greenhouse was lowered by approximately 2°C by removing some of the insulation layer on the greenhouse under the winter cultivation. As a result, neither leaf emergence nor overall plant weight was affected. As the air temperature continued to fall with the approach of winter, the dry matter ratio, Brix value, and sucrose content increased in ‘Winterbor’ leaves. The dry matter ratio, Brix value, and combined content of fructose, glucose, and sucrose tended to be higher under cold management (less insulation) than under standard management. Statistical analysis based on the air temperature conditions during the cultivation period indicated that leaf emergence and plant weight reached constant values at approximately 5°C and 3°C daily mean air temperature, respectively. However, the dry matter ratio, Brix value and total soluble sugar content (three soluble sugars) increased by 1.1%, 1.1, and 7.3 mg·g−1 fresh weight (FW) with every 1°C drop from the average daily minimum air temperature for 10 days before harvest, starting at approximately 6°C. These results suggest the possibility of estimating and controlling borecole plant growth and quality parameters and could contribute to the systematic production and distribution of highly valuable borecoles grown in unheated plastic greenhouses in winter in Hokkaido.