Several studies have reported that the nutritional quality of spinach is enhanced by application of cold stress to the root area. However, the relationship of the nutritional value of spinach with the root area temperature and duration of root area chilling has not been investigated. Here, we examine the ascorbic acid, nitrate ion, and soluble solid content of spinach subjected to root area chilling in a nutrient solution for 2, 4, 5, 6, and 7 days at various temperatures (4, 6, 10, and 14°C). Under all temperature conditions, the nitrate ion concentration significantly decreased within 2 days of the onset of chilling. Ascorbic acid and soluble solid content showed similar trends
significant change in concentrations of both the compounds occurred after 4 days at 4°C, 5 days at 6°C, 6 days at 10°C, and 7 days at 14°C. On the basis of these results, we developed the relational expression for the chilling duration and solution temperature. The results of this study indicate that adjusting the chilling duration and solution temperature according to the relational expression would help plant factories produce high value-added spinach.