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.

It has been proposed that cold acclimatization of the plant body enhances the nutritional value of leaf vegetables; however, little is known about the optimal duration of cooling required to enhance the quality of factory-processed vegetables. The ideal low temperature acclimatization period increases ascorbic acid and sugar content without increasing the nitrate ion concentration for a given root area. Here, we investigated the ascorbic acid and sugar contents in spinach with a given root area subject to chilling for different durations; 2, 4, 5, 6, and 7 days. Ascorbic acid and sugar content levels remained stable at the beginning of chilling; however, after 6 days, the levels of both parameters doubled compared to those in the control (no chilling). In comparison, nitrate ion concentration gradually decreased with increasing acclimatization period. Spinach plants acclimatized for 7 days showed similar results as those acclimatized for 6 days. Our results show that the optimal duration for root area chilling to produce high quality spinach is 6 days, when controlling the temperature of the nutrient solution under plant factory conditions.

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