Effect of Cold Exposure on Brix Value of Borecole Leaves Grown in an Unheated Greenhouse in Hokkaido, Subarctic Region

Abstract

Borecole (curly kale) was cultivated without heating during autumn and winter in a multi-film-covered greenhouse in Hokkaido, Japan. Borecole survived even when the greenhouse air temperature dropped to −6.2°C with an accompanying drastic increase in Brix value. This rapid increase of Brix value was observed under conditions where the minimum air temperature in the greenhouse was less than 0°C. Cold temperatures caused the plant to freeze, inducing extracellular freezing. Although the Brix value increased at low temperatures irrespective of leaf order, the Brix values for leaves of lower order, at the top of the plant, were much larger than the higher order leaves, closer to the base of the plant. Leaf Brix values started to increase when the average minimum temperature was 1.3°C or less for 10 days before harvest. Moreover, the nitrate concentration, reported to correlate with bitterness intensity, decreased at low temperatures. These results revealed that leaves with reduced bitterness and improved sweetness can be produced in low temperatures from the upper portions of borecole plants.