Cold stress modifies bioactive compounds of kale cultivars during fall–winter harvests

Kale is a plant known and valued since antiquity as a healthy vegetable crop, used for culinary, decorative, but also healing purposes. The aim of the study was to examine the effect of harvest date on physiological status and nutritional composition of two kale cultivars: ‘Winterbor’ F 1 (blue-green leaves) and ‘Redbor’ F 1 (red-purple leaves). The leaves were harvested in three periods: before frost (>0°C), after medium (−5.0°C) and heavy frost (−15.0°C). Content of dry weight, soluble sugars, l-ascorbic acid, carotenoids, chlorophylls, polyphenols, anthocyanins, as well as antioxidant activity and peroxidase activity were determined. Cold temperature significantly affected bioactive compounds of kale. The content of dry weight, soluble sugars, l-ascorbic acid, phenolics, and antioxidant activity increased after medium frosts for both cultivars. The level of anthocyanins also increased significantly for the ‘Redbor’ F 1 cultivar. After strong frost, most of the tested parameters (content of dry weight, soluble sugars, phenolics, anthocyanins, and total antioxidant and peroxidase activity) significantly increased. The chlorophyll a content was reduced by heavy frost in both seasons. Harvesting kale before and after frost may allow the level of biologically active ingredients to be regulated as cold also significantly affects the physiological status of the plants.