Harvest strategies to maximize the annual production of bioactive compounds, glucosinolates, and total antioxidant activities of kale in plant factories

Since kale (Brassica oleracea L. var. acephala) is one of the healthiest vegetables, its cultivation is increasing for either fresh consumption or as a source for functional foods and nutraceuticals. Plant factories are able to control the environment and trigger the accumulation of bioactive compounds with a stable supply by systematic cultivation methods. The objectives of this study were to evaluate the changes in the total phenolic compounds (TPCs), total flavonoid compounds (TFCs), glucosinolates (GLSs), and antioxidant capacity of kale in a plant factory and to determine an optimal harvest time for the maximum annual production. Two cultivars, namely ‘Manchoo collard’ and ‘Jangsoo collard’, were cultivated in a plant factory and thinned to avoid mutual shading. Both cultivars were harvested every week from 14 to 49 days after transplanting (DAT). The fresh weight (FW), dry weight (DW), projected leaf area (PLA), TPCs, TFCs, GLSs, and antioxidant capacity of both plants were measured every week. The annual production was calculated as follows: DW × the concentration × planting density × cultivation cycles per year. The optimal harvest time was determined based on the continuous phase of the production by modeling. The FW and DW of both cultivars exponentially increased, but the PLA hardly increased at 35 DAT. The TPCs, TFCs, and antioxidant capacity fluctuated or slightly changed, but the amount of substance per plant gradually increased. Their annual production increased with increasing harvest time, and only the production of TPCs in ‘Manchoo collard’ showed a local maximum when harvested at 35–42 DAT. Glucoiberin, sinigrin, and glucobrassicin were the major components of GLSs in both cultivars, and their contents fluctuated. The concentration of total GLSs was the highest at 42 DAT. Additionally, the annual production of the total and major GLSs showed the same results as the TPCs, TFCs, and antioxidant capacity. From the results, the optimum harvest time for production was determined to be 42 DAT.