Biomass production, nutritional quality, and phytochemical properties of hydroponic-grown kale (Brassica napus) and collard (Brassica oleracea) under different electrical conductivities of the nutrient solution

Authors: YANG, TENG - Kansas State University; SAMARAKOON, UTTARA - The Ohio State University; Altland, James; LING, PETER - The Ohio State University

Submitted to: Agronomy
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/13/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: Controlled environment agriculture (CEA) is an advanced and efficient method of producing leafy greens, including kale and collard. Despite the economic significance of CEA grown produce, there is a notable absence of scientific studies addressing cultural practices influencing their commercial production. Moreover, many hydroponic growers follow the recommendations for lettuce (Lactuca sativa) in kale and collard production, potentially constraining the yield and quality. EC has been used as a standard indicator of hydroponic nutrient solution concentration due to the ability of soluble salts in the solution to conduct electricity. Thus, the objective of current research was to determine the optimum electrical conductivity (EC) levels for high yield and quality hydroponic-grown kale and collard. Kale ‘Red Russian’ and Collard ‘Flash F1’ were grown for 4 weeks after transplanting under a double polyethylene-plastic covered greenhouse using a nutrient film technique (NFT) system with 18 channels. Kale and collard were alternately grown in each channel at four different electrical conductivity (EC) levels (1.2, 1.5, 1.8, and 2.1 mS·cm-1). It was observed that collard had the best growth performance with higher growth rate, nutritional and phytochemical contents under EC 1.8, while kale showed an increased growth performance with rising electrical conductivity. Mineral nutrients and phytochemicals did not change in similar pattern to yield, indicating the compromise between yield and qualitative parameters during EC based nutrient management.

Technical Abstract: Kale (Brassica napus) and collard (Brassica oleracea) are two leafy greens in the family Brassicaceae. The leaves are rich sources of numerous health-beneficial compounds and commonly used either fresh or cooked. With the aim of optimizing the nutrient management of kale and collard in hydroponic production system to obtain high productivity and crop quality, Kale ‘Red Russian’ and Collard ‘Flash F1’ were grown for 4 weeks after transplanting under a double polyethylene-plastic covered greenhouse. A nutrient film technique (NFT) system with 18 channels and 8 separate solution tanks was used. Kale and collard were alternately grown in each channel at four different electrical conductivity (EC) levels (1.2, 1.5, 1.8, and 2.1 mS·cm-1). Fresh and dry yields of kale increased linearly with increasing EC levels, while those of collard did not increase when EC was higher than 1.8 mS·cm-1. Kale leaves had significantly higher P, K, Mn, Zn, Cu, and B than that of the collard under all EC levels. Additionally, mineral nutrients (except N and Zn) in leaf tissue were highest under EC 1.5 and EC 1.8 in both of the kale and collard. However, the changing trend of total N and NO3- of the leaves showed a linear trend, which were highest under EC 2.1, then followed by EC 1.8 and EC 1.5. EC levels also affected phytochemical accumulation in leaf tissue. In general, the kale leaves had significantly higher total anthocyanin, vitamin C, phenolic compounds, and glucosinolates, but lower total chlorophylls and carotenoids than that of the collard. In addition, although EC levels neither affected total chlorophylls and carotenoids in kale, nor glucosinolates content in both kale and collard, other important health-beneficial compounds (especially vitamin C, anthocyanin, and phenolic compounds) in kale and collard leaves reduced with the increasing EC levels. In conclusion, the kale leaf had more nutritional and phytochemical compounds than the collard. EC of 1.8 mS·cm-1 was the optimum EC level for the collard, while the kale yielded more at 2.1 mS·cm-1. Further investigations are needed to optimize nitrogen nutrition for hydroponically grown kale.