Influence of Substrate and LED Light Spectrum on Growth, Yield, and Phytochemical Content of Ethiopian Kale (Brassica carinata A. braun) Microgreens

Abstract

Microgreens are plant products harvested shortly after the first true leaves emerge, usually between 7 and 21 days. Microgreens are rich in nutrients and other beneficial phytochemicals that play a major role in alleviating diet related illnesses. Therefore, they can play a role in addressing malnutrition and lifestyle diseases in Kenya. Microgreens are gaining popularity in human diets as functional foods that deliver superior nutritional value and health benefits to consumers compared to their mature counterparts. In Kenya awareness of microgreens, their production and utilization is extremely low due to limited information on microgreens. As a result, their benefits have not been fully harnessed. Since substrates and light conditions influence the quality of microgreens in terms of nutrients and phytochemicals content, it is necessary to determine appropriate substrates and optimum lighting for microgreen production. Therefore, the present study aimed at providing insights on the influence of different lighting treatments provided by LEDs, including Blue (B, 450nm), Red (R, 650nm), a cool White (W) and a combination of three color diodes (B+R+W) and substrates Cocopeat, Sand and Cocopeat-Sand mix (v:v) (1:1) on growth, yield and phytochemical content of Brassica carinata microgreens. The research was carried out at Tokyo University of Agriculture, Japan. Brassica carinata seeds were germinated in dark chambers and cultivated in growth chambers equipped with LED lighting systems in a factorial experimental setup in a split-plot design for 14 days. The plants were exposed to a fixed light intensity of 160 ± 2.5 µmol m−2 s−1 with a photoperiod of 12 h d-1. Light was considered as the main plot while substrate as the subplot. There were three replications for light spectra and twelve for substrate treatments. Growth parameters assessed included plant height, leaf area, canopy cover, fresh weight and dry weight of B. carinata microgreens. Selected phytochemicals including ascorbic acid (Vitamin C), chlorophyll and total flavonoids, carotenoids and total polyphenols all of which are associated with antioxidant activities were assessed. Anti-nutrients (nitrates) were also assessed. All data collected were subjected to ANOVA in R software at P≤0.05. Significant means were separated by Tukey’s HSD (Honestly Significant Difference). Best performance including statistically higher average yield (19.19 g plant-1), higher plant height (9.94 cm), leaf area (68.11 mm2) and canopy cover (55.9%) were found under combined Blue + Red + White (B+R+W) LEDs and Cocopeat + Sand mix. B+R+W LEDs enhanced carotenoids and flavonoid content, while Blue LED alone (B) increased total amount of chlorophyll (11880 mg kg-1). Brassica carinata microgreens grown using Red LED alone (R) and in cocopeat +Sand mix recorded the highest total phenols (8.1 mg kg-1). In addition, B+R+W LED and in cocopeat enhanced accumulation of Vitamin C content of B. carinata microgreens (1155.1 mg kg-1). For plants grown under B+R+W LED in cocopeat, high nitrate levels were observed. The results therefore suggest that substrate and LEDs are important factors for the growth, development and accumulation of secondary metabolites of B. carinata microgreens specifically supplemental irradiation with combined (B+R+W) LED or Blue LED alone and using combined sand and cocopeat substrates can improve growth and nutritional quality of B. carinata microgreens.