Effects of substrates and light-emitting diode illumination on the phytochemical content of roselle (Hibiscus sabdariffa L.) microgreens

A study was conducted to investigate the effect of substrates and illumination using light-emitting diodes on the total phenolic content (TPC) and flavonoid content (TFC) of roselle microgreens. Light-emitting diodes such as red, blue, a combination of red and blue (RB) and white were used as artificial lights of the roselle microgreens. Cocopeat, carbonized rice hull (CRH) and vermiculite are the substrates of microgreens. Newly sown seeds were exposed to 48 hr of darkness followed by light exposure. Light exposure of 10 hr was done, depending on the LEDs (red (R), blue (B), RB and white), while 14 hr of darkness until 7 days after sowing (DAS), the time of harvesting (with true leaves). Roselle microgreens were grown under ambient conditions (temperature: 28 ± 3 °C; relative humidity: 60 ± 5 %). For a fresh yield of microgreens, blue LED + CRH and red LED + vermiculite obtained the highest. Roselle microgreens grow best in RB LED + cocopeat to obtain higher TPC and TFC. The antioxidant activity was higher in roselle microgreens grown under cocopeat and vermiculite with either blue, red, or RB LEDs. The availability of the substrate may be considered when combining it with the LED, using the following combinations to obtain high TPC and TFC. For TPC, these include cocopeat + RB, CRH + RB or white and vermiculite + white. For TFC, the suitable substrates are cocopeat and CRH + RB, as well as vermiculite + white.