Comparison of LED and HPS illumination effects on cultivation of red pak choi microgreens under indoors and greenhouse conditions

The objective of our studies was to determine changes of growth and quality of red pak choi microgreens depending on different irradiance level of LED and HPS lighting under indoor and greenhouse conditions. Microgreens of red pak choi (Brassica rapa var. chinensis 'Rubi F₁') were grown in a peat substrate (pH 5-6) in 0.5 L plastic vessels for 10 days, from sowing to harvest. Day/night temperatures of 21±2/17±2°C in growth chambers and 20±3/16±3°C were established with a 16-h photoperiod and relative humidity of 50-60%. LED lighting equipment was composed of 20 units of custom-made lamps. Each lamp was designed using red LEDs (spectrum peak wavelength ' 662 nm) and blue LEDs (spectrum peak wavelength ' 451 nm). Reference microgreens were grown under HPS lamps. The photosynthetic photon flux density (PPFD) of LEDs and HPS lamps was 150, 200, 250 and 300 µmol m^‑2 s^‑1. For the LED lamps, red to blue light ratio was 91:9. ICP-OES method was used for determination of mineral elements, potentiometric method for nitrate content, and spectrophotometric for total anthocyanins and phenols, ascorbic acid, and DPPH. Our results revealed different effects of HPS and LED lamp illuminations under indoor and greenhouse conditions. LED lamps resulted in decrease of elongation of red pak choi microgreens under all PPFD in greenhouse, but opposite results were obtained under indoor conditions. LED lamps illumination decreased nitrate content under both growing conditions. PPFD had no effect on fresh weight, however higher PPFD of LED lamps increased it indoors. It was determined that all irradiance levels of LED lamps no effect on ascorbic acid, total phenols, DDPH; however, an increase in total anthocyanins and a decrease in macro elements were determined indoors. The opposite effect was observed in greenhouse grown plants.