Evaluation of the effects of root medium and light quality on morphology and nutritional quality of radish microgreen

Microgreens, are tender seedlings produced from seeds of different species of vegetables, aromatic herbs and herbaceous plants, including wild edible species. Microgreens are generally harvested 7-21 days after germination, when cotyledonary leaves are fully developed, with or without the emergence of a small pair of trueleaves. Increasingly used by chefs as edible garnish, microgreens are becoming popular also for their high content of bioactive compounds. Moreover, with the development of the urban farming industry there is increasing interest in their commercial production. Despite the short growing cycle, the commercial production of microgreens requires particular attention, and the choice of the growing medium and optimizing light intensity and quality represents one of the most critical aspects of the production process. They are recognized as health-promoting foods because of containing high phytonutrients and bioactive compounds such as phenolic compounds, flavonoids, pigments, vitamins and trace minerals (Xiao et al., 2012). Compared to fruit and other vegetables, they contain higher phytonutrients per calorie. Moreover, many phytochemicals also have medicinal properties playing important roles against various diseases such as stroke in the brain, cancer and Cardiovascular diseases and strengthening the immune system.

Experiments were performed at the horticultural department of university of Mohaghegh Ardabili, radish seeds were evenly broadcasted on the surface of the growing media (soil or coco-peat) in each tray. Seeds were kept under dark condition for 3 days. Every day trays were sprayed two times with deionized water. After germination, trays were subjected to different light quality conditions (red, blue, white and natural light). Air temperature ranged between18 to 20 °C. At the first appearance of the first true-leaves, 10 days after sowing, microgreens of each tray were harvested by cutting the seedling just above the surface of the growing media with a sterilized knife. Morphological and physiochemical traits including yield, height, leaf area, chlorophyll, phenolic compound, carotenoid and flavonoid content, antioxidant activity, peroxidase and ascorbate peroxidase were evaluated with 5 replications.

Results showed that light quality and media affects growth, physiology and nutritional value of the radish microgreen. Radish microgreen also performed differently in different growth media. According to the results there was significant difference among media and light treatments with respect to yield, height and leaf area. In microgreen production yield depend on plant height, leaf area and planting density and the quality of microgreens depend on their nutritional value, antioxidant activity and exterior appearance. According to the results, seedling grown in trays filled with garden soil performed better with respected to quality and yield compared to coco-peat. Plant grown under blue and red light showed more chlorophyll, carotenoid, phenol and flavonoid content, however plant grown under natural light condition showed more antioxidant activity and leaf area. There was a positive and significant correlation between leaf area and yield per area, while the correlation between plant height and yield was not significant. So in microgreen production any treatment that affect leaf area might be recommended for better exterior look and its yield. The highest chlorophyll, carotenoid, phenol and flavonoid content achieved in plant grown under blue and red light. Moreover, the highest antioxidant enzymes activity of peroxidase and ascorbate peroxidase measured in plant grown under blue light. As there was a positive and significant correlation between enzyme activity and chlorophyll and carotenoid content, so it might be concluded that blue and red light induce chlorophyll and carotenoid production in microgreens. There was also a high correlation among plant height, enzyme activity and chlorophyll and carotenoid content.

As described above, in microgreen production the quality of microgreen depends on seedling height, greenness of the seedling leaf, leaf area, and nutritional value and so on. It is better to use combination of light sources to produce more marketable microgreen with high quality. Moreover it is recommended that any media with no or low mineral element should be mixed with organic or inorganic compound to produce high quality microgreens.