Effects of Light Quality on the Morphology, Growth, and Quality of Common Microgreens

Microgreens are young seedlings of vegetable or herb species. Microgreens have created an emerging market in the food industry focusing on their high value. However, while microgreens are often grown under sole source electric lighting such as LEDs, limited information is available on effects of light quality on microgreens. Our study focuses on optimizing light quality for different microgreens to achieve desirable quality attributes and to improve the yield. Three microgreen species, ‘Korean Perilla’ (Perilla frutescens var crispa), ‘Savoy Tatsoi’ (Brassica rapa (Narinosa group)) and ‘Red Acre Cabbage’ (Brassica oleracea var capitate) were used in this study. The preliminary experiment was performed in a growth chamber set at 22˚C air temperature. The plants were grown under blue (469 nm, B) and red (661 nm, R) LEDs of 50:50 B:R or 30:70 B:R ratios. Another group of plants were also grown under white/UV-A LEDs with 47:25 B:R ratio or under cool white fluorescent (CWF) light (34:25 B:R). The DLI was 12 mol·m^-2·d^-1 for all treatments. Photosynthetic photon flux (PPF) was 210 μmol·m^-2·s^-1 (16 h photoperiod) for all treatments except for White/UV-A (144 μmol·m^-2·s^-1 for 21 h). The plants were harvested after 90% of their true leaves emerged, approximately 10-17 days after planting. All plants grown under white/UV-A or 50:50 B:R light increased hypocotyl length by 39-54% compared to those under CWF light. The hypocotyl length of plants grown under 30:70 B:R light was not significantly different compared with that under CWF light. In both Brassica species, plants grown under white/UV-A light showed greater fresh weight than other treatments. This however, was likely due to increased water content as their dry weight was similar to or less than other LED treatments. Light quality did not have a significant effect on the growth of Perilla. Of interest, the flavor of Brassica microgreens seemed to be correlated with the plant growth and morphology. Brassica that had a greater plant dry weight and a stronger flavor under 30:70 B:R or CFW light than under 50:50 B:R or white/UV-A light. In contrast, Perilla had an overwhelmingly strong flavor under 30:70 B:R or CWF light compared to those under 50:50 B:R or white/UV-A light. These results indicate the need for species specific light qualities for microgreens. Further research would be necessary for other types of microgreens.