Supplemental irradiation with far-red light-emitting diodes improves growth and phenolic contents in Crepidiastrum denticulatum in a plant factory with artificial lighting

The aim of this study was to analyze the growth and phytochemical composition of Crepidiastrum denticulatum, a valuable medicinal plant, in response to various periods of irradiation and ratios of far-red (FR) LED light combined with red (R) and blue (B) LED lights. Three-week-old C. denticulatum seedlings were transplanted to a hydroponic system in a plant factory equipped with R, B, and FR LEDs. The ratio of R to B LEDs was set at 8:2 (R8B2), and the ratio of R to FR LEDs was adjusted to 0.7, 1.2, 4.1, and 8.6. R8B2 (without FR LEDs) and commercial LEDs were used as control I and II, respectively. The plants were cultivated under these light treatments, and some of the control plants were subjected to four different R/FR ratios for 30 min before the end of the light period (EOL). We measured growth characteristics, total phenolic contents, and the levels of individual phenolic compounds (chlorogenic acid, caffeic acid, and chicoric acid) 6 weeks after treatment. Shoot fresh and dry weight, leaf area, leaf length, and leaf width under R/FR ratios of 0.7 and 1.2 were 1.8- to 2.4-times higher than those of the control plants. Continuous R/FR 0.7 and 1.2 irradiation reduced the total phenolic content per dry weight compared to the control, although this effect was not significant. The total phenolic content per plant under R/FR 0.7 and 1.2 increased more than 2-fold, and the shoot fresh weight increased 1.7- and 1.6-fold, compared to control I. Individual phenolic compound levels exhibited the same trends as total phenolic content. Under R/FR 0.7 and 1.2, chlorogenic acid, caffeic acid, and chicoric acid levels per shoot increased 1.3- to 1.8-fold compared to control I. FR EOL treatments did not have a significant effects on growth or the contents of bioactive compounds. These results suggest that supplemental irradiation with FR LEDs could be used to improve C. denticulatum growth and quality in terms of phytochemical composition when grown in a plant factory with artificial lighting.