Ki-Ho Son

Growth, photosynthetic and antioxidant parameters of two lettuce cultivars as affected by red, green, and blue light-emitting diodes

The addition of green light-emitting diodes (LEDs) to a combination of red and blue LEDs, which promote photosynthesis and growth in plants, is known to enhance plant growth in closed-type plant production systems. However, there is limited information on the effects of supplementary green light. This study aimed to determine the effect of red (R), green (G), and blue (B) LED ratios on the growth, photosynthetic, and antioxidant parameters in two lettuce (Lactuca sativa) cultivars, red leaf ‘Sunmang’ and green leaf ‘Grand Rapid TBR’. The seedlings were grown for 18 days and then cultivated in growth chambers equipped with LED lighting systems for 4 weeks. Combinations of six LED lighting sources (R:B = 9:1, 8:2, 7:3; R:G:B = 9:1:0, 8:1:1, 7:1:2) were manufactured to emit red (655 nm), blue (456 nm), or green (518 nm) lights under photosynthetic photon flux density of 173 ± 3 μmol·m-2·s-1. Red LEDs were found to improve growth characteristics such as fresh and dry weights of shoots and roots, and leaf area in combination with blue LEDs. The substitution of blue with green LEDs in the presence of a fixed proportion of red LEDs enhanced the growth of lettuce. In particular, the fresh weights of red leaf lettuce shoots under R8G1B1 were about 61% higher than those under R8B2. Furthermore, analysis of leaf morphology, transmittance, cell division rate, and leaf anatomy under treatments with green LEDs supported the enhanced growth of the two lettuce cultivars tested. Meanwhile, growth under blue LEDs led to the accumulation of antioxidant parameters in ‘Sunmang’. Thus, the results of this study suggest that the percentage of red, green, and blue LEDs is an important factor for the growth, development, and biosynthesis of secondary metabolites in plants and especially the supplemental irradiation of green LEDs based on the combination of red and blue LEDs can improve lettuce growth.

Increase in biomass and bioactive compounds in lettuce under various ratios of red to far-red LED light supplemented with blue LED light

The aim of this study was to analyze the growth and bioactive compounds of lettuce in response to far-red LED light supplemented with a combination of red and blue LED light. Sixteen-day-old red leaf lettuce seedlings were transplanted to a growth chamber equipped with red, blue, and far-red LEDs. After setting the ratio of blue (B) to red (R) LEDs to 2:8, the ratio of R to far-red (FR) LEDs was adjusted to 0.7, 1.2, 4.1, or 8.6 (B+R/FR 0.7, 1.2, 4.1, or 8.6). Additionally, plants were irradiated with combined B and R LEDs (B+R) and fluorescent lamps (control) for 24 days. Growth characteristics including cell division rate, epidermal cell density and thickness, and antioxidant phenolic compounds were measured. Supplementation with FR LED light improved shoot and root growth compared to plants under B+R and control treatment. B+R/FR 1.2 treatment resulted in the highest shoot fresh weight and leaf area on day 24 of treatment. Obvious activation of the G2M phase was not observed in plants under far-red treatment, and most far-red treatments besides B+R/FR 4.1 increased the epidermal cell size. Plants treated with B+R/FR ratios of only 0.7 and 1.2 had significantly higher total phenolic levels, antioxidant activity, chlorogenic acid contents, and caffeic acid contents per plant than the control. These results suggest that supplementing the existing visual light spectrum, such as red and blue light, with far-red LEDs improves lettuce growth and bioactive compound content in a closed-type plant production system.

Growth and bioactive compounds as affected by irradiation with various spectrum of light-emitting diode lights in dropwort

The aim of this study was to determine the effect of irradiation of various spectrums generated by light-emitting diodes (LEDs) on the growth and accumulation of bioactive compounds in dropwort (Oenanthe stolonifera). Dropwort plants with 2-3 offshoots were grown under three monochromatic LEDs: red (R; 654 nm), blue (B; 456 nm), and green (G; 518 nm), thirteen combinations of R and B (R:B = 9:1, 8:2, 7:3, and 6:4), RB with G (R:G:B = 9:1:0, 8:1:1, 7:1:2, and 6:1:3), and RB with white (W) (R:W:B = 8:2:0, 8:1:1, 7:2:1, 7:1:2, and 6:2:2) and fluorescent lamps (control) for 6 weeks. R LEDs improved growth characteristics, including plant height and fresh and dry weights of shoots. Combined LEDs with 70-80% red wavelength resulted in the highest values in fresh weight. The B LED treatment resulted in the highest total phenolic and anthocyanin content in dropwort leaves, which increased in the combined LEDs treatments as the proportion of blue wavelength increased. Further, the RWB treatments, regardless of the ratio, resulted in higher anthocyanin contents than that in the RB and RGB treatments. Persicarin content was also significantly higher in the B treatment than in the R and G treatments. However, compared to the RB treatments, persicarin content in the RGB and RWB treatments was decreased, on average, by 72 and 64%, respectively. We suggested that the light spectrum is closely related to the enhancement of growth and bioactive compounds. Our findings provide basic information for designing lighting systems in plant factories to improve the growth and bioactive compounds in dropwort.

Application of supplementary white and pulsed light-emitting diodes to lettuce grown in a plant factory with artificial lighting

Light-emitting diodes (LEDs) are currently undergoing rapid development as plant growth light sources in a plant factory with artificial lighting (PFAL). However, little is known about the effects of supplementary light and pulsed LEDs on plant growth, bioactive compound productions, and energy efficiency in lettuce. In this study, we aimed to determine the effects of supplementary white LEDs (study I) and pulsed LEDs (study II) on red leaf lettuce (Lactuca sativa L. ‘Sunmang’). In study I, six LED sources were used to determine the effects of supplementary white LEDs (RGB 7:1:1, 7:1:2, RWB 7:1:2, 7:2:1, 8:1:1, 8:2:0 [based on chip number] on lettuce). Fluorescent lamps were used as the control. In study II, pulsed RWB 7:2:1 LED treatments (30, 10, 1 kHz with a 50 or 75% duty ratio) were applied to lettuce. In study I, the application of red and blue fractions improved plant growth characteristics and the accumulation of antioxidant phenolic compounds, respectively. In addition, the application of green light increased plant growth, including the fresh and dry weights of shoots and roots, as well as leaf area. However, the substitution of green LEDs with white LEDs induced approximately 3.4-times higher light and energy use efficiency. In study II, the growth characteristics and photosynthesis of lettuce were affected by various combinations of duty ratio and frequency. In particular, biomass under a 1 kHz 75% duty ratio of pulsed LEDs was not significantly different from that of the control (continuous LEDs). Moreover, no significant difference in leaf photosynthetic rate was observed between any pulsed LED treatment utilizing a 75% duty ratio versus continuous LEDs. However, some pulsed LED treatments may potentially improve light and energy use efficiency compared to continuous LEDs. These results suggest that the fraction of red, blue, and green wavelengths of LEDs is an important factor for plant growth and the biosynthesis of bioactive compounds in lettuce and that supplementary white LEDs (based on a combination of red and blue LEDs) might be more suitable as a commercial lighting source than green LEDs. In addition, the use of suitable pulses of LEDs might save energy while inducing plant growth similar to that under continuous LEDs. Our findings provide important basic information for designing optimal light sources for use in a PFAL.