Myung-Min Oh

Growth and phenolic compounds of Lactuca sativa L. grown in a closed-type plant production system with UV-A, -B, or -C lamp.

BACKGROUND The production of high-quality crops based on phytochemicals is a strategy for accelerating the practical use of plant factories. Previous studies have demonstrated that ultraviolet (UV) light is effective in improving phytochemical production. This study aimed to determine the effect of various UV wavelengths on growth and phenolic compound accumulation in lettuce (Lactuca sativa L.) grown in a closed-type plant production system. RESULTS Seven days, 1 day and 0.25 day were determined as the upper limit of the irradiation periods for UV-A, -B, and -C, respectively, in the lettuce based on physiological disorders and the fluorescence parameter F(v)/F(m). Continuous UV-A treatment significantly induced the accumulation of phenolic compounds and antioxidants until 4 days of treatment without growth inhibition, consistent with an increase in phenylalanine ammonia lyase (PAL) gene expression and PAL activity. Repeated or gradual UV-B exposure yielded approximately 1.4-3.6 times more total phenolics and antioxidants, respectively, than the controls did 2 days after the treatments, although both treatments inhibited lettuce growth. Repeated UV-C exposure increased phenolics but severely inhibited the growth of lettuce plants. CONCLUSION Our data suggest that UV irradiation can improve the accumulation of phenolic compounds with antioxidant properties in lettuce cultivated in plant factories.

Antioxidant Phytochemicals in Lettuce Grown in High Tunnels and Open Field

Genotype along with growing and management conditions can affect the content and the composition of phytochemicals in plants. Two lettuce (Lactuca sativa L.) cultivars, ‘Baronet’ and ‘Red Sails,’ were grown in an open field and high tunnels to examine the effect of growing conditions on their phytochemical content. The total phenolic concentration and antioxidant capacity of lettuce increased in response to transplanting from greenhouse to both open field and high tunnels. However, the increase was much greater when seedlings were transplanted to the open field and was more than 4 fold over the pre-transplant stage. The concentrations of two major phenolic compounds, chicoric acid and chlorogenic acid, were about 2.5–5.5 times higher in both cultivars when grown in open field than in high tunnels. Also, growing lettuce in open field resulted in a greater activation of key genes (phenylalanine ammonia-lyase, L-galactose dehydrogenase and γ-tocopherol methyl transferase) involved in the biosynthesis of phenolic compounds, ascorbic acid and α-tocopherol. ‘Red Sails’ accumulated caffeic acid 4 times as much in open field as it did in high tunnels and overall contained higher amount of phenolic compounds, especially in open field, than did Baronet. Although lettuce plants grown in open field were richer in phytochemicals, a significant reduction in biomass accumulation occurred when the lettuce plants were grown in open field compared to high tunnels regardless of cultivar. These results show that growing conditions, in addition to genotype, can significantly affect the content of many phenolic compounds in lettuce and that growing lettuce under open field can have a positive impact on its health-promoting qualities.

Growth and phenolic content of sowthistle grown in a closed-type plant production system with a UV-A or UV-B lamp

This study was conducted to determine the effects of UV-A and UV-B wavelengths on the growth and content of antioxidant phenolic compounds in sowthistle (Ixeris dentata Nakai), a medicinal plant, grown in a closed plant-production system. In study I, sowthistle plants were continuously exposed to UV-A light. In study II, two UV treatments [repeated UV-B (R): 4 h·d−1 for 6 days, gradual UV-B (G): from 1 to 7 h·d−1 for 6 days] were applied to the sowthistle plants. As a result, contents of total phenolics and antioxidants in UV-A-treated plants were significantly (50 and 30%, respectively) higher than those in the control plants after 3 d of UV treatment without growth inhibition. Moreover, plants continuously exposed to UV-A for 5 d had 50% higher total flavonoid content than the control. The phenylalanine ammonia-lyase (PAL) activity supported the accumulation of phytochemicals in plants exposed to UV-A. The UV-B (R) treatment led to a more rapid decrease in the chlorophyll fluorescence ratio than UV-B (G) treatment. The UV-B (R) or UV-B (G) treatment produced more total phenolics, flavonoids, and antioxidants, although both UV-B treatments significantly inhibited plant growth measured at 2 days after treatment. The UV irradiation also enhanced PAL activity at 2 and 3 days after treatment, suggesting biosynthetic activation of secondary metabolites. Therefore, application of UV-A or UV-B light can be used as a strategy to improve antioxidant phenolic compounds of sowthistle plants grown in closed plant production systems.

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.

Growth and cell division of lettuce plants under various ratios of red to far-red light-emitting diodes

We investigated the effects of various ratios of red to far-red light-emitting diodes (LEDs) on growth characteristics, physiological response, and cell division of red leaf lettuce. Sixteen-day-old lettuce seedlings were transferred into growth chambers and cultivated under various ratios of red (R) and far-red (FR) LEDs (R/FR = 0.7, 1.2, 4.1, and 8.6), only red LEDs (RED), or fluorescent lamps (control) for 22 days. Growth characteristics were measured at 11 and 22 days of treatment. In addition, cell division rate, epidermal cell density, chlorophyll fluorescence, and photosynthesis of leaves were analyzed. Fresh and dry weights and leaf area in all R/FR treatments were higher than those in the control at 22 days of treatment. The R/FR 1.2 had the highest values among R/FR treatments. The number of leaves appeared to increase as R/FR ratio increased. The specific leaf weights in the R/FR ratio of 0.7, 1.2, and 8.6 were similar to the control at 22 days of treatment. The SPAD values in all R/FR treatments were lower than that in the control. All R/FR treatments led to a longer leaf shape than the control. The percentage of cells in the G2M phase, indicating the cell division rate, increased in the R/FR treatments after 4 days of treatment, which supported the growth improvement in the R/FR treatments. The Fv/Fm and the photosynthetic rate in all treatments decreased due to the absence of blue light. The results of this study suggest that the supplementation with far-red LEDs should be considered when designing artificial lighting systems for closed-type plant factories since far-red light affects the vegetative growth of leafy vegetables such as lettuce.

Growth and Antioxidant Phenolic Compounds in Cherry Tomato Seedlings Grown under Monochromatic Light-emitting Diodes

Light-emitting diodes (LEDs) can be used in closed-type plant production systems as an artificial light source. Here, we determined the effects of monochromatic LEDs on the growth and production of phenolic antioxidants in cherry tomato seedlings (Solanum lycopersicum L. ‘Cuty’). Two week-old seedlings germinated under normal growing conditions were transplanted into a growth chamber equipped with various monochromatic LEDs and fluorescent lamps (control), and cultivated for 4 weeks. Fresh weights of shoots and roots under LED treatment, especially, red or green, were higher than those under the control light at 4 weeks. The SPAD value of seedlings grown under blue LEDs was significantly lower than in seedlings grown under other LEDs. The plant height, stem length, and internode length of tomato seedlings grown under blue LEDs were the highest. Blue LEDs induced 1.5–2.2-fold higher stem length than red and white LEDs. Expansin gene expression was the highest under blue LEDs, consistent with the effect on stem length. Blue LEDs stimulated the biosynthesis of total phenolics, antioxidants, and total flavonoids in tomato seedlings. Specifically, the antioxidant capacity of seedlings grown under blue LEDs was 2.1-folds higher than that in seedlings grown under green LEDs. Thus, manipulating light quality using LEDs is a crucial factor for growth and antioxidant production in cherry tomato seedlings.

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.

Short-term low temperature increases phenolic antioxidant levels in kale

The objective of this study was to determine the effect of short-term low temperature on the concentration of phenolic antioxidant compounds in kale. For the low-temperature treatment, two kale cultivars (‘Manchoo Collard’ and ‘TBC’) grown for 3 weeks in a growth chamber were subjected to 4°C for 3 days, and subsequently allowed to recover for 2 days under normal growth conditions (20°C). Fresh and dry shoot and root weights, chlorophyll fluorescence (potential quantum yield in dark-adapted conditions), reactive oxygen species (O2·- and H2O2), total phenolic concentration, antioxidant capacity, individual phenolics, and phenylalanine ammonia-lyase (PAL) activity were measured before and after treatment. No significant difference was observed between the control and low-temperature treatments in the fresh or dry shoot or root weights of either cultivar. The Fv/Fm decreased during the low-temperature treatment in both cultivars, and O2·- and H2O2 were generated in ‘Manchoo Collard’ leaves treated with low temperature but not in ‘TBC’ leaves. ‘Manchoo Collard’ had a 15% higher total phenolic concentration than the control after 2 days of recovery, whereas that of ‘TBC’ was 16% lower than that of the control. Individual phenolic compounds, such as caffeic acid, ferulic acid, and kaempferol, exhibited a similar trend to the total phenolic concentration and antioxidant capacity. The increased PAL activity in ‘Manchoo Collard’ at low temperature was in accord with the total and individual phenolic content results. These results suggest that a short-term low temperature during cultivation of kale in a controlled environment is a potential strategy to increase the plant’s phenolic antioxidant compound content.

Concentrations of minerals and phenolic compounds in three edible sprout species treated with iron-chelates during imbibition

Iron (Fe) is an essential micronutrient involved in fundamental biological processes in both humans and plants. Iron deficiency is common in humans, making iron supplementation of foods an important area of research. Edible sprouts are a rich source of minerals and phenolic compounds beneficial to human health; our objective was therefore to investigate the effects of iron supplementation in sprouts. We supplemented iron concentrations in three species of edible sprouts (alfalfa, broccoli, and radish) by soaking the seeds in a high-iron solution, and subsequently measured the concentration of minerals and of phenolic compounds. Seeds were soaked in either Fe(III)-EDTA or Fe(III)-citrate at concentrations of 2.5, 5.0, or 10 mM for 5–8 h, and then were maintained with distilled water in a commercial sprouter for 5 days. The soaking treatment significantly increased the iron concentration in 5-day-old alfalfa sprouts by up to 1.8 times the concentration observed in the controls. For broccoli and radish sprouts, an insignificant trend toward higher Fe concentrations was observed. The accumulated iron in treated alfalfa sprouts was negatively associated with concentrations of other minerals such as Ca, Mg, Mn, and Na. Treated alfalfa sprouts showed a significant increase of 8.0–36.4% in total phenolic concentrations compared to the controls, whereas broccoli and radish sprouts showed no significant change in phenolic concentrations. In summary, soaking seeds with iron chelates enhanced the iron concentration of sprouts, especially alfalfa sprouts, and had a positive or neutral impact on the concentration of phenolic compounds, suggesting that this treatment could be used to improve the nutritional quality of some types of edible sprouts.

Air anions enhance lettuce growth in plant factories

This study was conducted to determine the effect of air anions on lettuce growth in a plant factory. Red leaf lettuce (Lactuca sativa L. cv. ‘Jeokchima’) seedlings grown under normal growth conditions (20°C, fluorescent lamp, 150 ± 3 μmol·m−2·s−1 PPFD, 12-h photoperiod) for 18 days were transplanted to hydroponic systems in a plant factory equipped with LEDs (red:blue = 78:22, 184 ± 2 μmol·m−2·s−1 PPFD, 12-h photoperiod). Three levels of air anions (low, 10 × 104 ion·cm−3; medium, 19 × 104 ion·cm−3; and high, 70 × 104 ion·cm−3) were applied to lettuce plants for 4 weeks. Lettuce plants exposed to air anions showed vigorous growth after 2 and 4 weeks of treatment. Both the medium and high levels of air anions improved growth characteristics such as leaf area and fresh weight of shoots, but there were no significant differences in the number of leaves and SPAD values were observed between the treatments. The medium level of air anions resulted in a 64% increase in shoot fresh weight compared to the control at 4 weeks after treatment. The photosynthetic rate of lettuce grown in the medium level of air anions after 3 weeks of treatment was 30% higher than that of the control. In addition, energy use efficiency in air anion treatments was higher than that in the control. In conclusion, this study demonstrated that the application of air anions in a plant factory imparts a positive effect on lettuce growth with low production cost.