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The low survival rates of in vitro-propagated plantlets under ex vitro conditions greatly inhibits the production of virus-free apple rootstock plantlets and necessitates tight control of ex vitro environments during plantlet acclimatization. Accordingly, this study investigated the effects of light intensity on the ex vitro acclimation of apple plantlets. In vitro-propagated ‘M9’ apple plantlets were acclimatized for 6 weeks under different light treatments: 60 μmol m−2 s−1 (L), 100 μmol m−2 s−1 (M), 140 μmol m−2 s−1 (H), 180 μmol m−2 s−1 (VH), 60 → 100 μmol m−2 s−1 at 2 weeks (L2M4) or 4 weeks (L4M2), 60 → 100 → 140 μmol m−2 s−1 (L2M2H2), and 60 → 140 μmol m−2 s−1 at 4 weeks (L4H2). Survival rate, maximum quantum yield of photosystem II (Fv/Fm), growth-related parameters, and photosynthetic rate were measured. The H and VH treatments yielded the lowest survival rates (78 and 71%, respectively), whereas the M treatment yielded the highest (95%). Meanwhile, the Fv/Fm ratio at 6 weeks after transplanting decreased with increasing light intensity at 4 and 5 weeks, whereas photosynthetic rate at 5 weeks after transplanting and stem diameter at 6 weeks after transplanting increased with increasing light intensity. Furthermore, the M treatment yielded greater relative growth rates than the other treatments at 2–4 weeks, and both the M and L2M2H2 treatments yielded significantly greater relative growth rates at 4–6 weeks. These results suggest that the M and L2M2H2 treatments are appropriate for the acclimatization of in vitro-propagated ‘M9’ apple plantlets.

This study aimed to evaluate the effect of the red (R)-to-blue (B) ratio of light on the growth and phenolic compounds of Crepidiastrum denticulatum under a phytochrome photostationary state (PSS) for growth in a plant factory with artificial light (PFAL) using light-emitting diodes (LEDs). Three-week-old C. denticulatum seedlings were transplanted into a PFAL where the air temperature, relative humidity, CO2 concentration, and light period were set at 20 °C, 60%, 1000 μmol·mol−1, and 16 h, respectively. Three controls were used with different ratios of R to B light without supplemental FR light: 8:2, 7:3, and 6:4 (based on chip number; 130 µmol·m−2·s−1 photosynthetic photon flux density). For the treatments, the same R to B light ratios as in the controls were used but supplemented with FR light, plus a treatment with only R light supplemented with FR, set to a PSS of 0.71. Growth characteristics and total phenolic and individual phenolic contents were measured after a 6-week treatment. When the R light ratio increased, shoot dry weight, leaf length, leaf width, and leaf area increased regardless of supplemental FR light, and the R8B2 with FR light treatment was the most effective, with significantly higher values (1.6–2 times) than the control. FR irradiation did not have any negative effect on total phenolic content, chlorogenic acid, caffeic acid, and chicoric acid per unit dry weight (g); thus, the R8B2 with FR treatment had significantly higher total phenolic and individual phenolic contents per shoot (43% and 52–62%, respectively). Thus, supplemental lighting with FR LEDs was found to be effective to enhance the growth and bioactive compounds of C. denticulatum in a PFAL installed with a R and B lighting system, and the effectiveness could be changed by the RB ratio, with 8:2 considered the proper ratio.

This study aimed to investigate the effects of UV-A light-emitting diodes (LEDs) and lamps on the growth and bioactive compounds of ice plant (Mesembryanthemum crystallinum). Three-week-old seedlings were germinated and grown at 23 °C air temperature, 60% relative humidity, 1000 μmol mol−1 CO2, 200 μmol m−2 s−1 (white LEDs), and a photo-period of 12 h for 3 weeks in a plant factory with artificial lighting. Plants were supplementally irradiated with different peak wavelengths of UV-A LEDs (395, 385, 375, and 365 nm) of 30 W m−2 and UV-A lamps of 15.5 W m−2 continuously for 7 days. Treatment with 395, 385, and 375 nm increased shoot fresh and dry weights than the control at 5 and 7 days of treatment. Fv/Fm value was significantly decreased at 12 h of all UV-A treatments and consistently showed lower levels compared to the control during the entire period. The photosynthetic rates of the 395 and 385 nm treatments were significantly higher than those of the other treatments. UV-A treatment enhanced total phenolic content and antioxidant capacity compared to those of the control after 3 days of treatment. Phenylalanine ammonia-lyase activity was also increased by UV-A light exposure. The content of pinitol, myo-inositol, and sucrose was increased by UV-A radiation, and the highest values were observed in the 395 nm treatment at 5 and 7 days of treatment. Our findings suggest that supplemental radiation of UV-A with a peak wavelength near 400 nm could increase the shoot biomass and antioxidant phenolics and sugar alcohols in ice plants.

High-energy ultraviolet (UV) light is an environmental stress that can be used to stimulate the biosynthesis of bioactive compounds in plants. This study aimed to comparatively determine the effects of UV-A, UV-B, and UV-C lamps or light-emitting diodes (LEDs) on the growth of dropwort (Oenanthe stolonifera) plants, and their contents of bioactive compounds. Dropwort seedlings with 2–3 offshoots were transplanted in a plant factory equipped with white LED and deep flow technique systems, and cultivated under standard growth conditions for 36 days. Thereafter, the dropwort plants were supplementally exposed to one of five UV treatments with energy equivalent to 10 W m−2: UV-C lamps for 2 days, UV-B lamps for 3 days, and UV-A lamps and LEDs with 370 nm or 385 nm peak wavelengths for 14 days. The variable fluorescence (Fv) to maximum fluorescence (Fm) ratio (Fv/Fm) of dropwort leaves began to significantly decrease 3 h after exposure to UV-C, and 6 h after UV-B exposure. Fluorescence in UV-C and UV-B-treated plants was lower than in control and UV-A-treated plants during the entire period of UV irradiation. The fresh weight of the shoots of plants treated with UV was not significantly different to those of the control plants during the entire UV irradiation period. The total phenolic content of dropwort shoots exposed to UV-A and UV-B treatments significantly increased compared to that of the control 1 day after treatment. The total phenolic content was highest in plants treated with the 370 nm UV-A LED, and this was significantly higher (33%) than the control. Plants treated with the 385 nm UV-A LED on day 3 of treatment had the highest total phenolic content compared to the other treatments. A similar trend was observed in contents of flavonoids and persicarin. UV light induced higher anthocyanin content than the control. The activity of phenylalanine ammonia-lyase after UV treatments was significantly higher than the control, supporting the findings of our bioactive compound assays. In conclusion, the results of this study suggest that irradiating vegetables with UV-A LEDs would be useful in plant factories with artificial light for improving vegetable quality without inhibiting growth.

The present study aimed to evaluate the effects of controlling the relative humidity (RH) and water content of the root-zone on the survival rate and growth of in vitro-propagated virus-free M9 apple plantlets in closed-type plant production systems. In the first experiment, three RH regimes were applied to pre-acclimated (PA) and non-PA apple plantlets for 6 weeks after transplantation. In the second experiment, the apple plantlets were transplanted into several growth media, including a mixture of peat moss and perlite (PP), rock wool (RW), and urethane sponge (SP), and in a deep flow technique (DFT) system for controlled root zone water content under controlled RH. In the first experiment, pre-acclimation improved the survival rate by preventing the loss of leaf water potential and promoting antioxidant capacity during the acclimation period. However, no clear difference was found among the three RH regimes. The antioxidant capacity was increased at 2 weeks after transplantation, followed by root initiation. The leaf water potential, which decreased continuously until 3 weeks after transplanting, tended to remain constant after root initiation. These results suggested that pre-acclimation is necessary for the survival of in vitro-propagated apple plantlets, and that the underdeveloped roots of apple plantlets have restricted water absorption under controlled RH. In the second experiment, the survival rate of plantlets grown in PP at 6 weeks after transplantation was only 70% accompanied by an increase in antioxidant capacity, whereas the survival rates of plantlets grown in RW, SP, DFT, and DFT-PP (replanted to PP from DFT 4 weeks after transplantation) were 98, 96, 93.8, and 93.8%, respectively. Most of the growth parameters of the plantlets grown in DFT were the highest among the growth media at 6 weeks after transplantation. The results of the second experiment implied that the application of DFT for in vitro-propagated apple plantlets can reduce the problems caused by poor root architecture during acclimation.

In the present study, the effect of light quality on endoreduplication and growth in Phalaenopsis ‘Spring Dancer’ plantlets was studied. The response of protocorm-like body (PLB)-derived plantlets subjected to monochromatic red (R60), blue (B60), and various combinations of both lights was investigated. Flow cytometry was used to investigate the effect of light on endocycle and growth, cell division, and endopolyploidy levels. In addition, the activities of stress-related enzymes such as catalase (CAT) and peroxidase (POD) were analyzed from leaves and roots. After 8 weeks, the leaf area of plants grown under monochromatic R60 and B60 light was found to be higher than that of plants grown under other wavelengths of light, except the control plants (fluorescent light). These results revealed monochrome blue (B60) light increased the ratio of endoreduplicated cells (4C–8C). CAT activity was highest in leaves grown under R60; however, the oxidized phenol concentration in the culture medium was lowest under R60 while it was the highest under B60 and fluorescent light (F). This indicates that plantlets were less stressed under R60 than B60 or F. The results of this study reveal that stress induced by monochromatic light stimulates endopolyploidy in leaves, which may subsequently increase Phalaenopsis leaf size.

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.

The objective of this study was to determine the effects of mild water deficit on the contents of bioactive compounds in dropwort (Oenanthe stolonifera). Dropwort plantlets with 2-3 offshoots were transplanted into plastic pots containing horticultural growing medium. The plantlets were grown at 22°C under a relative humidity of 60%, photosynthetic photon flux density (PPFD) of 285 μmol·m -2 ·s -1, 12-hour light period, and CO2 concentration of 600 μmol·mol -1 and subirrigated with nutrient solution for 3 weeks after transplanting. To induce mild water deficit, we placed wick culture systems (4- and 6-cm long wicks) between the bottom of each pot and the water surface at 3 or 4 weeks after transplanting. The control plants were subirrigated at the pot bottom without wick treatment. The soil water content of plants under water-deficit averaged 45% lower than that of the control at 5 weeks after transplanting. Moreover, mild water-deficit treatment reduced the leaf water potential by 7-25% compared to the control. However, mild water-deficit treatment did not significantly affect the fresh or dry weights of dropwort shoots at 6 weeks after transplanting. All treatments significantly inhibited the photosynthetic rate at 5 weeks after transplanting. In particular, water-deficit treatment using a 6-cm long wick for 2 weeks resulted in a 16% lower photosynthetic rate than that of the control. The total phenolic content and antioxidant capacity of dropwort stems did not significantly differ between treatments. However, in the leaves, total phenolic content and antioxidant capacity of all plants subjected to water-deficit treatments averaged 43 and 64% higher, respectively, than those of the control. The total anthocyanin content in all treatment groups, except for treatment with a 4-cm wick at 4 weeks, was significantly higher than that of the control. These results suggest that dropwort plants subjected to mild water deficit effectively accumulate antioxidant phenolic compounds, including anthocyanins, which is more reminiscent of reddish, small dropwort plants that grow under limited water conditions than of water dropwort.

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.

The medicinal plant Crepidiastrum denticulatum, which is found throughout East Asia, contains various health-promoting phytochemicals. The aim of this study was to evaluate the effects of a newly developed nutrient solution (referred to as NSC) on the growth of this plant and to determine the proper EC level of NSC for stable phytochemical production in plant factories. Three-week-old seedlings were transplanted to a wick culture system supplied with Hoagland solution (EC 1.0 dS·m -1) as a control or with five different concentrations of NSC (EC 0.5, 1.0, 1.5, 2.0, and 2.5 dS·m -1). We grew the plants under normal conditions (20°C, 310 ± 10 µmol·m -2·s -1 PPF, CO2 1,000 µmol·mol -1, and a 16 hours photoperiod) for 6 weeks and evaluated their photosynthetic rates and growth characteristics, such as the fresh and dry weights of shoots and roots, leaf area, number of leaves, and S/R ratios, at 6 weeks after transplanting. We also measured the total phenolic content, antioxidant capacity, and chicoric acid content each week for 6 weeks after transplanting. The fresh weights of shoots and roots, leaf area, and number of leaves significantly increased in plants supplied with 2.0 and 2.5 dS·m -1 NSC compared with the control, while the photosynthetic rate did not change under different concentrations of NSC. The total phenolic content and antioxidant capacity per shoot significantly increased with increasing in EC level of NSC; this trend became more pronounced over time. Moreover, the chicoric acid content significantly increased during growth up to 6 weeks after transplanting. These results suggest that NSC increases the biomass of C. denticulatum and that an EC level of 2.0 or 2.5 dS·m -1 is proper for accumulating high levels of phytochemicals, such as chicoric acid, in C. denticulatum grown in plant factories.

Iron (Fe) is essential for many vital processes in plants, including chlorophyll biosynthesis, DNA synthesis, nitrogen reduction, and photosynthetic electron transfer. However, free Fe ions also participate in the Fenton reaction, which generates reactive oxygen species that induce oxidative stress, leading to lipid peroxidation, protein oxidation, and DNA mutation. Accordingly, plants have developed strategies to prevent roots from absorbing excess Fe. Here, we investigated Fe homeostasis in hydroponically grown spinach and kale under various Fe concentrations (5-800 µM), as well as the resulting changes in bioactive compound levels. Spinach and kale Fe contents did not increase under Fe concentrations in the nutrient solution of up to 200 µM. When spinach plants (grown in nutrient solution containing 2 µM Fe-EDTA) were transferred to 50 µM Fe-EDTA, ferric-chelate reductase activity in roots rapidly decreased within 24 hours. In kale, ferric-chelate reductase activity also significantly decreased with increasing Fe-EDTA concentration. As Fe did not accumulate in spinach plants, no significant differences in plant growth were observed. However, in kale, root growth was slightly suppressed by high Fe (100-200 µM) levels in the rhizosphere. Comparisons to other studies suggest that variations in phenolic and flavonoid contents are dependent on plant species, but overall, Fe treatment did not result in a significant increase in these compounds in spinach or kale. Our results suggest that in spinach and kale roots, the regulation of Fe contents by ferric-chelate reductase is responsible for maintaining Fe homeostasis.

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.

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.

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.

Air anions affect plant growth by stimulating various biological mechanisms. We investigated the effect of atmospheric anion concentrations on plant growth and mineral concentration in kale (Brassica oleracea var. acephala cv. TBC) plants cultivated in a plant factory. Kale seedlings grown under normal growth conditions for two weeks were transplanted to a nutrient film technique (NFT) system in a plant factory equipped with light-emitting diodes (LEDs) [red:white:blue (RWB) = 8:1:1 and red:green:blue (RGB) = 8:1:1, 181 ± 4.0 µmol·m−2·s−1, 12 h photoperiod]. Three concentrations of air anions (low, 2.9 × 105 ions·cm−3; medium, 5.4 × 105 ions·cm−3; and high, 7.8 × 105 ions·cm−3) were applied to the kale seedlings for four weeks using high voltage air anion generators. The medium and high levels of air anions increased shoot fresh weight to approximately 1.5-fold compared to the control after four weeks. Medium and high-level air anion treatments led to significantly higher leaf area than the control. The medium level of air anions improved the photosynthetic rate at four weeks of treatment although there was no significant difference between air anion treatments and the control. In addition, transpiration rate and stomatal conductance were significantly increased in the low and medium levels of air anion treatments, which likely supported biomass accumulation. Air anions also increased mineral uptake. The content of macroelements (P, K, Ca, Mg, and S) was at least 1.5-fold higher for plants exposed to RGB LEDs and 1.3-fold higher under RWB LEDs exposure. Microelements (Fe, Mn, and Zn) were increased at least 1.6- and 1.3-fold in kale shoots treated with air anions under RGB and RWB LEDs, respectively. In conclusion, air anions had a positive effect on kale growth and air anion generators could be used as a new technology for enhancing plant growth in plant factories and greenhouses.

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.

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.

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.

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.

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.

The objective of this study was to determine the effect of physicochemical properties of inorganic supporting materials on the growth of potato plantlets in a nutrient-circulated photoautotrophic micropropagation (NCM) system. Nine mixtures consisting of combinations of perlite and/or vermiculite with three particle sizes (PL, PM, and PS = perlite large, medium, and small; VL, VM, and VS = vermiculite large, medium, and small, respectively) were used to cultivate potato plantlets in the NCM system. The ratios of vermiculite to perlite in the nine mixtures were determined by cation exchange capacity (CEC) and three distinct levels of water content. Among the nine mixtures with different values of porosity, bulk density, pH, and EC, the mixtures with high ratios of PS or VS had higher levels of water content, total porosity, and water-filled porosity than those with high ratios of PL or VL. Water holding capacity increased in mixtures with high levels of vermiculite. The EC and pH ranges of all mixtures were 1.2 to 2.5 mS·cm−1 and 6.3 to 7.2, respectively. Two mixtures containing 40 and 50% VS had exceptionally the highest EC and pH values. The optimum perlite and vermiculite content for potato plantlet growth was found in two treatments (PL:VL = 30:70 and PM:PS:VL:VM = 20:10:40:0) with a water content of 37% to 47% and CEC of approximately 17 cmol·kg−1. Regardless of the mixtures of supporting materials, conventional polycarbonate culture boxes retarded the growth of potato plantlet as compared with the NCM system. In conclusion, this study showed that the combinations of vermiculite and perlite with different particle sizes made different physical and chemical properties, and some of these mixtures improved growth of potato plantlets in the NCM system.

Appropriate temperature control of seeds leads to uniform germination and efficient management of the production of seedling grafts, which are required for successful cultural practices. In this study, the base, optimum, and maximum temperatures of four hot pepper cultivars were used as scions as well as four hot pepper cultivars used as rootstocks were estimated using a quadratic model. Seeds of the cultivars were germinated in growth chambers at constant temperatures of 20°C, 25°C, 30°C, and 35°C. Cumulative germination was described using a logistic function. The base, optimum, and maximum temperatures were estimated by regressing the inverse of the time to 50% germination (1/GR50) against temperature. Although germination rates varied according to cultivar and temperature, the highest germination rates were observed at temperatures of 25°C and 30°C. Wongang 1 was the most tolerant at low temperature, whereas Koregon PR-380 and Wongang 1 were the most tolerant at high temperature. Further, we suggest appropriate combinations of scion and rootstock cultivars based on our cardinal temperature results for the eight hot pepper cultivars.

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.