2021 年 90 巻 4 号 p. 357-364
Sweet basil (Ocimum basilicum L.), one of the most widely consumed herbs globally, is used in raw or processed food, and for aromatic essential oils. We investigated the effects of photosynthetic photon flux density (PPFD; 150, 225, and 300 μmol·m−2·s−1, herein, referred to as P150, P225, and P300) and red to blue light ratios (R/B ratio) (R:B = 1:4, 1:1, and 4:1, herein, referred to as R/B 0.25, 1.0, and 4.0) with a 16 h light period on the leaf shape and concentrations of functional and aromatic compounds in basil. Total leaf dry weight and leaf mass per area increased with increasing PPFD and R/B ratio. Total leaf area tended to increase with increasing R/B ratio at the same PPFD. Although the highest growth was noted when R/B was 4.0 at P300, the leaves showed ruggedness and curling. β-Carotene concentration based on the leaf dry weight and leaf area at the fourth node increased with decreasing R/B ratio, regardless of PPFD. Concentrations of aromatic compounds (eugenol and linalool) based on dry weight were significantly higher at P150 than at other PPFDs and in treatments with greater amounts of red light. These results suggest that basil growth, appearance, and functional and aromatic compound concentrations can be adjusted as needed by manipulating the PPFD and R/B ratio, although R/B 4.0 at P300 caused malformed leaves.
Stable production of crops can be achieved year-round in a plant factory with artificial light (PFAL) by controlling light, air temperature, humidity, carbon dioxide concentration, and air flow (Kozai and Niu, 2016). The main crops cultivated in PFALs are leafy vegetables; however, recently, the number of PFALs for cultivating herbs and strawberries has also increased in some countries. In most of the PFALs for production of leafy vegetables, the light source has changed from white fluorescent lamps (WFLs) to light emitting diode (LED) lamps, including white, mixed with blue and red lamps. Using blue and red LED lamps, growers control the photosynthetic photon flux density (PPFD) and ratio of red to blue light (R/B ratio) to optimize the growth and yield of leafy vegetables. Furuyama et al. (2013) reported that controlling the R/B ratio accelerated the growth rate of lettuce by causing changes in leaf area, thickness, and angle for light interception compared to the use of WFLs. Especially for herbs, yield and year-round stable production, as well as leaf morphology and quality (e.g., the concentration of functional and aromatic compounds), are important to receive a positive evaluation in food markets.
Sweet basil (Ocimum basilicum L.) is one of the most widely consumed herbs globally, and its leaves contain functional and aromatic compounds (Fig. 1). Demand and the prioritized characteristics of basil leaves vary depending on the usage by consumers because the leaves are used in raw, cooked, and processed foods, as well as for extracting essential oils. According to the eighth revised version of the Standard Tables of Food Composition in Japan (Ministry of Education, Culture, Sports, Science and Technology in Japan, 2020), basil contains abundant minerals, including K, Ca, Mg, and Cu. It also contains β-carotene, the precursor of vitamin A, and other vitamins, such as vitamins E and K. Among these, β-carotene is the most abundant and functional compound present in basil leaves. β-carotene is a photosynthetic, orange-colored pigment, which is converted into retinol, a terpenoid possessing antioxidant functions, in the small intestine (Bartley and Scolnik, 1995).
Functional and aromatic compounds in sweet basil leaves.
Furthermore, basil leaves contain some aromatic and bioactive compounds that affect the taste of food and health of humans. The main aromatic compounds present in basil are linalool and eugenol (Simon et al., 1990). Linalool and eugenol are synthesized through the mevalonate and shikimate pathways, respectively. Linalool is synthesized upstream of the terpenoid synthetic pathway as an intermediate compound of vitamins A and E. Eugenol is not only an important aromatic compound, but also a bioactive compound with sedative, spasmolytic, and anti-inflammatory activities. Sato and Sugawara (2003) identified relaxation effects and recovery of the ability to concentrate in individuals who inhaled the sweet smell of basil after a long period of constant work.
Conventionally, basil is commercially cultivated in open fields or greenhouses. It has been reported that the growth rate of basil and amount of oil are related to harvest timing, storage period, air temperature, drying duration (Chang et al., 2005), and solar radiation levels (Chang et al., 2008). However, it is difficult to understand the effects of environmental factors on the growth and quality of leaves and to maintain the same quality of basil year-round under natural light conditions.
Recently, the growth of basil and other herbs belonging the family Labiatae and their aromatic compound levels under a controlled environment have been investigated in a number of studies (Malayeri et al., 2010; Rehman et al., 2016; Goto et al., 2018). Pennisi et al. (2020) reported that a PPFD of 250 μmol·m−2·s−1 was optimal for growth and water, energy, and light use efficiency of lettuce and basil plants under a constant R/B ratio of 3.0 (R:B = 3:1). The effects of combinations of blue, red (Pennisi et al., 2019), green (Nguyen and Saleh, 2019), and other light qualities (Carvalho et al., 2016), including white light (Hosseini et al., 2018), on the growth (Frąszczak et al., 2014), concentrations of functional and aromatic compounds (Frąszczak et al., 2015), and antioxidant capacity (Carvalho et al., 2016) of basil under a constant PPFD have also been reported. Ohashi-Kaneko et al. (2013) cultivated some herbs under illumination with monochromatic red or blue light or mixed R/B light from LED lamps. They reported that the concentration of perillaldehyde in the leaves of perilla plants grown under blue light was higher than that under red light, while decanal concentration in coriander leaves under red light was higher than that under blue light. Thus, the PPFD and R/B ratio not only affects the growth of herbs, but also the concentrations of functional and aromatic compounds in these plants. Larsen et al. (2020) reported the effects of PPFD and light spectrum on basil growth, but not on secondary metabolites. Dou et al. (2018) reported the effects of daily light integrals on basil growth and nutritional compounds. However, the effects of the combination of PPFD and light quality on the aforementioned parameters in basil have not been studied.
Importantly, it has been surmised that the effects of the R/B ratio on the abovementioned parameters may be influenced by PPFD (Furuyama et al., 2013). To elucidate the effects of PPFD and the R/B ratio on plant growth and secondary metabolite levels, it is important to distinguish the factors affecting photosynthesis and photomorphogenesis. Experiments reported to date aimed at establishing a relation between growth rates and morphological changes at a fixed space and plant density. Therefore, the light environment, such as the PPFD on the top part of plants under treatment, and mutual shading conditions among plants may have changed during the experiments and remained unreported. Therefore, to understand the basic characteristics of plants and to ensure the reproducibility of the results, PPFD and light quality should be adjusted daily and any changes should be indicated clearly. In addition, allowing enough space between adjacent plants based on their growth is critical to avoid photomorphogenesis due to mutual shading.
In the present study, the effects of PPFD and the R/B ratio on the leaf shape and concentration of functional and aromatic compounds in sweet basil were investigated to determine suitable light conditions for meeting various consumer demands including appearance, yield, and compound quality.
This study was conducted in a controlled environment room at the Matsudo campus of Chiba University (Matsudo, Japan). Sweet basil (Takii & Co., Ltd., Kyoto, Japan) was used as the plant material. The seeds were sown in urethane sponges in a water-filled container. These germinated seedlings were grown under WFLs (FHF32-EX-N-H; Panasonic Corporation, Kadoma, Japan) set at PPFD 150 ± 5 μmol·m−2·s−1 until 20 days after sowing (DAS), one day before the start of the experiment. The light period was 16 h·d−1, air temperature was 25/20°C (light period/dark period), CO2 concentration was 1000 μmol·mol−1, and relative humidity was 70%. At 7 DAS, the water in the container was replaced with a nutrient solution, comprising 1/8 unit of Otsuka A prescription (16 mM NO3−, 4 mM H2PO4−, 4 mM Ca2+, 2 mM Mg2+, 8 mM K+, and 1.3 mM NH4+) (OAT Agrio Co., Ltd., Tokyo, Japan). The concentration of the nutrient solution was gradually increased from 1/8 to 1/2 unit, as the plant grew. At 21 DAS, 30 seedlings were transplanted into a container (0.54 m [W] × 0.38 m [D] × 0.65 m [H]) filled with 8 L of the 1/2-unit nutrient solution and the light treatment mentioned below was initiated.
Since basil is a wild herb and shows growth heterogeneity, seedlings with significantly different plant heights and leaf areas compared with the corresponding average values were thinned out to ensure that the size of seedlings at the start of treatment was uniform. A total of 20 and 10 plants were finally retained in each container at 30 and 37 DAS, respectively.
Experimental treatmentsThe treatment was started at 21 DAS, when the leaves at the second node emerged at a height of 2 cm. We used three PPFD levels (150, 225, and 300 μmol·m−2·s−1, herein, referred to as P150, P225, and P300, respectively) and three ratios of red to blue light (R/B ratio) (R:B = 1:4, 1:1, and 4:1, herein, referred to as R/B 0.25, 1.0, and 4.0) using tube-type LED lamps (DPT2RB120Q33 40 form; Showa Denko K. K., Tokyo, Japan), which could control the photon flux of red and blue lights independently. The blue and red LED chips had peak wavelengths at 466 nm and 665 nm, respectively. PPFD was measured using a light meter with a quantum sensor (LI-250 and LI-190; LI-COR, Inc., Lincoln, NE, USA). The spectral distribution of the light source was measured using a spectroradiometer (USR-45DA; USHIO Inc., Tokyo, Japan) (Fig. 2).
The relative spectral photon distribution of red and blue LED lamps (Showa Denko K.K., Tokyo, Japan) measured with a spectroradiometer (USR‐45DA; USHIO Inc., Tokyo, Japan). The blue and red LED chip had a peak wavelength at 466 and 665 nm, respectively. The maximum values were converted to 1.0.
In this experiment, we also used white LEDs (LDL40S·N/19/25; Panasonic) with three PPFD levels. However, the results obtained were quite similar to those for R/B 1.0 for each PPFD, and are presented as Supplemental data (Figs. S1–S6).
In this experiment, the PPFD at the top of the plant was adjusted to the set value every 3 days to maintain the value for all treatments. The light period, air temperature, relative humidity, and CO2 concentration were the same as the seedling cultivation period. We repeated this experiment twice. At 30 and 37 DAS, 5 plants were randomly selected to measure growth and analyze the concentrations of functional and aromatic compounds.
Plant growth measurementLeaf area, and fresh and dry weight of leaves at each node position were measured at 30 and 37 DAS. The leaves were numbered 1 through 5 from the bottom node. Subsequent to the measurement of leaf area using a leaf area meter (LI-3100; LI-COR), the leaf dry weight was measured after drying at 80°C in a convection oven (MOV-212F [U]; Panasonic) for over 72 h. Leaf mass per area (LMA) at the 4th node was then calculated using the leaf area and dry weight data.
β-carotene analysisβ-carotene analysis was performed by partially modifying the method described by Gilmore and Yamamoto (1991). Leaf discs were punched out from the leaves at the 4th node with a cork borer (ca. 0.04 g fresh weight, 4 cm2 × 2 pieces) at 37 DAS and stored in a freezer at −80°C for analysis. Based on our previous results, the β-carotene concentration in the leaves at the 4th node was higher and indicated a similar trend to the concentration in the leaves at other nodes.
During extraction, all processes were performed in a dark room. Six milliliters of pyrogallol/acetone solution (1/50 [w/v]) was added to the sample in a mortar, and the mixture was ground using a pestle and extracted. Thereafter, 1 mL of this solution was placed in a polypropylene tube, allowed to stand at 4°C for 12 h, and then centrifuged using an MX305 centrifuge (Tomy Seiko Co., Ltd., Tokyo, Japan) at 21900 × g for 10 min, after which the supernatant was collected. After adding 1 mL of pyrogallol/acetone solution to the sample remaining in the tube, the mixture was stirred and allowed to stand at 4°C. After 1 h, the sample tube was centrifuged again under the same conditions. The supernatant was mixed with the sample extract collected earlier, and the volume was adjusted to 7 mL. The resulting solution was filtered through a 0.20 μm polytetrafluoroethylene (PTFE) filter and analyzed using high-performance liquid chromatography (HPLC, LC-2010HT; Shimadzu Corporation, Kyoto, Japan).
A solvent containing methanol and hexane at a ratio of 5:1 (w/v) was prepared, filtered through a 0.20 μm membrane filter, and degassed with an ultrasonic cleaner for 5 min. An Inertsil ODS-2 column (particle size 5 μm, 4.6 × 250 mm; GL Sciences Inc., Tokyo, Japan) was used for chromatography. The column temperature was maintained at 30°C. Isocratic elution was performed at a flow rate of 1.5 mL·min−1 for 12 min. The injection amount was 10 μL, and the detection wavelength was 440 nm. HPLC-grade β-carotene (FUJIFILM Wako Pure Chemical Corporation, Osaka, Japan) was diluted in chloroform and used as the standard solution at 1, 2, 5, 10, and 20 mg·L−1. The standard solution was used to generate the calibration curve. After each analysis, the column was washed with 100% methanol for 1 min and the solvent was passed for 3 min before the next analysis. The concentration based on fresh weight was converted to that based on dry weight using the dry matter ratio.
Aromatic compound analysisLeaf discs from a leaf at each node (1–5 nodes) were punched out with a cork borer (ca. 0.08 g fresh weight, 4 cm2 × 4 discs) at 37 DAS and stored in a freezer at −80°C for analysis. Linalool and eugenol were assessed by the method of Leal et al. (2006), with some modifications. Briefly, a sample of the leaf blade (fresh weight, approximately 0.08 g) was homogenized in 0.5 mL of n-hexane using a mixer mill (MM200; Verder Scientific Co., Ltd., Tokyo, Japan) for 12 min. The homogenate was then centrifuged (micro-cooled centrifuge 1720; Kubota Corporation, Tokyo, Japan) at 21,900 × g for 10 min at 4°C.
The supernatant was filtered through a PTFE filter and subjected to gas chromatography using a Shimadzu GC-2014 gas chromatograph equipped with a hydrogen flame ionization detector (Shimadzu). The column used for gas chromatography was a Stabilwax (60 m × 0.25 mm) capillary column (Restek, Bellefonte, PA, USA); helium was used as the carrier gas at a flow rate of 1.7 mL·min−1. The temperature of the sample vaporization chamber was initially set at 50°C for 5 min; it was then increased to 180°C at a rate of 4°C·min−1, and maintained at that temperature for 20 min. The temperature was further increased to 240°C at a rate of 15°C·min−1 and maintained for 10 min. The total analysis time was 71.5 min.
To construct the calibration curve, linalool and eugenol standard solutions (FUJIFILM Wako Pure Chemical) for HPLC were used. Standard solutions at different concentrations were prepared by dilution with hexane to concentrations of 0.01, 0.05. 0.1, 0.5, and 1.0 mg·L−1. The measurement was repeated twice, and the concentration based on dry weight per plant was calculated using the dry matter ratio and leaf weight proportion for each node.
Statistical analysisAll data presented are mean values with standard errors. Treatment means were compared using the Tukey–Kramer test in Excel Statistical Analysis ver. 5.0 (ESUMI Co., Ltd., Tokyo, Japan), and the level of significance was set at P < 0.05.
In this study, leaves up to the 5th node emerged and expanded at 37 DAS in all treatment groups (data not shown). In the P150 treatment group, the plant height and shape at 30 DAS, 10 days after the start of the treatment, did not show significant differences among the R/B ratio treatments (Fig. 3). At 37 DAS, the plant height in the P150 group was almost double that at 30 DAS and was higher with increases in the R/B ratio. A similar tendency was observed for the P225 and P300 groups (data not shown).
Effect of the R/B ratio on plant growth at 30 and 37 DAS in the P150 treatment.
In all treatments, the total leaf dry weight increased with increases in PPFD and the R/B ratio at 37 DAS (Fig. 4A). This trend was similar for the leaf dry weight of leaves at the upper 4th and 5th nodes; however, it was the lowest among the P300 and R/B 4.0 treatments (Fig. 4B). Although the highest total leaf dry weight was recorded in the R/B 4.0 and P300 treatment, the leaves, especially those at the upper nodes, in this treatment showed physiological disorders, such as ruggedness and curling (Fig. 5).
Effects of photosynthetic photon flux density (PPFD) and the R/B ratio on total leaf dry weight (A) and leaf dry weight at the 4th node (B) at 37 DAS. Vertical bars indicate standard errors. Different letters indicate significant differences among the same PPFD at P < 0.05 determined using the Tukey–Kramer test (n = 5).
Appearance of leaves grown under R/B 4.0 treatment with P300. Red arrows show the malformed parts.
The total leaf area at 37 DAS was not significantly different among PPFDs, but tended to increase with increases the in R/B ratio at the same PPFD (Fig. 6A). The leaf mass per area (LMA) at the 4th node in the P150 and P225 treatments at 37 DAS increased with increases in the PPFD and R/B ratio; however, LMAs in the P300 treatment were the same among the treatments (Fig. 6B).
Effects of PPFD and the R/B ratio on total leaf area (A) and leaf mass per area (LMA) at the 4th node (B) at 37 DAS. Vertical bars indicate standard errors. Different letters indicate significant differences among the same PPFD at P < 0.05 determined using the Tukey–Kramer test (n = 5).
The β-carotene concentration per unit dry weight in leaves at the 4th node decreased with increases in the PPFD and R/B ratio in all treatments (Fig. 7A). On the contrary, the β-carotene concentration per unit leaf area increased with increases in PPFD and decreased with increases in the R/B ratio in all treatments (Fig. 7B).
Effects of PPFD and R/B ratio on the β-carotene concentration per unit leaf dry weight (A) and per unit leaf area (B) at the 4th node at 37 DAS. Vertical bars indicate standard errors. Different letters indicate significant differences among the same PPFD at P < 0.05 determined using the Tukey–Kramer test (n = 5).
The concentrations of both aromatic compounds, linalool and eugenol, per unit dry weight (Fig. 8) and per unit leaf area (data not shown) at all the nodes were significantly higher in the P150 treatment than in the other PPFD treatments. Among the P150 treatments, these concentrations increased with increases in the R/B ratio. The contents of aromatic compounds per plant were higher at R/B 4.0 in the P150 and P300 treatments than in the P225 treatment (data not shown).
Effects of PPFD and R/B ratio on the concentration of linalool (A) and eugenol (B) per unit leaf dry weight at all the nodes at 37 DAS. Vertical bars indicate standard errors. Different letters indicate significant differences among the same PPFD at P < 0.05 determined using the Tukey–Kramer test (n = 5).
In many plants, it is known that photosynthesis and growth are promoted with increases in PPFD and the R/B ratio (Furuyama et al., 2013). There are similar reports that increases in PPFD (Larsen et al., 2020; Pennisi et al., 2020) and in the ratio of red to blue light (Pennisi et al., 2019) promotes basil growth. Based on our results, it was also considered that higher PPFD and a larger amount of red light promoted the growth (total leaf dry weight and LMA) of basil, except for the R/B 4.0 treatment with P300. Since most of the leaves were fully expanded at 37 DAS, the total leaf area among all treatments and the LMA among the R/B ratios in the P300 treatment showed similar values. These values may be the maximum for basil leaves regardless of malformed leaves.
The leaves in the R/B 4.0 treatment with P300 showed malformation after 32 DAS (data not shown), and the entire plant became curly at 37 DAS. Fukuda et al. (2003) cultivated leafy vegetables using red LED lamps alone with a PPFD of 100 μmol·m−2·s−1. They reported that malformed or curly leaves under the red LED lamps were suppressed by supplementing blue light at 50 μmol·m−2·s−1. In contrast, Pennisi et al. (2020) reported that a PPFD of 250 μmol·m−2·s−1 was optimal for the growth of lettuce and basil plants compared with a PPFD of 300 μmol·m−2·s−1 under an R/B ratio of 3.0. Although they did not mention leaf shape in the treatment with a PPFD of 300 μmol·m−2·s−1, it is suggested that the photosynthetic rate and the accumulation of photoassimilates may be inhibited under the high PPFD of red light.
The treatment of R/B 4.0 with P300 used approximately 240 μmol·m−2·s−1 of red light and 60 μmol·m−2·s−1 of blue light. In contrast, no abnormality was found at R/B 4.0 in the P150 and P225 treatments, which used 120 and 180 μmol·m−2·s−1 of red light and 30 and 45 μmol·m−2·s−1 of blue light, respectively. Since the malformed leaves occurred only under R/B 4.0 in the P300 treatment among all treatments including the white LED exposure (data not shown), it is possible that excessive red light caused the abnormality rather than insufficient blue light or an imbalance in the R/B ratio. In other words, it was suggested that morphological abnormalities may occur in the leaves in the range of 180 (R/B 4.0 in P225) to 240 (R/B 4.0 in P300) μmol·m−2·s−1 of red light during basil cultivation. It appears that R/B 4.0 in the P300 treatment may not be suitable to meet the demand for basil leaves with a good shape and appearance.
β-carotene as a functional compoundβ-carotene is a carotenoid present on the surface of leaves, for example, in chromoplasts on the surface of the leaves (Bartley and Scolnik, 1995). Based on our results, it is possible that the accumulation of β-carotene is enhanced with increases in PPFD, whereas that at the site of accumulation, such as the leaf surface, is limited. Therefore, the β-carotene concentration based on the dry weight may be deceased by increasing the leaf weight or leaf thickness due to the increase in PPFD.
To elucidate the effect of photosynthetic radiant flux density (PRFD) on the β-carotene concentration, Oyama et al. (1999) cultivated spinach and Boston lettuce under different PRFDs using a light shading material in a greenhouse and reported that the β-carotene concentration per leaf fresh weight decreased with increases in PRFD and total leaf fresh weight. Hikosaka et al. (2014) investigated the influence of PPFD on the β-carotene content of rucola (Eruca vesicaria) grown under artificial light. They reported that the β-carotene concentration based on leaf dry weight decreased at higher PPFD, whereas the β-carotene content per plant increased. Their results also suggest that β-carotene production may be promoted at higher PPFDs; however, the rate of accumulation of photoassimilates in leaves was only higher than that of β-carotene accumulation in the leaf surface under high PPFD. Therefore, a high PPFD appears to be better for producing basil with a high β-carotene content.
Carotenoids, including β-carotene, are essential structural components that protect photosynthetic systems against potentially harmful photo-oxidative processes (Lawlor, 1993; Bartley and Scolnik, 1995) and high PPFD or short-wavelength light, such as blue and UV light (Ben-Amotz et al., 1989; White and Jahnke, 2002). It is possible that the increase in the amount of blue light absorbed by β-carotene in our experiment may have promoted the biosynthesis of β-carotene. There are many reports on the effect of blue light on the β-carotene concentration in leaves. Li et al. (2011) investigated the β-carotene content of three spinach cultivars grown under red and blue fluorescent lamps with PPFD levels of 100 and 300 μmol·m−2·s−1, and found that the β-carotene concentrations per leaf dry weight were higher under blue light than under red light in both PPFD conditions for all cultivars. According to this report and also as observed in this study, blue light is an effective light source for increasing β-carotene concentration based on both leaf dry weight and leaf area.
Eugenol and linalool as aromatic compoundsThere are many reports on the concentrations of aromatic compounds in sweet basil under artificial light environments. For example, the effects of five levels of PPFDs with the same R/B ratio (Pennisi et al., 2020), several R/B ratios with the same PPFD (Hosseini et al., 2018; Pennisi et al., 2019), and combinations of several light qualities (Carvalho et al., 2016) on the concentration of aromatic and bioactive compounds and antioxidant capacity have been investigated. However, no clear trend or effect resulting from PPFD or light quality was reported in these studies.
On the other hand, in some reports and review articles, it has been mentioned that red light treatment induces the biogenesis of essential oils (Sangwan et al., 2001). It was also observed that in geranium (Pelargonium graveolens), excised leaves exposed to red light enhanced the biogenesis of essential oil from radiolabeled precursors (Sangwan et al., 2003). These reports are in agreement with our finding that the concentrations of two aromatic compounds, eugenol and linalool, were significantly increased in treatments with greater amounts of red light than in those with blue light. Since leaf growth was promoted by red light rather than by blue light at the same PPFD, it was hypothesized that the synthesis of aromatic compounds was promoted by high growth rates. However, this hypothesis was rejected because the concentrations of both aromatic compounds at P150 were the highest among the PPFD treatments.
Considering the effect of PPFD, there are two possibilities for the high aromatic concentrations in the P150 treatment—the effect of leaf age and the negative effect of high PPFD. Shanker et al. (1999) reported that the concentration of essential oil in Japanese mint (Mentha arvensis) per unit fresh weight and per unit leaf area showed maximum values at an early stage of leaf unfolding and decreased gradually depending on the leaf age. As trichomes over-mature, subcellular integrity is compromised and cellular constituents/inclusions slowly disappear, leaving the secretory cell vacant and empty. In our study, although the number of emerged leaves did not differ significantly among the treatments, the leaf age in the P150 treatment may have been younger than in P225 and P300.
Increasing PRFD or PPFD, regardless of sunlight or artificial light conditions, enhanced the production of trichomes, which accumulate essential oils and often induce high yields of essential oils in many herbal plants (Sangwan et al., 2001; Fernandes et al., 2013). Fernandes et al. (2013) reported that the eugenol concentration did not change under several PPFDs, although the total essential oil yield increased. In addition, mint (Mentha arvensis) grown under 25% of PPFD produced an essential oil with the best commercial profile compared to plants grown under 100% of PPFD, which produced more essential oil (De Souza et al., 2016). Furthermore, light is considered to reduce essential oils by accelerating the autoxidation processes (Turek and Stintzing, 2013). Compositional changes proceeded considerably faster when illumination was used. Monoterpenes, including linalool, have been shown to degrade rapidly under light (Misharina et al., 2003).
Plants with high concentrations of aromatic compounds are generally receive a high evaluation in markets. Therefore, the best light condition for basil production with high concentrations of aromatic compounds may be R/B 4.0 at P150. Meanwhile, the best light condition for high yield of aromatic oils may be R/B 4.0 at both P150 and P300.
ConclusionsWhen the PPFD and R/B ratio (R/B ratio: 0.25–4.0) were adjusted, the growth of sweet basil leaves was promoted and thick leaves could be formed when the PPFD and the R/B ratio increased. However, when the amount of red light was approximately 240 μmol·m−2·s−1, morphological abnormalities were observed in the leaves. The concentration of β-carotene and aromatic compounds per unit dry weight of leaves increased with increasing amounts of blue and red light, respectively. Although the tendency of increase was different for these compounds, growers will be able to select optimal light conditions for leaf weight, good appearance, thickness, β-carotene, and aromatic compound concentration and/or content by controlling the PPFD and R/B ratio.
