Plant Growth Optimization Using Amber Light Supplemented with Different Blue Light Spectra

Abstract

Blue (400–500 nm) and red (600–700 nm) light regions have been investigated for their effects on photosynthesis and plant growth, yet evidence for specific blue light wavelengths in plant research is lacking. Investigations into amber (595 nm) light are similarly limited. To ‘shed light’ on these two important wavelengths, this study investigated the combined effects of blue and amber light on plant growth and development in two model plants: tomato (Solanum lycopersicum cv. Beefsteak) and lettuce (Lactuca sativa cv. Breen). Plant growth responses were determined with four light treatments: B+BA (blue + broad amber, 455–602 nm), RB-NA (royal blue + narrow amber, 430–602 nm), RB-BA (royal blue + broad amber, 423–595 nm), and high-pressure sodium at a PPFD of 250 µmol m⁻² s⁻¹. After 21 days, the highest fresh and dry mass for both plant species was obtained under the RB-BA light treatment. Shifting the blue wavelength from 430 nm to 455 nm with broad amber lighting led to 40% less fresh mass for tomatoes, whereas only an approximate 5% reduction in fresh mass was observed for lettuce plants. Our findings demonstrate that an alternate and combined blue + amber light spectrum is effective for optimizing plant productivity.

1. Introduction

Energy-efficient light-emitting diodes (LEDs) have impelled a marked shift in electrical lighting systems for controlled environment agriculture [1,2]. Low maintenance and relatively long lifespans render LEDs additionally advantageous, reducing vegetable production costs [3,4]. LEDs offer different wavelengths of emitted light that can be achieved by combining narrow bandwidths. This control over the spectral output allows for light photon wavelength selection that can promote photosynthetic activity, nutritional value, and leaf coloration [5,6]. Targeting the activity of specific pigments/photoreceptors with this approach can be achieved through the irradiance of their associated activation spectrum [7].

Blue (400–500 nm) and red (600–700 nm) LED wavelengths have been extensively studied in the context of improving biomass production [8,9,10], as these spectra are best aligned with the absorption peaks of chlorophyll a, which plays a major role in driving photosynthesis. Blue light influences plant morphology by increasing shoot and root biomass, chlorophyll a-to-b ratio, carotenoid levels, and other polyphenol and antioxidant content, while decreasing the shoot-to-root ratio and leaf areas in red lettuce seedlings compared to white, fluorescent lamps [11]. Various studies have highlighted the effect of blue on the accumulation of metabolites in plants [12,13]. Published data on lettuce showed that although blue light did not insignificantly increase the fresh mass of lettuce, blue light increased the chlorophyll a/b from 3.4 to 4.1 compared to the white, fluorescent light [11]. However, earlier research on the spectral response of photosynthesis concludes that 450–460 nm light is not the most efficient wavelength for inducing photosynthesis [14,15]. Quantum yield curves determined by McCree [14] and Inada [15] show that the most efficient wavelength range for inducing photosynthesis in the blue light region is between 400 and 430 nm. In tomato plants, blue light increases chlorophyll and flavanol content in certain genotypes [16]. It is contrarily reported that a higher percentage of blue light leads to growth suppression [8]. Blue light can lead to a chloroplast avoidance response [17] and reduce the quantum yield of the CO₂ fixation [18].

LED lighting with spectra in the green to amber range (500–600 nm) allows for deeper penetration depth in the leaf environment, which activates the lower chlorophyll layer [19,20]. The study on tomato plants examined the effects of LED spectra on the mycorrhizal symbiosis of the plants and concluded that a 66% red to 33% green light ratio enhanced soil urease activity, mycorrhizal development, and nitrogen content in leaves [20]. A study examining the effects of various light wavelengths on tomato plants concluded that green light promoted tall plants with large leaf areas [21]. The authors noted that red, amber, and white light displayed comparable results relating to height, leaf area, and biomass production, and that plants grown initially under green light, followed by red light, had the highest photosynthetic rate. Recent studies have examined the impact of amber light in tomato (Solanum lycopersicum cv. Beefsteak) [22], lettuce (Lactuca sativa, cv. Breen) [23], tomato (Solanum lycopersicum cv. Beefsteak) [24], and Arabidopsis [25]. These studies report that amber light leads to substantial biomass accumulation and high photosynthetic activity at lower light intensities (<1000 μmol m⁻² s⁻¹).

Our current knowledge of blue and amber lights is mainly built on combined blue and red LED light studies [26,27], in addition to the well-established spectral quality of photosynthesis [14,15]. Limited research studies have evaluated the impact of different blue light wavelengths and their interactive effects with amber light on plant growth and development. To test the effects of these two pivotal wavelength ranges, this study evaluated biomass accumulation (vegetative stage) under two different blue light wavelengths combined with amber lighting at different bandwidths. Specifically, the effects of royal blue (430 nm), blue (455 nm), broad amber (595 nm), and narrow amber (595 nm) on overall tomato and lettuce plant biomass production, chlorophyll content, and flower formation were examined.

2. Materials and Methods

2.1. Plant Materials

Tomato (Solanum lycopersicum cv. Beefsteak) and lettuce (Lactuca sativa cv. Breen) obtained from Johnny’s Selected Seeds (Winslow, ME, USA) were used. A total of 24 seeds per species were germinated in pre-soaked rockwool cubes (25 × 25 × 30 mm, Grodan, Etobicoke, ON, Canada), partially submerged in water on germination trays (0.28 × 0.54 m, Mondi Products, Vancouver, BC, Canada) inside of a growth chamber (TC30, Conviron Controlled Environment Ltd., Winnipeg, MB, Canada). The chamber’s environment was maintained at 23/21 ± 1 °C (day/night) and 50 ± 10% relative humidity, with a photoperiod of 16 h d⁻¹ supplied by cool white, fluorescent light bulbs (150 µmol m⁻² s⁻¹, 4200 K, F72T8CW OSRAM Sylvania Inc., Wilmington, MA, USA) for 20 days. After the first week of germination, the seedlings were provided with a half-strength Hoagland nutrient solution for two weeks [28], followed by a full-strength Hoagland solution containing the following nutrients: nitrogen (210 mg L⁻¹), phosphorus (31 mg L⁻¹), potassium (235 mg L⁻¹), calcium (200 mg L⁻¹), magnesium (48 mg L⁻¹), sulfur (64 mg L⁻¹), iron (1 mg L⁻¹), manganese (0.5 mg L⁻¹), zinc (0.05 mg L⁻¹), boron (0.11 mg L⁻¹), copper (0.01 mg L⁻¹), and molybdenum (0.11 mg L⁻¹).

2.2. Light Treatments and Greenhouse Growing Conditions

Four experimental light treatment zones were set up in a north-south oriented greenhouse bay at McGill University’s Macdonald Campus in Sainte-Anne-de-Bellevue, QC, Canada. Light treatments were based on a previously described method examining blue and narrow amber light on tomato plants [22]. The present study included royal blue + narrow amber light (RB-NA; 430–602 nm, 600 W, Shenzhen Idea Light Ltd., Shenzhen, China), royal blue + broad amber light (RB-BA; 423–595 nm, 600 W, VANQ Technology Co., Ltd., Shenzhen, China), blue + broad amber (B-BA; 455–602 nm, 150 W, U Technology Inc., Calgary, AB, Canada), and a single-ended HPS lamp (ED18 400W, Philips, Amsterdam, The Netherlands). The bandwidths for narrow and broad amber light were 20 nm and 80 nm, respectively. Spectral characteristics, including peak wavelengths and intensity ratios, were confirmed and determined with a spectroradiometer (PS-300, Apogee, Logan, UT, USA) (Figure 1 and Table 1). The full width at half maximum (FWHM) for the different light systems used for this study is presented in Table 2.

The spectral reading of the RB-NA treatment showed wavelength shifting toward shorter wavelengths for both peaks, approximately 2–5 nm shorter when compared to the manufacturer’s specifications sheet, which was attributed to an active cooling approach, leading to a lower junction temperature. Initial experimentation showed that this small wavelength discrepancy had little impact on plant growth between treatments. After installing each light in their individual shade cloth units, a light map with 12 grids was acquired using the spectroradiometer. Due to low photosynthetic photon flux density (PPFD) uniformity commonly conveyed with LED lighting, only six grids with the same PPFD level were used for the experiment, additionally ensuring proper spacing between plants. Before starting the experiment, stray light testing was conducted. Each light treatment zone was separated and closed off with a double-layered shade cloth (8MK808, Harnois, St-Thomas, QC, Canada). The shade cloth was 1.2 m high and open at the top. The cloth was 96% effective at preventing light from entering at the sides, reducing wavelength interference from other light treatment zones. This allowed for independent light treatment analyses. All lighting fixtures were suspended above the center of the hydroponic system in each light treatment zone below the double-layer shade cloth.

After the emergence of the second true leaf (20 days after sowing), plants were transplanted from the growth chamber into the experimental light treatment zones (six plants per treatment). Both tomato and lettuce plants were cultivated for 21 days under PPFD levels of 250 µmol m⁻² s⁻¹ with a 16 h d⁻¹ photoperiod, and this provided a 14.40 mol m⁻² d⁻¹ daily light integral. Consistent PPFD levels were provided throughout this plant growth period by adjusting the distance between the light source and the top of the plant canopy every 3 days. PPFD levels were recorded at each plant position after light adjustment using the spectroradiometer (LI-250A; LI-COR Inc., Lincoln, NE, USA) with an underwater quantum sensor (LI-192, LI-COR Inc.).

In each light treatment zone, temperature and humidity were recorded every 10 min with Hobo sensors (S-THB-M002, OnSet, Hobo, Bourne, MA, USA). Temperatures in the greenhouse were set at 20 ± 1 °C during the day and 17 ± 1 °C at night. Customized ebb and flow hydroponic systems were used to irrigate plants with a 10-min/h flooding period using full-strength Hoagland solution maintained at a pH and electrical conductivity (EC) of 6 and 1.6 mS cm⁻¹, respectively. The Hoagland solution was stored in 40 L basins below the growing trays that rested on a wire bench, 1 m above the floor, and replaced with a new solution every week. Upon replacement, water loss, pH, and EC were recorded. Air pumps (Elite 802 and Marina 200, Rolf C. Hagen, Baie d’Urfé, QC, Canada) were used to oxygenate the solution. Submersible pumps (Aquakingdom SP1200 submersible pump, Guangzhou, China) were used to accomplish the irrigation process and timing.

2.3. Plant Growth Parameters (Postharvest) and Statistical Analysis

Plants were harvested after 21 days of light treatment, and the aerial parts of the plants were assessed for this study. Plant fresh mass (FM), dry mass (DM), and chlorophyll content (Chl) were recorded. The Chl in leaves was measured using a SPAD [Soil Plant Analysis Development] meter (Spectrum Technologies, Chlorophyll Meter SPAD-502Plus, Konica Minolta, Sakai, Osaka, Japan) [29,30,31]. For tomato plants, flower count, stem diameter, and plant height were additionally recorded at harvest. The FM and DM of each plant were measured using a balance (APX-153, Denver Instruments, Bohemia, NY, USA). Plants were dried in an oven at 80 °C (S.D. ± 1 °C) until there were no significant (p < 0.05) changes in mass. Although destructive chlorophyll tests were not conducted, the total chlorophyll content per leaf area was calculated using the model (Equation (1)) generated previously for tomato plants under different light treatments [29]. The model (Equation (2)) on the relationships between SPAD and the total chlorophyll content was used to determine the total chlorophyll content (g of chlorophyll/100 g fresh tissue) for the lettuce plants under different light treatments [31].

SPAD = (0.0492 × total chlorophyll content per leaf area) + 25.084(1)

Total chlorophyll content (g of chlorophyll/100 g fresh tissue) = 3.4 × SPAD − 19(2)

An analysis of variance between the data among the four light treatments was achieved using a single factor (alpha = 0.05) ANOVA of means. The data considered included FM, DM, and Chl, as well as flower count, stem diameter, and plant height for tomato plants. This was accomplished using JMP software (version 17, JMP Statistical Discovery LLC, Cary, NC, USA). For statistically significant data, a post hoc test was accomplished using Tukey’s Honest Significant Difference (HSD) analysis to pinpoint the sources of significance within the data. In addition, Pearson correlation analysis was performed to assess the potential correlations between various wavebands of light (Table 1) and the growth parameters of tomato and lettuce. The entire experiment was repeated three times using six plants per light treatment for each experimental run.

3. Results

3.1. Growth Responses under Different Supplemental Blue Wavelengths

The effect of the four different light treatments (RB-NA, RB-BA, B-BA, and HPS) on tomato and lettuce growth parameters was examined. Figure 2 summarizes the growth responses (FM, DM, and Chl) of tomato and lettuce plants grown under different light treatments for 21 days. Light treatments differentially affected tomato and lettuce plant growth, particularly under RB-BA light treatment. RB-BA light led to a higher FM in tomato plants (156.4 ± 16.8 g), followed by HPS (139.9 ± 14.7 g), B-BA (95.8 ± 11.2 g), and RB-NA (76.4 ± 9 g). A similar trend in lettuce plant FM grown was observed, with a reversed order between HPS and B-BA light treatments. In lettuce plants, the highest FM obtained was with RB-BA light (51 ± 2.8 g), followed by B-BA (47.7 ± 2.72 g), HPS (45.9 ± 2.6 g), and RB-NA (35.5 ± 2.6 g). More distinct responses with RB-BA and HPS treatments were observed in tomato plants than in lettuce plants. These two treatments resulted in an 80–200% increase in tomato FM compared to the lowest tomato FM (RB-NA light), while there was only a 35–40% increase in FM for lettuce plants. Tomato and lettuce plant DM were similar to FM; however, there were no significant differences in lettuce DM between light treatments. In tomato plants, RB-BA light resulted in the highest DM in tomato plants (10.36 ± 1.10 g), followed by HPS (9.64 ± 1.04 g), B-BA (11.18 ± 6.39 g), and RB-NA (8.96 ± 5.53 g). In lettuce plants, the highest DM was observed under RB-BA light (1.57 ± 0.08 g), then followed by B-BA (1.48 ± 0.07 g), HPS (1.49 ± 0.14 g), and RB-NA (1.25 ± 0.12 g).

Light treatments had less impact on Chl in tomato plants than in lettuce plants, as statistical analyses showed no significant (p < 0.05) differences in Chl for tomato plants grown under the different light treatments (Figure 2C,D). Chl in tomato plants determined with the SPAD meter was highest under RB-NA (43.36 ± 1.33), followed by RB-BA (41.88 ± 0.73), HPS (41.41 ± 0.78), and B-BA (40.67 ± 0.90). The SPAD (Soil Plant Analysis Development) meter is a rapid, accurate, and non-destructive measurement of leaf chlorophyll levels in research and agricultural applications for various plants. Although non-destructive tests were not used for chlorophyll analyses, research has shown a strong correlation (R² = 0.9) between SPAD and chlorophyll content per leaf area for monocots and dicots [29,30,31]. Statistical analyses showed the different light treatments did not have a significant (p < 0.05) effect on the accumulation of chlorophyll in tomato plants. The highest Chl in lettuce observed was under RB-NA (26.50 ± 1.36) followed by RB-BA (23.97 ± 0.67), B-BA (22.37 ± 0.99), and HPS (20.57 ± 0.42). Similar observations were made for the total chlorophyll content derived using published models (Figure 3) [29,31]. Plants treated with RB-NA had the highest total chlorophyll content for tomato (371.46 µmol m⁻²) and lettuce (71.41 g of chlorophyll/100 g fresh tissue).

3.2. Other Growth Parameters in Tomato Plants

Flower count, stem diameter, and plant height were assessed for tomato plants under the four light treatments. Some light treatments resulted in distinct growth and development parameters for tomato plants, except for stem diameter (Table 3). RB-BA light yielded the highest flower counts, nearly 2-fold more than the lowest counted with RB-NA. No significant difference in stem diameter was observed between light treatments (7–8 mm). For plant height, RB-NA light yielded the shortest plant height among the light treatments (Table 3). The height of tomato plants grown under RB-NA light was 20–25% shorter than any other light treatments, and these stunted growth responses were emphasized when a higher fraction of 430 nm light was used.

3.3. Pearson Correlation Analysis

To further unravel the relationships between various wavebands and growth parameters, a Pearson correlation analysis was conducted (Figure 4 and Figure 5). Strong negative correlations were observed between FM and wavelengths within the 400–450 nm range, which corresponds to −0.71 for tomato and −0.94 for lettuce. Similarly, DM exhibited a negative correlation with the same wavelength range. The wavelength intervals of 551–600 nm, however, exhibited a contrasting and species-dependent response, where a positive correlation was observed with tomato FM and DM, as opposed to no correlation with lettuce FM and DM. The 651–700 nm and 701–750 nm wavelength ranges displayed positive correlations with FM for both tomato and lettuce plants. Strong positive correlations were found between the SPAD index and wavelengths within the 400–450 nm range for both experimental plant species. The SPAD index of tomato and lettuce plants displayed negative correlations with longer wavelengths, particularly in the 501–550 nm, 651–700 nm, and 701–750 nm ranges. Apart from biomass and Chl, the Pearson correlation analysis showed contrasting results regarding the major wavebands of interest. The 400–450 nm wavelength range displayed negative correlations with flower number, stem diameter (SD), and plant height (H), whereas the wavelength ranges of 551–600 nm, 651–700 nm, and 701–750 nm exhibited positive correlations with these growth and development parameters.

4. Discussion

The effects of blue wavelengths on lettuce growth and development have been scrutinized for many years [32,33,34]. The study focused on lettuce (‘Mid-season’) growth using sole blue LED light within the range of 432 to 466 nm (4–6 nm intervals) revealed that the shoot fresh and dry biomass of lettuce was significantly influenced by specific blue LED peaks, with the most substantial biomass observed at 432 nm [33]. However, contrasting results were reported under broad spectrum lighting in a recent study [35]; the impact of shifting the blue spectrum between 437 and 453 nm under full spectrum light conditions on lettuce biomass was explored, and data were consistent with prior findings, indicating that such spectral shifts did not significantly affect lettuce biomass [35]. These findings align closely with the outcomes observed with lettuce in our current study (Figure 2B and Figure 4). The highest biomass yields (FM and DM) were obtained under RB-BA light, and RB-NA light led to lower biomass yields for both plant species (Figure 2A,B). When blue wavelengths shifted from 430 nm (RB-BA light treatment) to 455 nm (B-BA light treatment), FM decreased by 40% in tomato plants and 5% in lettuce plants. As both treatments have nearly identical main peaks (BA, 595 nm light), this decrease in FM appears to result from variations in supplemental blue light wavelengths. While the environmental influence (light, temperature, etc.) over tomato flowering, flower/fruit abortion, and fruit set is well established [36,37,38], it is important to note that the observed increase in flower number in plants treated with RB-BA compared to the reference light, HPS, may not lead to greater biomass in harvestable fruit. Results for Fagopyrum esculentum showed that reducing the number of flowers by half significantly increased the number of developed embryos and seeds per plant [38]. How these findings translate in an extended greenhouse study for a given tomato variety grown to maturity is an imperative next step when evaluating the benefits of light recipes.

The effect of blue and narrow amber light treatment on the biomass of tomato plants was recently reported [22]. This study reported that supplementing narrow amber with narrow blue light resulted in a 39.9% and 29.6% increase in FM and DM, respectively, compared to the monochromatic narrow amber light treatment. Similar observations were made for the chlorophyll content in leaves and stem diameter. Other studies show that compared to narrow amber, broad amber promotes plant growth parameters [22,23]. These results complement the findings of this study.

Apart from lettuce, similar responses regarding shifting blue wavelengths were reported for various plant species. A study on kale varieties (‘Toscano’, ‘Redbor’, and ‘Winterbor’) under full spectrum light conditions, supplementing with different blue LED peaks at 400, 420, and 450 nm showed that the supplemental blue LED peaks did not exert any discernible impact on the biomass or physical characteristics of the kale plants [39]. Published data on perilla cultivars (Perilla frutescens var. acuta) showed that supplemental 415 nm and 430 nm similarly did not improve the growth of the plants [35]. Results for tomato plants highlighted the importance of the light intensity of blue light supplemented with amber light (445–595 nm) [24]. This study showed growth suppression above a PPFD of 1000 μmol m⁻² s⁻¹ with the highest DM of 2.4 g at 847 μmol m⁻² s⁻¹.

Our experiment with tomatoes highlighted an intriguing sensitivity of these plants to shifts in blue wavelengths, a response not previously documented in existing studies. This distinct reaction could be linked to the prevalent use of blue/red light or the incorporation of limited amounts of amber light in prior research. A particularly intriguing avenue for further exploration emerges in the form of a combined 430 nm blue light and broad amber light in different ratios. The distinct impact observed in this combination warrants more scrutiny to elucidate its underlying mechanisms and potential applications in tomato plant growth and development.

Although McCree [14] and Inada [15] action spectra show a higher photosynthetic action within the red region (650 nm), various studies show that monochromatic amber (595 nm) increases FM and DM when compared to red light [22,23,24,40,41]. Research studies show that the addition of red (635 nm) to amber can reduce the fresh mass of tomato plants by 33% at 800 PPFD [24,41]. However, the addition of 655 nm (red) to amber (595 nm) has a comparable photosynthetic rate to monochromatic amber (595 nm). Various studies have highlighted the influence of increased red/blue light ratio on plant growth and development [10,42,43]. Data for strawberry plants showed a higher fresh and dry mass for strawberry plants cultivated under 10:1 and 19:1 ratios of red-to-blue LEDs compared to HPS and a low red-to-blue ratio of 5:1 [42]. Increases in both lettuce and tomato fresh mass, flower count, and height of tomato plants may be attributed to the increased in the red/blue ratio in RB-BA (16.9), B-BA (9.2), and HPS (4.5) compared to RB-NA (1.9). Future studies must investigate the optimized ratio of red/blue light for maximum plant growth and development.

There are several reasons for using LEDs in plant photobiology studies, and the use of narrow light spectra is one important advantage [44,45]. However, the impact of light bandwidth with the same peak wavelength on plant growth and development has not yet been fully investigated, and cultivar-related effects require some assessment. In this work, we newly report that the observed impact of the bandwidth of amber light is greater in tomatoes than in lettuce plants. When the amber light bandwidth is narrowed from 80 nm to 20 nm, FM decreased by 50% in tomatoes, yet by only 30% in lettuce plants. It is worth noting that recent research has indicated conflicting results, in which narrowing the amber light bandwidth resulted in increased biomass of tomato plants when cultivated for 60 days [22]. The bandwidth of amber light might hold significant importance in the context of the leaf environment and the age of tomato leaves due to its unique interactions within the leaf structure. Green and amber light (500–600 nm) can be efficiently absorbed by unsaturated chlorophyll of high reflectance within leaf environments. This characteristic becomes particularly relevant when considering the diverse environments and developmental stages of tomato leaves. Different cultivation periods (21 and 60 days) result in varying levels of overall intra-leaf absorption (e.g., leaf thickness), influencing the efficiency of photosynthesis. This variance in intra-leaf absorption could potentially lead to contrasting outcomes in tomato plants when changing the amber light bandwidth, and such findings imply that the correlation between light bandwidth and plant growth is complex.

The importance of wavelength on plant growth and development is underlined in this work. Currently, lighting studies on plant performance mainly use single-wavelength LEDs, or researchers create mixed wavelength treatments with standard LED wavelengths for plant growth (e.g., 460 nm and 655 nm). With this approach, the narrow bandwidth of LEDs (~20–30 nm) allows for the construction and optimization of precise light recipes for plant growth. Typical LED spectra can cover a range of over 50 nm and even as great as 100 nm for some “narrow” spectrum LEDs. Our findings show that using specific blue wavelengths with background amber lighting led to lower plant productivity, and simply considering the peak wavelength and bandwidth in LED spectra could lead to misleading conclusions. These findings may prompt some discussion for narrower spectrum lighting system requirements in the blue wavelength region that might complement recent research exploring the use of blue laser diodes for plant production [46,47]. Further studies will explore cultivar-related responses to RB-NA, RB-BA, B-BA, and HPS using different cultivars of tomato and lettuce plants.

5. Conclusions

Light wavelength, intensity, and bandwidth play crucial roles in the photosynthetic activity and biomass of plants. A close examination of the blue wavelength region, with different light bandwidths and in combination with amber light, shows that blue LED light (450–460 nm) results in lower biomass production despite being commonly applied in plant production systems. Tomato and lettuce growth responses exhibited different sensitivities to blue light wavelengths. Narrowing the light spectrum can promote or negate biomass accumulation, depending on the wavelength range. This work provides the first documented evidence of the importance of light bandwidth. These findings could lead to a new research path when investigating light properties on plant growth and an alternate approach to increasing plant productivity. In addition, an intriguing sensitivity of tomato plants to shifts in blue wavelengths is reported. Data reinforce plant species-specific responses to light wavelength while presenting new information on how bandwidth influences plants’ responses to light.

Author Contributions

Conceptualization, K.T., B.-S.W., and M.L.; methodology, K.T.; software, K.T.; formal analysis, K.T., B.-S.W., and P.W.A.; investigation, K.T.; writing—original draft preparation, K.T., and B.-S.W.; writing—review and editing, P.W.A., S.M., and M.L.; visualization, K.T.; supervision, M.L.; funding acquisition, M.L. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

We extend our sincere gratitude to U Technology Corporation, Urban Barns Foods Canada Inc., and Sophie Anne Rufyikiri for their support to this project.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. The relative spectral photosynthetic photon flux density (PPFD) of each light experimental treatment. HPS (A): single-ended high-pressure sodium; B + BA (B): blue + broad amber (455–602 nm); RB-NA (C): royal blue + narrow amber (430–602 nm); RB-BA (D): royal blue + broad amber (423–595 nm) light treatment.

Figure 2. Effect of different light treatments [HPS: single-ended high-pressure sodium, B-BA: blue + broad amber (460–595 nm), RB-BA: royal blue + broad amber (430–595 nm), and RB-NA: royal blue + narrow amber (430–595 nm)] on tomato and lettuce plant growth parameters, including (A) fresh mass (FM) and dry mass (DM) for tomato plants, (B) fresh mass (FM) and dry mass (DM) for lettuce plants, (C) chlorophyll (Chl) content in leaves [SPAD value (Soil Plant Analysis Development)] for tomato plants, and (D) chlorophyll (Chl) content in leaves (SPAD value) for lettuce plants. Data represent the means of three replicates ± standard error (S.E.). Different superscript letters represent significant (p [less than] 0.05) differences using Tukey’s post hoc test.

Figure 3. Total chlorophyll content per leaf area (µmol m−2) measured for tomato plants (A), and total chlorophyll content (g of chlorophyll/100 g fresh tissue) measured for lettuce plants (B). Data represent the means of three replicates ± standard error (S.E.). Different superscript letters represent significant (p [less than] 0.05) differences using Tukey’s post hoc test.

Figure 4. Correlation among light waveband (50 nm intervals) and growth parameters of tomato plants, including fresh mass (FM), dry mass (DM), Chl (SPAD), flower number, stem diameter (SD), and height (H). The number in each cell is the correlation coefficient. Colors reflect changes in the correlation coefficient: red represents the correlation coefficient with high and positive correlation; blue indicates high and negative correlation.

Figure 5. Correlation among light waveband (50 nm intervals) and growth parameters of lettuce plants, including fresh mass (FM), dry mass (DM), and chl (SPAD). The number in each cell is the correlation coefficient. The colors reflect the changes in the correlation coefficient: red represents the correlation coefficient with high and positive correlation; blue indicates high and negative correlation.

Table 1

Spectral characteristics of the four experimental lighting treatments.

	Light Treatment
	HPS (Control)	B-BA	RB-NA	RB-BA
Single-band photon flux density (µmol m−2 s−1)
400–450 nm	4.24	4.96	47.53	6.38
451–500 nm	10.72	9.01	4.44	1.55
501–550 nm	5.60	25.16	1.76	7.11
551–600 nm	161.88	82.43	99.49	100.93
601–650 nm	53.66	89.91	94.45	101.08
651–700 nm	13.90	38.54	2.34	32.95
701–750 nm	7.27	12.79	1.59	8.55
751–800 nm	13.12	5.72	1.36	2.77
Integrated photon flux density (µmol m−2 s−1)
PPFD	250.00	250.00	250.00	250.00
TPFD	270.38	268.51	252.94	261.33

HPS: single-ended high-pressure sodium; B-BA: blue + broad amber (455–602 nm); RB-NA: royal blue + narrow amber (430–602 nm); RB-BA: royal blue + broad amber (423–595 nm); PPFD: photosynthetic photon flux density (400–700 nm); TPFD: total photon flux density (TPFD: 400–800 nm).

Table 2

Peaks and full width at half maximum (FWHM) for the light treatments used for this study.

Wavelength	Light Treatment
	HPS (Control)	B-BA	RB-NA	RB-BA
Blue	-	455 nm (28.5 nm)	430 (21.5 nm)	423.5 nm (24 nm)
Amber	-	602.5 (107.5 nm)	602 nm (20 nm)	594.5 nm (80.5 nm)
Green/amber/red	590 (30.5 nm)	-	-	-

Full width at half maximum (nm) is in parenthesis. HPS: single-ended high-pressure sodium; B-BA: blue + broad amber (455–602 nm); RB-NA: royal blue + narrow amber (430–602 nm); RB-BA: royal blue + broad amber (423–595 nm).

Table 3

Growth and development parameters (flower number, stem diameter, and plant height) for tomato plants grown under different light treatments [HPS: single-ended high-pressure sodium, B-BA: blue + broad amber (460–595 nm), RB-BA: royal blue + broad amber (430–595 nm), and RB-NA: royal blue + narrow amber (430–595 nm)].

Light Treatments	Flower Number(Count)	Stem Diameter(mm)	Height (cm)
HPS	6.88 ± 0.74 ab	8.03 ± 0.19 a	71.98 ± 3.12 a
B-BA	5.41 ± 0.76 ab	7.69 ± 0.22 a	67.44 ± 3.36 a
RB-NA	4.41 ± 0.99 b	7.68 ± 0.24 a	53.11 ± 4.01 b
RB-BA	8.19 ± 0.94 a	8.06 ± 0.23 a	71.89 ± 2.84 a

Data represent the means of three replicates ± standard error (S.E.). Different superscript letters represent significant (p < 0.05) differences using Tukey’s post hoc test.

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