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Article

Optimization of Light Quality for Plant Factory Production of Brassica campestris (Pakchoi)

by
Chengbo Zhou
1,†,
Kangwen Zhou
1,2,†,
Jiangtao Hu
1,
Xu Zhang
3,* and
Qingming Li
1,*
1
Institute of Urban Agriculture, Chinese Academy of Agricultural Sciences, Chengdu 610213, China
2
College of Horticulture, Sichuan Agricultural University, Chengdu 611134, China
3
College of Horticulture, Shanxi Agricultural University, Jinzhong 030801, China
*
Authors to whom correspondence should be addressed.
†
These authors have contributed equally to this work.
Agriculture 2025, 15(3), 347; https://doi.org/10.3390/agriculture15030347
Submission received: 24 December 2024 / Revised: 30 January 2025 / Accepted: 3 February 2025 / Published: 6 February 2025
(This article belongs to the Special Issue Research on Plant Production in Greenhouse and Plant Factory Systems)
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Abstract

:
Light is a key factor influencing the growth and quality of crops in plant factories. To explore the optimal light quality for pakchoi production, five light formulations were applied to ‘Youguan NO.3’ pakchoi: white LEDs (W; CK); white/red = 4:1 (WR); white/blue = 4:1 (WB); white/red/blue = 3:1:1 (WRB); and white/green = 4:1 (WG), all with a light intensity of 250 ± 10 µmol·m−2·s−1. The results showed significant variations in growth indices, nutritional quality, enzyme activity, and other parameters under different light qualities. The best growth results were observed under the WRB treatment. Chloroplasts under WRB treatment appeared well-developed, with clear grana lamellae. The thylakoids in the chloroplast grana of the WRB plants were densely stacked, and a large number of starch grains were detected. The contents of total sugar, soluble sugar, soluble protein, and protein nitrogen were significantly higher under the WB, WRB, and WR treatments compared to the CK treatment, along with a significant reduction in nitrate content. Among all the treatments, WRB treatment resulted in the highest levels of total sugar, starch, free amino acids, soluble protein, total nitrogen, protein nitrogen, and ascorbic acid (AsA). Enzyme activity assays revealed that the activities of sucrose phosphate synthetase (SPS), nitrate reductase (NR), glutamine synthetase (GS), glutamate synthetase (GOGAT), and glutamate dehydrogenase (GDH) were highest under WRB treatment. Therefore, supplemental red-blue mixed light can effectively improve the growth and nutritional properties of pakchoi grown under white light. This supplementary lighting strategy provides a new way to enhance the nutritional value of leafy vegetables in plant factories.

1. Introduction

Pakchoi (Brassica campestris L.) is one of the most widely consumed vegetables in Asian countries, known for its high content of minerals, vitamins, and glucosinolates [1]. It can be produced in plant factories, enabling year-round cultivation [2]. In recent years, the intensive cultivation of facility-grown vegetables has led to increased soil salinization, resulting in reduced yield and quality. Excessive nitrate accumulation in salinized soils has become a major challenge hindering the development of the greenhouse vegetable industry in China [3]. At the same time, with improving living standards, consumers are increasingly favoring high-quality vegetables. Therefore, it is essential to explore efficient cultivation methods and quality enhancement technologies for pakchoi.
Plant factories are modern agricultural technologies designed to achieve the efficient and sustainable production of crops. Light, as both an energy source and a signal, plays a crucial role in plant growth and development through factors such as light quality, light intensity, and photoperiod [4,5]. In terms of light quality, red (R) and blue (B) light have garnered the most attention due to their significant absorption by photosynthetic pigments, which affect the development of various organs in plants [6]. Previous studies have shown that red light notably increases the content of carbon metabolites in plants [4,7], while blue light regulates physiological processes such as protein accumulation, stomatal opening, and chloroplast development [8,9]. Appropriate blue light treatment is also beneficial for increasing the content of important nutrients such as phenolic acids, glucosinolates, and flavonoids [10]. Additionally, research has demonstrated that supplemental green light can significantly contribute to photosynthetic carbon assimilation and enhance both the yield and nutritional quality of plants [3,11,12]. However, plants growing under monochromatic light often experience physiological problems such as decreased photosynthetic capacity and growth inhibition [13]. For example, due to the short-wavelength and high-energy radiation properties of blue light, strong exposure to monochromatic blue light can cause plant leaves to shrink and grow slowly [14]. To avoid this phenomenon, our research supplements white light with red, blue, and green light.
The growth and quality of plants are closely linked to carbon and nitrogen metabolism [15,16]. Key quality indicators, such as sugar, starch, protein, and nitrate, are all carbon and nitrogen metabolites, and their metabolism and accumulation are regulated by enzymes like sucrose phosphate synthetase (SPS), sucrose synthase (SS), nitrate reductase (NR), glutamine synthetase (GS), glutamate synthetase (GOGAT), and glutamate dehydrogenase (GDH) [17,18]. Light quality plays a crucial role in regulating carbon and nitrogen metabolism in plants [19]. The activities of enzymes involved in carbon metabolism are influenced by monochromatic red (R) light and the combination of red and blue (R + B) light, which in turn affects the content of soluble sugars in plants [20,21]. Additionally, light quality (R, B, G, or R + B) also impacts the activity of enzymes like NR and GS by regulating gene expression, ultimately affecting nitrogen metabolite accumulation [17,18,22].
Plant factories face challenges such as low light efficiency and high energy consumption. In accordance with previous research, optimizing light quality conditions is crucial for improving both yield and quality. Red and blue light have been widely studied in plant factory cultivation experiments, and green light has also been partially studied. However, research on supplementing white light with red, blue, and green light is still rare. In this experiment, LED lights were used as the light source, with light quality supplementation based on white light, while maintaining the same photon quantum, to identify an optimal light formulation for enhancing the growth and quality of pakchoi. We investigated various growth indicators of pakchoi, observed the ultrastructure of pakchoi leaves, and measured the contents of carbon and nitrogen metabolism products such as total sugar, soluble sugar, sucrose, free amino acids, soluble protein, total nitrogen, and nitrate, along with the activities of key enzymes involved in carbon and nitrogen metabolism. Additionally, we compared and analyzed the content of major nutrient indicators of pakchoi under a controlled environment and field cultivation. This research provides insights into the effects of supplementing light quality with white light on pakchoi’s growth and nutritional properties. The results are expected to offer a theoretical basis and technical parameters for supplemental lighting in agricultural facilities.

2. Materials and Methods

2.1. Plants Growth Conditions and Light Treatments

The test material used was pakchoi ‘Youguan No.3’. Seeds were planted in a 50-hole tray (52 cm × 26 cm, with 4.8 cm × 4.8 cm potholes) filled with a seedling substrate (soil/substrate/chicken manure = 4:2:1) and placed in a greenhouse for seedling cultivation. When the seedlings developed two leaves, they were transferred to pots (12 cm in diameter, 10 cm in height, filled with the same substrate), with one seedling per pot. After recovery, the seedlings were moved to an artificial climate chamber (Zhejiang Qiushi Artificial Environment Co., Ltd., Hangzhou, China) for further cultivation. Each treatment was repeated 3 times, with 18 pots per replicate. Samples were collected 25 days after treatment in the chamber.
The light source used in this experiment was a new type of LED tubular plant growth light (Jishi Technology Lighting Co., Ltd., Zhongshan, China), with each tube measuring 1.2 m in length and 4 cm in diameter. The tube contained white (W), red (R), blue (B), and green (G) light, with each lamp having a power rating of 18 W. Five light formulations were tested: white LEDs (W; CK); white/red = 4:1 (WR); white/blue = 4:1 (WB); white/red/blue = 3:1:1 (WRB); and white/green = 4:1 (WG). The number of LED tubes and the height of the culture rack were adjusted to maintain a photosynthetic photon flux density (PPFD) of (250 ± 10) µmol·m−2·s−1 for each formulation (Figure 1, Table 1). The PPFD was measured using a 3415FX photometer (Spectrum Technologies, Aurora, lllinois, USA). The environmental conditions in the artificial climate chamber were as follows: a day temperature of (20 ± 1) °C, a night temperature of (15 ± 1) °C, an air relative humidity of 60–80%, a CO2 concentration of 380–400 µL·L−1, and a photoperiod of 12 h per day.

2.2. Determination of Carbohydrates

The Maness method was used to extract and determine the content of soluble sugar in the pakchoi leaves [23]. After homogenization, 0.1 g of fresh leaves were boiled in 1 mL of deionized water. The mixture was then centrifuged at 8000 rpm for 15 min, and the supernatant was collected. An appropriate amount of deionized water and supernatant were mixed with the anthrone reagent and then placed in a water bath at 95 °C for 10 min. After cooling, the absorption spectrum was measured at 620 nm to determine the soluble sugar content. Starch content was measured using a kit (Grace Biotechnology, G0551W, Suzhou, China) [24]. To remove grease, 0.1 g of the fresh sample was ground with 0.5 mL of petroleum ether. The sample was transferred to a 2 mL Eppendorf tube, and the volume was adjusted to 1 mL. The tube was shaken in a water bath at 50 °C for 30 min, followed by centrifugation at 12,000 rpm for 5 min at room temperature to collect the precipitate. After removing the sugar with ethanol, 1 mL of DMSO was added to the mixture and vortexed. A 0.1 mL sample solution was then mixed with 0.9 mL of diluent solution and boiled for 15 min before starch content was determined using an enzyme meter. Li’s method was used for total sugar extraction [25]. Sucrose concentration was determined using the anthrone colorimetric method, and the absorbance was measured at 620 nm [26].

2.3. Determination of Nitrate, Free Amino Acids, Soluble Protein, Total Nitrogen, and Protein Nitrogen

The protein content was extracted and determined following the method of Li et al. [27]. Fresh pakchoi leaf tissue was thoroughly ground and homogenized with an extraction buffer in an ice water bath. The homogenate was then centrifuged at 4 °C and 15,000 rpm for 25 min. The concentration of soluble protein was measured using the Coomassie Plus Protein Assay Reagent (Pierce, Rockford, IL, USA). Nitrate content was determined using the salicylic acid method. A 0.1 g sample of leaf tissue was added to 10 mL of deionized water, boiled in a water bath for 25 min, and then the supernatant was diluted to a final volume of 20 mL. A 0.1 mL aliquot of the supernatant was mixed with 0.4 mL of salicylic acid and sulfuric acid solution, shaken well, and left to stand for 20 min. Afterward, 9.5 mL of NaOH solution was added, and the absorbance was measured at 410 nm [9]. The free amino acid content in the pakchoi leaves was extracted and determined according to the method of Cao et al. [28]. Total nitrogen and protein were determined using a kit. The BCA Protein Assay Kit (Shanghai Enzyme-linked Biotechnology, Shanghai, China) was used, and total nitrogen and protein nitrogen were extracted and determined following the manufacturer’s instructions.

2.4. Determination of AsA Content

Ascorbic acid (AsA) content was determined using a modified method from Chen et al. [29]. A total of 0.2 g of dried pakchoi leaves were mixed with 15 mL of 4.5% aqueous phosphoric acid. After shaking at 300 rpm for 30 min in the dark, the mixtures were centrifuged at 16,000 rpm for 10 min. The AsA content was determined by analyzing the supernatants using an HPLC system (Agilent, model-1100, Santa Clara, CA, USA) equipped with a C18 column (4.6 mm inner diameter, 250 mm length, 5 μm particle size, Restek, Mount Ayr, PA, USA). Then, 0.21% phosphoric acid was used to elute the extract at a flow rate of 0.8 mL/min, and concentrations were measured at 254 nm against AsA standards (Standard Substance Center, Beijing, China).

2.5. Determination of Enzyme Activity

The activities of glutamate synthetase (GOGAT) and glutamine synthetase (GS) were measured following the method of Fan et al. [2]. A total of 0.5 g of leaf tissue was homogenized in 50 mmol·L−1 buffer (containing 1% (w/v) insoluble polyvinylpyrrolidone, 1.5% (w/v) soluble casein, 2 mmol·L−1 dithiothreitol, and 2 mmol·L−1 EDTA) in an ice water bath. The mixture was then centrifuged at 18,000 rpm and 4 °C for 20 min. The supernatant was retained for the determination of GOGAT and GS enzyme activity.
The activity of nitrate reductase (NR) was determined according to the method of previous studies [30]. A 0.1 g sample of leaf tissue was added to 4 mL of phosphate buffer and homogenized, followed by centrifugation at 4 °C and 18,000 rpm for 15 min. After that, 0.5 mL of 100 mM KNO3 and 0.3 mL of 2.5 mM nicotinamide adenine dinucleotide were added to 200 µL of the supernatant and incubated at 25 °C for 30 min. Then, 1 mL of 30% trichloroacetic acid was added to stop the reaction.. Finally, 2 mL of sulfonamide reagent and 2 mL of 1-naphthylamine were added. After 15 min of reaction, absorbance was measured at 520 nm to determine the nitrate reductase activity.
The activities of sucrose phosphate synthetase (SPS) and sucrose synthase (SS) were determined following the method of Solomakhin and Blanke [31]. The activity of glutamate dehydrogenase (GDH) was measured calorimetrically at 340 nm using the GDH Assay Kit (Comin Biotechnology Co., Ltd., Suzhou, China). The enzyme activity was defined as the number of moles of NADH consumed per minute.

2.6. Observation of Chloroplast Ultrastructure

The observation of chloroplast ultrastructure was based on the methods of our predecessors, with some modifications. After treatment, the third fully expanded true leaf, free from disease spots and with uniform size, was selected. Tissue blocks (1 mm × 1 mm) were cut from both sides of the main vein and immediately immersed in 1% glutaraldehyde prepared with 0.1 mol·L−1 phosphate-buffered saline (PBS, pH 6.8). The tissue was subjected to vacuum infiltration for about 5 min until the blocks sank, followed by fixation in 4% glutaraldehyde prepared with PBS at 4 °C for 3 h. The tissue was then rinsed with PBS 4–6 times, each rinse lasting 30 min, and postfixed with 1% osmium tetroxide prepared with PBS at 4 °C for 2 h. After rinsing with PBS 4–6 times, each for 30 min, excess fixative was removed. The tissue was dehydrated through a graded acetone series (30%, 50%, 70%, 80%, 90%, and 95% acetone), with each gradient lasting for 30 min, followed by three changes of 100% acetone, each for 30 min. After dehydration, the samples were infiltrated and embedded. Permeation was carried out using a mixture of 100% acetone and embedding agent at ratios of 3:1 for 3 h, 1:1 for 5 h, and 1:3 for 12 h. The samples were then embedded in pure embedding agent for 24 h. The embedding agent consisted of 10 mL of epoxy resin Epon812, 4 mL of DDSA hardener, 7 mL of MNA softener, and 0.3 mL of DMP-30 catalyst. Polymerization occurred at 30 °C for 12 h and 60 °C for 48 h. Representative polymerized blocks were selected, and 70 nm thick ultrathin sections were cut using a Leica Ultracut UCT ultramicrotome (Leica Microsystems, Wetzlar, Germany). The sections were stained with a double staining method using uranyl acetate and lead citrate. The leaf ultrastructure was observed and photographed with a JEM-1400 Plus transmission electron microscope (JEOL Ltd., Tokyo, Japan) at magnifications of 25,000 rpm to examine starch grains and chloroplasts and 50,000 rpm to analyze grana thickness and the number of grana lamellae.

2.7. Statistical Analysis

The data were processed using SigmaPlot 10.0 and one-way ANOVA was performed with SPSS 22.0 [32]. Duncan’s test was applied for multiple comparisons to assess significant differences (p < 0.05).

3. Results

3.1. Effects of Supplementary Light Quality on the Growth Morphological Indices in Pakchoi

As shown in Table 2, under the WRB treatment, the growth morphological indices (plant height, leaf area, fresh shoot weight, dry shoot weight) were significantly higher than those of the other treatments. Across all treatments, the growth indices followed a consistent downward trend, with the order from highest to lowest being WRB > WR > WG > CK > WB. This indicates that the WRB treatment resulted in the best growth performance for pakchoi.

3.2. Effects of Supplementary Light Quality on Chloroplast Ultrastructure in Leaves of Pakchoi

Under CK treatment, chloroplasts were spindle-shaped, closely adhering to the intact cell wall, with orderly thylakoid lamellae and small starch grains (Figure 2, Table 3). Under WB treatment, the grana lamellae were numerous but less stacked, with narrower lamellae. Under WRB treatment, chloroplasts were plump and tightly adhered to the cell wall, with well-defined grana lamellae, tightly stacked thylakoids, and larger starch grains. Under WR treatment, the chloroplasts had clear grana lamellae, more stacked thylakoids, and many large starch grains. Under WG treatment, chloroplasts were blurred, with a disintegrated envelope, unclear thylakoid stacking, and no starch grains. These results suggest that supplemental red-blue mixed light added to white light most effectively promoted chloroplast development in pakchoi leaves.

3.3. Effects of Supplementary Light Quality on Quality of Pakchoi

The total sugar and soluble sugar contents were significantly increased under the WB, WRB, and WR treatments compared to CK, with sucrose content also found to be higher in the WRB, WR, and WG treatments (Table 4). Starch content was significantly enhanced only under WRB treatment. The addition of green light to white light led to a reduction in sucrose accumulation in pakchoi leaves. Notably, the highest total sugar and starch contents were observed under the WRB treatment. When compared to the CK treatment, soluble protein and protein nitrogen content were significantly increased under the WB, WRB, WR, and WG treatments, while nitrate content was significantly reduced. Analysis of free amino acids and total nitrogen content revealed that free amino acids slightly decreased and total nitrogen significantly decreased under WG treatment. However, under WRB treatment, free amino acids, soluble protein, total nitrogen, and protein nitrogen content were all the highest among the treatments, while nitrate content was the lowest. AsA content in pakchoi was significantly higher under the WRB, WR, and WG treatments compared to CK, with no significant difference observed in the WB treatment. These results suggest that blue light had no significant effect on AsA accumulation, but the addition of red or green light to white light effectively promoted AsA accumulation and improved the nutritional quality of pakchoi.

3.4. The Influence of Supplemental Light Quality on the Activities of Key Enzymes

The activity of SPS was 46.9%, 22.3%, and 14.7% higher in the WRB, WR, and WB treatments, respectively, compared to CK (Figure 3A). No significant difference was observed between the WG and CK treatments. Additionally, SPS activity was significantly higher under WRB treatment than in the other treatment groups. In contrast, SS activity was significantly decreased in all treatments compared to CK, with the WRB and WB treatments showing the lowest activity (Figure 3B). The highest activity of nitrate reductase (NR) was observed in the WRB treatment group, with increases of 94%, 61.9%, and 29.9% compared to CK in the WRB, WR, and WB treatments, respectively. No significant difference was found between the WG and CK treatments (Figure 3C). The activity of glutamine synthetase (GS) followed the order WRB > WR > WB > WG > CK (Figure 3D). The activity of glutamate synthetase (GOGAT) was significantly higher in WRB treatment than in the other treatments (Figure 3E). No significant difference was found between the WR and WB treatments, both of which were significantly higher than CK, while the difference between WG and CK was not significant. The highest activity of glutamate dehydrogenase (GDH) was observed in the WRB treatment, followed by WR, while the CK treatment showed significantly lower activity compared to the other treatments (Figure 3F).

3.5. Quality of Pakchoi Cultivated in Different Conditions

To compare the content of typical quality indices in pakchoi grown under different conditions, average values for soluble sugar, soluble protein, AsA, and nitrate were calculated and are shown in Figure 4. Previous studies have indicated that pakchoi cultivated in controlled environments had significantly higher levels of AsA and soluble protein compared to field-grown pakchoi, while soluble sugar and nitrate levels were lower. In this experiment, pakchoi grown under controlled environmental conditions exhibited significantly higher levels of AsA compared to other cultivation methods, while soluble protein, soluble sugar, and nitrate levels were lower.

4. Discussion

Light quality is a crucial environmental factor influencing the growth and nutritional quality of green horticultural crops [33]. With the increasing demand and advancements in technology, LEDs with controllable dynamic spectra have become widely used. These LEDs offer a broad range of wavelengths, including red, blue, green, and far-red, which improve plant light conversion and utilization efficiency [34,35]. Therefore, optimizing the combination of photosynthetically effective spectra for each crop can enhance light efficiency, save energy, and improve both yield and nutritional quality. In this study, pakchoi plants were exposed to white LED light with different supplementary light qualities. The effects of these light treatments on growth and quality were assessed through growth indicators and various biochemical parameters. The results indicated that supplementing white light with red-blue mixed light was the most effective regimen for optimizing pakchoi growth and enhancing its quality (Table 2 and Table 4).
The growth and development of plants are influenced by light quality. Plants grown under red light have been shown to improve light energy capture efficiency, leading to the increased accumulation of photosynthetic products in photosynthetic structures, which boosts carbohydrate content and promotes plant growth [4,36]. This was confirmed in our experiment. Compared to the CK treatment, pakchoi under red light supplementation (WR) exhibited more and larger starch grains in chloroplasts, along with higher carbohydrate content (total sugar, soluble sugar, and sucrose) in their leaves (Figure 2 and Table 4). At the same time, supplementing white light with blue light (WB) only increased the content of soluble sugars; however, after supplementing white light with red-blue mixed light, the carbohydrate content was significantly higher than other treatment groups, and the activity of sucrose metabolism-related enzymes such as SPS was significantly increased (Figure 2). This indicates that the combination of red and blue light is not a simple additive effect, but can regulate the production and accumulation of carbohydrates by regulating the activity of carbon metabolism-related enzymes. According to Ren et al.’s research, the combined treatment of red and blue light can increase the expression of various carbon metabolism-related genes, including the SPS-related gene SPS1 [18]. It is interesting that the trend of SS activity changes under WRB treatment is completely opposite to SPS, which may be due to the fact that most SS was used to decompose sucrose rather than synthesize sucrose. Additionally, supplementing red and blue light with white light was more beneficial for thylakoid development. WRB treatment resulted in a greater number of thylakoid layers and larger thylakoid sizes (Table 3). This improved photosynthetic apparatus enhanced leaf photosynthesis and promoted carbohydrate accumulation in pakchoi, which contributed to the higher biomass observed under WRB treatment.
Different light qualities have varying effects on plant nutrient biosynthesis [29]. Pakchoi is particularly susceptible to excess nitrate accumulation during production, especially under hydroponic conditions [37]. Nitrate metabolism in plants is significantly influenced by nitrogen-metabolizing enzymes such as nitrate reductase (NR), glutamine synthetase (GS), and glutamate synthetase (GOGAT). NR directly reduces nitrate to nitrite, which is then reduced to ammonium by nitrite reductase, while GS and GOGAT indirectly regulate nitrate metabolism by participating in ammonium assimilation [22,38]. Sun et al. [39] found that the combination of blue and red light enhanced the activities of GS and GOGAT in tomato leaves, increasing the contents of total nitrogen, nitrate nitrogen, free amino acids, and soluble protein. In this experiment, the WB, WRB, and WR treatments significantly reduced nitrate content, with WRB treatment showing the lowest nitrate content and the highest activities of NR, GS, GOGAT, and GDH. These effects may be attributed to blue and red light promoting the synthesis of soluble proteins, thereby increasing the supply of enzymes that reduce nitrate and NADPH in the leaves, and the accumulation of high levels of amino acids also negatively regulating nitrate absorption [2,40]. The combination of red and blue light further enhances the activity of nitrogen metabolism enzymes, resulting in more synthesis of amino acids and soluble proteins, leading to lower nitrate content. However, the molecular mechanism of how the combination of red and blue light enhances the activity of nitrogen metabolism enzymes still needs further research. At the same time, the supplementation of green light can effectively reduce nitrate content, which is consistent with the research results of Bian et al. [22]. However, in this experiment, green light had no significant effect on NR activity, which is speculated to be related to the synthesis of soluble proteins.
AsA is a key antioxidant in plants that effectively scavenges reactive oxygen species, playing a crucial role in preventing scurvy and maintaining healthy skin, gums, and blood vessels. Since the human body cannot synthesize or store AsA, it must be obtained through the consumption of fruits and vegetables [41,42]. Light is an important environmental factor that regulates AsA levels in plants. However, in plant factory conditions, light radiation is much lower than in natural environments, resulting in reduced AsA levels in vegetables [43,44]. Optimizing light conditions for leafy vegetables not only increases AsA levels and prolongs shelf-life but also enhances their nutritional value, which is crucial for advancing facility-based cultivation [45,46]. Ma et al. [47] showed that red light treatment can increase the AsA content in broccoli by delaying the degradation of AsA, while blue light treatment has no effect on AsA metabolism. In this study, the AsA content in pakchoi under WRB treatment was significantly higher than in the other treatments. Furthermore, AsA content under red and green light conditions was significantly higher than under control and blue light conditions. These findings suggest that green light also enhances AsA content, and that a combination of red and blue light is more effective than red light alone for increasing AsA levels in pakchoi. This suggests that mixing red, blue, and green light may yield better results.
Soluble sugars are a primary source of energy and flavor in plants, and an important nutritional parameter. In studies on apple leaves treated with different light qualities, it was found that blue light promoted an increase in soluble sugar content, while red light inhibited starch accumulation [48]. Di et al. [49] observed that the carbon metabolism level in eggplant was higher under the combination of red and blue light treatment compared to monochromatic or white light treatments. Lin et al. [9] further demonstrated that, in hydroponic lettuce, the combination of red and blue light with white light significantly increased soluble sugar content compared to red and blue light alone. In our study, the WR and WB treatments significantly increased total sugar and soluble sugar contents in pakchoi leaves, but had no significant effect on starch content. WRB treatment promoted starch accumulation while also increasing sugar content. The WG treatment showed no significant effect on sugar metabolism but inhibited starch accumulation. This may be due to the lower absorption of green light during photosynthesis [50]. Previous studies have shown that soluble carbohydrates and nitrates in plants complement each other in maintaining cell osmotic pressure [51]. In this study, the addition of red and blue light reduced the nitrate content in cabbage leaves, which may also be the reason for the increase in soluble sugar content. The soluble sugar content in pakchoi in this study was lower than that found in field-grown plants, which may contribute to a less favorable flavor, but it was similar to that in greenhouse cultivation. This could be due to the absence of adverse conditions, and to increase sugar content in production, short-term pre-harvest stress treatments, such as supplemental light and nitrogen cutoff, can be applied [52].

5. Conclusions

In summary, our results suggest that supplementing white light with red and blue light is the optimal light regimen for pakchoi cultivation. This light combination significantly increased the activities of sucrose phosphate synthase and sucrose synthase, resulting in a higher biomass. It also enhanced chloroplast morphology, carbon assimilation and transport in the source leaves, and carbohydrate accumulation in the sink. Additionally, red and blue mixed light influenced the activities of NR, GS, GOGAT, and GDH in nitrogen metabolism, enhancing the conversion of inorganic nitrogen into protein. Our research results further validate the functional effects of red, blue, and green light on plants, providing relevant technical guidance for artificial light configurations for leafy vegetables in plant factories, and improving the commercial economic benefits of pakchoi in plant factories. Therefore, supplementing white light with red and blue light is a suitable option for pakchoi cultivation in plant factories or greenhouses. However, the effectiveness of this supplementary light scheme in improving the growth and quality of other non-leafy vegetables remains to be verified, and the molecular mechanism of red-blue mixed light regulating carbon and nitrogen metabolism also needs further detailed research.

Author Contributions

Q.L. supervised the project and conceived this project. C.Z. and X.Z. designed the research study and performed the experiments. J.H. and K.Z. analyzed the data. C.Z. and K.Z. wrote the manuscript and edited the work. C.Z. and K.Z. contributed equally to this work. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Key Research and Development Program (Grant NO: 2023YFF1001500); the Sichuan Province Science and Technology Plan Project (Grant NO: 2023YFN0003); the Key R&D Program Project of Xinjiang Province (Grant NO: 2023B02020); the Sichuan Science and Technology Program (Grant NO: 2024NSFSC1221); the Agricultural Science and Technology Innovation Program (Grant NO: ASTIP: CAAS-ZDRW202415); the Central Public-Interest Scientific Institution Basal Research Found (Grant NO: S2024004); and the Tianchi Talent Introduction Plan.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

The authors declare no conflicts of interest.

Correction Statement

This article has been republished with a minor correction to the correspondence contact information. This change does not affect the scientific content of the article.

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Agriculture 15 00347 g001
Figure 1. Relative spectral values of four treatments.
Figure 1. Relative spectral values of four treatments.
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Figure 2. Effect of supplementary light quality on ultrastructure of chloroplasts of pakchoi leaves. Left side is amplified by 2500 times and right side is amplified by 5000 times. CW: cell wall; SG: starch grain; GL: grana lamella.
Figure 2. Effect of supplementary light quality on ultrastructure of chloroplasts of pakchoi leaves. Left side is amplified by 2500 times and right side is amplified by 5000 times. CW: cell wall; SG: starch grain; GL: grana lamella.
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Figure 3. The effect of supplementary light quality on the activities of a variety of enzymes of pakchoi leaves. (A) Sucrose phosphate synthetase (SPS); (B) sucrose synthase (SS); (C) nitrate reductase (NR); (D) glutamine synthetase (GS); (E) glutamate synthetase (GOGAT); and (F) glutamate dehydrogenase (GDH). Different letters indicate significant differences using Duncan’s multiple range test (p < 0.05; n = 5).
Figure 3. The effect of supplementary light quality on the activities of a variety of enzymes of pakchoi leaves. (A) Sucrose phosphate synthetase (SPS); (B) sucrose synthase (SS); (C) nitrate reductase (NR); (D) glutamine synthetase (GS); (E) glutamate synthetase (GOGAT); and (F) glutamate dehydrogenase (GDH). Different letters indicate significant differences using Duncan’s multiple range test (p < 0.05; n = 5).
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Figure 4. Quality of pakchoi cultivated in different conditions. Red dots indicate content of quality indicators in this study. Black dots indicate content of quality indicators in previous studies. Field: pakchoi grown in field; CEA: pakchoi grown in controlled environmental agriculture conditions; (A) Ascorbate content; (B) Soluble protein content; (C) Soluble sugar content; (D) Nitrate content.
Figure 4. Quality of pakchoi cultivated in different conditions. Red dots indicate content of quality indicators in this study. Black dots indicate content of quality indicators in previous studies. Field: pakchoi grown in field; CEA: pakchoi grown in controlled environmental agriculture conditions; (A) Ascorbate content; (B) Soluble protein content; (C) Soluble sugar content; (D) Nitrate content.
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Table 1. Light quality parameters of four treatments.
Table 1. Light quality parameters of four treatments.
TreatmentsPPFD of W (µmol·m−2·s−1)PPFD of B (µmol·m−2·s−1)PPFD of R (µmol·m−2·s−1)PPFD of G (µmol·m−2·s−1)Total PPFD
(µmol·m−2·s−1)
CK250\\\250
WB20050\\250
WRB1505050\250
WR200\50\250
WG200\\50250
W: white light; B: additional blue light supplement; R: additional red light supplement; G: additional green light supplement.
Table 2. Effect of supplementary light quality on growth of pakchoi.
Table 2. Effect of supplementary light quality on growth of pakchoi.
TreatmentsPlant Height (cm)Leaf Area (cm2)Shoot Fresh Weight (g)Shoot Dry Weight (g)
CK10.80 ± 0.54 b236.07 ± 34.91 c8.62 ± 1.37 c0.606 ± 0.08 c
WB10.10 ± 0.26 c216.91 ± 19.60 c7.98 ± 0.93 c0.558 ± 0.08 c
WRB12.25 ± 0.44 a333.99 ± 28.06 a13.84 ± 1.03 a0.898 ± 0.07 a
WR11.17 ± 0.66 b294.97 ± 28.44 b11.62 ± 1.43 b0.762 ± 0.08 b
WG10.88 ± 0.26 b245.49 ± 14.59 c9.19 ± 0.72 c0.632 ± 0.06 c
Different letters indicate significant differences using Duncan’s multiple range test (p < 0.05; n = 5).
Table 3. The structural characteristics of the thylakoids in pakchoi leaves under different light conditions.
Table 3. The structural characteristics of the thylakoids in pakchoi leaves under different light conditions.
TreatmentsNumber of Thylakoid LayersThylakoid Layer Width
(nm)		Length of Thylakoid Layers (nm)Thylakoid Size (nm2)
CK4.17 ± 1.6 bc71.54 ± 12.07 bc309.46 ± 30.92 c4533.4 ± 845.8 a
WB3 ± 0.63 c59.42 ± 4.29 c450.92 ± 40.62 ab3116.2 ± 497.7 b
WRB6.5 ± 2.26 a134.61 ± 60.46 a504.43 ± 61.79 a3802 ± 487.5 a
WR5.5 ± 1.52 ab106.17 ± 28.69 ab427.85 ± 19.06 b3497.6 ± 498.1 b
WG\\\4511.6 ± 469.3 ab
Different letters indicate significant differences using Duncan’s multiple range test (p < 0.05; n = 4).
Table 4. Effect of supplementary light quality on sugar content of pakchoi leaves.
Table 4. Effect of supplementary light quality on sugar content of pakchoi leaves.
CompoundCKWBWRBWRWG
Total sugar/mg·g−1 (DW)91.43 ± 1.85 c100.33 ± 4.25 b110.05 ± 2.46 a99.17 ± 3.81 b95.53 ± 2.35 bc
Soluble sugar/mg·g−1 (DW)51.81 ± 1.49 c55.00 ± 0.34 a54.33 ± 0.82 ab54.51 ± 1.10 ab53.25 ± 0.60 bc
Sucrose/mg·g−1 (DW)9.58 ± 0.91 c9.35 ± 0.17 c11.00 ± 0.54 b13.40 ± 0.23 a11.28 ± 1.10 b
Starch/mg·g−1 (DW)95.43 ± 1.13 b93.80 ± 4.38 b106.63 ± 0.42 a93.06 ± 0.56 b83.79 ± 1.31 c
Free amino acids/mg·g−1 (FW)0.168 ± 0.01 bc0.178 ± 0.01 b0.195 ± 0.01 a0.178 ± 0.01 b0.16 ± 0.01 c
Soluble protein/mg·g−1 (FW)4.57 ± 0.24 d5.38 ± 0.23 b6 ± 0.11 a5.51 ± 0.17 b4.87 ± 0.26 c
Nitrate/mg·g−1 (FW)0.292 ± 0.01 a0.157 ± 0.01 c0.106 ± 0.01 d0.112 ± 0.01 d0.175 ± 0.01 b
Total nitrogen/mg·g−1 (DW)25.26 ± 0.58 ab25.87 ± 1.37 ab27.61 ± 1.70 a24.03 ± 1.80 b18.67 ± 1.03 c
Protein nitrogen/mg·g−1 (DW)16.92 ± 0.69 d20.89 ± 0.07 b21.88 ± 0.39 a20.84 ± 0.06 b19.93 ± 0.43 c
AsA/mg·g−1 (FW)2.32 ± 0.02 c2.3 ± 0.02 c2.43 ± 0.01 a2.38 ± 0.02 b2.36 ± 0.01 b
Different letters indicate significant differences using Duncan’s multiple range test (p < 0.05; n = 5). DW: dry weight; FW: fresh weight; AsA: ascorbic acid.
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Zhou, C.; Zhou, K.; Hu, J.; Zhang, X.; Li, Q. Optimization of Light Quality for Plant Factory Production of Brassica campestris (Pakchoi). Agriculture 2025, 15, 347. https://doi.org/10.3390/agriculture15030347

AMA Style

Zhou C, Zhou K, Hu J, Zhang X, Li Q. Optimization of Light Quality for Plant Factory Production of Brassica campestris (Pakchoi). Agriculture. 2025; 15(3):347. https://doi.org/10.3390/agriculture15030347

Chicago/Turabian Style

Zhou, Chengbo, Kangwen Zhou, Jiangtao Hu, Xu Zhang, and Qingming Li. 2025. "Optimization of Light Quality for Plant Factory Production of Brassica campestris (Pakchoi)" Agriculture 15, no. 3: 347. https://doi.org/10.3390/agriculture15030347

APA Style

Zhou, C., Zhou, K., Hu, J., Zhang, X., & Li, Q. (2025). Optimization of Light Quality for Plant Factory Production of Brassica campestris (Pakchoi). Agriculture, 15(3), 347. https://doi.org/10.3390/agriculture15030347

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