Light Modulation of Photosynthate Accumulation in Microgreens Grown in a Controlled Environment During Storage

Abstract

Light intensity and spectral composition are the main parameters that may be modulated to further affect plant nutritional value and shelf life. The current study aimed to assess how variations in spectral composition and light intensity affect sugar accumulation during the storage of two popular microgreens cultivated in a greenhouse under controlled conditions. Thus, in this study, amaranth (Amaranthus tricolor) and mustard (Brassica juncea) microgreens were grown in a greenhouse at 17/20 ± 3 °C and a 16 h photoperiod was maintained. (I) Four LED light intensities were set: 100, 150, 200, and 250 µmol m⁻² s⁻¹ while using 4000 K white LED lighting. (II) Maintaining 250 µmol m⁻² s⁻¹ the effect of spectrac composition: B_75.6%:R_24.2%:W_0.02%/R_88.9%:B_11.1%/and R_77.6%:W_9.9%:B_3.5% was evaluated. After 10 days from germination, microgreens were harvested and stored in the dark or under white LED light at +4 °C. Samples were collected on D₀, D₁, D₃, and D₅ days of postharvest storage. The results revealed that a wide spectrum of 250 µmol m⁻² s⁻¹ PPFD and R_88.9%:B_11.1% growing conditions produced the highest sugar content, achieving a balance between increased sugar accumulation and reduced deterioration during storage, ultimately extending shelf life.

1. Introduction

Numerous factors can influence the nutritional value and shelf life of plants, but lighting is a key determinant in the production of mustard (Brassica juncea) and amaranth (Amaranthus tricolor) microgreens, affecting their yield, morphology, and phytochemical composition [1]. A key challenge facing the microgreen industry is the rapid decline in quality that occurs shortly after harvest, leading to high prices and limiting sales to local markets. After harvesting, microgreens are prone to dehydration, wilting, decay, and a quick loss of certain nutrients. Research has investigated various preharvest and postharvest strategies, including calcium treatments, modified atmosphere packaging, temperature regulation, and light management to preserve quality, enhance nutritional value, and prolong shelf life [2].

The manipulation of light may result in different outcomes depending on the plant species. Light serves as an energy source for plants through photosynthesis processes in which energy drawn from light is applied to produce sugars. These soluble sugars further initiate cellular activities [3]. The quality of the light affects crop development and phytochemical content, and it is even more relevant because of the microgreen’s tendency to accumulate higher amounts of bioactive compounds compared to mature plants [4].

Microgreens are a new class of edible vegetables that demand minimal resources and can be easily cultivated [5]. Because of the high content of bioactive compounds, which are beneficial to human health, they might be considered functional foods [6]. Microgreens are 7 to 14 days from germination and are usually harvested at the first true leaf stage [7]. These greens are often used to enhance the flavor, texture, or color of the dishes [8]. Even though microgreens are beneficial to human health, their short shelf-life and rapid product deterioration challenge post-harvest product management [7]. Although, by providing proper storage conditions, they are safe for human consumption [9], research shows that microgreen growth and quality depend on light exposure parameters [10]. When compared to other microgreens, amaranth is frequently used by consumers and contains high concentrations of antioxidants and other useful people nutrients [11]. One of the mentioned nutrients is sugars, of which the dominant ones are sucrose and glucose with galactose [12,13]. Even though there is a lack of information about sugar contents and their deterioration dynamics in amaranth microgreens during growth and storage, it may be predictable that microgreens would contain these phytonutrients as they are derived from the seeds [14]. Another well-accepted microgreen that is rich in nutrients is mustard [9]. Mustard leaves are considered to be a healthy alternative to most of the winter season leafy vegetables [15,16]. Specific sugar contents vary significantly among different plant organs from which the highest amounts are found in alabastrum at the bolting stage, although these findings suggest that sugar content in mustard microgreens may be the highest in the early stages of growth [17]. Furthermore, different lighting conditions may influence the contents of bioactive compounds in microgreens. The use of red/blue light ratios can enhance the contents of sugars in amaranth microgreens [18,19] and additional application of UV-A light may influence the sugar contents in mustard microgreens [20]. Although spinach baby leaves under different spectral composition lights did not show significant differences in sugar content [21], lettuce treated with white-red light showed a tendency to accumulate higher amounts of sugars compared to white-blue light. In addition, a study with lamb’s lettuce shows a greater response to 90% red and 10% blue light regarding sugar contents when compared to monochromatic red, monochromatic blue, and red–blue treatments [22]. These findings suggest that red light is beneficial to the accumulation of sugars [4]. Additionally, the amount of red light may influence the contents of specific sugars: in a study with green and red leaf lettuce, when 120 µmol m⁻² s⁻¹ of PPFD was applied, this resulted in increased sucrose contents in both investigated species and when 112 µmol m⁻² s⁻¹ of PPFD was applied, this showed enhanced fructose contents. Glucose and maltose seemed to be resistant to spectral light changes in green lettuce and red lettuce, and glucose contents seemed to decrease under supplemental orange light. Supplemental UV-A and green light exhibited the ability to increase the maltose content in red-leaf lettuce [23]. As well as spectral composition, light intensity may influence the nutritional quality of microgreens [24]. In a study with Chinese kale microgreens, the results showed PPFD of 90 μmol m⁻² s⁻¹ to increase sugar contents, although authors stated the optimal light intensity was 70 μmol m⁻² s⁻¹, which they found to be an increased antioxidant capacity along with slightly increased sugar contents. Contrary to these findings, there were no significant differences in sugar contents when applied to cabbage microgreens [25]. Broccoli microgreens showed the highest amounts of sugar contents when grown under 70 μmol m⁻² s⁻¹ PPFD, even though 50 μmol m⁻² s⁻¹ of PPFD resulted in an enhancement in growth; the phytochemicals were largely improved only under 70 μmol m⁻² s⁻¹ of PPFD [26].

The main functions of sugar in food products are energy, preservation, fermentation, color, and texture [27]. Carbohydrates, mainly sugar and starch, are essential parts of energy storage, metabolic intermediation, and the partly structural framework of RNA and DNA [28]. Sugars such as glucose are the primary source of energy for cellular metabolism in the human body [29]. Other widely found sugars are fructose and galactose, which are epimers of D-glucose. Sugars are vital to prevent stress in the body and to provide such tissues as the brain and red blood cells with energy. Additionally, the hygroscopic nature of sugar exhibits a low antioxidant effect by decreasing the availability of water to oxidants [28]. The literature also states that sugars do interact with other natural antioxidants (e.g., vitamin E) and work synergistically to prevent the oxidation of lipids, resulting in the extended shelf life of food [30].

Thus, the present study aimed to evaluate the effect of different spectral compositions and the influence of light intensity on sugar accumulation during the storage of two popular microgreens grown in a greenhouse under controlled conditions. Specifically, Amaranth (Amaranthus tricolor) and mustard (Brassica juncea) were selected due to their nutritional value to shed light on specific sugars involved in prolonged shelf-life.

2. Materials and Methods

2.1. Growing Conditions

Amaranth (Amaranthus tricolor) and mustard (Brassica juncea) microgreens were grown in a greenhouse (55°05′08.4″ N 23°48′03.5″ E, at an altitude of 51 m; moderate climate zone of the northern hemisphere), Lithuania. A temperature of 17/20 ± 3 °C and 16 h photoperiod was maintained throughout the experiments. Seeds for the experiments were purchased from CN seeds (Pymoor, Ely, Cambridgeshire, UK). Seeds of microgreens were sowed and grown in a peat substrate and harvested after 10 days after germination.

Seeking to evaluate the effect of PPFD on microgreens sugar accumulation during cultivation/growth, four PPFD levels of wide spectrum 4000 K LED light (Elektros taupymo sprendimai, Lithuania) were maintained: 100, 150, 200, and 250 µmol m⁻² s⁻¹. To identify the effect of the spectral composition on selected microgreens, three LED light treatments consisting of red (R), blue (B), and white (W) were applied: B_75.6%:R_24.2%:W_0.02%; R_88.9%:B_11.1%; and R_77.6%:W_9.9%:B_3.5%. The PPFD level of 250 µmol m⁻² s⁻¹ was maintained. PPFD levels were measured and regulated at the plant level using a photometer–radiometer (RF-100, Sonopan, Poland). Ten days after germination, plants were harvested and stored in the dark or under white LED light (10 µmol m⁻² s⁻¹ PPFD) at +4 °C temperature in plastic containers (15 × 20 × 5 cm). Samples were collected on the harvest day (D₀), after one (D₁), three (D₃), and five (D₅) days of postharvest storage (Figure 1). Samples were frozen with liquid nitrogen and freeze-dried before performing phytochemical analysis. This experiment was carried out in a controlled environment and replicated three times in the area and each treatment in experimental replication consisted of three cultivation systems. Microgreens from 1.5 g of seeded were cultivated in each system. All plant material from all treatment replications (n = 9 per treatment) was analyzed as a conjugated sample.

2.2. Phytochemical Analysis

Soluble sugar contents were evaluated using the HPLC method with evaporative scattering detection (ELSD). About 0.05 g of freeze-dried plant tissue was homogenized and diluted with 2 mL of deionized warm water. The extraction was carried out for 2 h at room temperature and centrifuged at 14,000× g for 15 min. A cleanup step was performed before the chromatographic analysis: 1 mL of the supernatant was mixed with 1 mL 0.01% (w:v) ammonium acetate in acetonitrile and incubated for 30 min at 4 °C. After incubation, samples were centrifuged at 14,000× g for 15 min and filtered through a 0.22 µm syringe filter, nylon (BGB Analytik, Boeckten, Switzerland). Analysis was performed on the Shimadzu Nexera (Japan) system. Separation was performed on a Supelcosil 250 × 4 mm NH₂ column (Supelco, PA, USA) using 77% acetonitrile as the mobile phase at 1 mL min⁻¹ flow rate. A calibration method was used for sugar quantification (mg g⁻¹ in dry plant weight (DW)).

2.3. Statistical Analysis

All data are presented as mean ± standard deviation (n = 3 replications) and expressed on a dried weight (DW) basis. Statistical analyses were performed using MS Excel Version 2010 and XLStat 2019 Data Analysis and Statistical Solution for Microsoft Excel Antioxidants 2024, 13, x FOR PEER REVIEW 5 of 12 180 (Addinsoft, Paris, France 2019) software (2024.2.2 version), using one-way ANOVA and Tukey’s HSD at the confidence level p ≤ 0.05. The statistical comparisons were conducted within each day’s samples to assess treatment effects, rather than across different days. This approach enabled a detailed evaluation of treatment impacts within daily variations, ensuring a focused assessment of immediate treatment effects on microgreen quality. Different letters in the figures represent significant differences between treatments on each day.

3. Results

Sugar Accumulation in Microgreens

Two main soluble sugars, glucose and fructose, were identified in amaranth microgreens. On harvest day (D₀), 100 µmol m⁻² s⁻¹ of PPFD seemed to result in the highest amounts of glucose, while 250 µmol m⁻² s⁻¹ PPFD treatment seemed to result in the highest amounts of fructose (Figure 2). Fructose levels seemed to show no significant differences to spectral composition, but glucose levels were highest under R88.9%:B11.1%.

During postharvest storage, the highest amounts of glucose and fructose were found to be while grown under 250 µmol m⁻² s⁻¹ PPFD white light treatment independent if they were stored in the light or dark. This tendency seemed to be present in all storage days (Figure 3).

For the microgreens grown under different spectral compositions, during storage on D₁ the B_75.6%:R_24.2%:W_0.02%, the treatment seemed to result in the highest contents of glucose. On day D₃, R_88.9%:B_11.1% growing conditions treatment resulted in higher contents of glucose but on D₅ of storage, glucose remained highest in amaranth grown under B_75.6%:R_24.2%:W_0.02% treatment (Figure 4A). Light treatment during all storage days seemed to show higher contents of sugars when compared to dark storage. As for fructose, a similar tendency is observed where B_75.6%:R_24.2%:W_0.02%; light treatment on D₁ resulted in the highest contents of fructose and on D₃ and D₅, R_88.9%:B_11.1% treatment remained most effective in fructose retention independently from the light or dark treatment (Figure 4B).

Mustard microgreens on harvest day showed similar results to amaranth microgreens, where higher intensity white light resulted in elevated levels of glucose. Contrary to these findings, 100 µmol m⁻² s⁻¹ PPFD of white light resulted in the highest fructose levels. Under the white light treatment, maltose was also detected and was present in all white light treatments but seemed to reach the highest amounts under 250 µmol m⁻² s⁻¹ PPFD (Figure 5). This sugar has not been detected in mustard grown under different spectral compositions treatments. However, R_88.9%:B_11.1% treatment resulted in the highest amounts of both detected sugars—fructose and glucose.

During storage, microgreens grown under 250 µmol m⁻² s⁻¹ PPFD white light were shown to accumulate the highest contents of glucose and fructose. Both identified sugars maintained the tendency to reach the highest levels when grown under the highest intensity of white light but during storage seemed to differ regarding light or dark storage treatments, although light treatment managed to result in higher contents of both sugars by D₅ of storage (Figure 6A,B). Another sugar was identified in mustard grown under white LED light—maltose. Even though the contents were much lower, it seemed to favor 250 µmol m⁻² s⁻¹ PPFD treatment during growth and storage, except for D3 when 100 µmol m⁻² s⁻¹ PPFD treatment and dark storage conditions resulted in the highest levels of maltose and stored in the dark (Figure 6C).

Microgreens grown under B_75.6%:R_24.2%:W_0.02%; LED Light showed the highest glucose contents on D₁ but on D₃ and D₅, R_88.9%:B_11.1% light treatment seemed to result in the highest amounts of glucose. Those results seemed to be independent of light or dark storage treatments (Figure 7A). Fructose seemed to exhibit similar patterns of accumulation, but seemingly both R_77.6%:W_9.9%:B_3.5% and R_88.9%:B_11.1% resulted in the highest amounts of fructose during storage (Figure 7B).

4. Discussion

Our study demonstrated that the spectral quality and the intensity of light significantly affected the accumulation of photosynthates in amaranth and mustard microgreens. Specifically, wide spectrum 250 µmol m⁻² s⁻¹ PPFD and R_88.9%:B_11.1% resulted in the highest levels of glucose and fructose, which aligns with previous studies showing that blue light enhances the efficiency of photosynthesis by boosting the activity of photosystem II and photoreceptors such as cryptochromes and phototropins [31]. Furthermore, by optimizing the light intensity or optimal nutritional quality of microgreens, the financial benefit might be achieved. The differential responses in photosynthate accumulation and degradation under varying spectral treatments could be explained by several physiological mechanisms.

The increased photosynthate content under blue and red light can be attributed to the stimulation of chlorophyll biosynthesis and increased stomatal conductance, which promotes higher carbon fixation rates [32]. Similar results have been observed in crops such as dill, parsley, and kale [33,34]. This finding suggests that optimizing the light spectrum, particularly by emphasizing blue and red light, could be a key strategy for enhancing the yield and nutritional quality of microgreens in indoor farming systems. According to the literature, blue light is known to activate cryptochromes, enhance chloroplast development, and increase carbon fixation, leading to higher photosynthate accumulation [35]. It influences several enzymes of the carbohydrate dissimilation pathways, such as glucose-6-phosphate dehydrogenase, nicotinamide adenine dinucleotide (NAD)-dependent glyceraldehyde-3-phosphate dehydrogenase, pyruvate kinase, and some tricarboxylic acid (TCA) cycle enzymes such as isocitrate dehydrogenase, succinate dehydrogenase, or fumarase [36]. The alteration of these enzymes may be responsible for sugar content differences between the treatments. In agreement, some studies with blue light show increased glucose and fructose levels in broccoli [34] and radish [37]. Other studies have shown a slight increase in soluble sugars related to the influence of red light during cultivation in all varieties of tomato leaves [38]. Although the effects of both red and blue light supplementation vary, some studies reported increased sugar levels [35], while others found reduced total soluble solids under red–blue ratios of 2:1 [4]. These findings suggest that optimal light spectra for enhancing nutritional quality may be species-specific and should be tailored accordingly [39].

In contrast, light may drive an increase in the activity of enzymes involved in carbohydrate metabolism, particularly those associated with the breakdown of starches and sugars, contributing to the observed degradation [40]. Our results are consistent with varying findings from studies on other controlled-environment crops, such as basil, spinach [40], leaf lettuce and komatsuna [39], and broccoli [34]. Researchers mostly agree that higher light intensities generally increase sugar production and plant growth [41] and insufficient low light impairs photosynthesis and reduces signaling sugar content [42]. Research shows that in Setaria viridis [43] and cabbage microgreens [42], low light-impaired photosynthesis and reduced signaling sugar content occur, while high light-induced sugar accumulation occurs without repressing photosynthesis [25]. In agreement with these findings, the literature states that higher light intensity increases sugar content in Brassica microgreens, with different optimal PPFD levels for cabbage and Chinese kale [25]. Although our study shows no strong tendency to favor either dark or light storage conditions, other experiments show light exposure during storage to increase the photosynthetic capacity of radish microgreens during postharvest storage, which may result in the production of glucose [44].

Our study underscores the critical role of both spectral quality and light intensity in influencing the accumulation of photosynthates, which are vital for enhancing the postharvest shelf life of microgreens [45]. In our research, we identified that specific light conditions—specifically a broad-spectrum intensity of 250 µmol m⁻² s⁻¹ PPFD or a light ratio of R_88.9%:B_11.1%—consistently yielded the highest levels of these critical sugars during growth in agreement with our findings in the literature states that pre-harvest treatments are beneficial in nutritional value enhancement [46]. Researchers agree that these sugars act as key osmoprotectants and energy sources, stabilizing cell membranes and delaying senescence, which directly contributes to their ability to extend the shelf life of microgreens [44]. The accumulation of glucose and fructose was markedly higher under these tailored light conditions, demonstrating that light quality not only influences photosynthetic efficiency but also determines the metabolic pathways that prioritize sugar synthesis and storage. Moreover, the study revealed that these sugars degraded at a slower rate during storage when plants were grown under optimal light conditions, highlighting their importance in maintaining nutritional quality, textural integrity, and visual appeal over time. This delayed degradation can be attributed to the synergistic effects of light-induced metabolic priming, which enhances the resilience of microgreens against oxidative and cellular stress during postharvest storage [45].

These findings are especially significant for amaranth and mustard microgreens, which exhibited pronounced benefits in terms of sugar accumulation and preservation under the identified conditions. By leveraging the interplay between spectral light quality and intensity, growers can effectively manipulate the biochemical composition of microgreens to meet the demands of longer supply chains and reduce postharvest losses. Furthermore, this research provides a framework for optimizing light strategies in controlled environments, paving the way for sustainable and efficient microgreen production systems that deliver superior nutritional and sensory attributes to consumers. These insights reinforce the value of precision light management as a tool for improving both the immediate and long-term quality of microgreens in commercial applications.

5. Conclusions

Our study shows that the spectral quality and the intensity of light significantly affected the accumulation of photosynthates and their important role in shelf-life prolongation. This research found a wide spectrum of 250 µmol m⁻² s⁻¹ PPFD or R_88.9%:B_11.1% growing conditions to result in the highest amount of sugars, which determined higher and prolonged postharvest quality. These specific conditions predetermined the increased glucose and fructose accumulation and delayed degradation of siding storage in amaranth and mustard microgreens.

However, while previous research has focused predominantly on mature crops, our study expands the understanding of spectral modulation effects on microgreens, which have a much shorter growth cycle and a different metabolic profile. This highlights the importance of tailoring light conditions specifically for microgreens to optimize both yield and nutrient retention. By modulating the light spectrum and light intensity, growers can potentially manipulate the balance between increased sugar accumulations and prolonged deterioration during storage, which leads to prolonged shelf life.