Introduction
Microgreens are immature plants whose aerial part (stem and leaves) is harvested at an early stage of development, between 10 and 20 days after emergence (Mir et al., 2016; Xiao et al., 2012). Its commercial production can be carried out with few inputs and without special facilities, which facilitates its cultivation in sites with natural light or artificial lighting (Castagnino et al., 2020). Therefore, they represent an alternative cultivation option in relatively short periods, with a low initial investment and a high potential for economic returns (Bautista-Olivas et al., 2023). In addition to their nutritional value, they contribute to agri-food sustainability due to their genetic diversity and their potential for generating functional and innovative foods with high added value (Castagnino et al., 2020).
Light is one of the most important parameters in the production of microgreens, since photosynthesis and photomorphogenesis depend, to a large extent, on the intensity, duration, and quality (wavelength) of the incident light (Alrifai et al., 2019; Casierra-Posada & Peña-Olmos, 2015). Morphogenesis in plants is regulated by photoreceptors sensitive to different wavelengths, mainly in the blue, red, and far-red regions of the visible light spectrum (Toscano et al., 2021). In this sense, lighting with light-emitting diodes (LEDs) offers the possibility of applying specific spectra, such as red or blue monochromatic light, which can influence the synthesis of photosynthetic pigments (chlorophyll a, b, total, and carotenoids), phenolic compounds, and other secondary metabolites (Mendoza-Paredes et al., 2021).
Several studies have investigated the regulation of the light spectrum in the production of microgreens from species of the Brassicaceae, Lamiaceae, Apiaceae, Boraginaceae, and Chenopodiaceae families, all of which have high nutritional value. These studies report that light can modify seedling growth and increase the content of bioactive compounds. However, they have been limited to the effect of monochromatic, blue, red, and red-blue light (Toscano et al., 2021), and there are little-studied species such as chia (Salvia hispanica L.) and amaranth (Amaranthus hypochondriacus L.).
Chia seeds are characterized by having a high content of omega-3 fatty acids, fiber, protein, antioxidants and other compounds recognized for their health benefits (González-Solano et al., 2019). These characteristics have increased commercial and industrial interest in the production of various food and medicinal products; however, its consumption as a microgreen is recent and has been associated with nutritional and antioxidant properties (Gómez-Favela et al., 2017). Amaranth is an important source of protein, calcium, iron, amino acids (such as lysine and niacin) and vitamins (B and E) (Mapes-Sánchez, 2015). The consumption of this species at the microgreen stage has a nutritional and nutraceutical contribution; in addition, the tender and juicy young red amaranth leaves have a subtle nutty flavor, which makes them an ideal complement for sandwiches, salads, sauces and other dishes (Xiao et al., 2015).
Because the consumption of these microgreen species is emerging, there is little information on their bioactive compound content and their response to supplemental LED lighting. Therefore, the objective of this study was to evaluate the effect of LED lighting on the nutritional and nutraceutical quality of chia and amaranth microgreens grown under hydroponic and greenhouse conditions.
Materials and methods
Experimental site
The field phase was carried out in a tunnel type greenhouse of 5 × 12 m and 4 m height, with manual lateral and frontal ventilation, thermal polyethylene cover, light diffuser and east-west orientation. The greenhouse is located at the Experimental Agricultural Field at the Universidad Autónoma Chapingo, Mexico (19° 29’ 35’’ N and 98° 52’ 19’’ W, at 2 250 m a. s. l.).
Agronomic management
Chia seeds were purchased from Dinat® (AlBlanco Branding Co.) while amaranth seeds var. Nutrisol were provided by the Instituto Nacional de Investigaciones Forestales, Agrícolas y Pecuarias, Valle de México unit. Both seeds were harvested in the previous cycle and were not treated.
The seeds were sown in a mixture of peat and vermiculite (1:1, v/v) at a depth of 3 mm, in polystyrene trays with 200 cavities (25 mL volume per cavity), cut to 9 × 13 cavities and with a density of 292 plants per tray (2.5 seeds on average per cavity). The trays were maintained in a floating root hydroponic system in 30 × 50 cm polyethylene tubs containing a Nutrient Solution (NS). Steiner's (1984) SN was used diluted to 50% with the following concentrations (mg∙L-1): N = 168, P = 31, K = 273, Ca = 180, Mg = 48 and S = 130. Micronutrients were provided at the following concentrations (mg∙L-1): Fe = 3, Mn = 1, B = 0.5, Cu = 0.1 and Zn = 0.1 (Sánchez-del Castillo & Escalante-Rebolledo, 1988). The water used was purified by activated carbon filtration, softening, UV irradiation and ozonation (Aqua Clyva®).
Artificial light management
Lamps sized 120 × 5 cm of 24, 20 and 18 W were used for white, blue and red light, respectively. The lamps were placed in pairs 15 cm apart, supported from the greenhouse structure. Red lamps (1 000 lm) were placed 10 cm above the plants, blue lamps (4 000 lm) were at 30 cm, and white lamps (4 200 lm) were at 40 cm.
Artificial lighting was activated by a timer from 7:00 PM to 3:00 AM. The photoperiod was 20 h of light and 4 h of darkness (12 h of natural light and 8 h of LED light), while the control period was 12 h of light and 12 h of darkness. Double-layer curtains (black/white) were placed at night to prevent light from entering between treatments; these, as well as the lamps, were removed during the day to prevent shadows.
Design and experimental unit
The experiment was conducted using a completely randomized design with a 2 × 4 factorial arrangement, corresponding to two species (chia and amaranth) and four light levels (no LED light, and with red, blue, and white LED light), with four replicates per treatment. The experimental unit consisted of a tray with 292 plants.
Response variables
Morphological variables
Plant height (from the stem base to the apex), fresh weight, and dry weight were measured 18 days after sowing (das). Ten plants, with full competence, were considered the sampling unit.
Nutritional quality
Mineral quantification. Ca, K, Mg, P, Fe, Mn, B, Na, Cu, and Zn contents were determined according to the method described by Alcántar-González and Sandoval-Villa (1999). A total of 0.25 g of dried, ground plant material was taken from each sample and subjected to wet digestion with a diacid mixture (H2SO4:HClO4; 4:1, v/v) and hydrogen peroxide. Determinations were performed using an inductively coupled plasma atomic emission spectrophotometer (ICP-AES) (Liberty II, Varian, USA).
Proximal analysis. The aerial parts (leaves and stems) of the microgreens were dried in a convection oven at 55 °C for 72 h (FE-291, Felisa®, Mexico). Moisture, lipid, and ash contents were determined according to the methods of the Association of Official Analytical Chemists (AOAC, 2005). Results were expressed as percentages. Total carbohydrate (TC) content was calculated using the following formula (Audu & Aremu, 2011):
where CP is the crude protein content (%), L is the lipid content (%), and A is the ash content (%). The calorie content was calculated by multiplying the grams of carbohydrate and protein by four. Total nitrogen was determined by micro-Kjeldahl digestion (AOAC, 2005).
Nutraceutical quality
Chlorophyll (a, b) and carotenoids. Nutraceutical quality was determined by the method of Yang et al. (1998), with modifications. One hundred milligrams of fresh tissue were macerated with 10 mL of acetone-water (4:1, v/v), sonicated for 3 min, and centrifuged at 2 100 × g for 10 min. The supernatant was made up to 10 mL with 80 % acetone, and absorbance was measured at 664, 647, and 470 nm in a spectrophotometer (Genesys 10s, Thermo Scientific, USA). Results were expressed as milligrams per gram of fresh weight (mg∙g-1).
Bioactive compounds and antioxidant capacity
For the determination of these compounds, a methanolic extract was prepared. One gram of fresh plant material was mixed with 10 mL of 80 % (v/v) MeOH, vortexed, sonicated for 15 min at room temperature, allowed to stand for 24 h, and centrifuged for 10 min at 1 461 × g (Cruz-de la Cruz et al., 2021). The supernatant was used for the analyses, and all measurements were performed on a spectrophotometer (Genesys 10s, Thermo Scientific, USA).
Total phenols. They were quantified by the method described by Waterman and Mole (1994), with some modifications. First, 0.5 mL of the methanolic extract, 0.5 mL of the Folin-Ciocalteu reagent (0.2 N), and 4 mL of a 0.7 M Na2CO3 solution were mixed. The mixture was vortexed and incubated in the dark for 2 h. The absorbance was measured at 765 nm. The concentration was calculated from a standard curve of gallic acid: y = 0.0079x + 0.0239; R2 = 0.9966. The results were expressed as milligrams of gallic acid equivalents per 100 g fresh weight (mgGAE∙100 g-1).
Total flavonoids. They were determined using the methodology proposed by Chang et al. (2002). First, 0.5 mL of the methanolic extract, 1.5 mL of methanol (95 %, v/v), 0.1 mL of an AlCl3 solution (10 %, w/v), 0.1 mL of a 1 M potassium acetate solution, and 2.8 mL of distilled water were mixed. The solution was incubated for 30 min, and the absorbance was read at 415 nm. For quantification, a quercetin standard curve was performed: y = 0.0047x - 0.0023; R² = 0.9987. The results were expressed as micrograms of quercetin equivalents per 100 g fresh weight (µgQE∙100 g-1).
Antioxidant capacity. It was quantified using the methodology described by Kuskoski et al. (2004). The ABTS•+ radical (2,2’-azinobis[3-ethylbenzothiazoline-6-sulfonic acid]) was obtained by the reaction of ABTS (7 mM) with potassium persulfate (2.45 mM, final concentration). The mixture was incubated at room temperature (± 25 ºC) in the dark for 16 h. Once the ABTS•+ radical was formed, it was diluted with ethanol until it reached an absorbance value of 0.70 ± 0.1 at 754 nm. On the other hand, 1 mL of the ABTS•+ solution was mixed with 10 µL of the methanolic extract and incubated in a water bath at 30 °C in the dark for 7 min. Absorbance was measured at 734 nm. Antioxidant activity was calculated using a Trolox (6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid) standard curve: y = 38.975x + 2.5678; R² = 0.9963. Results were expressed in millimoles Trolox equivalents per 100 g of fresh weight (mmolTE∙100 g-1). The percentage inhibition of the ABTS•+ free radical was calculated using the following formula:
where IA is the initial absorbance of the free radical at 734 nm and FA is the final absorbance of the reaction with the sample.
Data analysis
For morphological variables, four replicates were considered, and nutritional variables were expressed as the mean ± standard error of three replicates. The results for all variables were subjected to analysis of variance and Tukey's mean comparison test (α = 0.05) in SAS version 9.1 software (SAS Institute, 2002).
Results and discussion
The analysis of variance showed significant differences (p ≤ 0.05) between species (amaranth and chia) in morphological, nutraceutical, and nutritional variables. The coefficients of variation were less than 10 %, except for lipid content (14.83 %). Furthermore, when analyzing the data for each species separately, no statistical differences were observed in the important variables (information not presented).
Morphological characteristics
Significant differences (p ≤ 0.05) were found in the three morphological variables among the species harvested at the microgreens stage (Table 1). Amaranth was 45 % taller than chia; however, chia microgreens had higher fresh (21 %) and dry (33 %) weight than amaranth microgreens. The height and fresh weight per plant of amaranth microgreens (6.04 cm and 0.104 g, respectively) were higher than the values reported by Toscano et al. (2021), who obtained 3.9 ± 0.2 cm and 0.0185 g, respectively. In contrast, the height of chia microgreens (4.16 cm) was lower than that obtained by Junpatiw and Sangpituk (2019), who documented values between 5.67 and 7.62 cm; However, the fresh weight of chia (0.126 g) exceeded that reported by these authors (0.009 to 0.018 g).
Table 1.
| Treatment | Plant height (cm) | Fresh weight per plant (g) | Dry weight per plant (g) |
|---|---|---|---|
|
|
|||
| Chia | 4.16 b | 0.126 a | 0.019 a |
| Amaranth | 6.04 a | 0.104 b | 0.014 b |
| HSD | 0.082 | 0.007 | 0.004 |
|
|
|||
| Control | 5.10 a | 0.10 a | 0.014 b |
| Red | 5.05 a | 0.12 a | 0.016 a |
| Blue | 5.10 a | 0.11 a | 0.016 ab |
| White | 5.16 a | 0.11 a | 0.015 ab |
| HSD | 0.15 | 0.01 | 0.002 |
Regarding the supplementary LED light treatments (Table 1), significant differences (p ≤ 0.05) were only observed in dry weight between the control and the red-light treatment in both species. The red-light treatment (0.016 g) was statistically superior to the control (0.014 g), but without significant differences with the blue and white light treatments. This result suggests a positive effect of red light on biomass accumulation. The observed effect contrasts with that reported by Lau et al. (2019) and Toscano et al. (2021), who point out that plants grown under monochromatic red light have lower biomass content compared to white light. However, Toscano et al. (2021), when calculating the height/biomass ratio, observed that in amaranth, the red-light treatment presented a better ratio than that with white and blue light due to the observed increase in height under red light.
Mineral concentration
Statistically significant differences were detected in the concentrations of most of the minerals evaluated, both between species and between light treatments (Tables 2 and 3). Amaranth microgreens showed the highest concentrations of all minerals except Mn compared to chia microgreens. These differences can be attributed to the genetic variability between the two species.
Table 2.
| Treatment | N | P | K | Ca | Mg |
|---|---|---|---|---|---|
| Specie | |||||
| Chia | 37 200 b | 9.91 b | 133.77 b | 40.64 b | 40.55 b |
| Amaranth | 44 300 a | 10.32 a | 253.11 a | 59.47 a | 83.93 a |
| HSD | 700 | 0.27 | 5.427 | 0.665 | 12.78 |
|
|
|||||
| Control | 41 200 ab | 9.62 b | 221.94 a | 48.51 c | 62.23 b |
| Red | 40 400 ab | 9.89 b | 193.02 b | 49.73 bc | 60.65 b |
| Blue | 41 400 a | 10.40 a | 184.32 bc | 50.76 ab | 61.44 b |
| White | 39 900 b | 10.56 a | 174.50 c | 51.20 a | 64.65 a |
| HSD | 1 300 | 4.29 | 10.26 | 1.26 | 2.42 |
Table 3.
| Treatment | B | Cu | Fe | Mn | Na | Zn |
|---|---|---|---|---|---|---|
| Specie | ||||||
| Chia | 0.34 b | 0.06 b | 1.69 b | 1.75 a | 9.81 b | 0.26 b |
| Amaranth | 0.35 a | 0.09 a | 1.81 a | 1.29 b | 14.65 a | 0.34 a |
| HSD | 0.01 | 0.00 | 0.04 | 0.02 | 0.26 | 0.09 |
|
|
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| Control | 0.36 a | 0.07 a | 1.73 a | 1.48 c | 11.84 b | 0.29 b |
| Red | 0.35 a | 0.08 a | 1.80 a | 1.44 c | 12.91 a | 0.29 b |
| Blue | 0.34 b | 0.08 a | 1.75 a | 1.56 b | 12.11 b | 0.28 b |
| White | 0.33 b | 0.08 a | 1.72 a | 1.62 a | 12.06 b | 0.33 a |
| HSD | 0.01 | 0.01 | 0.08 | 0.04 | 0.48 | 0.02 |
Toscano et al. (2021), in their study of microgreens of other amaranth species (A. tricolor), reported higher contents of P, K, Ca, Mg, Cu, Na and Zn, but lower contents of Fe and Mn, than those found in this work. It is important to note that, to the best of our knowledge, there are no studies on the mineral content of A. hypochondriacus microgreens. Likewise, no N and B concentrations have been reported in microgreens of any species, which positions this research as an initial reference for these parameters, particularly for chia microgreens.
The mineral composition of microgreens depends on both the species and the availability of minerals in the culture medium and the nutrient solution. Di Gioia and Santamaria (2015) and Ghoora et al. (2020a) point out that microgreens with high contents of essential macro and microelements, or low contents of undesirable elements (nitrates and sodium), can be obtained by modifying the culture system and the formulation of the nutrient solution.
Supplementary light significantly influenced (p ≤ 0.05) the concentrations of some elements (such as N, P, K, Ca, Mg, B, Mn, Na and Zn) in both species (Tables 2 and 3). White light increased the concentrations of P, Ca, Mg, Mn and Zn, while the control favored the levels of K and B. Blue light promoted an increase in the concentration of N, and red light favored the Na content. No significant differences (p ≤ 0.05) were observed in the concentrations of Cu and Fe.
The observed differences in mineral absorption could be due to specific factors such as transpiration, stomatal aperture, growth rate and biomass accumulation at different stages of plant development (Pennisi et al., 2019). Also, mineral absorption and accumulation in plants may be modulated by factors such as species, light, temperature, CO2 level, water stress, phytohormones, and interaction with other nutrients (Sakuraba & Yanagisawa, 2018).
There is scarce information on the effect of different wavelengths of LED light on the dynamics of nutrient absorption, particularly during the commercial maturity stage of microgreens. In this sense, the present research provides novel information on the effect of different types of LED light on young seedlings (microgreens) of chia and amaranth.
Proximal analysis
The results of the proximal analysis varied among species, except for lipid content (Table 4). Amaranth microgreens had the highest percentages of protein, ash, and moisture content, but lower carbohydrate and calorie content than chia.
Table 4.
| Treatment | Protein (%) | Lipids (%) | Ash (%) | Carbohydrates (%) | Moisture (%) | Calories (Kcal∙100 g-1) |
|---|---|---|---|---|---|---|
| Specie | ||||||
| Chia | 23.20 b | 3.28 a | 10.25 b | 63.21 a | 85.45 b | 345.85 a |
| Amaranth | 27.70 a | 3.20 a | 17.94 a | 51.15 b | 86.48 a | 315.43 b |
| HSD | 0.44 | 0.35 | 0.55 | 0.84 | 0.30 | 2.37 |
|
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| Control | 25.70 ab | 3.84 a | 14.98 a | 55.41 c | 85.68 b | 324.67 c |
| Red | 25.30 ab | 3.77 a | 13.84 b | 57.08 b | 86.33 a | 329.53 b |
| Blue | 25.90 a | 2.82 b | 14.11 ab | 57.15 b | 85.82 ab | 332.25 a |
| White | 24.90 b | 2.52 b | 13.44 b | 59.07 a | 86.03 ab | 336.12 a |
| HSD | 0.84 | 0.66 | 1.04 | 1.59 | 0.57 | 4.48 |
Due to the limited availability of proximal analysis data on microgreens of the studied species, the results were compared with studies conducted on their respective seeds. In the case of chia, the protein content in microgreens (23.20 g∙100 g-1) was similar to that reported for seeds (23.17 g∙100 g-1) by Scapin et al. (2016). In contrast, amaranth microgreens had a considerably higher protein content (27.70 g∙100 g-1) than that reported for their seeds (1.40 g∙100 g-1) by Trino et al. (2017).
Regarding the lipid content in chia microgreens (3.28 g∙100 g-1) and amaranth (3.20 g∙100 g-1) it was lower than that observed in seeds (28.35 and 4.2 g∙100 g-1, respectively) by Scapin et al. (2016) and Trino et al. (2017). The carbohydrate concentration also showed important variations, since the value of this metabolite in chia microgreens was higher (63.1 g∙100 g-1) than that reported in seeds (8.6 g∙100 g-1) by Jiménez et al. (2013). In contrast, amaranth microgreens had a lower carbohydrate content (51.1 g∙100 g-1) than that observed in seeds (74.4 g∙100 g-1) by Trino et al. (2017). Finally, the caloric energy observed in amaranth microgreens was lower (315 Kcal∙100 g-1) than the value found in seeds (386 Kcal∙100 g-1) by Trino et al. (2017).
The observed differences may be related to the fact that seeds contain reserve substances for germination and seedling development, which are transformed and translocated to generate new and diverse metabolites according to the demands of the new plant. Therefore, the content of primary metabolites (carbohydrates, proteins, and lipids) tends to decrease with plant growth (Ghoora & Srividya, 2017).
LED light treatments significantly influenced (p ≤ 0.05) all variables in the proximate analysis in both species (Table 4). Control microgreens (without artificial light) showed higher concentrations of lipids and ash in both species. The blue light treatment maximized protein content, possibly due to the higher amount of N. Red light resulted in a higher percentage of moisture, which was reflected in lower plant dry weight. Finally, white light promoted carbohydrate content and, consequently, caloric value, since high light intensity favors the accumulation of carbohydrates in chloroplasts and the cytoplasm. Azcón-Bieto and Talón (2013) emphasize that the light environment during plant growth determines their photosynthetic, morphological, and physiological characteristics.
Nutraceutical composition and antioxidant activity
Amaranth microgreens showed higher chlorophyll a content, while chia microgreens were characterized by a higher accumulation of chlorophyll b (Table 5). The concentrations of chlorophylls a and b in both species were higher than those reported by Ghoora et al. (2020b) in species of the Lamiaceae and Amaranthaceae families, to which chia and amaranth belong, respectively. Likewise, Toscano et al. (2021) obtained lower concentrations of these pigments in A. tricolor.
Table 5.
| Treatment | Chlorophyll a (mg∙g-1) | Chlorophyll b (mg∙g-1) | Carotenoids (mg∙g-1) | Phenols (mgGAE∙100 g-1) | Flavonoids (µgQE∙100 g-1) | Antioxidant activity (mmolTE∙100 g-1) |
|---|---|---|---|---|---|---|
| Specie | ||||||
| Chia | 1.20 b | 0.44 a | 0.44 a | 126.60 a | 256.22 a | 2.71 a |
| Amaranth | 1.31 a | 0.31 b | 0.40 b | 114.20 b | 119.03 b | 1.01 b |
| HSD | 0.05 | 0.02 | 0.02 | 1.71 | 6.41 | 0.01 |
|
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| Control | 1.20 b | 0.37 a | 0.41 b | 119.40 b | 179.10 b | 1.84 b |
| Red | 1.29 ab | 0.38 a | 0.45 a | 119.60 b | 179.40 b | 1.84 b |
| Blue | 1.32 a | 0.38 a | 0.42 ab | 124.10 a | 208.70 a | 1.93 a |
| White | 1.21 a | 0.35 a | 0.40 b | 118.60 b | 183.06 b | 1.82 c |
| HSD | 0.10 | 0.04 | 0.10 | 3.24 | 12.12 | 0.01 |
From a nutraceutical perspective, chlorophyll consumption provides health benefits. Additionally, high levels of these pigments intensify the green color of leaves, which can improve consumer acceptance of the product (Ghoora et al., 2020b).
Regarding LED light treatments, no significant differences (p ≤ 0.05) were observed in chlorophyll b content, but differences were observed in chlorophyll a content (Table 5), where treatment with blue light caused an increase in this pigment. Toscano et al. (2021) obtained similar results in amaranth under blue LED light. On the other hand, in treatments without LED light, significant differences (p ≤ 0.05) were found in the carotenoid content between species (Table 5), which was higher than the values reported by Toscano et al. (2021). Treatments with red and blue light promoted a greater accumulation of these pigments compared to white light. In this sense, Toscano et al. (2021) observed that blue light promotes carotenoid biosynthesis (≈30 % more). The increase in photosynthetic pigments in microgreens is closely related to the species, the interaction between chlorophyll and carotenoid biosynthesis, as well as the duration and intensity of light (Mlinarić et al., 2020).
The content of phenolic compounds showed different behavior between species and light treatments (Table 5). The highest concentration of these metabolites was recorded in chia microgreens and was higher than that obtained by Ghoora et al. (2020b) in 10 species (14.06 - 73.6 mgGAE∙100 g-1), including one species from the Lamiaceae family and another from Amaranthaceae. On the other hand, the content of phenolic compounds (124.8 mgGAE∙100 g-1) in another amaranth species (A. tricolor) (Toscano et al., 2021) was higher than that obtained in amaranth microgreens (A. hypochondriacus) in the present investigation. These differences could be due to factors such as genetic variability, cultivation conditions or composition of the nutrient solution.
In both species, the content of phenolic compounds increased with blue light, which coincides with that reported by Toscano et al. (2021) in amaranth and turnip. Exposure of plants to blue light has been associated with the activity of the enzyme phenylalanine ammonia lyase (PAL), which is decisive in the biosynthesis of phenols through the shikimic acid metabolic pathway (Cuong et al., 2019). This enzyme is activated by abiotic stress and the accumulation of reactive oxygen species (ROS) (Eloy et al., 2018).
A similar trend was observed among species and light treatments in flavonoid concentrations (Table 5). Chia microgreens had higher flavonoid concentrations than amaranth microgreens (0.256 and 0.11 mgQE∙100 g-1, respectively), similar to that observed for phenolic compounds. These values are lower than those reported by Ghoora et al. (2020b) in 10 species of microgreens (1.1 to 6.5 mgQE∙100 g-1).
There are intrinsic and extrinsic factors that can affect the content of phenolic compounds and flavonoids, such as the species, growth conditions, maturity stage at harvest, abiotic stress, and sample preparation (Eloy et al., 2018; Podsędek, 2007). The reported flavonoid content in microgreens is scarce; however, some research has shown that blue light promotes their accumulation to a greater extent than white or red light (Nam et al., 2018). This effect has been linked to the expression of genes encoding enzymes responsible for the synthesis of some flavonoids, such as 4-cinnamic acid hydroxylase, chalcone isomerase, flavonol synthase II, anthocyanidin synthase (Thwe et al., 2014), flavonoid 3-hydroxylase and flavonol synthase (Kim et al., 2015).
Regarding antioxidant activity, the results followed a similar trend to that of phenolic and flavonoid compounds, highlighting chia microgreens and blue light treatment in both species (Table 5). The antioxidant activity in chia microgreens (27.1 μmolTE∙g-1) was higher than the range reported by Ghoora et al. (2020b) (10.9 to 22.8 μmolTE∙g-1) in 10 species evaluated. In contrast, amaranth microgreens had lower antioxidant activity (10.06 μmolTE∙g-1) compared to the mentioned range. The observed differences can be attributed to the defense mechanisms induced by lighting, which activate different metabolic pathways that favor the synthesis of antioxidant compounds, particularly phenolic ones (Toscano et al., 2021). According to the results, antioxidant activity is related to the concentrations of phenols, flavonoids, and carotenoids, which act as potent ROS neutralizing agents.
Conclusions
The chia microgreens stood out for their nutraceutical quality, evidenced by the higher concentration of carotenoids, phenols, flavonoids, and high antioxidant activity. The amaranth microgreens, meanwhile, stood out for their nutritional quality, presenting higher macro and micronutrient (except Mn) and protein contents.
Among the supplemental LED light treatments, white light improved nutritional content, while blue light increased the content of nutraceutical variables. Red light promoted dry matter accumulation in both species, while the treatment without light had little significant effect. Supplemental LED light generated changes in the nutritional and nutraceutical properties of the microgreens of both species evaluated.