The distinct impact of multi-color LED light on nitrate, amino acid, soluble sugar and organic acid contents in red and green leaf lettuce cultivated in controlled environment

Effects of Different LED Spectra on the Antioxidant Capacity and Nitrogen Metabolism of Chinese Cabbage (Brassica rapa L. ssp. Pekinensis)

Light quality optimization is a cost-effective method for increasing leafy vegetable quality in plant factories. Light-emitting diodes (LEDs) that enable the precise modulation of light quality were used in this study to examine the effects of red-blue (RB), red-blue-green (RBG), red-blue-purple (RBP), and red-blue-far-red (RBF) lights on the growth, antioxidant capacity, and nitrogen metabolism of Chinese cabbage leaves, while white light served as the control (CK). Results showed that the chlorophyll, carotenoid, vitamin C, amino acid, total flavonoid, and antioxidant levels of Chinese cabbage were all significantly increased under RBP combined light treatment. Meanwhile, RBG combined light treatment significantly increased the levels of amino acids but decreased the nitrite content of Chinese cabbage. In addition, RBF combined light treatment remarkably increased the amino acid levels but decreased the antioxidant capacity of Chinese cabbage. In conclusion, the addition of purple light to red-blue light was effective in improving the nutritional value and antioxidant capacity of Chinese cabbage. This light condition can be used as a model for a supplemental lighting strategy for leafy vegetables in plant factory production.

Photoperiod-Dependent Nutrient Accumulation in Rice Cultivated in Plant Factories: A Comparative Metabolomic Analysis

Plant factories offer a promising solution to some of the challenges facing traditional agriculture, allowing for year-round rapid production of plant-derived foods. However, the effects of conditions in plant factories on metabolic nutrients remain to be explored. In this study, we used three rice accessions (KongYu131, HuangHuaZhan, and Kam Sweet Rice) as objectives, which were planted in a plant factory with strict photoperiods that are long-day (12 h light/12 h dark) or short-day (8 h light/16 h dark). A total of 438 metabolites were detected in the harvested rice grains. The difference in photoperiod leads to a different accumulation of metabolites in rice grains. Most metabolites accumulated significantly higher levels under the short-day condition than the long-day condition. Differentially accumulated metabolites were enriched in the amino acids and vitamin B6 pathway. Asparagine, pyridoxamine, and pyridoxine are key metabolites that accumulate at higher levels in rice grains harvested from the short-day photoperiod. This study reveals the photoperiod-dependent metabolomic differences in rice cultivated in plant factories, especially the metabolic profiling of taste- and nutrition-related compounds.

Effects of Light Intensity on Growth and Quality of Lettuce and Spinach Cultivars in a Plant Factory

The decreased quality of leafy vegetables and tipburn caused by inappropriate light intensity are serious problems faced in plant factories, greatly reducing the economic benefits. The purpose of this study was to comprehensively understand the impact of light intensity on the growth and quality of different crops and to develop precise lighting schemes for specific cultivars. Two lettuce (Lactuca sativa L.) cultivars-Crunchy and Deangelia-and one spinach (Spinacia oleracea L.) cultivar-Shawen-were grown in a plant factory using a light-emitting diode (LED) under intensities of 300, 240, 180, and 120 μmol m-2 s-1, respectively. Cultivation in a solar greenhouse using only natural light (NL) served as the control. The plant height, number of leaves, and leaf width exhibited the highest values under a light intensity of 300 μmol m-2 s-1 for Crunchy. The plant width and leaf length of Deangelia exhibited the smallest values under a light intensity of 300 μmol m-2 s-1. The fresh weight of shoot and root, soluble sugar, soluble protein, and ascorbic acid contents in the three cultivars increased with the increasing light intensity. However, tipburn was observed in Crunchy under 300 μmol m-2 s-1 light intensity, and in Deangelia under both 300 and 240 μmol m-2 s-1 light intensities. Shawen spinach exhibited leaf curling under all four light intensities. The light intensities of 240 and 180 μmol m-2 s-1 were observed to be the most optimum for Crunchy and Deangelia (semi-heading lettuce variety), respectively, which would exhibit relative balance growth and morphogenesis. The lack of healthy leaves in Shawen spinach under all light intensities indicated the need to comprehensively optimize cultivation for Shawen in plant factories to achieve successful cultivation. The results indicated that light intensity is an important factor and should be optimized for specific crop species and cultivars to achieve healthy growth in plant factories.

Leaf morphology, optical characteristics and phytochemical traits of butterhead lettuce affected by increasing the far-red photon flux

Light and its spectral characteristics are crucial for plant growth and development. The far-red photon flux mediates many plant processes through the action of phytochrome and also accelerates the photosynthetic electron transfer rate. In this study, we assessed the effects of far-red addition on butterhead lettuce morphology, light use efficiency, optical properties, and phytochemical characteristics. Three-week-old lettuce plants (Lactuca sativa L. cv. Alyssa) were grown for up to 28 days under a 10% blue and 90% red light spectrum (200 µmol m-2 s-1, 16 h photoperiod) to which five different intensities of far-red light (peak at 735 nm) were added (0-9-18-36-72 µmol m-2 s-1). White light-emitting diodes were included as a proxy for sunlight. Increasing supplemental far-red photon flux from zero to 21% increased the light use efficiency (g per mol) by 37% on day 14; 43% on day 21; and 39% on day 28. Measurements of projected head area suggest that this was associated with an increase in leaf expansion and photon capture and not necessarily a direct effect on photosynthesis. Moreover, vegetation indices based on leaf reflectance showed a decrease in chlorophyll-related indices under a high far-red photon flux. This decrease in pigment content was confirmed by chemical analyses, suggesting that the plants may not reach their full potential in terms of photon capture, limiting the overall photosynthetic performance. Furthermore, the stress-related Carter 1 index increased in plants grown under a high far-red photon flux, indicating early plant stress. Far-red tended to decrease the content of total phenolics and increase soluble sugars. The higher sugar levels can be attributed to an improved photochemical efficiency due to photosystem I excitation by far-red wavelengths, also known as the Emerson Enhancement effect. Despite these higher sugar levels, no effect on foliar nitrate content was observed. Our results show that far-red supplementation has the potential to enhance light interception at the early growth stages, although higher intensities of far-red may cause plant stress.

Optimizing lettuce yields and quality by incorporating movable downward lighting with a supplemental adjustable sideward lighting system in a plant factory

Background

Lettuce is a vegetable that is increasingly consumed globally, given its nutritional quality. Plant factories with artificial lighting can produce high-yield and high-quality plants. High plant density in these systems speeds up leaf senescence. Wasted energy and lower yield raised labor expenses are some of the bottlenecks associated with this farming system. In order to increase lettuce yields and quality in the plant factory, it is essential to develop cultivating techniques using artificial lighting.

Methods

Romaine lettuce was grown under a developed "movable downward lighting combined with supplemental adjustable sideward lighting system" (C-S) and under a system without supplemental sideward lighting (N-S) in a plant factory. The effects of C-S on lettuce's photosynthetic characteristics, plant yield, and energy consumption relative to plants grown under a system without N-S were studied.

Results

Romaine lettuce growth and light energy consumption in the plant factory were both influenced favorably by supplementary adjustable sideward lighting. The number of leaves, stem diameter, fresh and dry weights, chlorophyll a and b concentration, and biochemical content (soluble sugar and protein) all increased sharply. The energy consumption was substantially higher in the N-S treatment than the C-S.

Fixed vs. variable light quality in vertical farming: Impacts on vegetative growth and nutritional quality of lettuce

Lettuce (Lactuca sativa) is commonly produced in vertical farms. The levels of nutritionally important phytochemicals such as beta-carotene (precursor to vitamin A) are generally low in lettuce. In this study, we investigated the benefits of variable lighting strategy (i.e., varying the light quality during production) on maintaining plant growth and increasing the biosynthesis of beta-carotene and anthocyanin. We tested two variable lighting methods, using green and red romaine lettuce, namely (i) providing growth lighting (supports vegetative growth) initially (21 days) followed by a high percentage of blue light (supports biosynthesis of phytochemicals) at final stages (10 days) and (ii) providing a high percentage of blue light initially followed by growth lighting at final stages. Our results indicate that the variable lighting method with initial growth lighting and high percentage of blue at final stages can maintain vegetative growth and enhance phytochemicals such as beta-carotene in green romaine lettuce while both variable lighting methods were not effective in red romaine lettuce. In green romaine lettuce, we did not observe a significant reduction in shoot dry weight but there was an increase in beta-carotene (35.7%) in the variable compared to the fixed lighting method with growth lighting for the entire duration. The physiological bases for differences in vegetative growth and synthesis of beta-carotene and anthocyanin in the variable and fixed lighting methods are discussed.

RNA-Seq Analysis Demystify the Pathways of UV-A Supplementation in Different Photoperiods Integrated with Blue and Red Light on Morphology and Phytochemical Profile of Kale

As an indispensable element in the morphology and phytochemical profile of plants, UV-A has proved to help promote the growth and quality of kale. In this study, UV-A supplementation in different photoperiods (light period supplemental UVA = LS, dark period supplemental UVA = DS, and light-dark period supplemental UVA = LDS) contributed to yielding greater biomass production (fresh weight, dry weight, and plant moisture content), thus improving morphology (plant height, stem diameter, etc.) and promoting higher phytochemicals content (flavonoids, vitamin c, etc.), especially glucosinolates. To fathom its mechanisms, this study, using RNA-seq, verified that UV-A supplementation treatments signally generated related DEGs of plant hormone signal pathway, circadian rhythm plant pathway, glucosinolate pathway, etc. Moreover, 2047 DEGs were obtained in WGCNA, illustrating the correlations between genes, treatments, and pathways. Additionally, DS remarkedly up-regulated related DEGs of the key pathways and ultimately contributed to promoting the stem diameter, plant height, etc., thus increasing the pigment, biomass, vitamin c, etc., enhancing the antioxidant capacity, and most importantly, boosting the accumulations of glucosinolates in kale. In short, this study displayed new insights into UV-A supplementation affected the pathways related to the morphology and phytochemical profile of kale in plant factories.

The Effect of LED Light Spectra on the Growth, Yield and Nutritional Value of Red and Green Lettuce (Lactuca sativa)

Controlled Environment Agriculture (CEA) is a method of increasing crop productivity per unit area of cultivated land by extending crop production into the vertical dimension and enabling year-round production. Light emitting diodes (LED) are frequently used as the source of light energy in CEA systems and light is commonly the limiting factor for production under CEA conditions. In the current study, the impact of different spectra was compared with the use of white LED light. The various spectra were white; white supplemented with ultraviolet b for a week before harvest; three combinations of red/blue lights (red 660 nm with blue 450 nm at 1:1 ratio; red 660 nm with blue 435 nm 1:1 ratio; red 660 nm with blue at mix of 450 nm and 435 nm 1:1 ratio); and red/blue supplemented with green and far red (B/R/G/FR, ratio: 1:1:0.07:0.64). The growth, yield, physiological and chemical profiles of two varieties of lettuce, Carmoli (red) and Locarno (green), responded differently to the various light treatments. However, white (control) appeared to perform the best overall. The B/R/G/FR promoted the growth and yield parameters in both varieties of lettuce but also increased the level of stem elongation (bolting), which impacted the quality of grown plants. There was no clear relationship between the various physiological parameters measured and final marketable yield in either variety. Various chemical traits, including vitamin C content, total phenol content, soluble sugar and total soluble solid contents responded differently to the light treatments, where each targeted chemical was promoted by a specific light spectrum. This highlights the importance of designing the light spectra in accordance with the intended outcomes. The current study has value in the field of commercial vertical farming of lettuce under CEA conditions.

The red/blue light ratios from light-emitting diodes affect growth and flower quality of Hippeastrum hybridum 'Red Lion'

Light quality strongly impacts the growth and flower quality of ornamental plants. The optimum light quality for the growth and flowering of Hippeastrum remains to be validated. In the present study, we investigated the effect of the red/blue light ratio of LEDs on the growth and flowering quality of H. hybrid 'Red Lion'. Two LEDs with red/blue light ratio of 1:9 (R₁₀B₉₀) and 9:1 (R₉₀B₁₀) were designed. LEDs of white light were the control. In the earlier vegetative and reproductive growth phase, R₉₀B₁₀ increased the biomass of the bulbs, leaves, and flowers. Compared with the control and R₁₀B₉₀ group, R₉₀B₁₀ LEDs delayed flowering by 2.30 d and 3.26 d, respectively. Based on chlorophyll contents, photosynthetic capacity, chlorophyll fluorescence parameters, and carbohydrate contents, the photosynthesis rate was higher in the R₁₀B₉₀ group. Optimal red and blue light intensity promoted the accumulation of carbohydrates and early flowering and prolonged the flowering period of H. hybrid. Microscopic analysis showed that stomatal density was high, and the number of chloroplasts was large in the R₁₀B₉₀ treatment group, which enhanced photosynthesis. Particularly, R₁₀B₉₀ promoted the expression of seven key genes related to chlorophyll synthesis. R₁₀B₉₀ also promoted early overexpression of the HpCOL gene that promotes early flowering. Thus, higher blue light and 10% red light intensities promote early and extended flowering, while higher red light and 10% blue light promote vegetative plant growth but delay flowering.

Effects of Pre-Harvest Supplemental UV-A Light on Growth and Quality of Chinese Kale

The effects of supplemental UV-A (385 nm) period and UV-A intensity for 5 days before harvest (DBH) on growth, antioxidants, antioxidant capacity, and glucosinolates contents in Chinese kale (Brassica oleracea var. alboglabra Bailey) were studied in plant factory. In the experiment of the UV-A period, three treatments were designed with 10 W·m^-2 UV-A supplement, T1(5 DBH), T2 (10 DBH), and no supplemental UV-A as control. In the experiment of UV-A intensity, four treatments were designed with 5 DBH, control (0 W·m^-2), 5 w (5 W·m^-2), 10 w (10 W·m^-2), and 15 w (15 W·m^-2). The growth light is as follows: 250 μmol·m^-2·s^-1; red light: white light = 2:3; photoperiod: 12/12. The growth and quality of Chinese kale were improved by supplemental UV-A LED. The plant height, stem diameter, and biomass of Chinese kale were the highest in the 5 W·m^-2 treatment for 5 DBH. The contents of chlorophyll a, chlorophyll b, and total chlorophyll were only highly increased by 5 W·m^-2 UV-A for 5 DBH, while there was no significant difference in the content of carotenoid among all treatments. The contents of soluble sugar and free amino acid were higher only under 10 DBH treatments than in control. The contents of total phenolic and total antioxidant capacity were the highest in 5 W·m^-2 treatment for 5 DBH. There was a significant positive correlation between total phenolic content and DPPH and FRAP value. After 5 DBH treatments, the percentages and contents of total aliphatic glucosinolates, sinigrin (SIN), gluconapin (GNA), and glucobrassicanapin (GBN) were highly increased, while the percentages and contents of glucobrassicin (GBS), 4-methoxyglucobrassicin (4-MGBS), and Progoitrin (PRO) were significantly decreased, especially under 10 W·m^-2 treatment. Our results show that UV-A LED supplements could improve the growth and quality of Chinese kale, and 5 W·m^-2 UV-A LED with 5 DBH might be feasible for Chinese kale growth, and 10 W·m^-2 UV-A LED with 5 DBH was better for aliphatic glucosinolates accumulation in Chinese kale.

Light emitting diodes improved the metabolism of rosmarinic acid and amino acids at the transcriptional level in two genotypes of Melissa officinalis L

In the present study, we used different LEDs to evaluate their effect on metabolic and transcriptional reprogramming of two genotypes (Ilam and Isfahan) of lemon balm grown under narrow-band LED lighting. Lemon balm plants were grown in four incubators equipped with artificial lighting and subjected to four LED lamps [White, Blue, Red, and mixed RB (Red+Blue) (70%:30%)] and in greenhouse conditions for 7weeks. The results showed significant increases in leaf number, pigment and soluble sugar contents, secondary metabolites, and calcium, magnesium, potassium and amino acid contents achieved in growth under mixed RB LEDs. As observed for the content of total phenolics, rosmarinic acid, and amino acids, the expression of genes involved in their production, including TAT , RAS , and DAHPS were also enhanced due to the mixed RB LED lighting. The best condition for both the plant growth and expression of genes was under the mixture of Red+Blue LED lamps. These observations indicate that the increase in secondary metabolites under mixed Red+Blue lights may be due to the increase in primary metabolites synthesis and the increased expression of genes that play an essential role in the production of secondary metabolites.

Pretreatment with LEDs regulates antioxidant capacity and polyphenolic profile in two genotypes of basil under salinity stress

In the present study, we evaluated a pretreatment with four LED light sources (red, blue, red + blue, and white) in two genotypes (green and purple) of basil on the growth parameters, stress oxidative markers, non-enzymatic antioxidants, osmoprotectant compounds, ion content, and polyphenolic profile under both control and salinity stress conditions. The results indicated that 150 mM of NaCl decreased biomass, RWC, and K⁺/Na⁺ ratio but increased the content of proline and antioxidant capacity in the leaves of both genotypes of basil grown under GH (greenhouse) conditions. The results suggested that RB LED-exposed plants in the green genotype and R LED-exposed plants in the purple genotype improved accumulation of shoot biomass, K⁺/Na⁺ ratio, proline and soluble sugars, glutathione and ascorbate, polyphenolic profile, and thioredoxin reductase activity in the leaves of basil under both control and salinity stress conditions. NaCl stress (150 mM) increased oxidative markers, which are responsible for disturbance of routine functions of various plant cellular modules. LED light pretreatments diminished these markers under both control and salinity stress conditions. It could be concluded that intensification of non-enzymatic antioxidant systems during light-mediated priming can diminish the deleterious effects of ROS induced by NaCl stress (150 mM) through preventing the lipid peroxidation, scavenging cytotoxic H₂O₂, and enhancement of antioxidant potentials. Therefore, usage of LED lighting systems as a pretreatment or to supplement natural photoperiods under both control and salinity stress conditions may be advantageous for increasing biomass and phytochemical accumulation in basil.

Light-emitting diode combined with humic acid improve the nutritional quality and enzyme activities of nitrate assimilation in rocket (Eruca sativa (Mill.) Thell.)

Belonging to the Brassicaceae family, rocket (Eruca sativa (Mill.) Thell.), is considered to be a nitrate-accumulating leafy vegetable. Many studies show that light-emitting diode (LED) lights can be a suitable tool to decrease anti-nutritional compounds (e.g., nitrate (and enhance antioxidant and nutritional quality for phytochemical-rich vegetable production. The positive influence of humic acid on health-promoting compounds in different crops is also well documented. This study aimed to investigate the effects of supplemental LED lights of various spectral compositions, namely 25-100% red, 25-100% blue, and 100% white, as well as their combination with humic acid on the physiological and biochemical responses of rocket plants. ANOVA results showed that almost all the measured traits were significantly affected by LED and humic acid treatments. Generally, LED combined with humic acid improved the accumulation of nutritional compounds (e.g., polyphenols, flavonoid, ascorbic acid, carbohydrate, tannin), increased the activity of key enzymes involved in nitrogen metabolism (e.g., nitrate reductase, nitrite reductase, and glutamine synthetase), and lowered nitrate and ammonium concentrations. The results of principal component analysis indicated that the combination of LED lights, regardless of the spectra, with humic acid was the most effective treatment to enhance the nutritional value and activity of enzymes involved in nitrate assimilation. In sum, these findings may be used as a reference in rocket production for supplemental LED light optimization and its combination with humic acid.

Impact of Ultraviolet Radiation on the Pigment Content and Essential Oil Accumulation in Sweet Basil (Ocimum basilicum L.)

In this study, we investigated the effects of additional ultraviolet radiation (UV) on the main growth fluorescent lamp light on pigment content and essential oil accumulation in sweet basil (Ocimum basilicum L.). Three different UV light sources from light-emitting diodes and discharge lamps, which emit UV in the UV-A (315–400 nm), UV-B (280–315 nm) and UV-C (100–280 nm) ranges, were tested for basil plant growing. The plants, growing under additional UV-A and UV-B from mercury lamps, on the 60th growing day were higher than control plants by 90% and 53%, respectively. The fresh leaf mass of the UV-A irradiated basil plants was 2.4-fold higher than the control plant mass. The dry mass/fresh mass ratio of the UV-A and UV-B irradiated plants was higher by 45% and 35% in comparison to the control plants. Leaf area was increased by 40% and 20%, respectively. UV-C affected the anthocyanin content most strongly, they increased by 50%, whereas only by 27% and 0% under UV-A and UV-B. Any UV addition did not affect the essential oil total contents but altered the essential oil compositions. UV-A and UV-B increased the linalool proportion from 10% to 20%, and to 25%, respectively, in contrast to UV-C, which reduced it to 3%. UV-C induced the eugenol methyl ether accumulation (17%) and inhibited plant growth. Moreover, UV increased the proportion of α-guaiene, β-cubebene and α-bulnesene and decreased the proportion of sabinene and fenchone. Thus, we concluded that UV (except UV-C) used jointly with main light with PPFD 120 ± 10 μmol photons·m−2·s−1 for sweet basil cultivation may be justified to stimulate basil growth and optimize the essential oil accumulation.

Effect of Supplemental UV-A Intensity on Growth and Quality of Kale under Red and Blue Light

Different intensities of UV-A (6, 12, 18 μmol·m^-2s^-1) were applied in a plant factory to evaluate the combined influences of supplemental UV-A and red and blue light (Red:Blue = 1:1 at PPFD of 250 μmol·m^-2 s^-1) on the biomass, antioxidant activity and phytochemical accumulation of kale. Supplemental UV-A treatments (T1: 6 μmol·m^-2 s^-1, T2: 12 μmol·m^-2 s^-1 and T3: 18 μmol·m^-2 s^-1) resulted in higher moisture content, higher pigment content, and greater leaf area of kale while T2 reached its highest point. T2 treatment positively enhanced the antioxidant capacity, increased the contents of soluble protein, soluble sugar and reduced the nitrate content. T1 treatment markedly increased the content of aliphatic glucosinolate (GSL), whereas T2 treatment highly increased the contents of indolic GSL and total GSL. Genes related to GSL biosynthesis were down-regulated in CK and T3 treatments, while a majority of them were greatly up-regulated by T1 and T2. Hence, supplemental 12 μmol·m^-2 s^-1 UV-A might be a promising strategy to enhance the growth and quality of kale in a plant factory.

Effect of light quality on the growth and main chemical composition of Bletilla striata

This study aimed to investigate the effects of light quality on the morphological traits, photosynthetic pigment content, protective enzyme (superoxide dismutase, peroxidase, and catalase) activity, and bioactive compound (BSP, total phenol, and militarine) content in Bletilla striata. Plants of B. striata were grown under light filtered through three differently colored films. The treatments were red film (RF), yellow film (YF), and blue film (BF), and an uncovered treatment was included as a control (CK). Compared with the B. striata plants in the RF, YF, and CK treatment groups, those receiving BF treatment showed significantly promoted growth of the aerial parts. Meanwhile, the total phenol and militarine contents in B. striata tubers were increased without affecting the accumulation of B. striata polysaccharides. These results show that growing B. striata plants under blue film could be a useful technique to improve quality and production. This technique is conducive to achieving large-scale sustainable production of high-quality plant materials.

ClPIF3-ClHY5 Module Regulates ClPSY1 to Promote Watermelon Fruit Lycopene Accumulation Earlier under Supplementary Red Lighting

Lycopene content is one of the important factors for determining watermelon fruit quality. In this study, a small-type watermelon was grown in a greenhouse with supplementary red lighting for 10 h per day. The results showed that the content of lycopene in the flesh was increased 6.3-fold after 25 days of supplementary red lighting. qRT-PCR analysis showed that PHYTOENE SYNTHASE 1(ClPSY1) is the major gene that responds to red light within the lycopene synthesis pathway. Moreover, we identified two key transcription factors that were involved in light signal transduction PHYTOCHROME INTERACTING FACTORS 3 (ClPIF3) and LONG HYPOCOTYL 5 (ClHY5) in watermelon flesh. The interaction experiments showed that ClHY5, a potent ClPIF3 antagonist, regulated ClPSY1 expression by directly targeting a common promoter cis-element (G-box). Collectively, our findings identified that ClHY5 and ClPIF3 formed an activation-suppression transcriptional module that is responsive to red light and, through this model, regulated watermelon lycopene accumulation in greenhouse winter cultivation.

Physiological Characteristics and Transcriptomic Dissection in Two Root Segments with Contrasting Net Fluxes of Ammonium and Nitrate of Poplar Under Low Nitrogen Availability

To investigate physiological and transcriptomic regulation mechanisms underlying the distinct net fluxes of NH4+ and NO3- in different root segments of Populus species under low nitrogen (N) conditions, we used saplings of Populus × canescens supplied with either 500 (normal N) or 50 (low N) μM NH4NO3. The net fluxes of NH4+ and NO3-, the concentrations of NH4+, amino acids and organic acids and the enzymatic activities of nitrite reductase (NiR) and glutamine synthetase (GS) in root segment II (SII, 35-70 mm to the apex) were lower than those in root segment I (SI, 0-35 mm to the apex). The net NH4+ influxes and the concentrations of organic acids were elevated, whereas the concentrations of NH4+ and NO3- and the activities of NiR and GS were reduced in SI and SII in response to low N. A number of genes were significantly differentially expressed in SII vs SI and in both segments grown under low vs normal N conditions, and these genes were mainly involved in the transport of NH4+ and NO3-, N metabolism and adenosine triphosphate synthesis. Moreover, the hub gene coexpression networks were dissected and correlated with N physiological processes in SI and SII under normal and low N conditions. These results suggest that the hub gene coexpression networks play pivotal roles in regulating N uptake and assimilation, amino acid metabolism and the levels of organic acids from the tricarboxylic acid cycle in the two root segments of poplars in acclimation to low N availability.

Effects of Green Light on Elongation Do Not Interact with Far-Red, Unless the Phytochrome Photostationary State (PSS) Changes in Tomato

Simple Summary

This paper focuses on the role of phytochromes (phys) in the interaction between green light and far-red light effects on “shade avoidance syndrome”. We grew wild type and phy mutants of tomato under a set of light conditions with different combinations of green, blue, red, and far-red light. Partial (20%) replacement of red/blue by green light in the absence of far-red light hardly affected the tomato plant morphology. However, when the spectrum contained far-red light, partially replacing red/blue by green light resulted in more elongation, which was associated with a lower phytochrome photostationary state (PSS) value. There was no effect of partial substitution of red/blue with green light when the PSS was kept constant. Thus, this study has revealed an interaction between green and far-red light effects on elongation unless PSS was kept constant. Green light was often a bit neglected in photobiology, but now an increasing number of researchers are realizing that green light deserves more attention. This study advances the understanding of light quality and plant growth and finding the optimal spectrum when growing plants under LED lighting in controlled environment agriculture.

Abstract

Green light (G) could trigger a “shade avoidance syndrome” (SAS) similarly to far-red light. We aimed to test the hypothesis that G interacts with far-red light to induce SAS, with this interaction mediated by phytochromes (phys). The tomato (Solanum lycopersicum cv. Moneymaker) wild-type (WT) and phyA, phyB1B2, and phyAB1B2 mutants were grown in a climate room with or without 30 µmol m⁻² s⁻¹ G on red/blue and red/blue/far-red backgrounds, maintaining the same photosynthetically active radiation (400–700 nm) of 150 µmol m⁻² s⁻¹ and red/blue ratio of 3. G hardly affected the dry mass accumulation or leaf area of WT, phyA, and phyB1B2 with or without far-red light. A lower phytochrome photostationary state (PSS) by adding far-red light significantly increased the total dry mass by enhancing the leaf area in WT plants but not in phy mutants. When the background light did not contain far-red light, partially replacing red/blue with G did not significantly affect stem elongation. However, when the background light contained far-red light, partially replacing red/blue with G enhanced elongation only when associated with a decrease in PSS, indicating that G interacts with far-red light on elongation only when the PSS changes.

Beyond the PAR spectra: impact of light quality on the germination, flowering, and metabolite content of Stevia rebaudiana (Bertoni)

BACKGROUND

Stevia rebaudiana is a high value crop due to the strong commercial demand for its metabolites (steviol glycosides) but has limited geographical cultivation range. In non-native environments with different daylength and light quality, Stevia has low germination rates and early flowering resulting in lower biomass and poor yield of the desired metabolites. In this study, artificial lighting with light-emitting diodes (LEDs) was used to determine if different light quality within and outside of the photosynthetically active radiation (PAR) range can be used to improve germination rates and yields for production of steviol glycosides for the herbal supplement and food industry.

RESULTS

Plants treated with red and blue light at an intensity of 130 μmol m^-2  s^-1 supplemented with 5% of UV-A light under a 16-h photoperiod produced the most desirable overall results with a high rate of germination, low percentage of early flowering, and high yields of dry leaf, stevioside and rebaudioside A, 175 days after planting.

CONCLUSION

While red and blue light combinations are effective for plant growth, the use of supplemental non-PAR irradiation of UV-A wavelength significantly and desirably delayed flowering, enhanced germination, biomass, rebaudioside A and stevioside yields, while supplemental green light improved yield of biomass and rebaudioside A, but not stevioside. Overall, the combination of red, blue and UV-A light resulted in the best overall productivity for Stevia rebaudiana. © 2021 Society of Chemical Industry.

Combination of Selenium and UVA Radiation Affects Growth and Phytochemicals of Broccoli Microgreens

Addition of selenium or application of ultraviolet A (UVA) radiation for crop production could be an effective way of producing phytochemical-rich food. This study was conducted to investigate the effects of selenium and UVA radiation, as well as their combination on growth and phytochemical contents in broccoli microgreens. There were three treatments: Se (100 μmol/L Na₂SeO₃), UVA (40 μmol/m²/s) and Se + UVA (with application of Se and UVA). The control (CK) was Se spraying-free and UVA radiation-free. Although treatment with Se or/and UVA inhibited plant growth of broccoli microgreens, results showed that phytochemical contents increased. Broccoli microgreens under the Se treatment had higher contents of total soluble sugars, total phenolic compounds, total flavonoids, ascorbic acid, Fe, and organic Se and had lower Zn content. The UVA treatment increased the contents of total chlorophylls, total soluble proteins, total phenolic compounds, and FRAP. However, the Se + UVA treatment displayed the most remarkable effect on the contents of total anthocyanins, glucoraphanin, total aliphatic glucosinolates, and total glucosinolates; here, significant interactions between Se and UVA were observed. This study provides valuable insights into the combinational selenium and UVA for improving the phytochemicals of microgreens grown in an artificial lighting plant factory.

Effects of Different Light Spectra on Final Biomass Production and Nutritional Quality of Two Microgreens

To improve microgreen yield and nutritional quality, suitable light spectra can be used. Two species-amaranth (Amaranthus tricolor L.) and turnip greens (Brassica rapa L. subsp. oleifera (DC.) Metzg)-were studied. The experiment was performed in a controlled LED environment growth chamber (day/night temperatures of 24 ± 2 °C, 16 h photoperiod, and 50/60% relative humidity). Three emission wavelengths of a light-emitting diode (LED) were adopted for microgreen lighting: (1) white LED (W); (2) blue LED (B), and (3) red LED (R); the photosynthetic photon flux densities were 200 ± 5 µmol for all light spectra. The response to light spectra was often species-specific, and the interaction effects were significant. Morphobiometric parameters were influenced by species, light, and their interaction; at harvest, in both species, the fresh weight was significantly greater under B. In amaranth, Chl a was maximized in B, whereas it did not change with light in turnip greens. Sugar content varied with the species but not with the light spectra. Nitrate content of shoots greatly varied with the species; in amaranth, more nitrates were measured in R, while no difference in turnip greens was registered for the light spectrum effect. Polyphenols were maximized under B in both species, while R depressed the polyphenol content in amaranth.

The Effect of Plant Growth Compensation by Adding Silicon-Containing Fertilizer under Light Stress Conditions

The effects of different spectral compositions of light-emitting diode (LED) sources and fertilizer containing biologically active silicon (Si) in the nutrient solution on morphological and physiological plant response were studied. Qualitative indicators and the productivity of plants of a red-leaved and a green-leaved lettuce were estimated. Lettuce was grown applying low-volume hydroponics in closed artificial agroecosystems. The positive effect of Si fertilizer used as a microadditive in the nutrient solution on the freshly harvested biomass was established on the thirtieth day of vegetation under LEDs. Increase in productivity of the red-leaved lettuce for freshly harvested biomass was 26.6%, while for the green-leaved lettuce no loss of dry matter was observed. However, being grown under sodium lamps, a negative impact of Si fertilizer on productivity of both types of plants was observed: the amount of harvested biomass decreased by 22.6% and 30.3% for the green- and red-leaved lettuces, respectively. The effect of using Si fertilizer dramatically changed during the total growing period: up to the fifteenth day of cultivation, a sharp inhibition of the growth of both types of lettuce was observed; then, by the thirtieth day of LED lighting, Si fertilizer showed a stress-protective effect and had a positive influence on the plants. However, by the period of ripening there was no effect of using the fertilizer. Therefore, we can conclude that the use of Si fertilizers is preferable only when LED irradiation is applied throughout the active plant growth period.

Effect of LED Spectrum on the Quality and Nitrogen Metabolism of Lettuce Under Recycled Hydroponics

Light quality optimization is an efficient method for improving the growth and quality of lettuce in plant factories. In this study, lettuce seedlings were illuminated under different light-emitting diode (LED) lights, namely, red-blue (RB), red-blue-green (RBG), red-blue-purple (RBP), and red-blue-far-red (RBF) LED lights, to investigate the effect of light quality on growth, quality, and nitrogen metabolism. The combination of 75% red and 25% blue light was set as the basic light source, and 20% of green, purple and far-red light were added to basic light source, respectively. All the treatments were set to 200 μmol m^-2 s^-1. Results showed that the fresh weight and dry weight of aboveground lettuce under RBG, RBP, and RBF treatments were significantly lower than those under the RB treatment because of the decrease in the effective photon flux density for chlorophyll absorption. The vitamin C content of the lettuce leaves was increased by about 23% with the addition of purple light. For nitrate reduction, the addition of green light significantly increased the nitrite content of the lettuce leaves. It also promoted the reduction from nitrite to ammonium through the activation of the nitrite reductase (NiR) expression and enzyme activity. The nitrate and ammonium content decreased with the addition of purple light because of the inhibited NR and NiR expression and enzyme activity. For nitrogen assimilation, individual (e.g., Asp, Glu, and Leu) and total amino acids were induced to increase by adding green, purple, and far-red light. The addition of light was hypothesized to have inhibited protein biosynthesis, thereby causing the accumulation of amino acids. Correlation analysis showed that the relative expression levels between HY5 and NR/NiR presented a significantly negative correlation. Transcription factor HY5 might mediate the regulation of light quality on nitrogen metabolism by inhibiting NR and NiR expressions. It might also exert a negative effect on nitrate reduction. Further studies via genome editing techniques on the identification of HY5 functions for nitrate assimilation will be valuable. Nevertheless, the results of this work enrich the understanding of the effect of light quality on nitrate metabolism at the level of gene expression and enzyme activity.

Regulation of the Photon Spectrum on Growth and Nutritional Attributes of Baby-Leaf Lettuce at Harvest and during Postharvest Storage

The photon flux density (PFD) and spectrum regulate the growth, quality attributes, and postharvest physiology of leafy vegetables grown indoors. However, limited information is available on how a photon spectrum enriched with a broad range of different wavebands regulates these factors. To determine this, we grew baby-leaf lettuce ‘Rouxai’ under a PFD of 200 µmol m⁻² s⁻¹ provided by warm-white (WW; control) light-emitting diodes (LEDs) supplemented with either 30 µmol m⁻² s⁻¹ of ultraviolet-A (+UV30) or 50 µmol m⁻² s⁻¹ of blue (+B50), green (+G50), red (+R50), or WW (+WW50) light. We then quantified growth attributes and accumulated secondary metabolites at harvest and during storage in darkness at 5 °C. Additional +G50 light increased shoot fresh and dry weight by 53% and 59% compared to the control. Relative chlorophyll concentration increased under +UV30, +G50, and especially +B50. At harvest, +B50 increased total phenolic content (TPC) by 25% and anthocyanin content (TAC) by 2.0-fold. Additionally, +G50 increased antiradical activity (DPPH) by 29%. After each day of storage, TPC decreased by 2.9 to 7.1% and DPPH by 3.0 to 6.2%, while TAC degradation was less pronounced. Principal component analysis indicated a distinct effect of +G50 on the lettuce at harvest. However, concentrations of metabolites before and during storage were usually greatest under the +B50 and +R50 treatments.

The Sustainable Use of Cotton, Hazelnut and Ground Peanut Waste in Vegetable Crop Production

The environmental burden from crop production byproducts is gradually increasing and necessitates the sustainable management of waste towards a circular economy approach. In the present study, three byproducts (cotton ginning waste (CGW), ground hazelnut husks (GHH) and ground peanut husks (GPH)) were evaluated in lettuce cultivation. For this purpose, the tested materials were incorporated in soil at two different rates (25% and 50% of total substrate volume) while a control treatment (no addition of byproducts) was also considered. Fresh weight per plant and total yield was the highest for the GHH50% treatment. The highest fat, protein, carbohydrates and energy content were observed for the CGW25% treatment. Chemical composition also differed among the tested byproducts where CGW25% treatment had the highest total tocopherols, sugars (sucrose, fructose, trehalose and total sugars) and organic acids content. The most abundant fatty acids were α-linolenic, linoleic and palmitic acid in all the tested treatments, while the highest antioxidant activity was observed for the GHH50% treatment. Regarding polyphenols, phenolic acids content was the highest in the GHH treatments, whereas flavonoids were the highest for the CGW25% treatment. No cytotoxicity against the PLP2 non-tumor cell line was observed, whereas only the GPH50% treatment showed moderate efficacy against HeLa, HepG2 and MCF-7 cell lines. The tested extracts also showed moderate antibacterial activities and only the extracts from the CGW50% treatment were more effective than the positive control against Trichoderma viride. In conclusion, the present results showed the great potential of using the tested byproducts as soil amendments for vegetable crops production, since they may improve the nutritional parameters, the chemical profile and the bioactivities of the final product. The suggested alternative use of the tested byproducts not only will increase the added value of crops but will also alleviate the environmental burden from bulky agroindustry byproducts.

Red Light Is Effective in Reducing Nitrate Concentration in Rocket by Increasing Nitrate Reductase Activity, and Contributes to Increased Total Glucosinolates Content

Rocket cultivation is increasing to supply the expanding ready-to-eat market because of its unique taste, but crops are often over fertilized to avoid nitrogen deficiencies. This leads to nitrate accumulation in leaves, and the products of their degradation (nitrites and nitrosamines) have been related to several health problems. Nitrate concentrations in rocket and other leafy vegetables are subject to limits by the EU legislation, yet rocket holds a great nutritional value. Degradation products of glucosinolates (isothiocyanates) have been consistently linked with benefits to human health. We investigated the influence of nitrogen application (1 and 8 mM), species [Eruca sativa (L.) Cav. and Diplotaxis tenuifolia (L.) DC.] and light spectrum (full spectrum, red, blue and red + blue) on the nitrate concentration, nitrate reductase activity and glucosinolate content of rocket grown in a soil-less system. Red light decreased the nitrate concentration with respect to the blue spectrum (4,270 vs. 7,100 mg⋅kg-1 of fresh weight, respectively), but such reduction was influenced by the species and the nitrogen level (significantly higher in D. tenuifolia and with the higher concentration of N). The nitrate reductase activity increased under red light in D. tenuifolia, with the lower N concentration. Rocket is known to contain several health-promoting compounds mainly antioxidants and glucosinolates, as secondary metabolites that act as part of plant defense mechanisms. The total content of glucosinolates was mainly affected by the species (D. tenuifolia showed the highest concentrations). Our results will help growers to tailor light spectra with the aim of reducing nitrate concentration and to remain within EU legislative limits, without any detrimental influence on other qualitative parameters in rocket.