Total Photosynthetic Photon Flux Research Articles

Growth, leaf anatomy, and photosynthesis of cotton (Gossypium hirsutum L.) seedlings in response to four light-emitting diodes and high pressure sodium lamp

BackgroundLight is a critical factor in plant growth and development, particularly in controlled environments. Light-emitting diodes (LEDs) have become a reliable alternative to conventional high pressure sodium (HSP) lamps because they are more efficient and versatile in light sources. In contrast to well-known specialized LED light spectra for vegetables, the appropriate LED lights for crops such as cotton remain unknown.ResultsIn this growth chamber study, we selected and compared four LED lights with varying percentages (26.44%–68.68%) of red light (R, 600–700 nm), combined with other lights, for their effects on growth, leaf anatomy, and photosynthesis of cotton seedlings, using HSP lamp as a control. The total photosynthetic photon flux density (PPFD) was (215 ± 2) μmol·m−2·s−1 for all LEDs and HSP lamp. The results showed significant differences in all tested parameters among lights, and the percentage of far red (FR, 701–780 nm) within the range of 3.03%–11.86% was positively correlated with plant growth (characterized by leaf number and area, plant height, stem diameter, and total biomass), palisade layer thickness, photosynthesis rate (Pn), and stomatal conductance (Gs). The ratio of R/FR (4.445–11.497) negatively influenced the growth of cotton seedlings, and blue light (B) suppressed stem elongation but increased palisade cell length, chlorophyll content, and Pn.ConclusionThe LED 2 was superior to other LED lights and HSP lamp. It had the highest ratio of FR within the total PPFD (11.86%) and the lowest ratio of R/FR (4.445). LED 2 may therefore be used to replace HPS lamp under controlled environments for the study of cotton at the seedling stage.

The Effect of the Daily Light Integral and Spectrum on Mesembryanthemum crystallinum L. in an Indoor Plant Production Environment

The effect of artificial lighting with different light spectra and photoperiods/daily light integrals (DLIs) on the yield, bioactive compounds and antioxidant capacity of the common ice plant (Mesembryanthemum crystallinum) was studied. Four-week-old seedlings were selected and subjected to four different light spectra made up of different combinations of blue (400–500 nm), green (500–600 nm) and red light (600–700 nm), with a total photosynthetic photon flux density (PPFD) of 180 µmol.m−2.s−1. Concurrently, the effect of the daily light integral (DLI) was also studied, with the light treatment photoperiod set at 18 h and 21 h. Biometric parameters such as fresh mass weight, leaf area, leaf width, and dry mass, together with plant metabolite contents such as total antioxidant capacity (TAC), vitamin C, chlorophyll a and b content, and total carotenoids and nitrates, were investigated. It was found that the plants grew better when exposed to light with a higher proportion of the red and blue spectrum, with the highest fresh mass of 68 g observed at a photoperiod of 18 h. On the other hand, green spectrum light was not found to yield any significant improvement in shoot weight, leaf area, or leaf size. It was also found that dry mass, chlorophyll b and nitrates were not influenced by the light spectrum but were influenced by the photoperiod duration. While both the dry mass and nitrate content increase as the photoperiod increases, a longer photoperiod had a negative effect on chlorophyll a, chlorophyll b and total carotenoids, with their content decreasing by as much 29% for chlorophyll a, 59% for chlorophyll b and 29% for total carotenoids. TAC content was seen to increase by more than 24% under the influence of 66% more green light, and 38% more under the 21 h photoperiod.

How does supplementary green light and UV-radiation affect biomass and rutin content in Levisticum officinale?

Due to several benefits regarding human health, the flavonoid rutin gains interest in nutrition and pharmaceutical industry. In order to satisfy high quality standards during cultivation and the final product, plants are grown increasingly in controlled environments with LED-technology as artificial light source. In this study the effect of various light settings on rutin content and biomass of Levisticum officinale was investigated. For continuous tracking of the biomass during cultivation, RGB-Images were taken. The actual biomass after harvest showed a strong positive correlation with the number of leaf-pixels detected via image processing (R2 = 0.937). Concerning the effect of UV-B radiation on rutin synthesis, time of synthesis was investigated. Two days after UV-B treatment a significant increase in rutin was observed. A short exposure time in combination with a high irradiance of 1 W m2 also showed positive effects on the rutin content in lovage. No significant effect of UV-B light on fresh weight was shown, but the combination of supplementary green light and high total photosynthetic photon flux density (PPFD) resulted in an increase of biomass.

Several short-day species can flower under blue-extended long days, but this response is not universal

Sole-source LED lighting enables spectral flexibility to achieve desirable plant characteristics and product quality. An earlier study from our lab showed that short-day plant chrysanthemum flowers normally under long days with dynamic lighting of 11 h dichromatic red-blue LED light extended with 4 h sole blue LED light. Such dynamic LED lighting is possible in vertical farms and opens the possibility to supply a higher daily light integral (DLI) to short-day plants by providing more hours of light and thus increase growth rate. This study aims to investigate for several short-day species whether normal flowering is obtained when 11 h of red-blue is extended with 4 h of sole blue LED light. Twelve genotypes of nine short-day plants species (kalanchoe - Kalanchoe blossfeldiana, perilla - Perilla frutescens, stevia - Stevia rebaudiana, artemisia - Artemisia annua, chrysanthemum - Chrysanthemum seticuspe and Chrysanthemum morifolium, cosmos - Cosmos bipinnatus, poinsettia - Poinsettia pulcherrima and wild tomato - Solanum habrochaites) were grown at three light conditions in a climate room: 11 h red-blue short day, 11 h red-blue extended with 4 h of sole blue, and 15 h red-blue long day. Mixed red and blue light (ratio 60:40) was provided at a total photosynthetic photon flux density (PPFD) of 100 µmol m−2 s−1 and sole blue light was provided at 40 µmol m−2 s−1 PPFD. Flowering response differed among species: kalanchoe, perilla and stevia flowered only in red-blue short day. Artemisia, chrysanthemum, cosmos, poinsettia, and wild tomato flowered in red-blue short day, and blue-extended long day but not in red-blue long day. However, there was a flowering delay in cosmos, poinsettia, and wild tomato under blue-extended long days compared to short days. In blue-extended long days plants received 15 % higher DLI resulting in a 4 to 36 % increase in total dry weight compared to short days. This study shows that increasing growth rate under light-limiting growth conditions through daylength-extension with sole blue light, without compromising flowering and quality, is possible for some, but not all short-day species.

The influence of LEDs with different blue peak emission wavelengths on the biomass, morphology, and nutrient content of kale cultivars

Light-emitting diodes (LEDs) enhance plant production in vertical farms by regulating photosynthetic rate and phytochemistry. Specific light recipes can be formulated using LEDs for high output by fine-tuning spectral composition and irradiance. In this study, the growth, development, and nutritional quality of three kale cultivars (‘Toscano’, ‘Redbor’, and ‘Winterbor’) were examined under different blue peak emission wavelengths (λpeak). Photosynthetic photon flux density (PPFD) was maintained at 200 ± 10% μmol m − 2s − 1 over a 16-hr photoperiod. The LED light treatments had blue λpeak centered at 400, 420, and 450 nm wavelengths, all with spectral ratios of 20% blue, 20% green, 60% red in the visible light region, and 15% of total PPFD in the far-red region. The control light was cool-white fluorescent (CWF) light with blue λpeak at 436 nm and a slightly higher amount of PPFD in the blue region (23%). The biomass yield and leaf physical characteristics were largely unaffected by the light treatments with different blue λpeak. However, the concentration of carotenoids and chlorophylls in kale leaves was influenced by the type and amount of blue light during growth. Future research should investigate the effect of different blue light percentages in pre- and post-harvest LED treatments (continuous or pulsed) on high-value, nutritious crops.

Evaluation of the enhancement of photosynthetic rate in a komatsuna (Brassica rapa L. var. perviridis) canopy with upward lighting using an optical simulation in a plant factory with artificial light.

In a plant factory with artificial light (PFAL), upward lighting is expected to prevent senescence and decrease in the photosynthetic capacity of the lower leaves in the canopy. Upward lighting may also increase the photosynthetic rate of a canopy by improving its photosynthetic photon flux density (PPFD) distribution. However, the net photosynthetic rate (Pn) of leaves is lower when the abaxial surface is irradiated than that when the adaxial surface is irradiated. The aim of this study was to estimate the PPFD in a PFAL and the Pn of plants using three-dimensional plant models and optical simulation. First, we measured the Pn of komatsuna (Brassica rapa L. var. perviridis) leaves under different conditions of the proportion (pad ) of PPFD on the adaxial surface to total PPFD on both surfaces and developed an equation for the light response curve of photosynthesis considering pad . When PPFD was low, except when it was 30 and 70 µmol m-2 s-1, Pn increased as pad increased, because the absorptance also increased with pad . Under high PPFD conditions, Pn was maximized at 67-83% of pad because the light would be distributed more efficiently for photosynthesis. Next, using optical simulation and the developed equation, we estimated the photosynthetic rate of a komatsuna canopy (CPn) under downward and upward lighting. The CPn increased by 1.08-1.13 times by combining downward and upward lighting due to the increase in the photosynthetic photon flux (PPF) of light incident on the canopy and the decrease in the spatial variation of PPFD on the leaves in the canopy. As the depreciation of lamps for upward lighting accounts for 7.5-9.0% of the production cost in a PFAL, even if the depreciation of lamps for upward lighting increased, enhancement of CPn by upward lighting would be cost-effective. We performed optical simulations under 220 conditions and evaluated them using CPn as an index. Moreover, we provided the proportion of PPF of upward lighting that improved CPn and discussed the reason for this improvement. The result shows that optical simulation is useful for evaluating the lighting design in a PFAL and analyzing the effects of the lighting design on the light environment and photosynthesis.

Spindly growth of watercress observed under the monochromatic light of a red light-emitting diode improved with slight changes in light quality using white light-emitting diode

The suppression effect of white light-emitting diode (LED) on spindly growth observed under monochromatic light of the red LED in watercress (Nasturtium officinale R. Br.) was investigated for production in plant factory. Watercress was cultured in a growth chamber under total photosynthetic photon flux density (PPFD) of 100 ?mol?m?2?s?1 using three differential treatments; the ratio of red LED to white LED based on PPFD was 100:0 (R100W0), 90:10 (R90W10), and 75:25 (R75W25). Watercress was harvested 33 days after sowing. The longest internode length was observed with R100W0. Watercress in R100W0 was observed lowest value for node number, maximum leaf length, shoot fresh weight, shoot dry weight, dry matter rate, and SPAD. R90W10 recorded higher values for maximum leaf length, shoot fresh weight, shoot dry weight, dry matter rate, and SPAD than R100W0. Therefore, in watercress cultivation, replacing the red LED with a white LED by 10% was effective for morphological changes and dry matter production.

Response of Strawberry to the Substitution of Blue Light by Green Light in an Indoor Vertical Farming System

Indoor production systems with light emitting diode (LED) lamps are a feasible alternative for increasing strawberry productivity by reducing the incidence of pests and diseases and the damage caused by extreme weather events. Blue (BL) and red (RL) LED light are considered the most important light spectra for photosynthesis and crop yield; however, recent studies have demonstrated that the beneficial effects of green light (GL) have been underestimated. This information would be of particular importance for strawberry production in controlled-environments/vertical farming systems as it may lower input costs and enhance production efficiency and quality and marketability. The present study aimed to define the effect of GL in combination with BL in strawberry. A proportion of 20% GL (20% BL + 60% RL) of total photosynthetic photon flux density was beneficial for plant growth and productivity; however, a 27% GL (12% BL + 61% RL) proportion was detrimental or comparable to that with 6% GL (36% BL + 58% RF). Total dry mass increased 51% when plants were illuminated with 20% GL lamps compared to those with 6% GL; the most impacted plant part was the root as it increased by 155%. The higher yield was observed with GL at 20%, but further increasing GL to 27% resulted in reduced yield. GL at 20% and 27% exhibited higher photosynthesis but reduced transpiration, stomatic conductance, and internal CO2, which in turn increased instantaneous and intrinsic water-use efficiency. Plants with the highest yield (20% GL) exhibited lower total soluble solids in fruits but still the values obtained were acceptable (8.25 °Brix); these fruits contained a high total sugars and phenolics concentration but a reduced antioxidant scavenging capacity. High proportions of GL were associated with a higher leaf and fruit Ca and a higher leaf P and K, which may be due to the increased allocation of biomass to the roots. In conclusion, GL at 20% and BL at 20% resulted in the best growth and yield parameters, enhanced net photosynthesis rate, water-use efficiency and fruit quality attributes. The effects of GL observed in this study may also be important for other high-value horticultural crops suitable for indoor vertical farming.

Effects of supplementing green light to red and blue light on the growth and yield of lettuce in plant factories

Red and blue light-emitting diodes (LEDs) are widely used as light sources in plant factories. Plants grown under red and blue light only produces purplish-gray light environment where plant leaves look purplish-gray. Under that light condition, it makes difficulty to check health status of plants specially insect and disease infected leaves by human naked eye. But the leaf color turns green when inspected under full spectrum of light environment. In this regard, the use of green light to red and blue create a white light environment which is congenial for the grower. Likewise, CO2 assimilation of green light is usually lower compared to red and blue light because of its lower absorptance under low PPFD. But at higher PPFD (≥500 µmol m−2 s−1), green light shows higher CO2 assimilation rate compared to red and blue light through uniform distribution of full spectrum (red, green and blue) of light into the plant canopy and the lower leaves. In this study, optimal intensity of green light supplementation to the red and blue LEDs was evaluated for better growth, yield, and quality of lettuce. In experiment I and II, green light was added with red and blue light where total photosynthetic photon flux density (PPFD) was increased for the additional green light. In experiment III, 0, 10, 20, 30, 40, 50, 60, 70, 80 µmol m−² s−1 green (G) light was supplemented to 235 µmol m−² s−1 red (R) and 59 µmol m−² s−1 blue (B) light maintaining 294 µmol m−² s−1 total PPFD. Lettuce plants were cultivated hydroponically in three-step vertical grow beds using half-strength of Enshi nutrient solution. The temperature was maintained at 20 ± 2 °C at day and night and the day length was 16 h. In experiment I and II, the fresh mass of lettuce grown under the combination of R, B and G LEDs was found higher than those cultivated under R and B LEDs only. In experiment III, lettuce plants produced high fresh masses when 30 µmol m−² s−1 of G light was supplemented with R and B lights maintaining the ratio 211:30:53 (R:G:B). By supplementing excessive G light (≥50 µmol m−² s−1) with R and B lights, lettuce shoot fresh mass declined. The various combinations of irradiation of R, G and B LED did not significantly affect the contents of ascorbic acid and mineral nutrients. It is recommended to supplement 30 µmol m−2 s−1 of green light to red and blue light for a higher yield of lettuce. Therefore, the suitable LED (R:G:B) combination would be 72% of red, 10% of green and 18% of blue for lettuce cultivation following recycled hydroponics in plant factories.

Green Light Partial Replacement of Red and Blue Light Improved Drought Tolerance by Regulating Water Use Efficiency in Cucumber Seedlings

Light is one of the most important environmental signals in plant growth, development, and stress response. Green light has been proved to enhance plant defense against biotic and/or abiotic stress. To illustrate the effects of green light partially replaced red light and blue light on the plant under drought condition, cucumber (Cucumis sativus L. cv. Xinchun No. 4) seedlings were treated with short-term drought stress and were concomitantly exposed to four treatments, which were set up by adjusting the relative amount of green light as 0 (RB), 25 (RBG25), 50 (RBG50), and 75 (RBG75) μmol m−2 s−1, respectively, with a total photosynthetic photon flux density of 250 μmol m−2 s−1 and a fixed red-to-blue ratio of 4:1. The results showed that compared with RB, RBG50 significantly increased shoot fresh weight (FW) and dry weight (DW), root DW, plant height, stem diameter, leaf area, and leaf dry mass per unit area (LMA) by 10.61, 7.69, 66.13, 6.22, 10.02, 4.10, and 12.41%, respectively. Also, the addition of green light significantly increased the root volume and root tip number. Moreover, green light partial replacement of red light and blue light increased total water content, especially free water content, improved leaf water status, and alleviated water loss in plants caused by drought stress. Also, the addition of green light increased net photosynthetic rate (Pn), reduced both stomata conductance (gs) and transpiration rate (E), enhanced the intrinsic water-use efficiency (WUE) and instantaneous water-use efficiency (iWUE) of leaves, and increased the content of chlorophylls a and b. Green light substituting a proportion of blue and red light regulated stomatal aperture by significantly increasing abscisic acid (ABA) and γ-aminobutyric acid (GABA) content. In addition, the increase of GABA was resulted from the upregulation of Glutamate Decarboxylase 2 (CsGAD2). However, the relative electrolytic leakage and contents of malondialdehyde (MDA), superoxide anion (), and hydrogen peroxide (H2O2) vigorously decreased as the intensity of green light was added to the spectrum under drought. Conclusively, green light partially replaced red light and blue light and improved drought tolerance of cucumber seedlings by upregulating the expression of CsGAD2 gene and promoting the synthesis of GABA. The increase in GABA content further downregulated the expression of aluminum-activated malate transporter 9 (CsALMT9) gene, induced stomata to close, improved water utilization, and alleviated damage caused by drought. This study highlights a role of green light in plant physiological processes. Moreover, analyzing the function of green light on improving drought tolerance of plants could open alternative avenues for improving plant stress resilience.

Morphology, Photosynthetic Traits, and Nutritional Quality of Lettuce Plants as Affected by Green Light Substituting Proportion of Blue and Red Light.

Green light, as part of the photosynthetically active radiation, has been proven to have high photosynthetic efficiency once absorbed by plant leaves and can regulate plant physiological activities. However, few studies have investigated the appropriate and efficient way of using the green light for plant production. Thus, the objective of this study was to investigate a moderate amount of green light, partially replacing red and blue light, for plant growth and development. In this experiment, four treatments were set up by adjusting the relative amount of green light as 0 (RB), 30 (G30), 60 (G60), and 90 (G90) μmol m−2 s−1, respectively, with a total photosynthetic photon flux density of 200 μmol m−2 s−1 and a fixed red-to-blue ratio of 4:1. Lettuce (Lactuca sativa cv. ‘Tiberius’) plant growth and morphology, stomatal characteristics, light absorptance and transmittance, photosynthetic characteristics, and nutritional quality were investigated. The results showed that: (1) shoot dry weight increased by 16.3 and 24.5% and leaf area increased by 11.9 and 16.2% under G30 and G60, respectively, compared with those under RB. Plant stem length increased linearly with increasing green-to-blue light ratio; (2) light transmittance of lettuce leaf under treatments employing green light was higher than that under RB, especially in the green region; (3) stomatal density increased, whereas stomatal aperture area decreased with the increase in the relative amount of green light; and (4) carbohydrate accumulation increased under G60 and G90. Soluble sugar contents under G60 and G90 increased by 39.4 and 19.4%, respectively. Nitrate contents under G30, G60, and G90 decreased by 26.2, 40.3, and 43.4%, respectively. The above results indicated that 15–30% green light replacing red and blue light effectively increased the yield and nutritional quality of lettuce plants.

Red and blue wavelengths affect the morphology, energy use efficiency and nutritional content of lettuce (Lactuca sativa L.)

Since red (R) and blue (B) LED light has different quantum efficiency and photoelectric conversion efficiency, mixed RB with different proportions of R and B results in varied energy consumption. In order to improve the energy use efficiency of the closed-type plant production systems, the effects of R and B proportions on the electric use efficiency (EUE), light use efficiency (LUE) as well as the quality of butter leaf lettuce were evaluated in this study. Lettuce seedlings were cultivated in a plant factory with artificial lighting (PFAL) and subjected to eleven combinations of R and B (100%R, 90%R, 80%R, 70%R, 60%R, 50%R, 40%R, 30%R, 20%R, 10%R, 0%R; the rest of the photons in each treatment were B) with the same total photosynthetic photon flux density (PPFD) and photoperiod (200 ± 3 μmol·m−2·s−1, 16 h) for 35 days. The results showed that palpable petiole distortion appeared when R proportion was more than 70% and the distortion was aggravated with the increase of R proportion. The highest EUE and LUE were both detected in lettuce under 90%R treatment, which were respectively 3.64% and 1.20%. The least number of photons and the least electricity amount required to produce 1 g dry weight of lettuce was respectively 2.92 mol and 1.67 MJ, which were both detected in lettuce treated with 90%R. The sucrose content in lettuce treated with more than 50%R was significantly higher than those treated with less than 50%R (50%R included). Lettuce treated with 80%R possessed the highest soluble sugar content as well as the lowest crude fiber and nitrate content (not significantly different with the minimum values). R proportion exceeding 50% in mixed RB light was beneficial to the accumulation of hexose and sucrose, as well as the decomposition of nitrate in lettuce. The vitamin C content in lettuce treated with 100%R was significantly higher than that in lettuce under other treatments in the study. On the whole, the study indicated that the proportions of R and B affected the energy use efficiency and quality of lettuce in closed plant factory, however the responses of plants to the proportions of R and B varied according to different indexes. Thus, some indexes of top priority should be determined before choosing the optimal proportions of R and B.

Unraveling the effects of blue light in an artificial solar background light on growth of tomato plants

While the use of narrowband irradiance regimes containing different blue light fractions has proven useful to unravel blue light effects on plants at a fundamental level, it does not quantify the responses to blue light under natural daylight conditions. The objective of this study is to understand the blue light growth responses by combining photosynthetic measurements with measurements of whole plant light absorption in a simulated daylight spectrum enriched with different levels of blue light. To achieve this, tomato plants were grown under six different combinations of artificial solar light and blue LED light. Light treatments were defined by the blue light (400–500 nm) fraction of total photosynthetic photon flux density (400–700 nm) and included 27 % (no additional blue LED), 28 %, 31 %, 38 %, 43 % and 61 % blue light with a total photosynthetic photon flux density of 100 μmol m−2 s-1 in all treatments. Whole plant light absorption was estimated by using ray tracing simulation combined with measured 3-dimensional structure of the plant and optical properties of the leaves. The total dry weight of the plants decreased linearly with the increase of blue light fraction; the dry weight of the plants grown under 27 % blue being 1.6 times greater than that of the plants grown under 61 % blue. This large difference was related to lower light absorption by the plants when fraction blue light increased, due to more compact morphology, i.e. lower leaf area, leaf length/width ratio and shorter stem. Light-limited quantum yield and maximum photosynthetic capacity were not affected by blue light fraction. In the case of the latter, which in other studies has often been found to be positively related to blue light fraction, it may be that the blue light fraction already present in the daylight source had saturated this response. Overall, increasing the blue light fraction in a solar light background decreases growth mainly through its effect on plant morphology and light interception. It remains to be elucidated whether the responses observed using the low growth light intensity in the present study are maintained in high light growth environments more characteristic for tomato growth and production.

Growth and Appearance Quality of Four Microgreen Species under Light-emitting Diode Lights with Different Spectral Combinations

To investigate plant growth and quality responses to different light spectral combinations, cabbage (Brassica oleracea L. var. capitata f. rubra), kale (Brassica napus L. ‘Red Russian’), arugula (Eruca sativa L.), and mustard (Brassica juncea L. ‘Ruby steak’) microgreens were grown in a controlled environment using sole-source light with six different spectra: 1) FL: cool white fluorescent light; 2) BR: 15% blue and 85% red light-emitting diode (LED); 3) BRFRL: 15% blue, 85% red, and 15.5 µmol·m−2·s−1 far-red (FR) LED; 4) BRFRH: 15% blue, 85% red, and 155 µmol·m−2·s−1 FR LED; 5) BGLR: 9% blue, 6% green, and 85% red LED; and 6) BGHR: 5% blue, 10% green, and 85% red LED. For all the light treatments, the total photosynthetic photon flux density (PPFD) was set at ≈330 µmol·m−2·s−1 under a 17-hour photoperiod, and the air temperature was ≈21 °C with 73% relative humidity (RH). At harvest, BR vs. FL increased plant height for all the tested species except arugula, and enlarged cotyledon area for kale and arugula. Adding high-intensity FR light to blue and red light (i.e., BRFRH) further increased plant height for all species, and cotyledon area for mustard, but it did not affect the fresh or dry biomass for any species. Also, BRFRH vs. BR increased cotyledon greenness for green-leafed species (i.e., arugula, cabbage, and kale), and reduced cotyledon redness for red-leafed mustard. However, BGLR, BGHR, and BRFRL, compared with BR, did not affect plant height, cotyledon area, or fresh or dry biomass. These results suggest that the combination of 15% blue and 85% red LED light can potentially replace FL as the sole light source for indoor production of the tested microgreen species. Combining high-intensity FR light, rather than low-level (≤10%) green light, with blue and red light could be taken into consideration for the optimization of LED light spectral quality in microgreen production under environmental conditions similar to this experiment.

Applying Blue Light Alone, or in Combination with Far-red Light, during Nighttime Increases Elongation without Compromising Yield and Quality of Indoor-grown Microgreens

To facilitate machine harvest for labor savings, the height of microgreens needs to reach ≈5 cm. Recent studies indicate that monochromatic blue light (B) can promote stem elongation similar to far-red light (FR). To examine whether nighttime B treatments can promote plant elongation without compromising yield and quality, mustard (Brassica juncea) and arugula (Eruca sativa) microgreens were grown under different light-emitting diode (LED) lighting regimes in a growth chamber. The 16-hour daytime lighting comprised 20% B and 80% red light (R), and had a total photosynthetic photon flux density (PPFD) of 300 µmol·m–2·s–1 at canopy level. During the 8-hour nighttime, the plants were exposed to the following treatments: 1) dark (D) as one control; 2) 4 hours of B at 40 µmol·m–2·s–1 followed by 4 hours of darkness (40B-D); 3) 4 hours of darkness followed by 4 hours of B at 40 µmol·m–2·s–1 (D-40B); 4) 8 hours of B at 20 µmol·m–2·s–1 (20B); 5) 8 hours of B + FR, and each of them at 20 µmol·m–2·s–1 (20B20FR); and 6) 8 hours of FR at 20 µmol·m–2·s–1 (20FR) as another control. The plants were harvested after 11 days of treatment. Nighttime B treatments (40B-D, D-40B, and 20B), compared with D, increased plant height by 34% and 18% for mustard and arugula, respectively, with no difference among the three B treatments. The combination of B and FR (20B20FR), compared with B alone, further increased plant height by 6% and 15% for mustard and arugula, respectively, and showed a similar promotion effect as 20FR. Plant height did not meet the machine harvest requirement for both species with the D treatment, but did so for mustard with the nighttime B treatments and for arugula with the 20B20FR treatment. There was no difference in biomass among all treatments except that 20B, compared with D, increased the fresh weight (FW) of arugula by 12%, showing a similar promotion effect as 20FR. Despite a greater promotion effect on elongation than B alone, 20FR reduced the leaf index compared with D. However, B alone or the 20B20FR treatment increased leaf thickness compared with D, and increased chlorophyll content index (CCI), leaf index, dry matter content, and leaf thickness to varying degree with species, compared with 20FR. Overall, nighttime B alone, or its combination with FR, promoted microgreen elongation without compromising yield and quality.

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

In this study we explore the effects of multi-colour LED lighting spectrum on nutritive primary metabolites in green (‘Lobjoits green cos’) and red (‘Red cos’) leaf lettuce (Lactuca sativa L.), cultivated in controlled environment. The basal lighting, consisting of blue 455 nm, red 627 and 660 nm and far red 735 nm LEDs, was supplemented with UV-A 380 nm, green 510 nm, yellow 595 nm or orange 622 nm LED wavelengths at total photosynthetic photon flux density of 300 μmol m−2 s−1. Supplemental lighting colours did not affect lettuce growth; however had distinct impact on nitrite, amino acid, organic acid, and soluble sugar contents. Orange, green and UV-A light had differential effects on red and green leaf lettuce metabolism and interplay with nutritional value and safety of lettuce production. The metabolic response was cultivar specific; however green light had reasonable impact on the contents of nutritive primary metabolites in red and green leaf lettuce.

Responses of yield and appearance quality of four Brassicaceae microgreens to varied blue light proportion in red and blue light-emitting diodes lighting

To optimize blue light proportion in red and blue light-emitting diode (RB-LED) lighting for microgreen production, the yield and appearance quality of cabbage, kale, arugula and mustard were investigated under RB-LED lightings with 5%, 10%, 15%, 20%, 25% and 30% blue light. For each lighting treatment, the total photosynthetic photon flux density was set at ≈ 300 μmol m-2 s-1, and the air temperature during light/dark was set at ≈ 21/ 17 °C. As a result, neither fresh nor dry yield was affected by blue light percentage for the tested species except cabbage, which showed quadratic (peaking at 15%) responses in crop yield. Hypocotyl length and cotyledon area of kale and mustard decreased linearly with increasing blue light percentage which, however, did not affect arugula or cabbage in these two traits. For plant color, cotyledons were darker red for mustard and less pure green for the other three species under higher blue light percentage. This was indicated by a negative linear response of hue angle or green chromaticity to increasing blue light percentage. These findings suggested that responses to blue light percentage varied with plant traits and microgreen species. To reach a balance of yield and appearance quality, 15% blue light was recommended for indoor production of cabbage microgreens, while 5% blue light for the other three species, under similar environmental conditions.

Physiological and growth strategies of two Cariniana species in response to contrasting light availability

Cariniana legalis and C. estrellensis are tropical emergent trees of the Lecythidaceae family that occur naturally in different forest ecosystems and belong to different ecological groups. With the aim of providing information to better manage these two congeneric Cariniana species in secondary forest enrichment plantations, an experiment was conducted using potted plants for comparing the physiological and growth responses of young C. legalis and C. estrellensis plants under contrasting light radiation availabilities. Considering that these two congeneric species occur naturally in different forest ecosystems and belong to different ecological groups, we tested the hypothesis that young plants of C. legalis have more plastic responses of photosynthetic light response curves, anthocyanin content, growth and biomass allocation to contrasting light environments than C. estrellensis. Plants were grown for 105 days under three light conditions (26, 10 and 1 mol photons m−2 day-1), simulating the daily total photosynthetic photon flux density (PPFD) in large and small canopy gaps and in the forest understory, respectively. The photosynthetic responses to contrasting light availabilities were very similar for both species. Higher values for anthocyanin reflectance index (ARI), leaf mass per area (LMA), and changes in biomass allocation indicate that C. legalis has a greater capacity for acclimation to contrasting light environments than did C. estrellensis. In addition, a greater number of smaller leaves in C. legalis could be an advantage in the understory of forests subjected to constant changes in canopy openness through the opening and closing of small- and medium-sized canopy gaps. From a practical point of view, seedlings of C. estrellensis should be planted in more open forests with less dynamic changes in canopy gaps, and seedlings of C. legalis should be planted in forests with more fluctuations in the opening and closing of small and large canopy gaps.

Pulsed LED light increases the phytochemical level of basil microgreens

The effects of pulsed light-emitting diode (LED) lighting on antioxidant and antiradical activities of basil microgreens were investigated. Basil (Ocimum basilicum L. 'Sweet Genovese') was grown in a controlled-environment growth chamber (21/17±2°C day/night temperature; 55±5% relative air humidity) for 14 days after sowing in peat substrate. The main lighting system of high-pressure sodium (HPS) lamps (SON-T Agro, Philips) was supplemented with 20±2 µmol m‑2 s‑1 flux of monochromatic LEDs with peak emission of royal blue 455 nm, blue 470 nm, cyan 505 nm, yellow 590 nm, or red 627 nm (Luxeon® III Star, Philips Lumileds Lighting Company, USA), and the total photosynthetic photon flux density (PPFD) was 200±10 µmol m(‑2) s(‑1) (16 h day(‑1)). Frequencies at 2, 32, 256, and 1024 Hz with a duty cycle of 50% of monochromatic LEDs were applied. Reference plants were grown under HPS lamps supplemented with continuous light of monochromatic LEDs. The results showed that the effects of supplemental pulsed light on total phenolic compounds, total anthocyanins, 2,2-diphenyl-1-picrylhydrazyl (DPPH) free-radical scavenging activity and ascorbic acid depended on the wavelength of monochromatic LEDs and frequency. Supplemental lighting of blue 470 nm or red 627 nm at 1024 Hz was the most effective for accumulation of total phenolic compounds as well as total anthocyanins. The highest antiradical activity in basil microgreens treated under all supplemental LEDs, was determined, except red 627 nm at 256 Hz. Accumulation of ascorbic acid was promoted by pulsed light at all frequencies, but higher contents of ascorbic acid were determined under the treatments of 32 and 256 Hz in comparison with other frequencies.

Pulsed Light-Emitting Diodes for a Higher Phytochemical Level in Microgreens.

A novel research of pulsed light-emitting diode (LED) lighting versus continuous lighting was conducted by analyzing phytochemical levels in microgreens. Red pak choi (Brassica rapa var. chinensis), mustard (Brassica juncea L.), and tatsoi (Brassica rapa var. rosularis) were grown indoors under HPS lamps supplemented with monochromatic (455, 470, 505, 590, and 627 nm) LEDs [total photosynthetic photon flux density (PPFD) of 200 ± 10 μmol m-2 s-1, for 16 h day-1]. For pulsed light treatments, the frequencies at 2, 32, 256, and 1024 Hz with a duty cycle of 50% monochromatic LEDs were applied. The results were compared to those under the continuous light (0 Hz) condition in terms of total phenolic content, anthocyanins, and antiradical activity (DPPH). The summarized data suggested that pulsed light affected accumulation of secondary metabolites both positive and negative in microgreens. The significant differences in the response of phytochemicals between pulsed light at several frequencies and continuous light were determined. The most positive effects of 2, 256, and 1024 Hz for total phenolic compounds in mustard under all wavelength LEDs were achieved. The LED frequencies at 2 and 32 Hz were the most suitable for accumulation of anthocyanins in red pak choi and tatsoi. The highest antiradical activity under the treatments of 32, 256, and 1024 Hz in mustard and under the 2 Hz frequency in red pak choi and tatsoi was determined.