LED irradiance level affects growth and nutritional quality of Brassica microgreens

Central European Journal of Biology

* E-mail: [email protected]

Research Article

1Institute of Horticulture, Lithuanian Research Centre for Agriculture and Forestry, 54333 Babtai, Lithuania

2Institute of Applied Research, Vilnius University, 10222 Vilnius, Lithuania

Giedrė Samuolienė1,*, Aušra Brazaitytė1, Julė Jankauskienė1, Akvilė Viršilė1, Ramūnas Sirtautas1, Algirdas Novičkovas2, Sandra Sakalauskienė1, Jurga Sakalauskaitė1, Pavelas Duchovskis1

LED irradiance level affects growth and nutritional quality of Brassica microgreens

1. IntroductionNowadays “functional foods” (food enriched with health promoting additives) have gained in popularity due to the known health promoting or disease preventing properties that can be added to enhance the nutritional quality of regular vegetables. Microgreens are very specific types of vegetables and herbs that are harvested with two fully developed cotyledon leaves with or without the emergence of a rudimentary first pair of true leaves [1]. Due to a higher levels of phytochemical compounds found in these early shoots, these plants are considered to belong to a group known as “functional foods”. Microgreens are commonly grown as cabbage, beet, kale, kohlrabi, mizuna, mustard, radish, swiss chard and amaranth [2]. Microgreens provide a large array of intense flavours, vivid colours and tender textures. Moreover, microgreen cotyledon leaves possess

higher nutritional densities compared to the nutritional concentrations found in mature leaves [1].

Growing, harvesting and postharvest handling conditions may have a considerable impact on the synthesis and degradation of microgreen phytonutrients [1]. Thus, it is important to ensure appropriate agronomic practices when handling these food products. The light environment plays a significant role in influencing physiological changes and secondary metabolite production in plants [3]. Variations in both the artificial lighting spectra [4-7] and irradiance levels [8,9] can affect growth and nutrition.

Light emitting diodes (LEDs) are, to date, one of the most promising energy – efficient and rapidly developing plant lighting technologies. Combinations of red, blue and far red LED light wavelengths are reported to be efficient for sprouted seed [4], microgreen [3], wheatgrass [10] and mature lettuce plant [5,11]

Cent. Eur. J. Biol. • 8(12) • 2013 • 1241-1249DOI: 10.2478/s11535-013-0246-1

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Received 13 November 2012; Accepted 07 July 2013

Keywords: Light • Functional foods • Antioxidants • Chlorophylls • Leaf area • Nitrates • Sucrose

Abstract: This study examines the effect of irradiance level produced by solid-state light-emitting diodes (LEDs) on the growth, nutritional quality and antioxidant properties of Brassicaceae family microgreens. Kohlrabi (Brassica oleracea var. gongylodes, ‘Delicacy Purple’) mustard (Brassica juncea L., ‘Red Lion’), red pak choi (Brassica rapa var. chinensis, ‘Rubi F1’) and tatsoi (Brassica rapa var. rosularis) were grown using peat substrate in controlled-environment chambers until harvest time (10 days, 21/17°C, 16 h). A system of five lighting modules with 455, 638, 665 and 731 nm LEDs at a total photosynthetic photon flux densities (PPFD) of 545, 440, 330, 220 and 110 μmol m-2s-1 respectively were used. Insufficient levels of photosynthetically active photon flux (110 μmol m-2 s-1) suppressed normal growth and diminished the nutritional value of the Brassica microgreens studied. In general, the most suitable conditions for growth and nutritional quality of the microgreens was 330–440 μmol m-2 s-1 irradiation, which resulted in a larger leaf surface area, lower content of nitrates and higher total anthocyanins, total phenols and 2,2–diphenyl–1–picrylhydrazyl (DPPH) free-radical scavenging capacity. High light levels (545 μmol m-2 s-1), which was expected to induce mild photostress, had no significant positive impact for most of investigated parameters.

© Versita Sp. z o.o.

PPFD impact on Brassica microgreens internal quality

cultivation. However, there is still a lack of information regarding the optimal light spectral conditions and irradiance levels for growth and nutitional value of plants of different genotypes and maturity stages. While low light can cause photoinhibition [12] or nitrate accumulation [13], high irradiance levels may create mild photostress and activate photoprotective mechanisms which influence the production of primary and secondary metabolites in plants. Plants acclimate to high light levels by increasing photosynthesis [12,14]

and, compounds such as ascorbate, glutathione, carotenoids, or α-tocopherol [15,16] that have antioxidant capacities which are important for human nutrition. Therefore, the photophysiological processes in green vegetable plants must be ascertained in order to select lighting strategies that are optimal for both plants and humans. The objective of the current study was to examine the effects of light intensity produced by LED on the growth and antioxidant properties of microgreens from the Brassicaceae family and to determine lighting conditions needed to induce higher nutritional values in plants during production.

2. Experimental Procedures2.1 Growth and lighting conditionsMicrogreens of Brassicaceae family, kohlrabi (Brassica oleracea var. gongylodes, ‘Delicacy Purple’) mustard (Brassica juncea L., ‘Red Lion’), red pak choi (Brassica rapa var. chinensis, ‘Rubi F1’) and tatsoi (Brassica rapa var. rosularis), were grown in peat substrate (Profi 1, Durpeta, Lithuania) (pH 5-6) in 0.5 L plastic vessels (18x11x6 cm) for 10 days from sowing to harvest. The following amounts of nutrients were available in the substrate N 110, P2O5 50, K2O 160 (used as mg L–1); microelements – Fe, Mn, Cu, B, Mo, Zn (used as mg L–1).Depending on size and weight from we used 1 and 2 g of seeds were seeded per vessel. Each light treatment contained four replicate vessels per species.

Experiments were performed in controlled-environment growth chambers. Day/night temperatures of 21±2/17±2°C were established with a 16 h photoperiod and a relative air humidity of 50-60%.

LED lighting units were originally designed by Tamulaitis et al. [17] light emitting diode (LED) based lighting units, consisting of commercially available LED’s with emission wavelengths of blue (LXHL–LR3C, l=455 nm), red (LXHL-LD3C, l=638 nm and LXM3-PD01-0300, l=660 nm) (Philips Lumileds, USA) and far-red (L735-05-AU, l=735 nm) (Epitex, Japan) were used for microgreen lighting. The surface area under the lighting unit was approximately 0.5 m2. Different irradiance levels, expressed as photosynthetic photon flux densities (PPFD) were set in each lighting unit (545, 440, 330, 220 and 110 μmol m-2 s-1, Table 1). In our study we used 220 μmol m-2 s-1 as the normal irradiance level. PPFD was measured and regulated at the vessel level using a photometer – radiometer (RF-100, Sonopan, Poland).

2.2 SamplingAt harvesting time (10 days after sowing), microgreen stems with cotyledons were cut. Conjugated biological samples of the randomly selected plants (0.5-1 g per sample) were used for each analysis. Three replicates were performed for each phytochemical measurement and ten replicate biometric measurements were performed for each treatment. All data are expressed as fresh weight.

2.3 Phytochemical analysis2.3.1 Determination of DPPH• radical-scavenging

activityThe antioxidant activity of microgreen green matter (80% methanol extracts (1:10)) was evaluated using a spectrophotometer (Genesys 6, Thermospectronic, USA) as the 2,2–diphenyl–1–picrylhydrazyl (DPPH) (Sigma-Aldrich, Germany) free radical scavenging ability (µmol g-1) [18]. The absorbance was measured for 16 minutes at 515 nm.

%Total PPFD Blue, 445 nm Red, 638 nm Red, 665 nm Far-red, 735 nm

µmol m-2 s-1

100 545 41 225 275 4

80 440 34 181 221.5 3.5

60 330 25 136 166 3

40 220 17 90 111 2

20 110 8 45 55 1

Table 1. Combinations of light-emitting diodes photosynthetic photon flux densities (PPFD).

PPFD-photosyntheticphotonfluxdensities

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G. Samuolienė et al.

2.3.2 Determination of total phenolic compounds The total content of phenolic compounds was determined from microgreen 80% methanol (POCh, Poland) extracts (1:10) using the calorimetric Folin-Ciocalteau method [18]. The absorbance was measured at 765 nm with a Genesys 6 spectrophotometer (Thermospectronic, USA) against water as a blank. Total phenolics were calibrated using gallic acid as a standard.

2.3.3 Determination of total anthocyanins The total amount of anthocyanins was determined using the spectrophotometric pH-differential method [19]. The absorption values were measured at 420, 520 and 700 nm wavelengths. Anthocyanins were expressed as cyanidin 3-glucoside equivalents, mg g-1 in fresh plant weight, using a molar extinction coefficient of 25,740 mol-1 cm-1 and a molecular weight of 485 g mol-1.

2.3.4 Determination of α-tocoperol Alpha tocopherol (Supelco, USA) content in hexane (Merck, Germany) extracts (1:10) were evaluated using a high-performance liquid chromatography (HPLC) method. Samples were centrifuged (5 min, 349xg) and filtrated through a 0.45 µm PTFE syringe filter (VWR International, USA). The HPLC 10A system, equipped with a RF-10A fluorescence detector (Shimadzu, Japan) and Pinacle II silica column, 5 µm particle size, 150x4.6 mm (Restek, USA) were used for analysis. The mobile phase was 0.5% isopropanol (Merck, Germany) in hexane, with a flow rate of 1 ml min-1. The peak was detected using an excitation wavelength of 295 nm and an emission wavelength of 330 nm.

2.3.5 Determination of ascorbic acid Ascorbic acid (Penta, Check rep.) content was evaluated using a spectrophotometric method [20] based on methyl viologen (Aldrich, Germany) reduction. A Genesys 6 spectrophotometer was used for this analysis (Thermospectronic, USA). The colored radical ion was measured at 600 nm and ascorbic acid contents were evaluated using calibration curve.

2.3.6 Determination of chlorophyll index Non-destructive measurements of leaf chlorophyll content were performed in the laboratory at ambient temperature and light using flavonols and a chlorophyll-meter (Dualex®

4, Scientific, USA). The absorption index was calculated as the difference between the optical transmission at two different wavelengths in the near infrared region.

2.3.7 Determination of sucrose Sucrose was measured using high performance liquid chromatography (HPLC). About 1 g of fresh plant

tissue was ground and diluted with 70ºC bi-distilled water. The extraction was carried out for 24 h. Samples were centrifuged (5 min, 349xg) and filtrated through a 0.45 µm PTFE syringe filter (VWR International, USA). The analyses were performed on a Shimadzu HPLC (Japan) chromatograph with a refractive index detector (RID 10A). Separation of carbohydrates was performed on a SC-1011 column (300x4.6 mm) (Shodex, USA) with mobile phase – bi-distilled water. The oven temperature was maintained at 80ºC.

2.3.8 Determination of nitrates Nitrate concentration was measured by a potentiometric method [21] using an ion meter (Oakton, USA) with a nitrate-selective electrode (Cole-Parmer, USA). Fresh microgreen matter was dried at 105°C for 24 h and ground using a mortar and pestle. The ionic strength adjustor (ISA) contained 0.02 mol L-1 Al2(SO4)3 (Poch, Poland), 0.01 mol L-1 Ag2SO4 (DeltaChem, Czech Republic), and 0.02 mol L-1 H3BO3 (Poch, Poland). The 0.2 g of the dry sample was diluted in a 20 ml water–ISA solution (50/50% v/v) and extracted in an ultrasound bath for 10 min. All measurements were performed after the sensor signal had been stabilized for 3 min.

2.4 Biometric measurementsThe hypocotyl length of 10 randomly selected plants was measured (cm). Dry weight (%) of the microgreens was evaluated by drying plants in drying oven (Venticell, MBT, Czech Republik) at 105°C for 24 h. Leaf area was measured using an automatic leaf area meter (AT Delta-T Devices, UK).

2.5 Statistical analysis. All data are presented as mean values ± standard deviation. Data was analyzed using one-way analysis of variance ANOVA, the Fisher’s LSD test to normal irradiance level (220 µmol m-2 s-1) with an alpha level 0.05. Data were processed using MS Excel software (version 7.0).

3. ResultsPhotosynthetic photon flux densities in the region of 110–545 µmol m-2 s-1 had no apparent effect on the external quality of the different microgreen species. However, light effects on microgreen internal quality and biometric parameters were evident (Table 2). Inhibition of elongation in tested microgreens was observed under higher irradiance levels: hypocotyls of mustard (P=0.27), tatsoi (P=0.39) and kohlrabi (P=0.26) were significantly shorter under 545-440 μmol m-2 s-1 treatments as

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compared to normal 220 µmol m-2 s-1 PPFD illumination. The lowest irradiance level investigated at 110 μmol m-2 s-1

had slightly elongated hypocotyls, however these results were only statistically significant in red pak choi (P=0.27) and kohlrabi (P=0.25). Irradiances also increased plant dry weight percentage in red pak choi (P=0.48) and tatsoi (P=0.36) at 545 and 440 μmol m-2 s-1.Leaf area also increased with increasing irradiance level in mustard (P=0.37) under 440-300 and in red pak choi (P=0.27) under 545-440 μmol m-2 s-1 PPFD levels(Table 2).

Nutrient contents also varied depending on the LED irradiance level. Leaf chlorophyll index increased at higher irradiance levels 545-440 μmol m-2 s-1 in red pak choi and mustard as compared to normal 220 μmol m-2 s-1

irradiance (Table 3). Sucrose accumulation in microgreen leaves was also related to LED irradiation

level (Table 3): the lowest PPFD levels resulted in lower sucrose contents,. Highest sucrose production was detected at different irradiation levels for different microgreen species: in kohlrabi (11.4 times – at 545 μmol m-2 s-1; in red pak choi (6 times)– at 440 μmol m-2 s-1; and in tatsoi (9.5 times higher as compared to normal 220 μmol m-2 s-1 irradiance) – at 330 μmol m-2 s-1. A significant (P<0.05) negative correlation (r=-0.93–-0.98) was determined between nitrate content and LED irradiation PPFD level in all microgreen species: the lowest light levels resulted in the highest nitrate concentrations in all microgreens that decreased with increasing light level (Table 3).

LED irradiance level had only slight effect on the DPPH free radical activity in microgreens (Figure 1A). However, significantly lower DPPH free-radical scavenging activity was determined at a PPFD level of

Varieties PPFD, µmol m-2 s-1 Hypocotyl length, cm Dry weight, % Leaf area, cm2

Mustard

545 2.3±0.3B 5.4±0.5 1.9±0.4

440 2.1±0.4B 5.3±0.6 3.1±0.5A

330 2.7±0.3 5.4±0.2 2.6±0.4A

220 2.9±0.3 4.9±0.4 2.0±0.6

110 3.2±0.2 5.4±0.7 1.1±0.3B

LSD05 0.3 0.5 0.4

Red Pak Choi

545 4.1±0.3 6.3±0.0A 3.4±0.4A

440 3.9±0.4 6.3±0.2A 3.4±0.9A

330 4.0±0.4 5.9±0.6A 2.8±0.6

220 4.1±0.3 4.7±0.5 2.6±0.4

110 4.5±0.3A 4.9±0.5 2.0±0.9

LSD05 0.3 0.5 0.6

Tatsoi

545 4.2±0.3B 5.8±0.4A 1.8±0.4

440 4.2±0.6B 5.6±0.3A 2.0±0.6

330 4.2±0.5B 5.7±0.7A 2.0±0.5

220 4.7±0.4 4.8±0.8 1.8±0.5

110 5.1±0.2 4.6±0.6 1.2±0.4B

LSD05 0.4 0.4 0.5

Kohlrabi

545 2.6±0.3B 6.7±0.5 5.8±1.0

440 2.6±0.3B 6.6±0.6 5.3±0.8

330 2.4±0.4B 6.5±0.9 5.6±0.9

220 3.2±0.4 6.2±0.7 5.3±1.0

110 3.6±0.5A 4.9±0.3B 4.5±0.7

LSD05 0.3 0.6 0.9

Table 2. Growth parameters of Brassica microgreens cultivated under different irradiation levels (means ± standard deviations).

Avaluesaresignificantly(P≤0.05)higherthannormal220µmolm-2s-1irradiancelevel; Bvaluessignificantly(P≤0.05)lowerthannormal220µmolm-2s-1irradiancelevel. PPFD-photosyntheticphotonfluxdensities

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Table 3. Nutrient quality of Brassica microgreens cultivated under different irradiation levels.

Avaluesaresignificantly(P≤0.05)higherthannormal220µmolm-2s-1irradiancelevel; Bvaluessignificantly(P≤0.05)lowerthannormal220µmolm-2s-1 irradiancelevel.

Varieties PPFD, µmol m-2 s-1 Chlorophyll, index Sucrose, mg g-1 Nitrates, mg kg-1

Mustard

545 20.5±2.1A 0.83±0.04 1474±93B

440 19.5±2.0 0.81±0.41 1437±33B

330 19.3±1.3 0.65±0.07 2014±59B

220 18.5±0.7 0.53±0.19 2377±150

110 19.1±2.5 0.34±0.09 2366±155

LSD05 1.7 0.41 213.8

Red Pak Choi

545 39.2±4.0A 0.62±0.05 503±98B

440 37.8±4.0A 3.47±0.29A 841±34B

330 33.1±4.6 0.94±0.25A 1222±68B

220 33.1±2.4 0.58±0.06 2297±148

110 30.2±1.5 0.61±0.26 3079±106A

LSD05 3.5 0.26 197.5

Tatsoi

545 31.5±5.0 0.92±0.15 598±20B

440 31.1±4.0 0.91±0.30 517±27B

330 28.2±3.4 5.71±0.28A 753±51B

220 28.2±2.1 0.60±0.17 1509±72

110 25.6±4.9 0.71±0.11 1818±65A

LSD05 4.7 0.44 103.3

Kohlrabi

545 21.4±3.7 2.62±0.33A 581±45B

440 23.9±2.7 0.77±0.07A 1464±59B

330 21.8±2.1 0.35±0.07 2099±43B

220 21.6±2.2 0.23±0.10 2432±58

110 19.0±2.8 0.19±0.05 3755±211A

LSD05 2.6 0.33 162.5

110 μmol m-2 s-1 and the highest – at 545-400 μmol m-2 s-1.Lower DPPH free radical activity coincidenced with a decrease in the amount of phenolic compounds (Figure 1B) at 110 μmol m-2 s-1. Peak production of total phenols occurred under high, 440 and 545 μmol m-2 s-1,irradiance for tatsoi and kohlrabi, and under 440-330 μmol m-2 s-1 for red pak choi. In mustard, contents of phenolic compounds under highest irradiance levels did not differ from normal 220 μmol m-2 s-1 illumination. Peak anthocyanin production occurred only at moderate LED irradiation levels: the irradiance of 330 μmol m-2 s-1 led to significantly higher contents of total anthocyanins in red pak choi and tatsoi (1.3 and 1.5 times higher than under 220 μmol m-2 s-1 treatment) and 440 μmol m-2 s-1 led to the highest anthocyanin concentration in kohlrabi. In mustard, 220 μmol m-2 s-1 irradiance resulted in lower anthocyanin content as compared to lower or higher irradiance levels (Figure 1C). α-tocopherol contents

also varied under different light intensities (Figure 1D). The lowest investigated PPFD levels led to significantly higher α tocopherol accumulation in mustard, red pak choi and kohlrabi, when in tatsoi both 220 and 110 μmol m-2 s-1 irradiance had significant positive effects on α tocopherol content.

The higher investigated PPFD level had uneven effect on α tocopherol and ascorbic acid accumulation. Mustard grown under 545 μmol m-2 s-1 had an α-tocopherol concentration 1.6 times higher than plants grown at 220 μmol m-2 s-1. Red pak choi and tatsoi grown at 545 μmol m-2 s-1 had significantly lower concentrations of α tocopherol compared to normal light. Negligibly low concentrations of α-tocopherol were detected in kohlrabi. Ascorbic acid content (Figure 1E) did not differ remarkably under different LED light intensities, except in red pak choi and tatsoi, where the lowest 110 μmol m-2 s-1 PPFD level resulted in significantly

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Figure 1. Changes in antioxidant properties of Brassica microgreens cultivated under different irradiation levels. A- the influence of different doses of examined compounds on DPPH free radicals scavenging activity; values are significantly (P≤0.05)

higher than normal 220 µmol m-2 s-1 irradiance level; B- the influence of different doses of examined compounds on total phenols concentration (mg g-1); values significantly (P≤0.05) lower than normal 220 µmol m-2 s-1 irradiance level; C- the influence of examined compounds on total anthocyanins level (mg g-1); D- the influence of examined compounds on α-tocopherol concentration (mg g-1); E- the influence of examined compounds on ascorbic acid concentration (mg g-1);

A

0

2

4

6

8

10

12

545 440 330 220 110 545 440 330 220 110 545 440 330 220 110 545 440 330 220 110

Mustard Red Pak Choi Tatsoi KohlrabiDPP

H fr

ee r

adic

al sc

aven

ging

act

ivity

, µm

ol g

-1

µmol m-2s-1

A BB

A A B

A A A B

LSD0.5=0.2 LSD0.5=0.3 LSD0.5=0.4 LSD0.5=0.2

B

0,00

0,10

0,20

0,30

0,40

0,50

0,60

0,7 0

545 440 330 220 110 545 440 330 220 110 545 440 330 220 110 545 440 330 220 110

Mustard Red Pak Choi Tatsoi Kohlrabi

Tot

al p

heno

ls, m

g g-1

µmol m-2s-1

B

B

AA A A

*

A A

B

LSD0.5=0.03 LSD0.5=0.03 LSD0.5=0.03 LSD0.5=0.03

B*

B

C

0,00,20,40,60,81,01,21,41,61,82,0

545 440 330 220 110 545 440 330 220 110 545 440 330 220 110 545 440 330 220 110


Tot

al a

ntho

cyan

ins,

mg

g-1

µmol m-2s-1

A

BB

A

AA

B

LSD0.5=0.07 LSD0.5=0.13 LSD0.5=0.16 LSD0.5=0.13

A

A

A

A

B

D

00,05

0,10,150,2

0,250,3

0,350,4

545

440

330

220

110

545

440

330

220

110

545

440

330

220

110

545

440

330

220

110

Mustard Red Pak Choi Tatsoi KohlrabiA

lpha

toco

pher

ol, m

g g

-1

µmol m-2s-1

A

A

A

B B B

B B

B

A

A

LSD0.5=0.007 LSD0.5=0.005 LSD0.5=0.003 LSD0.5=0.003

B

A

B

A

E

0,0

1,0

2,0

3,0

4,0

5,0

6,0

545

440

330

220

110

545

440

330

220

110

545

440

330

220

110

545

440

330

220

110


Asc

orbi

c ac

id, m

g g

-1

µmol m-2s-1

A

AB B

LSD0.5=0.34 LSD0.5=0.25 LSD0.5=0.65 LSD0.5=0.15

A

higher ascorbic acid content (3.8 and 3.5 times higher than under normal 220 μmol m-2 s-1 light level).

4. DiscussionLighting systems should provide sufficient photosynthetic photon flux for optimal efficiency of plant photosynthetic processes and also meet the constraints for light power [12]. It is, therefore, necessary to select the optimal irradiance level from both an agronomic and economic perspective. A minimum level of irradiance is necessary for sufficient synthesis and activity of photosynthetic pigments, biomass, leaf area formation or nitrogen investment between photosynthetic

components [12,22]. In our experiments, low LED light levels negatively affected brassica microgreen growth parameters: irradiance levels of 110 or 220 μmol m-2 s-1

resulted in hypocotyl elongation, and a decrease in dry weight and leaf area, whereas higher PPFD levels (up from 330 μmol m-2 s-1) resulted in proper growth parameters. However, lowest PPFD levels resulted in high accumulated nitrate contents in all investigated microgreen species, which is consistent with other studies of different green vegetables under low irradiance [13,23,24]. Higher LED light intensity gradually reduced nitrate content in microgreens, whereas sucrose content remarkably increased. However, peak sucrose production matched the highest applied PPFD only in mustard and kohlrabi, whereas sucrose content in red

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pak choi and tatsoi were the highest under moderate applied PPFD levels. Sugars are important not only for growth, but also function as signaling molecules which are able to modulate light signaling pathways. Moreover, in some plants, intense sucrose accumulation in leaves is associated with a reduction in nitrates [13] or decreased photosynthetic rates [25].

Other authors have demonstrated that low light results in more effective functioning of PSII [12], higher contents of chloroplast pigments and electron carriers [26], whereas high light causes degeneration of pigments as they participate in photoprotection. However, Fu and coauthors [23] suggest that at lower light levels (200 μmol m-2 s-1), light use efficiency is the highest, whereas at high, but tolerable light (400-600 μmol m-2 s-1) levels, lettuce plant yield is the highest. Our data showed that in microgreens grown under lower irradiance levels (110–330 μmol m-2 s-1), significantly lower chlorophyll index was determined, but were of similar values to those found in other studies [3]. We showed that increasing PPFD level led to an increase in the content of chlorophylls and carotenoids and also improved biometric parameters.

The lowest applied LED light level (110 µmol m-2 s-1)also resulted in lower antioxidant activities of microgreen tissues. We suggest that higher, 220-330 µmol m-2 s-1 PPFD levels should be used by growers seeking normal growth and superior nutritional quality of brassica microgreens. It has been previously reported that high light (>400 μmol m-2 s-1) treatment, as a mild environmental stress, was useful in enhancing nutritional properties in sprouts [27] and mature plants [15,28]. Plants typically respond to environmental stressors by inducing antioxidant production as a defense mechanism. High light treatments also resulted in increased contents of phenolic compounds and antioxidant activity with no adverse effect on growth or yield [28]. However, the effects of higher light levels was not consistent across all of the antioxidants

studied. Peak anthocyanin accumulation was observed under moderate 330–440 μmol m-2 s-1 LED light levels, whereas α-tocopherol (in all microgreen species) and ascorbate (in red pak choi and tatsoi) contents were the highest at the 110–220 μmol m-2 s-1 PPFD levels. Anthocyanins, α-tocopherol and ascorbate are involved in the photoprotection mechanisms caused by high light stress [29,30]. However, variation in the amount of these antioxidants in brassica microgreens suggests that there are complex relationships among the effects of environmental stress on plant antioxidant compounds that are likely mediated through a variatey of genetic, developmental, environmental and metabolic signal transduction pathways [31].

5. ConclusionsInsufficient levels of photosynthetically active photon flux (110 µmol m-2 s-1) suppressed normal growth and diminished the nutritional value of Brassica microgreen plants. Depending on the species, the most suitable conditions for growth and nutritional quality of microgreens was 320–440 µmol m-2 s-1

irradiation, conditions which resulted in larger leaf area higher total anthocyanins, total phenols and DPPH free-radical scavenging capacity and lower content of nitrates. High light (545 µmol m-2 s-1) had no significant positive impact on most of the investigated parameters. Moderate light levels were more effective at enhancing the nutritional value of microgreen plants through altered action of the protective antioxidant system.

AcknowledgementsThis research was funded by a grant (No. SVE-03/2011) from the Research Council of Lithuania.

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[3] Kopsell D.A., Sams C.E., Increase in shoot tissue pigments, glucosinolates and mineral elements in sprouting broccoli after exposure to short-duration blue light from light emitting diodes, J. Amer. Soc. Hort. Sci., 2013, 138, 31-37

[4] Samuolienė G., Urbonavičiūtė A., Brazaitytė A., Šabajevienė G., Sakalauskaitė J., Duchovskis P., The impact of LED illumination on antioxidant properties of sprouted seeds, Cent. Eur. J. Biol., 2011, 6, 68-74

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LED irradiance level affects growth and nutritional quality of Brassica microgreens