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118 Page 1 of 11
ISSN 1021-4437, Russian Journal of Plant Physiology, 2023, Vol. 70:118. © Pleiades Publishing, Ltd., 2023.
Continuous Lighting Increases Yield and Nutritional Value
and Decreases Nitrate Content in Brassicaceae Microgreens
T. G. Shibaeva a, *, A. A. Rubaevaa, E. G. Sherudiloa, and A. F. Titova
a Institute of Biology, Karelian Research Center, Russian Academy of Sciences, Petrozavodsk, Russia
*e-mail: shibaeva@krc.karelia.ru
Received March 13, 2023; revised April 10, 2023; accepted April 12, 2023
Abstract—Microgreens of four species of the family Brassicaceae (broccoli, mizuna, radish, and arugula) were
grown under 16- and/or 24-h photoperiod conditions. In the first series of experiments, the daily light inte-
gral (DLI) was different (15.6 and 23.3 mol m–2 day–1 at PAR 270 μmol m–2 s–1), while it was the same
(15.6 mol m–2 day–1 at PAR 270 μmol m–2 s–1 and 180 μmol m–2 s–1) in the second. In the third series of
experiments, continuous lighting was used only in the last three days before harvesting. The results obtained
showed that broccoli, mizuna, radish, and arugula plants in the early phases of growth are resistant to contin-
uous lighting and do not show typical signs of leaf photodamage. In all three series of experiments, micro-
greens of all four species grown under 24-h photoperiod had a higher yield and nutritional value (higher con-
tent of substances with antioxidant properties—anthocyanins, flavonoids, carotenoids, and proline—as well
as increased activity of antioxidant enzymes) and a lower content of nitrates compared to plants grown under
16-h photoperiod. It was concluded that it is possible through the use of continuous lighting without increas-
ing energy costs (while maintaining the DLI) to increase the yield and nutritional value of the studied species
of microgreens and reduce their nitrate content compared to the standard 16-h photoperiod. In addition, an
increase in nutritional value and a decrease in nitrate content is also possible with the use of continuous light-
ing (as an agricultural practice) for several days immediately before harvesting.
Keywords: antioxidants, continuous lighting, microgreens, photoperiod
DOI: 10.1134/S1021443723601337
INTRODUCTION
Microgreens are a special crop representing the lat-
est trend in global crop production. Its production has
become an attractive line of business due to the
steadily growing consumer demand and due to its high
market value [1]. In addition, many species as micro-
greens have a beneficial effect on human health since
they contain a high concentration of beneficial nutri-
ents [2]. These young plants have a higher content of
antioxidant compounds (polyphenols, carotenoids,
ascorbic acid) compared to mature plants; therefore,
microgreens are classified as “functional food prod-
ucts” [1, 2]. In addition to high nutritional value, one
of the main criteria for a “functional product” is its
biological safety, and excessive mineral nutrition or
low light (or other reasons) can lead to an increase in the
content of nitrate ions ( ). Nitrates, being necessary
components for the vital activity of plants, when reduced
to nitrites in the human body cause various metabolic
disorders and even pathological processes [3]. The
results of single studies of the effects of 24-h photope-
riod (CL, from continuous lighting) on the content of
nitrates in plants are rather contradictory. There is evi-
dence that the content of nitrates in arugula leaves sig-
nificantly decreased under CL conditions, regardless
of the spectral composition of light [4]. At the same
time, higher light intensity had little effect on the
content of nitrates in rapeseed, cabbage, arugula,
mustard [5], and mizuna [6] microgreens. Therefore,
more research is needed to study the effect of lighting
conditions on nitrate content in different species in
order to grow microgreens with nitrate content in
amounts that are safe for humans.
Currently, microgreens are grown in greenhouses
and plant factories using artificial lighting (PFAL:
plant factories with artificial lighting). Such plants are
closed production systems that integrate modern
industrial technologies for year-round production of
various crops. The cost of the products obtained in this
case is primarily determined by the cost of electricity,
and since energy prices around the world are steadily
rising, it is necessary to find reliable solutions to
reduce energy consumption in the production of cer-
tain fresh products and to increase the efficiency of
converting electricity into yield and its quality. Cur-
rently, most of the ongoing work on the study of the
effect of light conditions on productivity and produc-
tion efficiency is aimed at selecting the optimal spec-
tral composition of light [7] and the intensity of pho-
tosynthetically active radiation (PAR) [8, 9], while
3
NO
−
RESEARCH PAPERS
118 Page 2 of 11
RUSSIAN JOURNAL OF PLANT PHYSIOLOGY Vol. 70:118 2023
SHIBAEVA et al.
insufficient attention is paid to the effect of photope-
riod under LED lighting, although it undoubtedly
plays an important role in regulating the processes of
plant growth and development. In particular, the dura-
tion of the photoperiod affects some molecular mecha-
nisms associated with the perception and transmission
of light signals by leaves, circadian rhythms that regulate
growth, flowering time, response to abiotic and biotic
stresses, and plant metabolism in general.
One of the possible ways to improve production
efficiency in plant factories is to redistribute the daily
light integral (DLI = illumination × photoperiod)
over time. The use of long photoperiods, including
CL, with lower light intensities reduces initial lighting
costs and lighting operating costs by using lower night
electricity rates. In addition, the use of CL opens up
significant additional opportunities for increasing
yield and crop quality without increasing energy costs
[4, 10, 11]. In general, CL provides a constant supply of
energy for carbon assimilation, which implies greater
biomass accumulation and higher yields in theory. But
the economic benefits of using CL can only be achieved
if such a light regimen does not lead to leaf injuries since
long light periods are known to cause plant photodam-
age (chlorosis and/or necrosis) of the leaves and
reduced yields of vegetable crops, such as tomatoes,
eggplants, cucumbers, and peppers, which limits the
use of this cost-effective lighting regime when growing
certain types of vegetables in greenhouses [11, 12]. Our
previous studies have shown that CL increases the yield
and nutritional value of microgreens of four species of
the family Brassicaceae [13]. However, these results
were obtained under conditions when plants were
grown at photoperiods of 16 and 24 h with the same
light intensity, which led to a higher DLI in the variants
with CL, and the increase in yield due to an increase in
DLI is energy-consuming, i.e., ineffective.
Given the above, the aim of this study was to study
the physiological mechanisms of plant response to CL
in four species of microgreens of the family Brassica-
ceae (broccoli, mizuna, radish, and arugula) and test-
ing whether microgreens can increase yield and nutri-
tional value and reduce nitrate content compared to a
standard 16-h photoperiod by: (1) using CL without
increasing energy costs (while maintaining DLI) and
(2) the application of CL at the end of the production
period (before harvesting).
MATERIALS AND METHODS
The objects of study were four species of plants of
the family Brassicaceae: broccoli (Brassica oleracea
var. italica Plenck), mizuna (Brassica rapa ssp nipposi-
nica (LH Bailey) Hanelt), radish (Raphanus sativus
var. radical Pers.), and arugula (Eruca vesicaria sp.
sativa Mill.). Plants were grown under controlled envi-
ronmental conditions. The average air temperature
and relative air humidity were 22 ± 1°С and 60 ± 5%,
respectively.
Microgreens were grown in plastic containers on
10 cm2 coir mats. In the first 3 days after sowing, con-
tainers with microgreens were placed in the dark and
watered for germination. Plants at the age of 4 days
from sowing (the phase of fully opened cotyledons)
were exposed to different light regimes, moistening the
substrate with half Hoagland-Arnon nutrient solution
(pH 6.2–6.4).
Plants were illuminated with LED lamps (LED GL
V300, China), and the ratio (%) of red : green : blue
light LEDs was 50.3 : 21.1 : 17.6. In the first series of
experiments with different daily light integral (DLI),
plants were grown at photoperiods of 16 or 24 h and
PAR 270 μmol m –2 s
–1 with DLI of 15.6 and
23.3 mol m–2 day–1, respectively. In the second series of
experiments with the same DLI, plants were grown at
photoperiods of 16 or 24 h at PAR 270 μmol m–2 s–1 and
180 μmol m–2 s–1, respectively. DLI in both variants
was 15.6 mol m–2 day–1. In the third series of experi-
ments, plants were grown at a photoperiod of 16 h and,
starting from 9 days (last 3 days of the experiment),
part of the plants were exposed to a 24-h photoperiod
at PAR 270 μmol m–2 s
–1 (Table 1). PAR was mea-
sured using an LI-250A light meter (Li-COR Biosci-
ences, Lincoln, NE, United States). In each experi-
ment, plants grown at a photoperiod of 16 h were taken
as control.
Plants were analyzed on the 12th day after sowing
in the phase of the appearance of the first true leaf.
The length of the hypocotyl, the length of the first true
leaf, and the fresh and dry weight of the shoots were
measured in ten seedlings of each species of all vari-
ants. The length of the hypocotyl was measured from
the base of the hypocotyl to the shoot apical meristem.
To determine the fresh weight, the plants were
weighed, and then the samples were dried in an oven
at a temperature of 105°C to constant weight.
The leaf mass per unit area (LMA) was calculated
as the ratio of the dry weight of four cuttings of cotyle-
don leaves with a diameter of 4 mm to their area. The
dry weight of the cut-outs was determined after their
drying at 105°C.
Robustness index was determined as the ratio of the
product of the dry mass of the shoot and the diameter
of the hypocotyl to the length of the hypocotyl.
The content of photosynthetic pigments (chloro-
phyll (Chl) a and b, carotenoids (Car)) was deter-
mined using an SF-2000 spectrophotometer (Spektr,
Russia) in an extract of 96% ethanol and calculated
using known formulas [14].
The content of malondialdehyde (MDA), the end
product of lipid peroxidation, was determined by a
method based on the formation of a trimethine com-
plex with an absorption maximum at 532 nm during
the interaction of these substances with thiobarbituric
acid [13].
RUSSIAN JOURNAL OF PLANT PHYSIOLOGY Vol. 70:118 2023
CONTINUOUS LIGHTING INCREASES YIELD AND NUTRITIONAL VALUE Page 3 of 11 118
To analyze the activity of antioxidant enzymes super-
oxide dismutase (SOD, EC 1.1.5.1.1), catalase (CAT,
EC 1.11.1.6 ), a scorb ate peroxidase (APO, EC 1.11.1.11),
and guaiacol peroxidase (GPOD, EC 1.11.1.7), plant
leaves were homogenized in 50 mM phosphate buffer
(pH 7.8), the homogenate was centrifuged at 15000 g
for 10 min at 4°C, and the activity of enzymes was
determined in the supernatant using an SF-2000 spec-
trophotometer (Spektr, Russia). APO activity was
determined in the presence of 0.5 mM ascorbic acid
and 0.25 mM H2O2 by the decrease in optical density
at 290 nm [13]. CAT activity was determined by the
enzymatic degradation of H2O2 at 240 nm; SOD activ-
ity was determined by the ability to inhibit the photo-
chemical reaction of nitro blue tetrazolium [13].
GPOD analysis was based on the oxidation of guaiacol
in the presence of H2O2 [13]. Optical density was mea-
sured at 470 nm. Enzyme activity was calculated per
1 g of leaf dry weight, and specific activity was calcu-
lated per 1 mg of protein. The total protein content was
determined by the Bradford method using bovine
serum albumin as a standard [13].
To determine the content of hydrogen peroxide, a
sample of plant tissue was homogenized on ice in 0.1%
trichloroacetic acid and centrifuged for 15 min at
12000 g and 4°С. We added 0.5 mL of 10 mM K-phos-
phate buffer (pH 7.0) and 1 mL of 1 M KI to 0.5 mL of
the supernatant. After keeping the mixture in a refrig-
erator in the dark for 1 h, the optical density was deter-
mined at 390 nm on an SF 2000 spectrophotometer
(Spektr, Russia) [13]. The content of hydrogen perox-
ide was calculated from a standard concentration
curve and expressed in μmol/g fresh weight.
The content of free proline in leaf tissues was esti-
mated by the ninhydrin method [13].
To determine the content of anthocyanins and fla-
vonoids, a sample of plant material was homogenized
in 4 mL of a cold mixture of ethanol and 1.5 N hydro-
chloric acid (85 : 15, v/v), followed by extraction for
14 h in a refrigerator in the dark. After 5 min centrifu-
gation of the extract at 10000 g and +4°С for anthocya-
nins, the optical density of the supernatant was deter-
mined at 530 and 657 nm on an SF 2000 spectropho-
tometer. When calculating the content of anthocyanins,
the absorption of Chl and its decomposition products
at 657 nm was taken into account. The content of fla-
vonoids was determined spectrophotometrically at
300 and 350 nm in the supernatant for anthocyanins,
preliminarily diluted ten times. The content of flavo-
noids was calculated as the ratio of optical density to
fresh weight:
Nitrate content was determined by the potentio-
metric method using an Anion-4100 pH meter
(Anion, Russia). A weighed portion of the dry matter
was placed in a 1% potassium aluminium sulphate
solution (Vekton, Russia) and stirred on a PE-6500
shaker (Ekroskhim, Russia) for 5 min. The readings of
the electromotive force (EMF) were then taken using
a potentiometer [16]. The content of nitrates was
expressed as mg/kg fresh weight of plant material.
The paper presents the mean values for two or three
independent experiments (four to six or more biologi-
cal replicates in each variant of a separate experiment)
and their standard errors. The significance of differ-
ences between the mean values was determined based
on the analysis of variance (LSD test) at R < 0.05 using
MS Excel software.
RESULTS
Experiment 1. Plants of all four species, when grown
under photoperiods of 16 and 24 h with the same PAR
(different DLI, experiment 1), had a shorter hypocotyl
length, greater fresh and dry shoot biomass, and
higher values of LMA and robustness index (Table 2,
Fig. 1a) under CL. In all studied species CL acceler-
ated development, which was seen by the earlier
appearance of the first true leaf (Table 2), which sug-
gests an earlier harvest time. None of the crops showed
signs of leaf photodamage. The content of Chl under
CL was somewhat lower than under 16-h photoperiod;
however, these changes were not visually noticeable
and the leaves had a normal green color. Car content
under CL significantly increased in radish and arugula
leaves but significantly decreased in mizuna leaves
(
)
()
[]
300
350
UB A g fresh weight and
UA
А g fresh weight 15 .
−
−
Table 1. Illumination conditions in experiments with different and the same daily light integral (DLI) as well as under con-
tinuous lighting (CL) at the end of the production period
Series of experiments Photoperiod, h PAR,
μmol/m–2 s–1
DLI,
mol/m–2 day–1
1. With different DLI 16 270 15.6
24 270 23.3
2. With the same DLI 16 270 15.6
24 180 15.6
3. CL at the end of the production period 16 270 15.6
16 (8 days) + 24 (3 days) 270 18.2
118 Page 4 of 11
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SHIBAEVA et al.
Table 2. Biometric parameters of plants (% of control)
Plants grown under 16-h photoperiod with light intensity of 270 μmol m–2 s–1 PAR. * Statistically significant differences with control.
Absolute values of control samples: hypocotyl length (mm): 51.2 ± 2.2 (broccoli), 47.3 ± 1.7 (mizuna), 52.3 ± 2.4 (radish), 36.8 ± 1.2
(arugula); first leaf length (mm): 2.5 ± 0.7 (broccoli), 5.4 ± 0.5 (mizuna), 5.5 ± 0.7 (radish), 16.5 ± 0.8 (arugula); shoot fresh weight (g):
0.07 ± 0.01 (broccoli), 0.076 ± 0.003 (mizuna), 0.23 ± 0.02 (radish), 0.052 ± 0.003 (arugula); shoot dry weight (mg): 10.0 ± 1.0 (broc-
coli), 7.7 ± 0.5 (mizuna), 21.0 ± 2.6 (radish), 2.9 ± 0.3 (arugula); LMA (mg cm-2): 5.2 ± 0.4 (broccoli), 3.3 ± 0.2 (mizuna), 2.9 ± 0.3
(radish), 2.5 ± 0.4 (arugula).
Series of
experiments Hypocotyl length First true leaf
length Shoot fresh weight Shoot dry weight LMA
Broccoli
1 87* 228* 129* 130* 128*
2 97 160* 110 124* 114
3 93 111 12 0 * 110 153 *
Mizuna
1 81* 128* 116* 128* 136*
2 87* 109 109 130* 118*
3 92 132*127*124*119*
Radish
1 80* 227* 130* 181* 169*
2 93 158* 122* 151* 135*
3 82* 145* 111* 150* 128*
Arugula
1 96 311*115*154*228*
2 99 100 10 9 114 130*
3 99 124* 110 135* 134*
(Table 3). All plant species treated by CL, except for
radish, had a higher content of hydrogen peroxide and
MDA (Table 3). It was noted that such plants accumu-
lated more anthocyanins, flavonoids, and proline
(Table 3) and had a higher activity of antioxidant
enzymes: CAT, SOD, APO, and GPOD (Table 4).
CL-treated plants of all four species had lower nitrate
content than plants grown under 16-h photoperiod
(Fig. 1b).
Experiment 2. In experiments with the same DLI,
i.e., when the light intensity at the 24-h photoperiod
was lower than at the 16-h photoperiod, the above dif-
ferences between plants grown under different photo-
periods were maintained, although they were less pro-
nounced in some cases. Thus, the height of the plants
grown under different photoperiods practically did not
differ (Table 2, Fig. 2), which indicates that this indi-
cator i s more inf luen ced by DLI . At the same tim e, t he
LMA and shoot dry weight values were generally
higher in plants grown under CL (Table 2), which
resulted in a higher robustness index in all four species
(Fig. 1a). The Chl content was somewhat higher under
CL in broccoli and mizuna, but lower in radish and
arugula (Table 3). The content of Car under CL was
higher only in mizuna, while it did not differ signifi-
cantly in other species from plants grown under 16-h
photoperiod. In contrast to the first series of experi-
ments, plants under CL conditions at lower light
intensity did not show a significant increase in the
content of hydrogen peroxide and MDA. An increase
in the content of hydrogen peroxide by 37% was
recorded only in mizuna, and the content of MDA was
higher by 17% in arugula. At the same time, an
increase in the content of anthocyanins in all species,
except mizuna, flavonoids in mizuna and radish, and
proline in all four plant species was recorded (Table 3).
An increase in the activity of antioxidant enzymes
under CL was also recorded, although it was less pro-
nounced than in the first series of experiments with
different DLIs (Table 4). The decrease in the content
of nitrates under CL was comparable to that in the first
experiment, only slightly less pronounced in arugula
(Fig. 1b).
Experiment 3. When plants were exposed to CL
only during the last 3 days of the experiment, a signif-
icant decrease in the length of the hypocotyl occurred
only in radish (Table 2); however, plants of all four
species had higher shoot biomass, higher LMA values,
and higher first true leaf length. As a result, the robust-
ness index in all species was higher than when plants
were grown under 16-h photoperiod. The robustness
index was lower than in experiments 1 and 2 only in
broccoli; in other plants, it was comparable to the pre-
vious variants of using the 24-h photoperiod (Fig. 1a).
The decrease in the content of Chl under CL was sim-
ilar to experiment 1 (Table 3). The content of Car was
RUSSIAN JOURNAL OF PLANT PHYSIOLOGY Vol. 70:118 2023
CONTINUOUS LIGHTING INCREASES YIELD AND NUTRITIONAL VALUE Page 5 of 11 118
Fig. 1. (a) Robustness index and (b) nitrate content in plants grown undedr photoperiods of (1) 16 h (control) and (2) 24 h with
different (series of experiments 1) and (3) the same (series of experiments 2) DLI and (4) when exposed to CL at the end of the
production period (series of experiments 3). Different L atin letters for each species indicate significant differences in mean values
at P < 0.05.
3000
2500
2000
1500
1000
500
0Broccoli Mizuna Radish Arugula
(b)
A
A
a
a
cc
c
b
b
d
СС
СС
B
B
Nitrate content, mg/kg
9
8
6
4
7
5
3
2
1
0Broccoli Mizuna Radish Arugula
(a)
AA
AAA
aaaaaa
bb
С
B
B
Robustness index, mg/cm
1
23
4
lower in broccoli and mizuna compared to plants
grown under 16-h photoperiod. A significant increase
in the content of hydrogen peroxide and MDA was
observed only in broccoli, while only an increase trend
can be noted in other species (Table 3). At the same
time, the content of anthocyanins, flavonoids, and
proline and the activity of antioxidant enzymes were
higher in most cases in plants exposed to CL at the end
of the production period (Tables 3, 4). In plants treated
by CL for 3 days, the content of nitrates decreased to a
lesser extent than in experiments 1 and 2, but, never-
theless, it ranged from 13 to 24% for different plant
species (Fig. 1b).
DISCUSSION
Growth and productivity. In all three series of exper-
iments in all studied species, fresh and dry plant bio-
mass and LMA values were higher under CL com-
pared to plants grown under 16-h photoperiod. In the
first experiment (with different DLI), such plant
response was expected since CL provided a higher
DLI, i.e., additional light for photosynthesis and,
hence, biomass accumulation. It is known that the dry
biomass of plants increases with an increase in DLI up
to the point of light saturation [17]. The higher pro-
ductivity of plants grown under CL supports the
118 Page 6 of 11
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SHIBAEVA et al.
Table 3. Physiological and biochemical parameters of plant leaves (% of control)
Plants grown under 16-h photoperiod with light intensity of 270 μmol m–2 s–1 PAR. * Statistically significant differences with control.
Absolute values of control samples: Chl (a+b) (mg/g dry weight): 9.0 ± 0.3 mm (broccoli), 5.4 ± 0.3 (mizuna), 6.5 ± 0.2 (radish),
12.3 ± 0.7 (arugula); carotenoids (mg/g dry weight): 1.2 ± 0.1 (broccoli), 0.4 ± 0.1 (mizuna), 0.8 ± 0.1 (radish), 0.7 ± 0.1 (arugula);
anthocyanins (c.u.): 1.3 ± 0.5 (broccoli), 0.30 ± 0.04 (mizuna), 0.40 ± 0.04 (radish), 1.7 ± 0.2 (arugula); flavonoids (c.u.): 23.0 ± 1.2
(broccoli), 18.9 ± 1.4 (mizuna), 23.9 ± 2.2 (radish), 30.7 ± 2.1 (arugula); proline (μmol/g wet weight): 26.5 ± 2.8 (broccoli), 11.5 ± 0. 3
(mizuna), 7.6 ± 1.3 (radish), 27.4 ± 2.5 (arugula); H2O2 (μmol/g wet weight): 0.71 ± 0.05 (broccoli), 0.63 ± 0.04 (mizuna), 0.40 ± 0.05
(radish), 0.54 ± 0.04 (arugula); MDA (μmol/g wet weight): 18.4 ± 1.3 (broccoli), 11.9 ± 0.5 (mizuna), 18.3 ± 1.2 (radish), 20.1 ± 0.7
(arugula).
Series of
experiments Chl (a + b)Caroti-
noids
Anto-
cyanins
Flavo-
noids Proline H2O2MDA
Broccoli
1 90 108 199* 136* 132* 131* 329*
2 117* 95 135* 109 108* 100 110
3 81* 67* 110 155* 155* 126* 188*
Mizuna
1 81* 50* 152* 125* 112 125* 130*
2 116* 114* 97 116* 134* 137* 109
3 92 81* 100 120* 123* 100 111
Radish
1 85* 125* 159* 136* 171* 117 98
2 80* 92 170* 119* 125* 89 101
3 76* 97 124* 125* 113 91 107
Arugula
1 78* 129* 348* 136* 561* 154* 276*
2 79* 94 166* 103 152* 100 117*
3 90 100 137* 155* 141* 106 107
Table 4. Activity of antioxidant enzymes in plant leaves (% of control)
Plants grown under 16-h photoperiod with light intensity of 270 μmol m–2 s–1 PAR. * Statistically significant differences with control.
Absolute values of control samples: CAT (μmol H2O2/(mg protein min)): 32.3 ± 2.6 (broccoli), 38.3 ± 5.5 (mizuna), 10.4 ± 2.8 (rad-
ish), 39.9 ± 3.2 (arugula); SOD (U/mg protein): 6.6 ± 1.2 (broccoli), 13.0 ± 1.0 (mizuna), 15.3 ± 1.4 (radish), 5.0 ± 1.0 (arugula); APO
(μmol/(mg protein min)): 79.4 ± 13.3 (broccoli), 96.4 ± 9.7 (mizuna), 143.70 ± 2.9 (radish), 113.1 ± 6.2 (arugula); GPOD (μmol/(mg
protein min)): 193.3 ± 31.4 (broccoli), 180.4 ± 20.3 (mizuna), 141.4 ± 14.8 (radish), 19.1 ± 2.7 (arugula).
Series of experiments Activity of antioxidant enzymes
CAT SOD APO GPOD
Broccoli
1 113 164* 165* 117*
2 86 136* 167* 112
3 164* 147* 118 100
Mizuna
1 128* 131* 118* 131*
288120*9089
3123*261*130*122*
Radish
1 289* 122* 132* 123*
2141*114114113*
3 148* 112 125* 117*
Arugula
1 118* 180* 132* 136*
2 103 115 117 108
3 120* 135* 125* 178*
RUSSIAN JOURNAL OF PLANT PHYSIOLOGY Vol. 70:118 2023
CONTINUOUS LIGHTING INCREASES YIELD AND NUTRITIONAL VALUE Page 7 of 11 118
hypothesis [11] that CL can lead to an increase in plant
productivity if this does not cause photodamage to the
leaves. In our study, all four plant species did not show
symptoms of leaf injury. Regarding the sensitivity of
the studied species to CL, there is only information in
the literature that arugula grown under CL for 30 days
did not have any damage [4]. The robustness index
serves as an integral morphophysiological indicator
that combines the volume of the hypocotyl (length and
diameter) with the dry biomass of the shoot, which is
used as an indicator of the quality of microgreens [1].
CL increased the robustness index in all four species,
mainly due to the increase in biomass. In addition, CL
significantly accelerated the formation of the first true
leaf. Since microgreens are usually grown to the first
true leaf stage, the use of CL can serve as a way to
reduce harvest time, i.e., production period. Thus, in
the first experiment, CL, providing a higher DLI,
increased the yield and accelerated the development of
all four species of microgreens.
In the second series of experiments (with the same
DLI), the light intensity in the variant with CL was
lower; however, the plant robustness index did not dif-
fer from that of plants in the first series of experiments
with a higher DLI at CL. This result is in good agree-
ment with the known data that, at the same DLI,
plants accumulate a greater biomass at longer photo-
periods [18–23]. The efficiency of the use of light
energy in the process of photosynthesis invariably
decreases with increasing light intensity and, therefore,
the yield of photosynthesis products per unit of PAR
will always be higher at lower illumination [19, 21]. The
decrease in the efficiency of using light energy at high
light intensity is partly due to the activation of photo-
protective processes, which convert part of the
absorbed light energy into heat, preventing it from
being used in light photosynthesis reactions. This pre-
vents light-induced damage to the photosynthetic
apparatus by initiating complementary photoprotec-
tive processes, including the xanthophyll cycle and
Fig. 2. Appearance of (a) broccoli plants (Brassica oleracea var. italica), (b) mizuna (Brassica rapa ssp nipposinica), (c) arugula
(Eruca sativa), and (d) radish (Raphanus sativus ssp radical) grown under photoperiods of 16 and 24 h with the same DLI (series
of experiments 2).
1 cm
(c) (d)
(a) (b)
16 h
24 h
16 h
24 h
16 h
24 h
16 h
24 h
118 Page 8 of 11
RUSSIAN JOURNAL OF PLANT PHYSIOLOGY Vol. 70:118 2023
SHIBAEVA et al.
molecular reorganization of antenna complexes and
reaction centers. The observed increase in biomass in
response to a longer photoperiod at the same DLI is
probably due to morphological changes and physio-
logical responses of plants. The “stretching” of DLI
over time using a lower PAR intensity has advantages
for growers, as it allows, firstly, to reduce initial capital
costs due to a decrease in the number of light sources
(lamps) to provide the required DLI. Secondly, by
increasing the efficiency of the use of light energy, the
yield of products per unit of funds spent increases.
Thirdly, the use of lower light intensity for a long time
reduces the amount of heat generated by lighting
sources, reducing the cost for air conditioning (cool-
ing, moisture removal) of the room [22]. Finally, the
use of long photoperiods allows one to take advantage
of lower night electricity rates.
In the third series of experiments, when using CL
in the last 3 days before harvesting, the robustness
index was slightly lower than in the first two series of
experiments, which is understandable, but it neverthe-
less significantly exceeded that of plants grown under
16-h photoperiod.
In principle, an increase in plant productivity
based on an increase in DLI is possible either by
increasing the light intensity or by lengthening the
photoperiod. In this work, an increase in productivity
was observed both in the first series of experiments
with an increase in the DLI, and in the second, when
the DLI was the same, which significantly increases
the efficiency of the use of resources, in this case, elec-
tricity. It should be noted that when growing amaranth
and green and purple basil microgreens under CL, the
efficiency of using electricity for lighting was 10–42%
higher than under 16-h photoperiod with the same
DLI, and the cost of electricity for the production of a
unit of production, on the contrary, decreased by 8–
38% [10].
Photosynthetic pigments. It is known that the effect
of light on the content of photosynthetic pigments var-
ies to a large extent, showing a positive or negative
response depending on the photoperiod [4, 12]. In our
work, the content of Chl in some species decreased in
response to CL but by no more than 20% and was not
visually noticeable. This is important, since the con-
tent of Chl is closely related to the perception by a per-
son (as a potential consumer) of the green pigmenta-
tion of leaves, which is one of the criteria for the qual-
ity of microgreens [1]. In adult arugula plants, a
decrease in the content of Chl was also noted under
CL [4]. At the same time, other authors noted an
increase in the content of Chl under CL in basil Oci-
mum basilicum, arugula, chicory Cichorium endivia
[24], as well as amaranth microgreens Amaranthus
tricolor and green and purple basil, compared to a
16-h photoperiod [10]. In kale Brassica oleracea var.
viridis microgreens, Chl content under CL increased
at DLI equal to 14 mol/(m2 day) but decreased at DLI
21 mol/(m2 days). A decrease in the total content of
Chl in leaves leads to a decrease in light absorption per
unit area of the leaf and serves as one of the mecha-
nisms of protection against excessive illumination.
Therefore, in the second series of experiments, where
the “excess” of light was created only by the duration
of the photoperiod but not by the number of photons
of light received by plants, the response of broccoli and
mizuna was different and the amount of Chl even
slightly increased. Previously [13], we noted an
increase in the ratio of Chl a and b (which suggests the
formation of LHC in PSII of a smaller size), as well as
the ratio of Car and Chl, indicating a relatively higher
concentration of Car in the pool of photosynthetic
pigments, which is associated with their protective
function in relation to excessive light since they are
effective physical and chemical quenchers of singlet
oxygen and other free radicals [25]. Thus, in CL-
treated plants, the Chl content was either comparable
or slightly higher than in plants grown under 16-h
photoperiod or the decrease was not significant
enough to affect the visual assessment of microgreen
quality. It should be noted that the reason for a slight
decrease in the content of photosynthetic pigments
may also be the so-called “dilution” effect since their
content was estimated in shoots, while the proportion
of stem biomass, in which the content of these pig-
ments is low, increases under CL conditions. At the
same time, the increased content of Car in arugula and
radish increases the nutritional value of microgreens
since Car has antioxidant properties that are beneficial
to human health.
Oxidative stress and antioxidants. The increased
content of MDA and hydrogen peroxide in plants
grown under CL indicates that the plants experienced
mild oxidative stress. In response to stress in plants, as
a rule, the formation of antioxidant bioactive sub-
stances is enhanced. Antioxidants in food are import-
ant compounds for human health since they play an
important role in neutralizing free radicals in the body.
Therefore, an increase in the content of substances
with antioxidant activity in microgreens increases its
nutritional value and makes the product more com-
petitive due to its high quality. In this work, in all three
series of experiments in all species of microgreens,
with some exceptions for certain indicators, an
increase in the content of proline, anthocyanins, and
flavonoids, as well as an increase in the activity of anti-
oxidant enzymes (CAT, SOD, APO, and GPOD), was
recorded. The fact that a higher DLI can increase not
only the yield of leafy vegetables but also the content of
substances with antioxidant activity in them has also
been shown in a number of studies, including those
using CL [9, 13, 17, 26, 27]. However, it was not clear
whether this would be the case if CL did not provide
the plants with additional light energy, i.e., using the
same DLI. In addition, the question of whether pho-
tooxidative stress, caused by CL, is a direct result of
continuous light input or is caused by excess light due
RUSSIAN JOURNAL OF PLANT PHYSIOLOGY Vol. 70:118 2023
CONTINUOUS LIGHTING INCREASES YIELD AND NUTRITIONAL VALUE Page 9 of 11 118
to a higher DLI. It should be noted that there is evi-
dence that the continuity of the light signal, photosyn-
thetic and photooxidative processes can cause oxida-
tive stress in plants under CL, even if the DLI values
are not higher than those usually required by plants
under shorter photoperiods [23]. In our study, despite
the fact that oxidative stress was not observed in most
cases (judging by the absence of an increase in the
content of MDA or hydrogen peroxide) in the second
series of experiments with the same DLI, nevertheless,
the content of low molecular weight antioxidants and
the activity of antioxidant enzymes increased. Previ-
ously, similar data on a higher activity of CAT, SOD,
and APO under CL compared to a 16-h photoperiod
at the same DLI were obtained on tomato plants [28].
As regards the third series of experiments, the
obtained results are of particular interest. According to
the content of antioxidants or the activity of antioxi-
dant enzymes, the data of the first and third series of
experiments are in some cases comparable or the
effects in the third series are somewhat weaker,
although the duration of the action of CL on plants
was 9 days in the first series of experiments and only
3 days in the third. In our work, we did not study the
dynamics of the content of anthocyanins, flavonoids,
and proline under CL, but there are works showing
that, in the initial period of CL action, plants experi-
ence stress, in response to which various protective
substances are synthesized and the activity of antioxi-
dant enzymes is increased, and subsequently adapta-
tion to CL may occur and, accordingly, the composi-
tion of substances in the plant changes. Thus, it is
shown in mung bean plants Vigna radiata L. that such
adaptation occurred after 6 days of the effect of CL,
when there was a decrease in the activity of CAT,
SOD, GPOD, and the content of proline against the
background of a decrease in the concentration of
MDA [29]. Maximum nitrate reduction in pak choi
Brassica campestris L. leaves under CL occurred
already after 24 h, and then some increase was
observed [30]. These results suggest that a short appli-
cation of CL just before harvest to improve plant
nutritional value and biosecurity may be even more
effective than applying CL throughout the growing
period. In addition, shorter application times for CL
reduce energy costs and increase the efficiency of
microgreen production.
On the whole, our data and analysis of the litera-
ture [10, 31] show that the sensitivity of plants to cer-
tain light exposures is quite species-specific. This is
especially manifested in responses when plants react
to one or another effect with the active formation of
protective metabolites. Thus, for example, when
studying the effect of various combinations of orange,
red, and blue light on eight species, subspecies, and
varieties of microgreens of the genus Brassica (two
varieties of mizuna Brassica rapa nipposinica, pak choi
Brassica rapa chinensis, two varieties of radish Rapha-
nus sativus, and three varieties of mustard Brassica jun-
cea), the authors identified three different types of
plant response, assessed by the synthesis of phenolic
substances and changes in antioxidant activity [32].
Under the action of CL on amaranth and kale micro-
greens, the content of phenolic compounds and
anthocyanins in them, as well as antioxidant activity,
increased, but the same conditions did not signifi-
cantly affect the biochemical composition of green
and purple basil [10].
The content of nitrates. Controlling the content of
nitrates in microgreens is an important component of
obtaining high-quality, safe products. Our results
show that the nitrate content was significantly reduced
when plants were grown under CL (series of experi-
ments 1 and 2), as well as when CL was used at the end
of the production period (series of experiments 3).
This is consistent with the data on a 17% decrease in
nitrate content in arugula plants grown under CL
compared to a 12-h photoperiod [4]. A significant (up
to 56%) reduction in nitrate content was also noted in
lettuce plants that were exposed to CL for 2−3 days
prior to harvest [3, 27]. In general, it is known that
light conditions, namely, light intensity, spectral com-
position of light, and photoperiod, are important fac-
tors influencing the content of nitrates in plants [4, 33,
34]. A rather pronounced species specificity was also
noted in plant responses to light in terms of the con-
tent of nitrates in plants [8, 34].
It is known that the content of nitrates in plants is
negatively correlated with the concentration of solu-
ble, nonstructural forms of carbon, such as sugars and
organic acids [35], because they play complementary
roles in maintaining cell turgor [36]. Therefore, a sig-
nificant decrease in the content of nitrates under CL
may be associated with an increase in the synthesis of
carbohydrates and an increase in the content of ferre-
doxin and NADPH, which are used in the reduction of
nitrates in leaves [11, 37]. However, with an increase in
the duration of CL, the synthesis of the enzyme nitrate
reductase (NR) and NADPH de novo may decrease
[38]. A decrease in the content of nitrates under CL can
also be caused by a higher NR activity as a result of an
increase in the level of NR gene expression [4, 39].
Nitrate reductase is the key enzyme that limits the rate
of reduction of the nitrate ion to the nitrite ion. Light
affects NR activity in plants by regulating NR gene
transcription, translation, and posttranslational activity
[40]. It is assumed that light can regulate NR activity in
two ways, namely, by regulating the expression of NR
genes by photosynthesis products and by regulating the
state of NR through NADPH [34].
In general, the results of our studies show that
broccoli, mizuna, radish, and arugula plants are resis-
tant to CL in the early growth phases and do not show
typical signs of leaf photodamage. Moreover, by using
CL without increasing energy costs (while maintain-
ing DLI), it is possible to increase the yield and nutri-
tional value of broccoli, mizuna, radish, and arugula
118 Page 10 of 11
RUSSIAN JOURNAL OF PLANT PHYSIOLOGY Vol. 70:118 2023
SHIBAEVA et al.
microgreens and reduce their nitrate content com-
pared to a standard 16-h photoperiod. Obviously,
experiencing moderate oxidative stress under CL,
plants accumulate more low molecular weight antiox-
idants (anthocyanins, flavonoids, carotenoids, pro-
line) and are characterized by increased activity of
antioxidant enzymes. This increases the nutritional
value of microgreens, which is recommended as a
functional food for a healthy diet. Increasing nutri-
tional value and reducing nitrate content is also possi-
ble by applying CL (as an agricultural practice) for sev-
eral days at the end of the production period (just
before harvest) while saving energy used for growing
microgreens.
FUNDING
The study was carried out with the financial support of
the Russian Foundation for Basic Research within the
framework of scientific project no. 20-016-00033a. The
work was done using scientific equipment of the Center for
Collective Use of the Karelian Research Center, Russian
Academy of Sciences, within the framework of state order
of the Karelian Research Center, Russian Academy of Sci-
ences (FMEN-2022-0004).
COMPLIANCE WITH ETHICAL STANDARDS
This article does not contain any studies involving
humans and animals as research subjects.
CONFLICT OF INTEREST
The authors declare that they have no conflicts of interest.
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