Continuous lighting can improve yield and reduce energy costs while increasing or maintaining nutritional contents of microgreens

Abstract

Microgreens represent a fast growing segment of the edible greens industry. They are prized for their colour, texture, and flavour. Compared to their mature counterparts, microgreens have much higher antioxidant and nutrient content categorizing them as a functional food. However, current production practices in plant factories with artificial light are energy intensive. Specifically, the lack of sunlight within the indoor structure means all of the light must be provided via energy consuming light fixtures, which is energy intensive and costly. Plant growth is usually increased with the total amount of light provided to the plants - daily light integral (DLI). Long photoperiods of low intensity lighting (greater than 18h) providing the desired/target DLI can reduce the capital costs for light fixtures and electricity costs. This is achieved by moving the electricity use from peak daytime hours (high price) to off-peak hours (low price) during the night in regions with time-based pricing scheme and lowering the electricity use for air conditioning, if plant growth is not compromised. However, lighting with photoperiods longer than tolerance thresholds (species/cultivar specific) usually leads to plant stress/damage. Therefore, we investigated the effects of continuous 24h white light (CL) at two DLIs (~14 and 21 mol m⁻² d⁻¹) on plant growth, yield, and antioxidant content on 4 types of microgreens - amaranth, collard greens, green basil, and purple basil to see if it compromises microgreen production. It was found that amaranth and green basil had larger fresh biomass when grown under CL compared to 16h when the DLIs were the same. In addition, purple basil had higher biomass at higher DLI, but was unaffected by photoperiods. Plants grown under the CL treatments had higher energy-use-efficiencies for lighting (10-42%) than plants grown under the 16h photoperiods at the same DLI. Notably, the electricity cost per unit of fresh biomass ($ g⁻¹) was reduced (8-38%) in all microgreens studied when plants were grown under CL lighting at the same DLIs. Amaranth and collard greens also had higher antioxidant content. Taken together, growing microgreens under CL can reduce electricity costs and increase yield while maintaining or improving nutritional content.

Full text

Continuous lighting can
improve yield and reduce
energy costs while increasing
or maintaining nutritional
contents of microgreens
Jason Lanoue, Sarah St. Louis, Celeste Little
and Xiuming Hao*
Harrow Research and Development Centre, Agriculture & Agri-Food Canada, Harrow, ON, Canada
Microgreens represent a fast growing segment of the edible greens industry.
They are prized for their colour, texture, and flavour. Compared to their mature
counterparts, microgreens have much higher antioxidant and nutrient content
categorizing them as a functional food. However, current production practices
in plant factories with artificial light are energy intensive. Specifically, the lack of
sunlight within the indoor structure means all of the light must be provided via
energy consuming light fixtures, which is energy intensive and costly. Plant
growth is usually increased with the total amount of light provided to the
plants - daily light integral (DLI). Long photoperiods of low intensity lighting
(greater than 18h) providing the desired/target DLI can reduce the capital costs
for light fixtures and electricity costs. This is achieved by moving the electricity
use from peak daytime hours (high price) to off-peak hours (low price) during
the night in regions with time-based pricing scheme and lowering the
electricity use for air conditioning, if plant growth is not compromised.
However, lighting with photoperiods longer than tolerance thresholds
(species/cultivar specific) usually leads to plant stress/damage. Therefore, we
investigated the effects of continuous 24h white light (CL) at two DLIs (~14 and
21 mol m
-2
d
-1
) on plant growth, yield, and antioxidant content on 4 types of
microgreens - amaranth, collard greens, green basil, and purple basil to see if it
compromises microgreen production. It was found that amaranth and green
basil had larger fresh biomass when grown under CL compared to 16h when
the DLIs were the same. In addition, purple basil had higher biomass at higher
DLI, but was unaffected by photoperiods. Plants grown under the CL
treatments had higher energy-use-efficiencies for lighting (10-42%) than
plants grown under the 16h photoperiods at the same DLI. Notably, the
electricity cost per unit of fresh biomass ($ g
-1
) was reduced (8-38%) in all
microgreens studied when plants were grown under CL lighting at the same
Frontiers in Plant Science frontiersin.org01
OPEN ACCESS
EDITED BY
Yuxin Tong,
Institute of Environment and
Sustainable Development in
Agriculture (CAAS), China
REVIEWED BY
Jung Eek Son,
Seoul National University, South Korea
Viktorija Vastakaite-Kairiene,
Lithuanian Research Centre for
Agriculture and Forestry, Lithuania
*CORRESPONDENCE
Xiuming Hao
Xiuming.Hao@agr.gc.ca
SPECIALTY SECTION
This article was submitted to
Technical Advances in Plant Science,
a section of the journal
Frontiers in Plant Science
RECEIVED 30 June 2022
ACCEPTED 12 September 2022
PUBLISHED 30 September 2022
CITATION
Lanoue J, St. Louis S, Little C and
Hao X (2022) Continuous lighting can
improve yield and reduce energy costs
while increasing or maintaining
nutritional contents of microgreens.
Front. Plant Sci. 13:983222.
doi: 10.3389/fpls.2022.983222
COPYRIGHT
© 2022 Lanoue, St. Louis, Little and
Hao. This is an open-access article
distributed under the terms of the
Creative Commons Attribution License
(CC BY). The use, distribution or
reproduction in other forums is
permitted, provided the original
author(s) and the copyright owner(s)
are credited and that the original
publication in this journal is cited, in
accordance with accepted academic
practice. No use, distribution or
reproduction is permitted which does
not comply with these terms.
TYPE Original Research
PUBLISHED 30 September 2022
DOI 10.3389/fpls.2022.983222

DLIs. Amaranth and collard greens also had higher antioxidant content. Taken
together, growing microgreens under CL can reduce electricity costs and
increase yield while maintaining or improving nutritional content.
KEYWORDS
continuous lighting, microgreens, antioxidant, phenolics, anthocyanin, energy
efficiency, plant factory with artificial light, indoor vertical farming
1 Introduction
Microgreens are a fast growing specialty crop within the
edible greens industry (Kyriacou et al., 2016). They are prized for
their colour, texture, and flavour. Microgreens are typically
harvested within 3 weeks of sowing and can be harvested
before or at the first true leaf stage depending on desired use.
Although smaller in size, microgreens have higher nutritional
content than their mature counterparts, and thus are considered
a functional food (Xiao et al., 2012;Choe et al., 2018;Kyriacou
et al., 2019).
Due to their small size and compact growing strategy,
microgreens are typically grown in indoor vertical farms or
plant factories with artificial light to maximize yield per unit of
land area (Graamans et al., 2018). The terms indoor vertical farm
and plant factory are typically used synonymously but usage
varies based on geographical location. Generally, indoor vertical
farm is used in North America whereas plant factories with
artificial light (PFAL) is used in Europe and Asia. Both refer to
the use of multi-layer growing platforms (i.e., vertical farming)
inside warehouses or insulated shipping containers for
production with artificial light as the sole light source (Kozai
and Niu, 2016). Throughout this manuscript we will use the
term plant factory. While this type of growing system can have
very high yield per unit of land area, it is energy intensive. All the
light required for plant growth and photosynthesis needs to
come from artificial lighting with the use of electricity (Shibaeva
et al., 2022b). Even if the adoption of the energy-efficient light-
emitting diode (LED) fixtures can reduce the electricity use (van
Delden et al., 2021), the electricity used by LED lighting still
represents upwards of 20% of operating costs in plant factories;
second only to labour (Kozai and Niu, 2019). Furthermore, this
type of growing system is also capital intensive. It not only uses
expensive LED lighting systems (even though their price has
come down) but also uses heating, ventilation, and air
conditioning (HVAC) to control temperature and humidity.
The majority of input electricity to the lighting system will
eventually becoming heat since plants typically only convert 1-
5% of the incoming radiation to biomass (Zhu et al., 2008;Kozai
and Niu, 2019). The excessive heat load and the high humidity
from plant transpiration requires its removal and
dehumidification via air conditioning systems to maintain an
optimal growing environment for the plants, which, in addition
to lighting, increases electricity costs (Goto, 2012;Kozai and Niu,
2016). These high input costs are in part why only 50% of plant
factories in Japan were profitable in 2018 (Kozai and Niu, 2019),
and why plant factories struggle to be used as a mainstream
producer of edible greens elsewhere in the world (Kozai, 2018).
Based on the poll conducted by Indoor AgTech Innovation
Virtual Summit 2021 (https://indooragcenter.org/indoor-
agtech-virtual-summit-2021/), high energy cost is the main
limiting factor for profitable production with plant factories.
Therefore, innovation in lighting systems and strategies is the
key to reduce energy costs and improve energy efficiency.
Plant growth and yield are usually determined by the
amount of light intercepted by the plant during a day - daily
light integral (DLI; photosynthetic photon flux density (PPFD) x
photoperiod duration). Both the increase in PPFD (Samuoliene
et al., 2013) and an extension in photoperiod (Demers and
Gosselin, 2002) can increase DLIs and have been shown to
increase biomass production up to a saturation point. With
respect to PPFD, beyond a certain species-specific limit, no
further increase in biomass is observed and further increases
to the PPFD can be detrimental to the plant (Demmig-Adams
and Adams, 1992;Szymanska et al., 2017) since as PPFD
increases, the quantum yield (i.e., the increase in CO
2
fixed
per additional photon) decreases (Lanoue et al., 2017).
Furthermore, the use of high PPFD can increase the
transpiration rate of plants, exacerbating the aforementioned
humidity issue (Goto, 2012;Lanoue et al., 2018).
Similar to an increase in PPFD, photoperiod extension can
be used to increase DLI and biomass (Demers and Gosselin,
2002).Theultimategoalinphotoperiodextensionis24h
lighting/continuous lighting (CL). It is more economical to use
long photoperiods (18h up to 24h) of low PPFD (<200 μmol m
-2
s
-1
) to achieve the target/desired DLIs because it reduces the
capital cost of light fixtures (Hao et al., 2018). The longer the
photoperiod, the lower the PPFD that can be used to reach the
desired DLI. It should be noted that the DLI requirements vary
between plant species and cultivars. Therefore the definition of a
long photoperiod, low PPFD lighting strategy will be
species-specific.
Lanoue et al. 10.3389/fpls.2022.983222
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In many regions of the world which employ time-of-use
pricing (TOUP) such as Ontario, Canada, some states in the
USA, 17 European countries including France, Sweden,
Germany, Finland, as well as South Korea, the price of
electricity is much higher in the peak hours during daytime
(when demand is highest) compared to the price in the off-peak
hours during the night (IRENA, 2019;IESO, 2022). It is
important to note that the form in which TOUP is utilized in
each country may be different (i.e., static time-of-use pricing,
real-time pricing, variable peak pricing, or critical peak pricing),
but regardless of strategy, off peak pricing is always cheaper than
on peak. Therefore, long photoperiod, low PPFD lighting such as
CL can also reduce electricity costs by moving part of electricity
use from daytime to nighttime when prices are at their lowest in
these regions (Hao et al., 2018;IESO, 2022). At lower PPFD,
both the heat load from lighting and plant transpiration
decrease, reducing the usage of electricity by the air
conditioning system to remove heat and moisture to maintain
optimal growing environment for plants (Kozai and Niu, 2016).
The use of long photoperiod lighting such as 24h CL means
constant photon energy is provided to the plant allowing for 24h
CO
2
fixation and growth. In this way, it has been hypothesized
that the use of CL can increase plant production (Sysoeva et al.,
2010;Velez-Ramirez et al., 2011;Velez-Ramirez et al., 2012;
Shibaeva et al., 2022a). However, exceeding the tolerable
photoperiod limits, which are species-specific, can lead to
diminished yield, photoperiod-related leaf injury, and an
economic disadvantage for growers (Demers and Gosselin,
2002;Hao et al., 2018). Some plant species such as tomato and
pepper have reduced yield and leaf injury characterized by
chlorosis when grown under CL (Murage and Masuda, 1997;
Velez-Ramirez et al., 2017). It is hypothesized that CL-injury is
due to a mismatch between environmental cues and endogenous
circadian rhythms (Velez-Ramirez et al., 2017;Marie et al.,
2022). Specifically, since the plant is under constant light, it
seems that components of the light harvesting complex are
negatively affected causing reduced transcription leading to
inadequate use and/or dissipation of light (Velez-Ramirez
et al., 2014). More recent research in these two crops have
provided evidence that dynamic CL, which involves a change in
light spectrum between daytime and nighttime, can result in
injury-free production (Lanoue et al., 2019;Lanoue et al., 2022).
Some cultivars of lettuces and some members of the Brassicaceae
microgreen family have also been shown to have positive
interactions with CL (Ohtake et al., 2018;Shibaeva et al.,
2022b). Since the production period of microgreens is short,
and CL-injury in tomatoes and peppers tends to take more than
a month to have noticeable reductions in yield (Lanoue et al.,
2021a), we hypothesize that CL may not compromise the
production of microgreens and could be a viable
production strategy.
However, CL can increase harmful reactive oxygen species
(ROS) due to the stress from constant light exposure to the plant
(Haque et al., 2015;Huang et al., 2019). Subsequently, the
concentration of ROS scavenging molecules such as
antioxidants can also be increased during CL (Haque et al.,
2015). The extent of injury is largely linked to the interplay
between ROS production and scavenging ability. If this
homeostatic balance becomes too heavily skewed by elevated
ROS production, then leaf injury will occur, resulting in
detrimental plant growth and yield. However, if one can
balance the oxidative pressure with the antioxidant synthesis,
injury-free production is feasible. Therefore, the prospect of
injury-free production under CL coupled with the hormetic
impact can increase antioxidants/health promoting
compounds in plants; which is an intriguing possibility for
microgreen production in plant factories.
As such, we studied the impact of PPFD and photoperiod
(including CL) on plant growth, yield and nutritional content of
4 types of microgreens in order to assess if long photoperiod
(CL) and low intensity lighting can be used to improve the
sustainability/energy efficiency in indoor production of
microgreens grown in plant factories.
2 Materials and methods
2.1 Plant materials and
lighting treatments
Four types of microgreens were used in the study. Two
hundred seeds each of amaranth (Amaranthus tricolor)cv.
‘Garnet Red’, collard greens (Brassica oleracea var. viridis) cv.
‘Vates’, as well as basil (Ocimum basilicum) cv. ‘Genovese’and
cv. ‘Red Rubin’(henceforth referred to as green and purple basil
respectively; Johnny’s Select Seeds, Fairfield, Maine, USA) were
sown into individual trays filled with Berger BM6 All-Purpose
potting soil (Berger, Saint-Modeste, Quebec, Canada). Once
sown, the trays were placed in a germination chamber at
Agriculture & Agri-Food Canada’s Harrow Research and
Development Centre with a constant temperature of 24°C and
a relative humidity of 90% in complete darkness. Amaranth and
collard greens remained in the chamber for 3 days while basil
was in the germination chamber for 5 days. Upon germination,
trays of each cultivar were placed into four different growth areas
(1.93 m
2
) within Conviron walk-in growth chambers (PGW40;
Conviron, Winnipeg, MB, Canada) each containing one of four
lighting treatments (Table 1). The growth chamber temperature
was maintained at 22°C (24 hour) while the relative humidity
was kept between 60-70%. Plants were irrigated as needed.
Harvest occurred 11 days after sowing for amaranth and
collard greens and 19 days after sowing for both basil cultivars.
The four light treatments consisted of 2 levels of DLIs (14
and 21 mol m
-2
day
-1
) and 2 photoperiods (16h and 24h;
Table 1). Throughout the manuscript, the lighting treatments
will be represented using the following notation: DLI/
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photoperiod. Lighting treatments were chosen based on similar
PPFD and photoperiods in other studies where between 200-300
μmol m
-2
s
-1
was the typical PPFD used with 16h photoperiods
(Samuoliene et al., 2013;Virsileet al., 2019;Pennisi et al., 2020;
Shibaeva et al., 2022b;Sutulieneet al., 2022;Vastakaite-Kairiene
et al., 2022). Using that as a baseline for the 14DLI/16h
treatment, the other treatments we determined by controlling
either DLI but extending the photoperiod or controlling the
PPFD and extending the photoperiod. All light treatments were
provided by Flexstar 645W dimmable LED fixtures (Flexstar,
California, USA) and were the same broad/white spectrum
(Figure 1). The growth chamber trials were replicated 3 times
in 2022.
2.2 Growth measurements
The plants were harvested by cutting them at the junction
where their base meets the growth media. Plant height was
measured on five random plant samples from each treatment of
each cultivar during each replicate (i.e., 15 total samples per
microgreen per treatment). Total fresh weight was obtained then
a subsample was flash frozen in liquid nitrogen and placed in a
-80°C freezer until analysis. Another subsample was weighed,
then placed in a 70°C oven for 1 week then re-weighed to obtain
the dry matter percentage of the sample. Energy-use-efficiency of
the lights (EUEL) only was calculated using the total fresh
biomass obtained and dividing it by the cumulative input of
energy into the lighting fixtures during the production period (g
FW MJ
-1
).
2.3 Photosynthetic pigment analysis
Frozen tissue was lyophilized then ground. One mL of 95%
ethanol was then added to the sample and the tube was placed in
a water bath at 50°C for 3 hours. The tube was centrifuged at
13000 rpm for 1 minute before the supernatant was removed
and placed in a clean tube. The process was repeated and both
aliquots were combined for a total extract volume of 2 mL.
FIGURE 1
Photon flux density (PFD) distribution of Flexstar 645W dimmable LED fixtures (Flexstar, California, USA) measured using a Li-180 spectrometer
(Li-COR Biosciences Inc. Lincoln, Nebraska, USA).
TABLE 1 Light treatments provided by the Flexstar 645W dimmable LED fixtures (Flexstar, California, USA) in Conviron walk-in growth chambers
(PGW40; Conviron, Winnipeg, MB, Canada) measured at the height of the top of the tray in 6 different locations within the chamber.
Treatment (DLI/photoperiod) PPFD (µmol m
-2
s
-1
) DLI (mol m
-2
d
-1
) Photoperiod (h)
14DLI/16h 250.8 ± 2.1 14.5 ± 0.1 16
14DLI/24h 166.6 ± 2.7 14.4 ± 0.2 24
21DLI/16h 376.8 ± 1.7 21.7 ± 0.1 16
21DLI/24h 247.6 ± 7.9 21.4 ± 0.7 24
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Samples were then analyzed at 664 nm, 649 nm, and 470 nm in a
UV/VIS spectrophotometer (UV-1600PC. VWR. Mississauga,
Ontario, Canada). Concentrations of chlorophyll a,b,and
carotenoids were determined using the equations from
Lichtenthaler (1987).
2.4 Antioxidant assays
2.4.1 DPPH (2,2-diphenyl-1-picrylhydrazyl)
assay
The antiradical activity in microgreen tissue was determined
based on a modified version of a previously reported method
(Alrifai et al., 2020). Tissue samples that were previously frozen
in liquid nitrogen and stored in a -80°C freezer were removed
and lyophilized. Lyophilized tissue was ground in a homogenizer
then 1 mL of 100% methanol was added to the microfuge tube.
The sample was then left on a nutator overnight at room
temperature. The next morning, the samples were centrifuged
at 13,000 rpm for 5 minutes. The supernatant was collected in a
clean tube before re-suspending the pellet in 1 mL of fresh 100%
methanol. The sample was placed on a nutator for 3 hours before
being centrifuged and having the supernatant removed. Both
supernatant fractions were mixed in a single tube and placed in
-20°C freezer until analysis. Fresh 2,2-diphenyl-1-picrylhydrazyl
(DDPH; 350 μM) was prepared immediately before analysis. In a
cuvette, 1 mL of DPPH was mixed with 125 μL of sample and
placed in the dark to incubate for 30 minutes before the
absorbance was measured at 517 nm. This procedure was
completed in duplicate. A standard curve was completed in
triplicate using the same assay technique with ascorbic acid used
in place of the tissue sample.
2.4.2 FRAP (ferric reducing antioxidant
power) assay
The ferric reducing antioxidant power (FRAP) assay of
microgreen tissue was determined using a modified version of
a previously reported method (Alrifai et al., 2020). Samples were
extracted using a method similar to the DPPH analysis. FRAP
reagent was made at the time of analysis and consisted of 300
mM acetate buffer (pH 3.6), 20 mM FeCl
3
, and 10 mM 2, 4, 6-
Tris (2-pyridyl)-s-triazine (TPTZ) in 40 mM HCl. 100 μL of
methanolic sample extract was mixed with 900 μL of FRAP
reagent and incubated at 37°C for 2h before reading the
absorbance at 593 nm. A standard curve was completed using
the same assay technique with ascorbic acid used in place of the
tissue sample.
2.4.3 Total phenolic content
Total phenolic content was determined using a modified
protocol from Ainsworth and Gillespie (2007). Briefly, 100 μL of
methanolic sample extract was combined with 200 μL of Folin-
Ciocalteu’s reagent (Thermo Fisher Scientific, MA, USA) and
800 μL of 700 mM sodium carbonate. The tubes were vortexed
for 30 seconds then allowed to stand at room temperature for 2h.
The absorbance was measured at 765 nm using a UV/VIS
spectrophotometer. Total phenolic content was expressed as
gallic acid equivalents.
2.4.4 Total anthocyanin
Determination of total anthocyanin content was done using
a slightly modified protocol from Lee et al. (2005). The assay
began by adding 100 μL of methanolic sample to both 1 mL of
potassium chloride (0.025 M; pH = 1.0) and 1 mL of sodium
acetate (0.4 M; pH = 4.5) separately. The mixtures were
incubated at room temperature for 30 minutes before the
absorbance was measured at both 520 nm and 700 nm.
Anthocyanin contents were then calculated using the following
equation:
Anthocyanin content =A*MW*DF*103

ϵ*l
Where Ais the absorbance (A=(A
520nm
-A
700nm
)
pH1.0
–
(A
520nm
-A
700nm
)
pH4.5
), MW is the molecular weight of
cyanidin-3-glucoside (449.2 gmol
-1
), DF was the dilution
factor, 10
3
is the factor to convert g to mg, ϵis the molar
extinction coefficient of cyanidin-3-glucoside (26900 L mol
-1
)
and lis the path length of 1 cm.
2.5 Electricity Cost Calculation
The electricity cost ($ g
-1
FW) from LED lighting only
during each production period of all microgreens was
calculated using the following equation:
Electicity cost =on
n=0 Lu
Lm

P
106

En

FW
Where nis the hour, Lu is the PPFD used, Lm is the
maximum PPFD of the fixture, Pis the input wattage of the
fixture (W), 10
6
is a conversion factor from W to MW, E
n
is the
electricity price at a given hour (n) as determined from IESO,
2022, and FW is the fresh weight (g) produced for a given
microgreen during a specific production period. Fresh weight
was a measure of total biomass produced by each microgreen
under each light treatment at the end of the growth period.
2.6 Statistics
For each microgreen, the experiment was replicated 3 times.
For each of the pigment analyses and antioxidant analyses, 2
subsamples were taken from each destructive harvest. All
statistics were performed using SAS studio 3.5. A two-way
Lanoue et al. 10.3389/fpls.2022.983222
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ANOVA was performed and a multiple means comparison was
done using a Tukey-Kramer adjustment with a p<0.05 indicating
a significant difference.
3 Results
3.1 Plant growth and yield
Amaranth plants were observed to be shortest in height
under the 21DLI/16h treatment which was a result of the high
PPFD used (Table 2). Both light treatments that ran for 24h
produced the tallest plants regardless of DLI (Table 2;Figure 2).
Fresh weight, a determination of yield for microgreens, was
highest under the 21DLI/24h light treatment andlowest under
the 14DLI/16h lighting treatment. Although the DLI was the
same, both lighting treatments which utilized a 24h photoperiod
produced more plant fresh biomass than did plants grown under
the 16h photoperiod (Table 2). The energy-use-efficiency of the
lights (EUEL) only is a measure of biomass accumulation
normalized for the input energy of the lighting fixture. For
amaranth, both 24h lighting treatments had the highest EUEL
indicating that the input energy produced a higher biomass than
the 16h treatment (Table 2). The percentage of dry matter was
also highest under the 21DLI/24h light treatment while both
light treatments with a low DLI had the lowest percentage of
dry matter.
Similar to amaranth, collard greens grown under 14DLI/24h
which utilized the lowest PPFD produced the tallest plants while
plants grown under the highest PPFD (21DLI/16h) were the
shortest (Table 3;Figure 3). Fresh weight was the lowest in
plants grown under the 14DLI/16h treatment and the highest
under the 21DLI/24h treatment (Table 3). Interestingly,
although the DLI was lower, plants grown under the 14DLI/
24h light treatment produced similar fresh weight to both
treatments with high DLIs of approximately 21 mol m
-2
d
-1
(Table 3). Collard green plants grown under the 14DLI/24h
treatment had the highest EUEL as more biomass was produced
with the least amount of input energy (Table 3). Plants grown
under both high DLI treatments had the lowest EUEL regardless
of photoperiods. Percentage of dry matter was the highest in
both 21DLI/16h and 21DLI/24h compared to treatments with
low DLIs of approximately 14 mol m
-2
d
-1
.
Green basil plants were tallest when grown under 21DLI/24h,
while those grown under 14DLI/16h were the shortest (Table 4;
Figure 4). The same trend was noticed in fresh weight production
where plants under 21DLI/24h produced the highest biomass
while those under 14DLI/16h produced the least (Table 4). In
green basil, both 24h lighting treatments had higher EUEL than
did their 16h counterparts which is both a factor of increased
biomass production and the lower input energy required by these
treatments (Table 4). Notably, plants grown under the 14DLI/24h
treatment produced the highest EUEL among all treatments.
While the fresh weight produced was not the highest, the input
energy required to produce the fresh weight was the lowest of all
treatments, resulting in the highest EUEL. However, while plant
height, fresh weight, and EUEL were impacted by the lighting
treatments, the percentage of dry matter was similar between all
treatments indicating no increase in water uptake under different
light treatments (Table 4).
Similar to green basil, purple basil plants were shortest
under 14DLI/16h and the tallest under 21DLI/24h (Table 5;
Figure 5). Total fresh weight was observed to be the highest
when plants were grown under the high DLI of approximately
21 mol m
-2
d
-1
and the lowest under the low DLI of
approximately 14 mol m
-2
d
-1
regardless of photoperiods.
Consistent with the green basil results, purple basil plants
grown under the 14DLI/24h light treatment had the highest
EUEL due to having the lowest input energy. Notably, both 24h
lighting treatments had higher EUEL than the 16h treatments
atthesameDLI(Table 5). Coinciding with the results from
green basil, the percentage of dry matter of purple basil was
similar regardless of treatments. Interestingly, although both
green and purple basils are the same species, both fresh weight
and percentage of dry matter were lower in purple basil
compared to green basil (Tables 4,5).
TABLE 2 Growth measurement summary of Amaranth cv. ‘Garnet Red’grown under various lighting DLI and photoperiods.
Daily Light Integral (mol m
-2
d
-1
) Photoperiod (h) Height (cm) Fresh Weight (g) EUEL (g FW MJ
-1
) % Dry Matter
Amaranth cv. ‘Garnet Red’
14 16 4.32 ± 0.16
AB
6.47 ± 0.29
C
0.26 ± 0.01
B
7.12 ± 0.12
C
24 4.65 ± 0.19
A
8.53 ± 0.58
B
0.35 ± 0.02
A
7.38 ± 0.25
BC
21 16 3.91 ± 0.32
B
8.90 ± 0.59
B
0.24 ± 0.02
B
7.92 ± 0.17
B
24 4.55 ± 0.09
A
12.47 ± 0.61
A
0.34 ± 0.02
A
8.76 ± 0.13
A
Daily Light Integral 0.0624 <0.0001 0.2441 0.0004
Photoperiod 0.0048 0.0001 0.0001 0.0111
Daily Light Integral*Photoperiod 0.2126 0.0628 0.5036 0.1062
Height was subsampled at five locations within the tray at the end of each of the three respective growth trials. Values presented are the means of three replicates, one from each trial ± the
standard error of the means. Different letter groups (A, B, C) represent statistical differences as determined by a two-way ANOVA within each parameter at p<0.05.
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FIGURE 2
Amaranth plants grown under various lighting DLI and photoperiods. Top photo is an overhead picture, while the bottom is a side profile. From
left to right, the lighting treatments are as follows (DLI/photoperiod): 14DLI/16h, 14DLI/24h, 21DLI/16h, and 21DLI/24h.
TABLE 3 Growth measurement summary of Collard greens cv. ‘Vates’grown under various lighting DLI and photoperiods.
Daily Light Integral (mol m
-2
d
-1
) Photoperiod (h) Height (cm) Fresh Weight (g) EUEL (g FW MJ
-1
) % Dry Matter
Collard Greens cv. ‘Vates’
14 16 3.44 ± 0.22
AB
20.40 ± 1.81
B
0.82 ± 0.07
B
9.87 ± 0.04
B
24 3.88 ± 0.06
A
23.37 ± 2.28
AB
0.95 ± 0.09
A
9.98 ± 0.47
B
21 16 3.09 ± 0.22
B
22.40 ± 2.29
AB
0.60 ± 0.06
C
11.95 ± 0.11
A
24 3.64 ± 0.22
AB
24.70 ± 2.39
A
0.67 ± 0.07
C
13.23 ± 0.24
A
Daily Light Integral 0.0888 0.0420 <0.0001 <0.0001
Photoperiod 0.0153 0.0066 0.0061 0.0425
Daily Light Integral*Photoperiod 0.7032 0.6248 0.2994 0.0757
Height was subsampled at five locations within the tray at the end of each of the three respective growth trials. Values presented are the means of three replicates, one from each trial± the
standard error of the means. Different letter groups (A, B, C) represent statistical differences as determined by a two-way ANOVA within each parameter at p<0.05.
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3.2 Photosynthetic pigments
Chlorophyll, as well as carotenoids, play a role in light
harvesting for photosynthesis. However, in microgreens, they also
provide vibrant green, yellow, and red colours which are sought
afterbychefs.Inamaranth,greenbasil,andpurplebasil,growth
under both 24h lighting treatments produced the highest
chlorophyll acontent (Figure 6A). In collard greens, chlorophyll
awas the highest in the 14DLI/24h treatment but observed to be the
lowest in the 21DLI/24h treatment. Chlorophyll bwas not affected
by light treatments in both amaranth and green basil (Figure 6B). In
collard greens, similar to chlorophyll a,chlorophyllbwas observed
to be the highest in the 14DLI/24h treatments and the lowest in the
21DLI/24h treatment. In addition, both high DLI treatments were
observed to have lower chlorophyll bcontent than the 14DLI/24h
light treatments. In purple basil, both 24h light treatments had
higher chlorophyll bcontent than did the 16h treatments
(Figure 6B). The chlorophyll a:bwas observed to be similar
between light treatments in amaranth and both basil microgreens
(Figure 6C). However, in collard greens, the chlorophyll a:bwas the
lowest in the 14DLI/16h treatment and the highest in both high DLI
treatments. Carotenoids were the highest in both 24h lighting
treatments in amaranth but the other three microgreens were
unaffected by light treatments (Figure 6D).
3.3 Antioxidants
Microgreens are prized for their antioxidant and nutrient
densities in comparison to their mature counterparts. Here we
see that the antioxidant activity as measured by 2,2-diphenyl-1-
picrylhydrazyl (DPPH) assay was increased in both 24h lighting
treatments compared to their 16h counterparts at the same DLI
in green basil (Figure 7A). Furthermore, plants grown under the
14DLI/24h light treatment had the highest DPPH activity of all
light treatments in green basil. DPPH activity was unaffected by
light treatments in all other microgreens. Similarly, ferric
reducing antioxidant power (FRAP) was observed to be
unaffected by light treatments in all microgreens (Figure 7B).
Phenolics, which can provide resistance against various biotic
and abiotic stress conditions the plant is under, were unaffected
by the different light treatments (Figure 7C). In amaranth, the
anthocyanin content was observed to be the highest under the
21DLI/16h treatment and the lowest under the 14DLI/16h
treatment (Figure 7D). Anthocyanin content was unaffected by
light treatments in all other microgreens (Figure 7D). A trend
which can be observed is that there is higher antioxidant activity
and phenolic and anthocyanin content in both basil microgreens
in comparison to amaranth and collard greens. Notably, purple
basil tends to have the highest antioxidant capacity as well as
FIGURE 3
Collard greens plants grown under various lighting DLI and photoperiods. Top photo is an overhead picture, while the bottom is a side profile.
From left to right, the lighting treatments are as follows (DLI/photoperiod): 14DLI/16h, 14DLI/24h, 21DLI/16h, and 21DLI/24h.
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phenolic and anthocyanin concentrations, which is in part a
cause of its deep purple colouration.
4 Discussion
4.1 Continuous lighting and
microgreen growth
Both PPFD and photoperiod are known to impact plant
morphology. Growth under low PPFD will increase leaf area in
order to maximize the area capable of intercepting incoming
light (Palmer and van Iersel, 2020). Conversely, high PPFD will
lead to a reduction in specific leaf area (i.e., smaller, thicker
leaves) to protect the plant from high irradiance levels in order to
minimize damage due to excessive light (Matos et al., 2009;Fan
et al., 2013). Extended photoperiods including CL have led to
smaller leaf area in tomatoes (Velez-Ramirez et al., 2014), which
is a similar attribute seen in plants grown under high PPFD in
order to avert damage due to excess light.
In this study, with the exception of collard greens, we
observed that plants grown under the 21DLI/24h treatment
were the tallest (Tables 2–5). Furthermore, and again with the
exception of collard greens, leaf area was visually larger when
plants were grown under the 21DLI/24h treatment (Figures 2–
5). Increases in both plant height and leaf area are traits typically
associated with growth in low light environments (Poorter et al.,
2019). However, there is an interplay between leaf expansion due
to low light and photosynthesis driven by adequate PPFD.
Compared to the 21DLI/16h treatment, the 21DLI/24h
treatment used lower PPFD which enabled greater leaf
expansion. In turn, the larger leaf expansion allowed for
greater light interception and thus higher overall plant
photosynthesis leading to increased biomass. This notation is
supported by an increased EUEL of plants grown under the 24h
photoperiods compared to their respective 16h counterparts. It
should also be noted that all 24h lighting treatments in
amaranth, green basil, and purple basil had elevated levels of
chlorophyll (Figure 5). Being the major photosynthetic pigment,
a strong correlation can be drawn between chlorophyll content
TABLE 4 Growth measurement summary of green basil cv. ‘Genovese’grown under various lighting DLI and photoperiods.
Daily Light Integral (mol m
-2
d
-1
) Photoperiod (h) Height (cm) Fresh Weight (g) EUEL (g FW MJ
-1
) % Dry Matter
Green Basil cv. ‘Genovese’
14 16 2.75 ± 0.12
C
16.33 ± 1.05
C
0.66 ± 0.04
B
10.17 ± 1.20
A
24 2.99 ± 0.13
B
20.30 ± 0.97
B
0.82 ± 0.04
A
10.29 ± 0.50
A
21 16 2.98 ± 0.08
B
19.30 ± 0.47
B
0.52 ± 0.01
C
11.73 ± 0.44
A
24 3.28 ± 0.15
A
23.37 ± 0.58
A
0.64 ± 0.02
B
10.93 ± 0.75
A
Daily Light Integral 0.0003 0.0010 0.0002 0.0378
Photoperiod 0.0003 0.0002 0.0005 0.4453
Daily Light Integral*Photoperiod 0.4284 0.9245 0.3046 0.3040
Height was subsampled at five locations within the tray at the end of each of the three respective growth trials. Values presented are the means of three replicates, one from each trial ± the
standard error of the means. Different letter groups (A, B, C) represent statistical differences as determined by a two-way ANOVA within each parameter at p<0.05.
TABLE 5 Growth measurement summary of purple basil cv. ‘Red Rubin’grown under various lighting DLI and photoperiods.
Daily Light Integral (mol m
-2
d
-1
) Photoperiod (h) Height (cm) Fresh Weight (g) EUEL (g FW MJ
-1
) % Dry Matter
Purple Basil cv. ‘Red Rubin’
14 16 3.02 ± 0.18
B
10.67 ± 0.50
B
0.43 ± 0.02
BC
7.20 ± 0.51
A
24 3.17 ± 0.13
AB
12.07 ± 0.32
B
0.49 ± 0.01
A
7.05 ± 0.88
A
21 16 3.21 ± 0.15
AB
14.77 ± 0.67
A
0.40 ± 0.01
C
7.53 ± 0.58
A
24 3.30 ± 0.10
A
15.53 ± 0.71
A
0.44 ± 0.01
B
7.75 ± 0.77
A
Daily Light Integral 0.0104 0.0001 0.0330 0.0398
Photoperiod 0.0329 0.0480 0.0197 0.8678
Daily Light Integral*Photoperiod 0.5624 0.4964 0.5373 0.3966
Height was subsampled at five locations within the tray at the end of each of the three respective growth trials. Values presented are the means of three replicates, one from each trial ± the
standard error of the means. Different letter groups (A, B, C) represent statistical differences as determined by a two-way ANOVA within each parameter at p<0.05.
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FIGURE 4
Green basil plants grown under various lighting DLI and photoperiods. Top photo is an overhead picture, while the bottom is a side profile.
From left to right, the lighting treatments are as follows (DLI/photoperiod): 14DLI/16h, 14DLI/24h, 21DLI/16h, and 21DLI/24h.
FIGURE 5
Purple basil plants grown under various lighting DLI and photoperiods. Top photo is an overhead picture, while the bottom is a side profile.
From left to right, the lighting treatments are as follows (DLI/photoperiod): 14DLI/16h, 14DLI/24h, 21DLI/16h, and 21DLI/24h.
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B
C
D
A
FIGURE 6
Photosynthetic pigment analysis of amaranth, collard greens, green basil, and purple basil grown under 14DLI/16h, 14DLI/24h, 21DLI/16h, and
21DLI/24h lighting treatments. Chlorophyll a, chlorophyll b, chlorophyll a:b, and carotenoids are shown in panels (A–D) respectively. Values
presented are the means of two subsamples from each of the three replicates ± the standard error of the means. Different letter groups (A–C)
represent statistical differences with microgreen type and panel as determined by a two-way ANOVA within each parameter at p<0.05. P-values
are shown to the right of their respective panels for each microgreen.
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and increased photosynthesis leading to greater biomass
accumulation (Buttery and Buzzell, 1977). It has been
observed that amaranth growth under a 20h photoperiod had
the highest fresh biomass while also having increased levels of
chlorophyll a(Meas et al., 2020). Interestingly, collard greens do
not show the same obvious enhancement in leaf size or biomass
accumulation under CL. This also coincided with similar or
lower chlorophyll content when comparing the 24h treatments
to the 16h treatments (Figure 6). Photosynthetic pigment
concentration was observed to increase in basil, rocket, and
chicory plants grown under a similar 24h light treatment as used
in this manuscript (Pennisi et al., 2020). Similar to the results in
amaranth and both basil cultivars, Weaver and van Iersel (2020)
observed that lettuce had increased leaf area and dry biomass
when the photoperiod was extended, but the DLI stayed the
same. In contrast to our study, Pennisi et al. (2020) did not
observe an increase in fresh biomass accumulation in basil when
grown under a 24h photoperiod with a DLI of 21.6 mol m
-2
d
-1
.
The difference in observations could be a result of differences in
plant age. In Pennisi et al. (2020) the 24h treatment began when
the plants were 21 days old whereas in our study, the 24h
treatment began when the plants were only 5 days old. In this
way, no mutual shading had occurred in our study (due to the
small size of the plants) allowing maximum photon capture by
the plant which resulted in greater biomass accumulation.
Accordingly, an increase in photon capture and biomass
accumulation would be negated when the canopy is fully
matured and vegetation is dense. Since microgreens are
typically harvested before or at the first true leaf stage, this
competitive advantage when plants are young would lead to
larger plants as observed in this study, or reduced production
times as the plants would reach the desired size more quickly
when grown under a long photoperiod with low PPFD (i.e.,
21DLI/24h) as opposed to a shorter photoperiod and higher
PPFD (i.e., 21DLI/16h).
In contrast to traditional morphological responses to CL,
generally speaking, the microgreens in this study grown under
24h lighting had visually larger leaves than those grown under
16h. Reduced leaf size due to CL has been seen in tomatoes
(Velez-Ramirez et al., 2014;Pham et al., 2019). However,
microgreens grown under CL have been shown to have
increased leaf area (Figures 2–4; Shivaeva et al., 2022b). This
may indicate that microgreens have a higher PPFD threshold
before morphological adaptation occurs to reduce light capture.
In a model analysis on tomatoes, a theoretical increase in yield of
22-26% (depending on PPFD) has been predicted when grown
under CL if injury could be averted (Velez-Ramirez et al., 2012).
In this study, yield increases were 92.7%, 21.1%, 43.1%, and
45.5% for amaranth, collard greens, green basil, and purple basil,
respectively when the photoperiod was extended from 16h to
24h at the same PPFD (i.e., comparing 14DLI/16h and 21DLI/
24h). Our results show equal or higher yield increases compared
to the theoretical model analysis for tomatoes. This difference in
result is likely two-fold. Firstly, as opposed to the complex
canopy of tomatoes, microgreens in this study had very little
mutual shading during their production and consequently all of
the leaf area was able to absorb light, maximizing
photosynthesis. Secondly, unlike tomatoes which produce fruit,
all of the above ground biomass of the microgreen is edible and
therefore all of the assimilated carbon contributes to yield
increase. Therefore, due to the simplicity of microgreens, a
greater yield return is observed during CL when compared to
the more complex crop of tomatoes.
4.2 Maintaining nutritional content
during CL production
Microgreens are prized for their nutrient profile, vibrant
colours, and flavour which often allow chefs to add new
dimensions to their dishes. Microgreens are also increasingly
becoming popular as an everyday leafy green due to their high
antioxidant content making them a functional food (Xiao et al.,
2012;Kyriacou et al., 2019). Here we see that, generally speaking,
antioxidant, phenolic, and anthocyanin content remained
similar or increased when microgreens were grown under a
high DLI or an extended photoperiod. Coupling this with the
reduced electricity cost of microgreens grown under 24h
lighting, utilizing CL for microgreen production can produce
plants at a reduced cost without sacrificing nutrient density.
Both amaranth and collard greens showed improved dry
matter content under the high DLI treatment regardless of
photoperiods (Tables 2 and 3). Additionally, amaranth was
observed to have improved dry matter content under the 24h
photoperiod compared to the 16h photoperiod at the high DLI.
Since DPPH and FRAP activities (Figures 7A,B) as well as
phenolic (Figure 7C)and anthocyanin (Figure 7D) content were
expressed on a dry weight basis and were similar, it then stands
to reason that under the high DLI treatment, both amaranth and
collard greens have improved antioxidant, phenolic, and
anthocyanin content due to their higher dry matter content.
Amaranth has also been shown to have increased anthocyanin
production when grown under 280 μmol m
-2
s
-1
compared to
lower PPFD values (Meas et al., 2020). What’s more is that for
amaranth, growth under CL at the high DLI also improved
overall nutrient content compared to the 21DLI/16h treatment.
Both an increase in DLI and photoperiod extension are known
to impact the secondary metabolite concentrations within plants
(Samuoliene et al., 2013). Increasing the PPFD during growth and/
or extending the photoperiod can cause an abiotic stress response
within plants as additional light is being provided, and, in this study,
the plant is under CL (Demmig-Adams and Adams, 1992;Sairam
et al., 2001;Brazaityte et al., 2015;Haque et al., 2015;Gharibi et al.,
2016;Szymanska et al., 2017). The stress response is characterized
by an increase in reactive oxygen species (ROS) and free radicals
which can be detrimental to plant health if not properly addressed,
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B
C
D
A
FIGURE 7
Antioxidant activities in microgreens as measured by 2,2-diphenyl-1-picrylhydrazyl (DPPH; Panel A), ferric reducing antioxidant power (FRAP;
Panel B), total phenolic content (Panel C), and total anthocyanin content (Panel D) of all microgreens grown under 14DLI/16h, 14DLI/24h, 21DLI/
16h, and 21DLI/24h lighting treatments. Values presented are the means of two subsamples from each of the three replicates ± the standard
error of the means. Different letter groups (A–C) represent statistical differences with microgreen type and panel as determined by a two-way
ANOVA within each parameter at p<0.05. P-values are shown to the right of the panel.
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causing damage to the photosynthetic machinery (Arora et al.,
2002;Pospisil et al., 2016;Huang et al., 2019). To counter the
increase in free radicals, an increase in antioxidants is needed. Here,
we see that under the high DLI conditions for collard greens and
under high DLI and CL in amaranth, an increase in DPPH and
FRAP activities was observed (Figures 7A,B). This, coupled with
increased phenolic and anthocyanin levels, helps to reduce oxidative
stress in the plant by channeling extra light energy away from the
light harvesting complex and removing free radicals (Huang et al.,
2019). Similar responses have been noted in tomatoes (Haque et al.,
2015), lettuces (Zha et al., 2019), mung beans (Kumar et al., 2022),
and Brassicaceae microgreens (Shibaeva et al., 2022b).
Notably, the increase in DPPH, FRAP, phenolics, and
anthocyanins was only observed to occur in amaranth and
collard greens and was absent in both basil microgreens with
the exception of DPPH in green basil which was higher in the
24h treatments than the 16h treatments. However, green and
purple basil had higher levels of all secondary metabolites
compared to amaranth and collard greens (Figures 7). Basil is
known for its incredible aroma and antioxidant concentration
(Ciriello et al., 2021). Due to its already high secondary
metabolic concentrations, further enhancement due to any
hormetic effect of increased PPFD/DLI or photoperiod was
not observed. Sutulieneet al. (2022) also observed no increase
in FRAP and DPPH activity as well as total phenolic and
anthocyanin content in basil when the PPFD was increased
from 150 to 250 μmol m
-2
s
-1
.Samuoliene et al. (2013) noted the
impact of PPFD is species-specific with respect to secondary
metabolite concentrations. This suggests that further studies
need to be done to identify species-specific secondary
metabolite responses to various lighting conditions.
While increases in antioxidants, phenolics, and anthocyanins
can have a beneficial response in plants under environment
stressors, they can also be advantageous to humans during
consumption. Similar to their radical-scavenging abilities in
plants, these compounds have been shown to reduce the risk of
cardiovascular disease, diabetes, cancer, and even mitigate age-
related diseases in humans (Lobo et al., 2010;Engwa, 2018).
Therefore, growing microgreens under high DLI and CL can
produce a hormetic effect in which the plant responds to the stress
of high light and/or a long photoperiod by increasing the
production of important secondary metabolites which also
happen to be health promoting compounds for humans.
4.3 Continuous lighting can improve
yield and lower electricity cost
Since microgreens are sold on a fresh weight basis, the main
goal of plant factories producing microgreens is to increase fresh
biomass while minimizing inputs and maintaining nutritional
content. A traditional way to increase biomass is to increase the
DLI either through increased PPFD or extended photoperiods
(Samuoliene et al., 2013;Meas et al., 2020). In this study, all
microgreens tested, with the exception of collard greens, showed
an increase in fresh weight when the DLI increased from
approximately 14 mol m
-2
d
-1
to approximately 21 mol m
-2
d
-1
when the photoperiod was 16h. In general, as the DLI increases, one
would expect biomass to increase as well (up to a saturation point)
becausemorelightmeansmorephoto-assimilation(Poorter et al.,
2019). The increase in biomass seen here with an increase in DLI is
in-line with results from previous works with other microgreen
species including broccoli, arugula, mizuna, radish, tatsoi, and red
pak choi (Samuoliene et al., 2013;Shibaeva et al., 2022b). However,
in all microgreens studied, an increase in production due to
increased DLI during a 16h photoperiod was associated with the
same or increased electricity cost due to the additional light needed
(Table 6). In this way, resource-use-efficiency actually decreased as
DLI increased.
In this study, the continuous 24h lighting uses low intensity
light throughout the production period of the plant. In this way, the
plant is under constant illumination and is continuously
photosynthesizing; thereby negating dark respiration and
therefore, no loss of carbon occurs during the night. In fact, most
plants are observed to have elevated leaf carbohydrate levels when
grown under CL when nighttime light intensities are above the light
compensation point (Globig et al., 1997;Matsuda et al., 2014;Pham
et al., 2019). In microgreens which do not export fixed carbon to a
growing fruit, CL-injury did not occur in any of the four
microgreens which were studied. The extended photoperiod
translated to an increase in biomass due to the accumulation of
carbohydrates in the leaves –increasing resource-use-efficiency of
the production system (Table 6). In fact, EUEL was increased by 10-
42% and electricity cost of the light fixtures was decreased by 8-38%
depending on DLIs and microgreens, when transitioning from a
16h to a 24h photoperiod.
Due to the nature of plant factories being inside buildings, all
lighting requirements needed by the plant must be achieved
through sole-source lighting such as LEDs. Electricity is then one
of the largest input cost components for plant factories (Kozai
and Niu, 2019;van Delden et al., 2021). CL can reduce the
number of lighting fixtures needed (compared to a similar DLI at
a 16h photoperiod) which will reduce the initial fixture cost –
often a large barrier to entry into controlled environment
agriculture, specifically plant factories (Hao et al., 2018).
Furthermore, CL can reduce electrical costs via the use of
lower nighttime electricity prices in regions of the world which
use TOUP (Supplementary Table 1). While the data provided in
Table 6 is for Ontario, Canada, other regions of the world such as
some US states, 17 European nations, and South Korea use time-
of-use electricity pricing, so this concept would also provide a
good potential to reduce electricity costs in those regions
(IRENA, 2019;IESO, 2022). In regions which do not utilize
TOUP, the reduction in initial fixture cost due to the lower
PPFD used during CL as well as the reduced need for heat and
humidity dissipation would still provide growers with financial
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gains (Goto, 2012;Kozai and Niu, 2016;Kozai et al., 2016;
Graamans et al., 2018). Plant factories typically require extensive
air conditioning in order to maintain proper temperature and
humidity for plants; mostly to overcome heat generated by the
LED fixtures (Wang et al., 2014;Graamans et al., 2018) and to
remove the moisture from plant transpiration. The use of CL not
only reduces the overall amount of fixtures, but allows the
grower to reduce the PPFD by 33% at the same DLI (Table 1).
Together, both the reduction in overall fixtures and lower PPFD
used means less heat and moisture will be produced within the
plant factory which in turn translates to less need for energy
consuming air conditioning. The use of CL and subsequent
reduction in fixtures needed, heat emittance, and moisture
generation can also be useful for space travel as power
consumption and heat loss can be large challenges in self-
supporting food production (Stutte, 2015).
In this study, at the same DLI, amaranth and green basil
produced higher fresh biomass when grown under CL when
compared to the 16h lighting treatments (Tables 2,4). Even in
collard greens and purple basil, the use of reduced electrical
prices during the night lowered electricity cost in 24h treatments
making them more cost effective than their 16h counterparts
(Table 6). The microgreens studied here join a growing list of
plants which can tolerate CL including other microgreen species
(Shibaeva et al., 2022b), lettuces (Ohtake et al., 2018), cucumbers
(Lanoue et al., 2021b), peppers (Lanoue et al., 2022), and
tomatoes (Haque et al., 2017;Lanoue et al., 2019).
5 Conclusion
Plant factories require the use of sole-source lighting with
intensive energy input. It usually requires high capital
investment due to the high costs of LED and HVAC
equipment. CL has been studied in many plant systems and
represents a potential strategy to lower fixture needs and reduce
PPFD during prolonged photoperiods; resulting in reduced
electricity costs. Here we show that four microgreens,
amaranth, collard greens, green basil, and purple basil have
increased fresh biomass accumulation and/or reduced electricity
costs when grown under CL regardless of DLIs. Furthermore, the
use of high DLI in collard greens and high DLI and CL in
amaranth increased DPPH and FRAP activities as well as
phenolic and anthocyanin content. Green basil and purple
basil maintained their secondary metabolite concentrations
while still having reduced electricity costs when grown under
CL. In this way, the use of CL for microgreen production can
improve energy efficiency while maintaining or increasing
antioxidants, phenolics, and anthocyanins making it a more
sustainable lighting strategy than high intensity short
photoperiod lighting.
Data availability statement
The original contributions presented in the study are
included in the article/Supplementary Material. Further
inquiries can be directed to the corresponding author.
Author contributions
JL and XH were involved in the conceptualization,
methodology development, and writing. JL, XH, and CL
edited the manuscript. JL SS and CL were involved in data
curation and day-to-day upkeep of the experiment. JL
performed data analysis. XH was responsible for funding
TABLE 6 Electricity cost from LED lighting only of microgreens under various DLI and photoperiods.
Daily Light Integral (mol m
-2
d
-1
) Photoperiod (h) Electricity Cost ($ g
-1
FW)
Amaranth Collard Greens Green Basil Purple Basil
14 16 0.41 ± 0.01
A
0.13 ± 0.02
B
0.65 ± 0.01
B
0.44 ± 0.03
AB
24 0.27 ± 0.02
B
0.10 ± 0.01
C
0.60 ± 0.01
C
0.34 ± 0.02
C
21 16 0.45 ± 0.02
A
0.18 ± 0.02
A
0.76 ± 0.02
A
0.48 ± 0.04
A
24 0.28 ± 0.02
B
0.14 ± 0.01
B
0.66 ± 0.02
B
0.39 ± 0.03
B
Daily Light Integral 0.2999 0.0001 0.0003 0.0085
Photoperiod <0.0001 0.0005 <0.0001 0.0002
Daily Light Integral*Photoperiod 0.5666 0.2023 0.1729 0.1824
All electrical prices are for Ontario, Canada and were obtained from the Independent Electricity System Operator (IESO, 2022) for the given production periods (Supplementary Table 1).
All prices are in Canadian dollars. Electricity usage was calculated using the Flexstar 645W with appropriate dimming capabilities to reach the desired light intensities (Table 1). Electricity
costs are calculated by using the total electricity costs related to lighting for each production period (i.e., replicate) and dividing that by the actually fresh biomass produced in said
production period. Values presented are the means of three growth trials, each representing one replicate ± the standard error of the means. Different letter groups (A, B, C) represent
statistical differences as determined by a two-way ANOVA within each parameter at p<0.05.
Lanoue et al. 10.3389/fpls.2022.983222
Frontiers in Plant Science frontiersin.org15

acquisition. All authors have read and agreed to the
submitted manuscript.
Funding
The project is funded by the Foundation Science Program of
Agriculture and Agri-Food Canada to XH (J-002228.001.04).
Conflict of interest
The authors declare that the research was conducted in the
absence of any commercial or financial relationships that could
be construed as a potential conflict of interest.
Publisher’s note
All claims expressed in this article are solely those of the
authors and do not necessarily represent those of their affiliated
organizations, or those of the publisher, the editors and the
reviewers. Any product that may be evaluated in this article, or
claim that may be made by its manufacturer, is not guaranteed
or endorsed by the publisher.
Supplementary material
The Supplementary Material for this article can be found
online at: https://www.frontiersin.org/articles/10.3389/
fpls.2022.983222/full#supplementary-material
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