Effect of Different Combinations of Red and Blue LED Light on Growth Characteristics and Pigment Content of In Vitro Tomato Plantlets

Abstract

The aim of this study was to evaluate the growth characteristics and pigment content of tomato plantlets grown under various ratios of red (R) (661 nm) and blue (B) (449 nm) LED light. In this study, three different ratios of R and B (RB) light such as 5:01, 10:01, and 19:01 along with R (100%) were used. The photosynthetic photon flux density (PPFD), and photoperiod of the growth chamber was 120 ± 5 μmol m−2s−1 and 16/8 h (day/night), respectively. Tomato plantlets were cultured for six weeks in the growth chamber. It was shown that tomato plantlets had higher photosynthesis rate, higher pigments content, higher growth characteristics (e.g., number of leaves, leaf area, shoot number, root number, root length, dry, and fresh mass), and greater surviving rate under the R:B = 10:01 ratio among the treatments. The plantlets showed at least a threefold decrease in photosynthesis rate, as well as a significant abnormal stem elongation when grown under 100% R light. It is concluded that the RB ratio of 10:01 showed excellent performance in all growth parameters. This result has shown that the optimum lighting environment improves tomato plantlet cultures in vitro.

Full text

agriculture
Article
Effect of Different Combinations of Red and Blue
LED Light on Growth Characteristics and Pigment
Content of In Vitro Tomato Plantlets
Most Tahera Naznin 1,*, Mark Lefsrud 2, Md Obyedul Kalam Azad 3and Cheol Ho Park 3, *
1Department of Biosystems and Technology, Swedish University of Agricultural Sciences, Box 103,
23053 Alnarp, Sweden
2Bioresource Engineering Department, Macdonald Stewart Building, McGill University, 21111 Lakeshore,
Ste-Anne-de-Bellevue, H9X 3V9, QC, Canada
3Department of Bio-Health Technology, College of Biomedical Science, Kangwon National University,
Chuncheon 24341, Korea
*Correspondence: naznin.most.tahera@slu.se (M.T.N.); chpark@kangwon.ac.kr (C.H.P.);
Tel.: +46-40-415019 (M.T.N.); +82-33-250-6473 (C.H.P.)
Received: 13 August 2019; Accepted: 5 September 2019; Published: 9 September 2019


Abstract:
The aim of this study was to evaluate the growth characteristics and pigment content
of tomato plantlets grown under various ratios of red (R) (661 nm) and blue (B) (449 nm) LED
light. In this study, three different ratios of R and B (RB) light such as 5:01, 10:01, and 19:01 along
with R (100%) were used. The photosynthetic photon flux density (PPFD), and photoperiod of the
growth chamber was 120
±
5
µ
mol m
−2
s
−1
and 16/8 h (day/night), respectively. Tomato plantlets
were cultured for six weeks in the growth chamber. It was shown that tomato plantlets had higher
photosynthesis rate, higher pigments content, higher growth characteristics (e.g., number of leaves,
leaf area, shoot number, root number, root length, dry, and fresh mass), and greater surviving rate
under the R:B =10:01 ratio among the treatments. The plantlets showed at least a threefold decrease
in photosynthesis rate, as well as a significant abnormal stem elongation when grown under 100% R
light. It is concluded that the RB ratio of 10:01 showed excellent performance in all growth parameters.
This result has shown that the optimum lighting environment improves tomato plantlet cultures
in vitro.
Keywords: light-emitting diode (LED); tomato plantlets; biomass; photosynthesis; pigments
1. Introduction
In vitro
plant culture is a valued method in achieving improved disease free identical plant
seedlings. It has enormous roles in the propagation of plants in large quantities with desired
characters [1]. Light is the most vital element to regulate in vitro plant growth and development.
Artificial light should provide photons in the spectral region that is involved in photosynthesis
and in the photomorphogenic responses of plant culture
in vitro
[
2
,
3
]. Light acts as a signaling
mechanism through different light receptors and provides the required energy for plant growth and
development [
4
]. The development of
in vitro
plantlets can be improved by tuning the light quality,
quantity, and photoperiod in the growth environment [5].
Artificial light emitting diode (LED) light has been strategically used for the growth of many plant
species including chrysanthemum [
5
], cabbage [
6
], cymbidium [
7
], rapeseed [
8
], and strawberry [
9
].
Among the different LED light qualities, blue (B) (420–450 nm) and red (R) (600–700) are the
most effective light spectra for the determination of plant growth and development [
10
]. It was
previously reported that the absorption percentage of B and R light is about 90% among the light
Agriculture 2019,9, 196; doi:10.3390/agriculture9090196 www.mdpi.com/journal/agriculture

Agriculture 2019,9, 196 2 of 9
spectra [
11
]. In addition, monochromatic B and R light alone could not meet the requirements of
plant growth and development [
12
]. The absence of one of the two light wavebands (B or R) creates
photosynthetic inefficiencies [
13
]. B light is predominantly absorbed by chlorophyll (chl)aand chl
b, which assimilates the photosynthetic CO
2
thus initiating the photosynthetic process. In contrast,
R light produces a narrow-spectrum light that regulates photomorphogenesis, energy distribution,
and the photosynthetic apparatus [
4
]. It is noted that artificial R light expressively enhances
in vitro
stem elongation of potato and Pelargonium plantlets [
14
,
15
]. Nhut et al. [
9
] demonstrated that growth
characteristics of strawberry increased when grown under 70% R and 30% B light.
Tomato (Solanum lycopersicum L.) is the second most crucial vegetable crop in the world after
potato [
16
]. It is grown in almost every country and achieved tremendous popularity due to its high
nutritional value [
17
]. Tomato seedling production by tissue culturing is an imperative approach to
improve the seedling quality [18].
Moreover, cultivated tomatoes suffer from many diseases that are caused by bacteria, fungi, viruses,
and nematodes [
19
,
20
]. It is reported that tomato plant grown under different combinations of LED
light has a confrontation to the various diseases and pathogen attack [
21
]. Plenty of research has already
been done on tomato
in vitro
culture [
22
–
24
]. However, information on the effect of a combination
of artificial R and B LED light application on
in vitro
tomato plantlets is still inadequate so far. It is
assumed that an efficient combination of RB light would improve the tomato plantlets quality.
2. Materials and Methods
2.1. Plant Materials and Culture Conditions
Tomato (Solanum lycopersicum L.) seeds was sterilized in 8% Clorox solution (sodium hypochlorite)
for 10 min followed by three times rinsing with distilled water. The nutrient agar medium was prepared
according to Murashige and Skoog [
25
] with slight modification (MS +IAA 0.2 mg/L+BAP 0.05 mg/L,
1
2
NH
4
NO
3
,
1
2
KNO
3
) at 26/22
◦
C (light/dark) temperatures adjusting pH at 5.8 and inoculated by
autoclaving (121 ◦C for 20 min).
Tomato explant was grown in a growth chamber equipped with fluorescent lamp (E15, Conviron,
Winnipeg, Canada) under photosynthetic photon flux density (PPFD) of 60
±
5
µ
mol m
−2
s
−1
,
16/8 light/dark regime, 23
±
1
◦
C temperature. After three weeks, tomato explants were excised (1–2 cm
hypocotyl segments) and cultured in the same nutrient medium (described above) supplemented with
30 g sucrose and 2 mg/L of BAP in order to grow tomato plantlets.
2.2. Plantlet Culture and LED Light Treatments
Tomato plantlets were cultured under different artificial red (R) (661 nm) and blue (B) (449 nm)
light treatment such as R:B =5:01; 10:01, 19:01, and R 100% in a growth chamber (General Electric
Lighting Solutions, Lachine, Quebec, Canada). The photosynthetic photon flux density (PPFD) of
120 ±5µmol m−2s−1
, photoperiod of 16/8 h (light/dark), and temperature of 23
±
1
◦
C was maintained
in the growth chamber. The above environmental parameters of the growth chamber were fixed based
on a previous study for the tomato plantlet cultures in vitro (data not published).
2.3. Evaluation of Plantlet Growth and Pigment Content
The tomato plantlets were cultured for six weeks and growth characteristics were analyzed by
measuring the stem height, number of leaves, leaf area, shoot number, root number, root length,
fresh, and dry weight. Leaf area was determined by taking a digital image of leaves and using
Image J software (Bethesda, Maryland, USA). The plantlet fresh mass (FM) was assessed immediately
after harvesting, and dry mass (DM) was determined after drying in an oven at 65
◦
C for 24 h.
Chlorophyll pigments were analyzed according to method described by Lichtenthaler [
26
] using
a spectrophotometer (PowerWave
™
XS Microplate Reader, BioTek Instruments, Inc. Vermont, USA).

Agriculture 2019,9, 196 3 of 9
The content of Chl a,Chl b, total Chl and carotenoid were calculated based on plant fresh weight.
Five tomato plantlets from each treatment were selected for the analysis of growth and pigment content.
2.4. Photosynthesis Measurement
The photosynthesis of the tomato plantlets was measured using an LI-6400 XT portable
photosynthesis system (LI-COR Inc., Lincoln, Nebraska, USA) on the day of harvesting. The cuvette
climate of the photosynthesis system was as follows: CO
2
400
µ
mol mol
−1
, airflow 200 ml min
−1
,
temperature 20
±
1
◦
C, vapor pressure deficit (VPD) 10
±
3 Pa, and RH 50
±
10%. Three plantlets from
each treatment were designated for the photosynthesis analysis. The wider leaves of tomato plantlets
were subjected to measurement for photosynthesis.
2.5. Statistical Analysis
The results were expressed as mean values and their standard errors (SE) using MS Excel software.
Least significant differences among the light treatments were evaluated by the Tukey’s HSD tests
(p<0.05). The experiment was repeated twice maintaining the same environment.
3. Results and Discussion
3.1. Plant Growth, Biomass, and Pigment Analysis
The morphological features of tomato plantlets are shown in Figure 1. It was observed that healthy
and vigorous tomato plantlets were attained when grown under the RB ratio of 10:01 among the light
treatments. Likewise, the survival rate of tomato plantlets was higher when cultured under RB light
than those under 100% R light. The highest survival rate (80%) of the tomato plantlets was attained
under an RB ratio of 10:1 compared to 100% R light (60%) (Figure 2).
The fraction of B light in the RB combination had significant influence on growth characteristics
and biomass of tomato plantlets (Table 1; Figure 3). It was observed that tomato plantlets had higher
leaf number, leaf area, shoot number, root number, and root length in the combination of RB treatment
compared to 100% R. In addition, the RB ratio of 10:01 showed the best performance considering
growth characteristics and biomass production (fresh and dry mass) of tomato plantlets.
Agriculture 2019, 9, x FOR PEER REVIEW 3 of 9
The content of Chl a, Chl b, total Chl and carotenoid were calculated based on plant fresh weight. Five
tomato plantlets from each treatment were selected for the analysis of growth and pigment content.
2.4. Photosynthesis Measurement
The photosynthesis of the tomato plantlets was measured using an LI-6400 XT portable
photosynthesis system (LI-COR Inc., Lincoln, Nebraska, USA) on the day of harvesting. The cuvette
climate of the photosynthesis system was as follows: CO2 400 µmol mol−1, airflow 200 ml min−1,
temperature 20 ± 1 °C, vapor pressure deficit (VPD) 10 ± 3 Pa, and RH 50 ± 10%. Three plantlets from
each treatment were designated for the photosynthesis analysis. The wider leaves of tomato plantlets
were subjected to measurement for photosynthesis.
2.5. Statistical Analysis
The
r
e
s
ult
s
w
e
r
e exp
r
e
ss
ed a
s
mean value
s
and thei
r
s
tanda
r
d e
rr
o
rs
(S
E
)
u
s
ing
M
S Excel
s
o
f
t
w
a
r
e. Least significant differences among the light treatments were evaluated by the Tukey’s HSD
tests (p < 0.05). The experiment was repeated twice maintaining the same environment.
3. Results and Discussion
3.1. Plant Growth, Biomass, and Pigment Analysis
The mo
r
phological features o
f
tomato plantlets are shown in Figure 1. It was observed that
healthy and vigorous tomato plantlets were attained when grown under the RB ratio of 10:01 among
the light treatments. Likewise, the
s
u
r
vival
r
ate o
f
tomato
plantlet
s was higher when
cultu
r
ed under
RB light
than
tho
s
e unde
r
100% R
light. The highest survival rate (80%) of the tomato plantlets was
attained under an RB ratio of 10:1 compared to 100% R light (60%) (Figure 2).
The fraction of B light in the RB combination had significant influence on growth characteristics
and biomass of tomato plantlets (Table 1; Figure 3). It was observed that tomato plantlets had higher
leaf number, leaf area, shoot number, root number, and root length in the combination of RB
treatment compared to 100% R. In addition, the RB ratio of 10:01 showed the best performance
considering growth characteristics and biomass production (fresh and dry mass) of tomato plantlets.
Figure 1. Tomato plantlets grown under different ratios of red (R) to blue (B) LED light. Photograph
showing six weeks cultured plantlets.
Figure 1.
Tomato plantlets grown under different ratios of red (R) to blue (B) LED light. Photograph
showing six weeks cultured plantlets.

Agriculture 2019,9, 196 4 of 9
Agriculture 2019, 9, x FOR PEER REVIEW 4 of 9
Figure 2. Surviving rate of tomato plantlets under different ratios of R to B LED light. Mean separation
within columns by Tukey’s HSD tests (n = 5). Values labeled with different letters in a column are
significantly different (p < 0.05).
However, the stem length was remarkably increased under 100% R light compared to other
treatments (
Table 1
). The stem length of the tomato plantlet was 1.2, 1.0, and 1.5 times shorter under
the RB ratio of 5:01, 10:01, and 19:01, respectively, compared to the 100% R light.
Table 1. Effects of various red-blue (RB) light ratios on the growth of tomato plantlets.
LED Light
Ratio
Stem Length
(cm)
Leaf
Number
Leaf Area
(cm2)
Shoot
Number
Root
Number
Root Length
(cm)
RB 5:01
9.3b
5.5a
14.3a
2.1b
3.0a
2.1a
RB 10:01
10.1a
5.9a
14.2a
2.8a
3.3a
2.4a
RB 19:01
7.8b
5.1a
12.9b
2.5a
3.2a
2.2a
R 100
12.1a
4.5b
13.3b
1.3c
2.6b
1.8b
Mean separation within columns by Tukey’s HSD tests (n = 5). Values labeled with different
letters in a column are significantly different (p < 0.05).
The results of the analysis of total chlorophyll, chl a, chl b, and carotenoids are presented in Table
2. It was observed that the chl a (565.02 µg/g), chl b (241.76 µ g/g), total chlorophyll (806.79 µ g/g), and
carotenoids (190.36 µg/g) of tomato plantlets were significantly higher in RB light compared to 100%
R light (412.71 µg/g, 181.66 µ g/g, 594.36 µ g/g, and 137.12 µ g/g, respectively). The highest content of
pigments was achieved at the RB ratio of 10:01 among the light combinations.
Table 2. Effects of various RB light ratios on the pigment contents of tomato plantlets.
LED Light Ratio
Chlorophyll a
(µg/g FW)
Chlorophyll a
(µg/g FW)
Total Chlorophyll
(µg/g FW)
Carotenoid
(µg/g FW)
RB 5:01
443.39 ± 12.47c
223.16 ± 5.51ab
667.61 ± 11.44c
182.91 ± 3.19a
RB 10:01
565.02 ± 7.25a
241.76 ± 8.68a
806.79 ± 7.31a
190.36 ± 7.48a
RB 19:01
497.51 ± 7.23b
212.66 ± 9.12b
710.16 ± 6.69b
172.14 ± 3.56a
R 100
412.71 ± 12.73d
181.66 ± 13.82c
594.36 ± 16.91d
137.12 ± 8.13b
Mean separation within columns by Tukey’s HSD tests (n = 5). Values labeled with different letters in
a column are significantly different (p < 0.05).
0
20
40
60
80
100
5:01 10:01 19:01 100
Survival rate (%)
Red to blue ratio
a
b
b
c
Figure 2.
Surviving rate of tomato plantlets under different ratios of R to B LED light. Mean separation
within columns by Tukey’s HSD tests (n=5). Values labeled with different letters in a column are
significantly different (p<0.05).
However, the stem length was remarkably increased under 100% R light compared to other
treatments (Table 1). The stem length of the tomato plantlet was 1.2, 1.0, and 1.5 times shorter under
the RB ratio of 5:01, 10:01, and 19:01, respectively, compared to the 100% R light.
Table 1. Effects of various red-blue (RB) light ratios on the growth of tomato plantlets.
LED Light
Ratio
Stem
Length (cm)
Leaf
Number
Leaf Area
(cm2)
Shoot
Number
Root
Number
Root Length
(cm)
RB 5:01 9.3b5.5a14.3a2.1b3.0a2.1a
RB 10:01 10.1a5.9a14.2a2.8a3.3a2.4a
RB 19:01 7.8b5.1a12.9b2.5a3.2a2.2a
R 100 12.1a4.5b13.3b1.3c2.6b1.8b
Mean separation within columns by Tukey’s HSD tests (n=5). Values labeled with different letters in a column are
significantly different (p<0.05).
The results of the analysis of total chlorophyll, chl a,chl b, and carotenoids are presented in
Table 2. It was observed that the chl a (565.02
µ
g/g), chl b (241.76
µ
g/g), total chlorophyll (806.79
µ
g/g),
and carotenoids (190.36
µ
g/g) of tomato plantlets were significantly higher in RB light compared to
100% R light (412.71
µ
g/g, 181.66
µ
g/g, 594.36
µ
g/g, and 137.12
µ
g/g, respectively). The highest content
of pigments was achieved at the RB ratio of 10:01 among the light combinations.
Table 2. Effects of various RB light ratios on the pigment contents of tomato plantlets.
LED Light Ratio Chlorophyll a
(µg/g FW)
Chlorophyll a
(µg/g FW)
Total Chlorophyll
(µg/g FW)
Carotenoid
(µg/g FW)
RB 5:01 443.39 ±12.47c223.16 ±5.51ab 667.61 ±11.44c182.91 ±3.19a
RB 10:01 565.02 ±7.25a241.76 ±8.68a806.79 ±7.31a190.36 ±7.48a
RB 19:01 497.51 ±7.23b212.66 ±9.12b710.16 ±6.69b172.14 ±3.56a
R 100 412.71 ±12.73d181.66 ±13.82c594.36 ±16.91d137.12 ±8.13b
Mean separation within columns by Tukey’s HSD tests (n=5). Values labeled with different letters in a column are
significantly different (p<0.05).

Agriculture 2019,9, 196 5 of 9
Agriculture 2019, 9, x FOR PEER REVIEW 5 of 9
(A)
(B)
Figure 3. Fresh (A) and dry (B) mass of tomato plantlets under different ratios of red to blue LED
light. Mean separation within columns by Tukey’s HSD tests (n = 5). Values labeled with different
letters in a column are significantly different (p < 0.05).
The fraction of B light in the RB combination showed a great influence on the growth
characteristics of tomato plantlets. It was reported that monochromatic light is not sufficient for the
normal growth and development of plants [27,28]. Plants cannot have functioning physiological and
morphological process without the combination of R and B light [29]. The potentiality of R and B light
on plant growth and development has been intensively studied [30,31].
Our investigations revealed that tomato plantlets were physically healthy when cultured under
various RB ratios and the highest growth was under the RB ratio of 10:01. In the absence of B light,
the plantlets were abnormal (elongated). Excessive elongation of plantlets reduced the survival rate
and hindered the process of transplanting and moving from one tray to another [32].
It was reported that the numbers of leaves of strawberry were higher when grown under RB
light than when grow under 100% R light [9]. It was reported that R light affected stem elongation
and decreased pigments content [33,34]. Our results are also in agreement with Appelgren [14] for
Pelargonium, in which blue light strongly inhibited stem elongation.
It was observed that the biomass and leaf growth of tomato plantlets were significantly increased
under RB light treatments as compared with monochromic R light treatments. This may be due to the
maximum photosynthetic efficiency of plants grown under RB light, since this light wavelength range
closely coincide with the absorption peaks of chlorophylls [35].
0
0.5
1
1.5
2
2.5
5:01 10:01 19:01 100
Fresh mass (g)
Red to blue ratio
a
b
b
c
0
0.2
0.4
0.6
5:01 10:01 19:01 100
Dry mass (g)
Red to blue ratio
a
b
c
d
Figure 3.
Fresh (
A
) and dry (
B
) mass of tomato plantlets under different ratios of red to blue LED light.
Mean separation within columns by Tukey’s HSD tests (n=5). Values labeled with different letters in
a column are significantly different (p<0.05).
The fraction of B light in the RB combination showed a great influence on the growth characteristics
of tomato plantlets. It was reported that monochromatic light is not sufficient for the normal growth
and development of plants [
27
,
28
]. Plants cannot have functioning physiological and morphological
process without the combination of R and B light [
29
]. The potentiality of R and B light on plant growth
and development has been intensively studied [30,31].
Our investigations revealed that tomato plantlets were physically healthy when cultured under
various RB ratios and the highest growth was under the RB ratio of 10:01. In the absence of B light,
the plantlets were abnormal (elongated). Excessive elongation of plantlets reduced the survival rate
and hindered the process of transplanting and moving from one tray to another [32].
It was reported that the numbers of leaves of strawberry were higher when grown under RB
light than when grow under 100% R light [
9
]. It was reported that R light affected stem elongation
and decreased pigments content [
33
,
34
]. Our results are also in agreement with Appelgren [
14
] for
Pelargonium, in which blue light strongly inhibited stem elongation.
It was observed that the biomass and leaf growth of tomato plantlets were significantly increased
under RB light treatments as compared with monochromic R light treatments. This may be due to the
maximum photosynthetic efficiency of plants grown under RB light, since this light wavelength range
closely coincide with the absorption peaks of chlorophylls [35].

Agriculture 2019,9, 196 6 of 9
In the current study, RB light significantly increased chlorophyll and carotenoid content compared
to 100% R. It was recognized that plant pigments have specific light absorption spectra [
36
]. For instance,
chl and carotenoid have high light absorption at 400–500 nm (B light) and at 630–680 nm (R light),
respectively [
36
]. The dose of RB light was plant species dependent; for instance, the appropriate
RB dose was 1:1, 4:1, and 3:7 for the growth of lilium, banana, and strawberry, respectively [
37
–
39
].
The combination of RB light promoted the growth of kale, capsicum, and pepper plantlets, but the
ideal proportion of RB light seemed to be plant species dependent [12].
B light was abundantly absorbed by photosynthetic pigments and a vital catalyst to increase the
chlorophyll and carotenoid [
6
]. Results from the current study showed that tomato plantlets have
high total chlorophyll and carotenoid pigments under RB ratio, which is consistent with the result
of brassica plantlets
in vitro
[
8
]. Ma et al. [
40
] illustrated that key gene activity of the enzyme in
chlorophyll and carotenoid pigments were stimulated by monochromatic B light resulting in higher
pigments accumulation.
3.2. Photosynthesis Analysis of Tomato Plantlets
The tomato plantlets grown under monochromatic R light had reduced photosynthesis rate
compared to different RB light treatments (Figure 4). The highest photosynthesis rate of the tomato
plantlets was observed at the RB =10:01 ratio. The photosynthesis rate of the tomato plantlets was
1.8, 2.6, and 1.6 times higher under RB ratio of 5:01, 10:01, and 19:01, respectively, compared to 100%
R light.
Agriculture 2019, 9, x FOR PEER REVIEW 6 of 9
In the current study, RB light significantly increased chlorophyll and carotenoid content
compared to 100% R. It was recognized that plant pigments have specific light absorption spectra
[36]. For instance, chl and carotenoid have high light absorption at 400–500 nm (B light) and at 630–
680 nm (R light), respectively [36]. The dose of RB light was plant species dependent; for instance, the
appropriate RB dose was 1:1, 4:1, and 3:7 for the growth of lilium, banana, and strawberry,
respectively [37–39]. The combination of RB light promoted the growth of kale, capsicum, and pepper
plantlets, but the ideal proportion of RB light seemed to be plant species dependent [12].
B light was abundantly absorbed by photosynthetic pigments and a vital catalyst to increase the
chlorophyll and carotenoid [6]. Results from the current study showed that tomato plantlets have
high total chlorophyll and carotenoid pigments under RB ratio, which is consistent with the result of
brassica plantlets in vitro [8]. Ma et al. [40] illustrated that key gene activity of the enzyme in
chlorophyll and carotenoid pigments were stimulated by monochromatic B light resulting in higher
pigments accumulation.
3.2. Photosynthesis Analysis of Tomato Plantlets
The tomato plantlets grown under monochromatic R light had reduced photosynthesis rate
compared to different RB light treatments (Figure 4). The highest photosynthesis rate of the tomato
plantlets was observed at the RB = 10:01 ratio. The photosynthesis rate of the tomato plantlets was
1.8, 2.6, and 1.6 times higher under RB ratio of 5:01, 10:01, and 19:01, respectively, compared to 100%
R light.
Figure 4. Photosynthesis of tomato plantlets under different ratios of red to blue led light. Mean
separation within columns by Tukey’s HSD tests (n = 5). Values labeled with different letters in a
column are significantly different (p < 0.05).
The combination of RB light has been proved to be effective in driving photosynthesis [29]. The
combination of R and B light had the most photosynthetically effective wavebands. The absence of
one of the two lights created photosynthetic inefficiencies for cucumber [13]. Our results indicated
that photosynthesis of tomato plantlets efficiently improved by increasing the B light fraction in the
RB light combination. The same trend of increasing photosynthesis was also observed in rice and
cucumber [30,41]. This result is supported by analyzing the chlorophyll pigment. B light increased
the chlorophyll molecule that makes photosynthesis possible [42]. A higher content of chlorophyll
improved the efficiency of light absorption, which directly enhances the photosynthesis process [42].
Good
quality tomato
seedlings s
hould be compact,
w
ith
s
ho
r
t inte
r
node
s
and
f
i
r
m
s
tem
s
, and la
r
ge and
inten
s
ive g
r
een leave
s
.
S
uch
seedlings
gua
r
antee optimal development o
f
t he
r
oot
s
y
s
tem a
f
te
r
t
r
an
s
pla
n
ting and have an e
ff
ect on the quality and quantity o
f
the plant yield [43].
4. Conclusions
Our result suggested that the fraction of B light in the RB combination has a crucial impact on
optimal growth and development of tomato plantlets in vitro. The higher and lower proportion of B
0
1
2
3
5:01 10:01 19:01 100
Photosynthesis
(mm CO2m-2 s-1)
Red to blue ratio
c
a
b
d
Figure 4.
Photosynthesis of tomato plantlets under different ratios of red to blue led light.
Mean separation within columns by Tukey’s HSD tests (n=5). Values labeled with different letters in
a column are significantly different (p<0.05).
The combination of RB light has been proved to be effective in driving photosynthesis [
29
].
The combination of R and B light had the most photosynthetically effective wavebands. The absence of
one of the two lights created photosynthetic inefficiencies for cucumber [
13
]. Our results indicated
that photosynthesis of tomato plantlets efficiently improved by increasing the B light fraction in the
RB light combination. The same trend of increasing photosynthesis was also observed in rice and
cucumber [
30
,
41
]. This result is supported by analyzing the chlorophyll pigment. B light increased
the chlorophyll molecule that makes photosynthesis possible [
42
]. A higher content of chlorophyll
improved the efficiency of light absorption, which directly enhances the photosynthesis process [
42
].
Good quality tomato seedlings should be compact, with short internodes and firm stems, and large
and intensive green leaves. Such seedlings guarantee optimal development of the root system after
transplanting and have an effect on the quality and quantity of the plant yield [43].

Agriculture 2019,9, 196 7 of 9
4. Conclusions
Our result suggested that the fraction of B light in the RB combination has a crucial impact on
optimal growth and development of tomato plantlets
in vitro
. The higher and lower proportion of B
light in the RB combination impede the growth and developmental process. The optimal RB ratio was
10:01 for tomato plantlet cultures
in vitro
. This findings would be helpful for the design of artificial
light for other plant species.
Author Contributions:
M.T.N. designed and conducted the experiment, analyzed the data, and drafted the
manuscript; M.O.K.A. and C.H.P. edited and revised the manuscript; M.L. supervised the research.
Funding:
This work was supported by General Electric Lighting Solutions Canada and the Natural Sciences and
Engineering Research Council of Canada (NSERC) (CRDPJ project no. 418919-11).
Acknowledgments:
We highly acknowledge Bioresource Engineering Department, McGill University, Quebec,
Canada for providing facilities to conduct this experiment.
Conflicts of Interest: The authors declare no conflict of interest.
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