Scientia Horticulturae 204 (2016) 1–7
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Scientia Horticulturae
journal homepage: www.elsevier.com/locate/scihorti
Acclimatisation of greenhouse crops to differing light quality
Karl-Johan Bergstrand a,b,∗ , Leiv M. Mortensen b , Aruppillai Suthaparan b ,
Hans Ragnar Gislerød b
a
b
Swedish University of Agricultural Sciences, Department of Biosystems and Technology, P.O. Box 103, SE-230 53 Alnarp, Sweden
Norwegian University of Life Sciences, Department of Plant Sciences, P.O. Box 5002, N-1432 Ås, Norway
a r t i c l e
i n f o
Article history:
Received 3 November 2015
Received in revised form 9 March 2016
Accepted 23 March 2016
Available online 31 March 2016
Keywords:
Artificial light
Chlorophyll fluorescence
Horticulture
Photosynthesis measurements
Rosa × hybrida
Solanum lycopersicum
a b s t r a c t
High-intensity discharge (HID) and light-emitting diode (LED) lights have been widely compared for
use in greenhouse plant production but the results are contradictory. In order to obtain more data on
the effects of different light sources on plant growth, growth chamber experiments with high pressure
sodium (HPS) or LED light and one treatment with alternating HPS and LED light (three days each) were
carried out using tomato and rose as model plants. The LED lamps used were composed of blue (B, peak
emissions 402, 419, and 445 nm) and red/far red (R/FR, peaks in 663 and 737 nm) LEDs. Plant growth
parameters were recorded, as were photosynthesis, chlorophyll fluorescence, chlorophyll content, leaf
temperature, leaf spectral properties and light penetration into the canopy. In roses, stem elongation
and leaf area were generally lower for plants grown under LED light while fresh and dry weight was
unaffected by the lamp type. For tomato, plants grown in alternating LED and HPS lamps had lower fresh
weight as compared with HPS. Specific photosynthetic capacity (Amax ) and maximum quantum yield of
PSII (Fv /Fm ) were higher in leaves developed under LED light than HPS. Leaf transmittance and reflectance
were higher for leaves grown in HPS light, which also gave better penetration of light into the canopy.
Plants subjected to alternating light regimes generally resembled LED treatment plants more than HPS
plants. Leaf temperature was higher under HPS (0.9–1.3 ◦ C) favouring plants growing in chambers with
HPS light. Leaf temperature and the amount of blue light supplied were concluded to be key factors for
plant performance.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
The use of solid-state technology light sources (e.g. lightemitting diodes, LED) for providing light in horticultural production
systems has attracted great interest in recent years. It was suggested already in 1966 that the artificial lighting used for plants
should be adapted to the peaks in sensitivity of the photosynthetic
apparatus (Hårdh, 1966). A generalised action spectrum for photosynthesis was suggested by McCree (1972). With lighting sources
based on LED technology, it is possible to tailor the spectral composition of the light in a way that is not possible with commonly
used high intensity discharge (HID) lamps. Thus, it has been suggested that using LED-based light sources matching the spectral
∗ Corresponding author at: Swedish University of Agricultural Sciences, Department of Biosystems and Technology, P.O. Box 103, SE-230 53 Alnarp, Sweden.
E-mail addresses: karl.johan.bergstrand@nmbu.no,
karl-johan.bergstrand@slu.se (K.-J. Bergstrand).
http://dx.doi.org/10.1016/j.scienta.2016.03.035
0304-4238/© 2016 Elsevier B.V. All rights reserved.
output of the lamps to the light response curve of photosynthesis
could improve growth and reduce the energy needed for assimilation lighting (Pinho, 2008; Deram et al., 2014). However, few
studies have reported an unambiguous positive growth response
when comparing LED lighting to HID lighting at the same PAR
light intensity. Dueck et al. (2012), Hernández and Kubota (2015)
and Hao et al. (2012) observed reduced growth when using LEDs,
which they attributed to lower leaf temperature due to low radiant
heat from LED light sources. Bergstrand and Schüssler (2013) also
observed lower biomass production when using LED light sources
compared with HID lighting. However, Currey and Lopez (2013)
reported increased leaf- and root mass for Petunia, but not for Impatiens or Pelargonium, when cultivated using a combination of red
and blue LEDs, compared with HPS.
Warrington et al. (1976) concluded that the efficiency of the
light source is more important than the quality of the spectrum for
a system’s light use efficiency. They also showed that long-term
biomass production is not as strongly influenced by light quality as
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K.-J. Bergstrand et al. / Scientia Horticulturae 204 (2016) 1–7
short-term photosynthesis. Similar results were reported by Terfa
et al. (2013). Acclimatisation of the plants to the light conditions
is a plausible explanation for this and probably part of the explanation for the relatively poor results often obtained when using
‘optimised’ spectra for plant lighting. Such acclimatisation may be
of various forms, e.g. changes in leaf size (Islam et al., 2012), leaf
thickness (Chabot et al., 1979), pigment content (Paradiso et al.,
2011), number of stomata (van Ieperen, 2012; Terfa et al., 2013) and
leaf positioning (Paradiso et al., 2011). In order to get data on the
acclimatisation effects due to different light sources, in this study
we performed a series of growth chamber experiments with differing light quality. The aims of the study were to: i) investigate
the importance of plant physiological acclimatisation to their light
environment and ii) evaluate alternating light quality as a way to
counteract acclimatisation and thus improve light use efficiency.
2. Materials & methods
2.1. Plant material
Stem cuttings of Rosa × hybrida ‘Toril’ were rooted in 12cm plastic pots with a peat-based growth medium (Degernes
Torvstrøfabrikk AS, Degernes, Norway) and seeds of Solanum
lycopersicum ‘Espero’ were sown in pots similar with the ones
used for Rosa. Before starting the different treatments, the plants
were kept in a greenhouse (heating temperature 18 ◦ C, ventilation
temperature 22 ◦ C, misting if relative humidity was below 70%).
Supplemental lighting, a mixture of high-pressure sodium and
high-pressure mercury lamps (HPS + HPI ratio 2:1, Gavita 400 W,
Gavita AS, Andebu, Norway) at a photon flux density (PFD) of
100–120 ␮mol m−2 s−1 was supplied for 16 h day−1 when natural outside irradiation was below 200 Wm−2 (corresponding to
∼460 ␮mol m−2 s−1 ).
2.2. Experimental conditions
The experiment was performed in controlled climate chambers
(2 m2 ) at the Centre for Plant Research in Controlled Climate (SKP),
Ås, Norway. Three-week-old plants grown as above were transferred to the climate chambers. At the time of transfer, the shoot
of the rose plants was pinched over five nodes. The tomato plants
were at stage 103 according to the BBCH scale (Feller et al., 1995)
at the start of the experiment (the third fully developed true leaf
on the main stem), with an average plant height of 51 ± 20 mm.
The climate in the chambers was set to 20 ◦ C and 70% RH. The
CO2 concentration was ambient (380 ± 20 ppm). The plants were
irrigated manually with respect to depletion using a nutrient solution composed of Kristalon Indigo (N-P-K 9-5–25 + micronutrients)
and Ca(NO3 )2 (Yara, Oslo, Norway) in the ratio 1:1 w/w, at
conductivity 2.5 mS cm−1 . Three different lighting regimes were
provided: A) HPS light (R:FR (660/730 nm) ratio ∼5); B) LED
light (Heliospectra L4A, Heliospectra AB, Gothenburg, Sweden.
Spectrum: R:FR-ratio ∼6, R:B ratio 2:1) (Fig. 1); and C) alternating HPS and LED light, with three days of each. A PFD of
200 ± 20 ␮mol m−2 s−1 (measured with a Li-Cor Li 250, Li-Cor, Lincoln, NE, USA) was supplied for 16 h day−1 , corresponding to a total
daily light integral of 11.5 mol m−2 day−1 . The plants were redistributed within each chamber once a week to compensate for any
irregularities in light distribution. The plants were grown in the
climate chambers for 55 (rose) or 23 (tomato) days.
area (Li-Cor LI-3100, Li-Cor, Lincoln, NE, USA) were measured. The
chlorophyll content of the leaves was measured at the end of the
experiment using a chlorophyll meter (Hansatech CL-01, Hansatech Instruments Ltd, King’s Lynn, UK). The first fully expanded leaf,
as well as the lowest leaf (for tomato) was used for measurements.
Leaf and stem fresh weight was measured at the end of the experiment. The dry weight was determined after 48 h of drying at 60 ◦ C.
The photosynthetic capacity (Amax ) of the leaves was measured
two weeks after start of the experiment using a leaf chamber photosynthesis meter (LC Pro, ADC Bioscientific, Hoddesdon, UK). The
capacity was measured at six different PFD levels in the range
0–1000 ␮mol m−2 s−1 using a light source composed of red and blue
LEDs (R:B ratio 5:2). Measurements were taken on the second fully
expanded leaf below the apex. The leaf temperature was adjusted
to 20 ◦ C during the measurements.
In addition, for roses, the photosynthetic capacity was measured for leaves exposed to full light level in the growth chamber
(200 ␮mol m−2 s−1 ) and for leaves where the light was filtered
through one leaf, to simulate conditions in the lower part of the
canopy. In this case, the lamp type used for the treatment was used.
The values presented are the mean of 10 measurements.
2.4. Physical analysis
The spectral output of the light sources used in the experiment was measured using a spectroradiometer (StellarNet Epp
2000, Apogee Instruments, Inc., Logan, UT. USA). Based on the measurements, the phytochrome photostationary state was calculated
as described by Sager et al. (1988). The temperature and relative
humidity in the chambers were logged every 5 min (Priva Office,
Priva, de Lier, the Netherlands). Leaf temperature was measured
regularly during the experiments using an IR thermometer (Raytek
Raynger ST, Raytek Corporation, Santa Cruz, CA, USA). The spectral
properties (transmittance and reflectance) of detached leaves were
measured (Ocean Optics SD2000, Ocean Optics, Dunedin, FL, USA)
on the third fully developed leaf below the apex using the method
described by Solhaug et al. (2010). Briefly, the leaf was illuminated
with light from a standardized light source (Halogen lamp) through
an optical fibre, and the transmitted/reflected light was analysed
with respect to its spectral composition.
Chlorophyll fluorescence was measured using a chlorophyll
fluorescence meter (PAM-2500, Heinz Walz GmbH, Effeltrich,
Germany) on dark-adapted leaves (basic fluorescence, F0 , maximal
fluorescence, Fm , and PS II Yield, Fv /Fm ) and in the presence of light
(maximal fluorescence, Fm ’, and incident fluorescence, Ft ). Chlorophyll fluorescence was measured on the second fully expanded leaf
below the apex.
2.5. Statistics
The experiment was run in duplicate, with 10 plants from each
species per repetition. Two-sided analysis of variance (ANOVA)
with Tukey’s multiple comparison test was used for data analysis (Minitab 16, Minitab Inc., State College, PA, USA). A value of
P ≤ 0.05 was considered significant. For leaf spectral property measurements, data were analysed at 20-nm intervals from 400 to
800 nm.
3. Results
2.3. Biometric analysis
3.1. Plant growth parameters
At the end of the experiment, plant height, width (plant diameter), number of lateral shoots, internode length (calculated as
total shoot length/number of nodes), number of leaves and leaf
Rose and tomato plants grown under LED light were generally more compact, with lower plant height and shorter internodes
(Table 1). Compared to plants grown with HPS-lamps, plant height
K.-J. Bergstrand et al. / Scientia Horticulturae 204 (2016) 1–7
3
Relative spectral distribution
Relative spectral irrandiance
1.2
1
0.8
HPS
0.6
LED
0.4
0.2
0
350 370 390 410 430 450 470 490 510 530 550 570 590 610 630 650 670 690
Wavelength (nm)
Fig. 1. Relative spectral distribution of the different light sources used in the experiment. HPS = high-pressure sodium lamp, LED = light-emitting diode light. The LED-spectrum
was composed of the following LEDs: Indium Gallium Nitride (InGaN, peak 402 nm), InGaN (peak 419 nm), InGaN (peak 445 nm), Aluminium Indium Gallium Phosphide
(AlInGaP peak 663 nm), and AlInGaP (peak 737 nm).
3.2. Photosynthesis and chlorophyll fluorescence
The specific photosynthetic capacity (maximum photosynthetic rate) of the leaves was higher for plants grown in LED
light, but the differences were only significant at light intensities >400 ␮mol m−2 s−1 , which was above the light level used
for growth during the experiment (Fig. 2). Chlorophyll content
was also generally higher for leaves developed under LED light
(Table 1). Photosynthesis, measured at 200 ␮mol m−2 s−1 , was
not significantly different between treatments for roses (6.26.9 ␮mol m−2 s−1 ). There were also no significant differences when
photosynthesis was measured with light filtered through another
leaf, but in this case there was a trend for lower photosynthetsis for
leaves in the LED treatment, with values below the compensation
point (−0.4 ␮mol m−2 s−1 ), whereas in the HPS treatment leaves
shaded by another leaf still contributed to the net photosynthesis
of the plants, although at low values (0.4 ␮mol m−2 s−1 ).
There were also differences between treatments with respect
to chlorophyll fluorescence for roses (Table 2). Minimum fluorescence was higher for dark-adapted leaves from the LED treatment,
but the maximum quantum yield of PSII (Fv /Fm ) was significantly
lower for leaves from plants grown with HPS light, indicating accli-
Tomato
30
*
*
*
Pn (µmol m-2 s-1)
25
20
15
A
10
B
C
5
0
0
200
-5
400
600
800
1000
Light intensity µmol m-2 s-1
Rose
30
*
25
Pn (µmol m-2 s-1)
was reduced by 13% for plants grown with LED light or alternating light. Moreover, the total leaf area was lower in tomatoes
and roses grown under LED light. However, for tomatoes this was
because average leaf size was lower for plants grown with LED light,
whereas for roses the main reason for the lower leaf area in the
LED treatment was fewer leaves and individual leaf size was not
different from plants grown with HPS light.
With regard to fresh weight, there were no significant differences between plants grown with HPS and LEDs. However, the plant
dry weight was significantly lower for tomato plants grown with
LED as compared with HPS, but in roses, the dry weight was not
differing with respect to treatment. Specific leaf area was significantly higher for plants grown under HPS lamps (27.8 m2 kg−1 )
than for plants grown under LED lamps (26.3 m2 kg−1 ). In general,
measured plant parameters for plants in alternating light conditions resembled those of plants in the LED treatment more than
those of plants in the HPS treatment. In fact, for most measured
parameters (plant height, leaf area, fresh weight), values for plants
grown with an alternating light regime were significantly different
from plants grown with HPS light, but not from plants grown with
LED light (Table 1).
*
*
*
20
15
A
10
B
C
5
0
0
-5
200
400
600
Light intensity (µmol
800
1000
m-2 s-1)
Fig. 2. Photosynthetic rate in rose (top) and tomato (bottom) grown under three
different light sources: (A) high-pressure sodium (HPS), (B) light-emitting diode and
(C) alternating HPS/LED at three days each. *indicates significant difference (P ≤ 0.05)
between treatments, N = 6).
matisation of the photosynthetic apparatus to the light conditions.
Fluorescence measured on light-exposed leaves also showed differences, with lower steady state fluorescence (Ft ) in leaves exposed
to LED light.
K.-J. Bergstrand et al. / Scientia Horticulturae 204 (2016) 1–7
14.5 ± 4.2 b
19.3 ± 5 a
22.3 ± 5.8 a
Table 2
Chlorophyll fluorescence in potted roses grown with three different light regimes:
(A) high-pressure sodium (HPS) light, (B) light-emitting diode (LED) red/blue light
and (C) alternating HPS/LED at three days each. Figures within columns with different letters are significantly different (Tukey’s multiple comparison test, P ≤ 0.05,
N = 12).
12.7 ± 1.9 b
15.9 ± 3.2 a
15.2 ± 3 a
5.17 ± 0.81 a
5.06 ± 0.68 a
5.07 ± 0.98 a
56.0 ± 6.5 a
53.2 ± 5 ab
50.6 ± 8.1 b
Treatment
A
B
C
Light-exposed
F0
Fm
Fv /Fm
Fm c
Ft
2629 a
1853 b
1961 b
6561 a
6656 a
6579 a
0.60 b
0.72 a
0.70 a
2589 a
2510 a
2589c a
1496 a
1076 b
1522c a
3.3. Climatic factors and physical properties of leaves
Leaf temperature was on average 0.9 ◦ C higher in the HPS treatment than in the LED treatment for tomato and 1.3 ◦ C higher in
HPS for roses (data not shown). The measured air temperature was
20.0 ± 0.28 ◦ C for treatment A, 20.0 ± 0.17 ◦ C for treatment B and
20.0 ± 0.21 ◦ C for treatment C.
In general, leaves from rose plants grown under HPS light had
higher light transmission and reflectance than those from plants
grown under LED light, especially in the green range (500–600 nm)
(Fig. 3). There were significant differences in both reflectance and
transmittance for most wavelengths in rose leaves. However, for
wavelengths >740 nm, wavelengths around 450 nm, and, in the case
of reflectance, wavelengths from 640 to 690 nm, there were no significant differences. For tomato leaves, there were less significant
differences between treatments (lamp types) with respect to transmittance. The reflectance was higher for leaves developed under
alternating LED/HPS light as compared with leaves developed in
LED-light except for wavelengths 540–580 nm and wavelengths
>700 nm.
The phytochrome photostationary state was calculated to be
0.84 for HPS and 0.83 for LED.
4.27 ± 0.61 a
4.34 ± 0.54 a
4.32 ± 0.79 a
9.3 ± 8.8 a
7.9 ± 7.7 a
7.5 ± 7.3 a
38.5 ± 5 a
39.6 ± 3.5 a
36.5 ± 5.9 a
4. Discussion
FW = fresh weight; DW = dry weight, Chl = chlorophyll content.
1071 ± 140 a
1038 ± 104 ab
970 ± 144 b
47.0 ± 5.7 a
36.2 ± 4.8 b
40.4 ± 5.9 b
422 ± 39 a
315 ± 40 b
330 ± 31 b
A (HPS)
B (LED)
C (Alter.)
Tomato
Dark-adapted
F0 = basic fluorescence, Fm = maximal fluorescence in darkness, Fv /Fm = PS II yield,)
Fm ’ = maximal fluorescence in the presence of light, Ft = incident fluorescence.
c
Measured in HPS light.
0.9 ± 0.25 a
0.72 ± 0.22 b
0.75 ± 0.25 ab
13.6 ± 5 b
17.2 ± 7.6 a
15.6 ± 7.7 ab
11.94 ± 4.01 a
9.32 ± 3.28 a
10.34 ± 3.57 a
53.2 ± 18.7 a
43.5 ± 14.6 a
47.7 ± 16.7 a
Chl bottom
Chl top
DW tot (g)
FW tot (g)
DW stem (g)
5.62 ± 2.56 a
4.07 ± 2.02 a
4.68 ± 2.7 a
6.32 ± 1.55 a
5.25 ± 1.49 a
5.66 ± 1.14 a
26.2 ± 12 a
20.8 ± 9.8 a
22.8 ± 13.2 a
DW leaf (g)
FW stem (g)
FW leaf (g)
27 ± 7.6 a
22.7 ± 6 a
24.9 ± 5 a
Leaf area (cm2 )
1990 ± 596 a
1342 ± 367 b
1555 ± 349 b
28.6 ± 5.4 a
25.8 ± 5.1 b
27.8 ± 5.2 ab
Internode length (mm)
A (HPS)
B (LED)
C (Alter.)
Rose
Plant height (mm)
392 ± 58 a
342 ± 42 b
343 ± 49 b
Treatment
Plant type
Table 1
Plant parameters measured at the end of the experiment (23 days after start of the experiment for tomato and 55 days for rose, respectively) for the two different plant species, rose and tomato. Three different light regimes
were used: (A) high-pressure sodium (HPS) light, (B) light-emitting diode (LED) red/blue light and (C) alternating HPS/LED at three days each. Numbers within columns for each plant species with different letters are significantly
different (Tukey’s multiple comparison test, P ≤ 0.05, N = 10).
4
As the R:FR-ratio and the phytochrome photostationary state
were similar for both the HPS and LED treatments, it does not
seem feasible to consider the reduction observed in cell expansion
a phytochrome-mediated effect. The lower leaf area and shorter
internodes in the LED treatment are likely to be the result of
cryptochrome-mediated blue light effects (Jenkins et al., 1995). It is
generally acknowledged that increasing the proportion of blue light
will lead to less elongation and smaller leaves, e.g. several studies
have reported a relationship between blue light and plant elongation (Islam et al., 2012; Hernández and Kubota, 2015). However,
some contradictory results have also been reported, e.g. Gautam
et al. (2015) found that the effect of blue light on stem elongation
is dependent on the presence of far red light. It has also been found
that using exclusively blue light induces increased stem elongation
in petunia (Fukuda et al., 2011), eggplant (Hirai et al., 2006) and sunflower (Bergstrand et al., 2014) compared with other narrow-band
wavelengths. Decreased stem length, but not leaf area, was demonstrated by Poulet et al. (2014). In the present study, both internode
length and leaf size were reduced in tomato when a spectrum rich
in blue light was supplied, whereas in rose internode length and leaf
area were reduced without a significant reduction in leaf size. These
results for leaf area indicate that the increased amount of blue light
in the LED treatment was responsible for the lower total leaf area in
tomatoes, which was caused by reduced leaf size, while lower leaf
area in roses was related to lower leaf temperature and decreased
K.-J. Bergstrand et al. / Scientia Horticulturae 204 (2016) 1–7
Transmiance Rose
Reflectance Rose
70
60
50
40
A
30
B
20
C
10
Relave reflectance (%)
Relave transmission (%)
70
0
60
50
40
A
30
B
0
Wavelength (nm)
400 425 450 475 500 525 550 600 625 650 675 700 725 750 775 800
Transmiance tomato
Reflectance tomato
70
60
60
50
A
30
B
20
C
10
0
Relave reflectance (%)
70
40
C
20
10
400425450475500525550600625650675700725750775800
Relave transmiance (%)
5
50
40
A
30
B
20
C
10
0
400425450475500525550600625650675700725750775800
400 425 450 475 500 525 550 600 625 650 675 700 725 750 775 800
Wavelength (nm)
Wavelength (nm)
Fig. 3. Spectral properties: transmittance (left) and reflectance (right) of leaves from rose (top) and tomato (bottom) cultivated under different light conditions: (A) highpressure sodium (HPS) light, (B) light-emitting diode (LED) red/blue light and (C) alternating HPS/LED at three days each. N = 10.
leaf unfolding rate with LED compared with HPS light. This has been
shown previously by Carlsson et al. (1991) and Milthorpe (1959).
As suggested by other authors (Wild and Holzapfel, 1980; Hao
et al., 2012), there seemed to be a relationship between increased
amounts of blue light and leaf chlorophyll content in this study.
High levels of chlorophyll are typical ‘sun-type’ characteristics
associated with leaves grown in environments rich in blue light
(Gautam et al., 2015). More chlorophyll and higher density of
chloroplasts in the leaves might be an explanation for the higher
specific photosynthesis capacity found in leaves grown under LED
light. Similar findings with respect both to chlorophyll content and
to specific photosynthetic capacity have been reported by Terfa
et al. (2013). The lower transmittance and reflectance for leaves
from LED treatments also indicates better absorption of light. However, as the green region in particular differs in transmittance, it
is likely that other pigments (carotenoids) are more abundant in
leaves developed under LED light. The lower photosynthetic capacity of leaves from plants grown under HPS light was confirmed by
the photosynthetic response curves and the Fv /Fm readings, which
indicated lower potential quantum efficiency of PSII (Maxwell and
Johnson, 2000), and by the higher values of steady state chlorophyll
fluorescence (Ft ) for the light-exposed leaves, indicating lower photosynthetic efficiency due to stress (Zarco-Tejada et al., 2003) when
light was supplied by an HPS lamp. A non-stressed plant normally has dark-adapted Fv /Fm values of around 0.83 (Björkman
and Demmig, 1987), whereas values in the range <0.8, as measured for plants grown under HPS lamps in this study, clearly
indicate photoinhibitory damage. It seems in this case Fv /Fm is
not useful as a predictor for plant growth as biomass accumulation was as high for the HPS-treatment with low Fv /Fm readings
as for the LED-treatments displaying non-stressed values of Fv /Fm .
The increased F0 values observed in plants grown in HPS light are
another indicator of photoinhibitory damage. It is generally known
that photoinhibitory damage can arise due to water stress, heat
stress, low temperature stress or stress due to excess light (Maxwell
and Johnson (2000), and references therein). However, stress due
to the spectral composition of the light is not a generally acknowledged effect, although suggested by Trouwborst et al. (2016). A
plausible explanation for the lower Fv /Fm readings in plants subjected to HPS light is that the chloroplast ultrastructure of those
plants adapted to low light conditions (Lichtenthaler, 1996) following the low exposure to blue light from the HPS lamps, thus
making them more sensitive to light stress. The fact that higher
photosynthesis was recorded for leaves grown in LED-light might
also indicate that these leaves are better acclimatized to absorbing
red/blue LED light, which was the type of light used for photosynthesis measurements. However, this is to some extent contradicted
by higher absorbance in the green spectral range for leaves developed in the green-deficient LED-light.
Previous experiments comparing HID lamps with LED-based
light sources in greenhouse environments have often shown lower
biomass production in the LED treatments (Dueck et al., 2012;
Islam et al., 2012; Bergstrand and Schüssler, 2013; Hernández and
Kubota, 2015). This is despite the fact that LED light sources in general are designed to target the peaks of the photosynthetic response
curve (Poulet et al., 2014). Lower leaf temperatures due to the
absence of radiant heat from solid state light sources seems a plausible explanation, as leaf temperatures are generally 1–2 ◦ C lower
for leaves in LED light compared with HID light (Islam et al., 2012;
Bergstrand and Schüssler, 2013; Hernández and Kubota, 2015). The
leaf temperature values measured in the present study fell within
this range. However, the differences in biomass production in this
study were not as pronounced as in previous studies. There are a
few possible explanations for this, but the most likely is the fact that
6
K.-J. Bergstrand et al. / Scientia Horticulturae 204 (2016) 1–7
this experiment was performed in growth chambers, whereas the
studies mentioned above were performed in greenhouses. Plants
grown exclusively in HPS light might have suffered lack of specific wavelengths (e.g. blue light), and the associated reduction
in chlorophyll synthesis might have reduced plant performance.
Another factor that might favour HID lamps in the greenhouse
but not growth chamber environments is the increase in air temperature in the canopy associated with the radiant heat warming
the leaves, as demonstrated by Bergstrand and Schüssler (2013).
Increasing air temperature in the canopy leads to air movement,
decreasing the boundary layer of the leaves, whereas a ‘cold’ light
source such as LED light causes less air movement and may create a thicker boundary layer depleted in CO2 but with increased
humidity, leading to lower transpiration and, in the longer run,
possible nutrient deficiency. In the growth chambers used in this
experiment, air change was forced with fans at a relatively high
air velocity, which probably decreased differences in temperature,
humidity and CO2 concentration between the stomata/plant and
the surrounding air.
As pointed out by Nelson and Bugbee (2014), commercial plant
lighting fixtures with radiation rich in photons in the 460 and
660 nm region are not optimal for plant growth, as the optimum for photosynthesis according to McCree (1972) is within
the 600–630 nm range, where HPS lamps have a major part of
their output. The photosynthetic action spectrum presented by
McCree (1972) only applies to one single leaf, however, and a whole
canopy will use light in the green area better than a single leaf, as
light transmitted through the top layer will be absorbed by lower
leaves, thereby contributing to their photosynthesis (Massa et al.
(2015) and references therein). The higher light transmission in the
green region for rose leaves developed in HPS light in the present
study might therefore not be negative. In fact, better distribution
of light within the canopy because of higher transmission of HPS
light might be beneficial, preventing top leaves from reaching light
saturation. However, this assumption was not confirmed by the
chlorophyll fluorescence (Ft ) measurements, which showed higher
steady state fluorescence in leaves exposed to HPS light compared
with LED light, indicating lower photosynthetic efficiency.
5. Conclusions
The effects of light quality on plant performance with respect to
fresh and dry weight were small, but stem elongation and leaf area
were lower when using LEDs. The specific photosynthetic capacity of the leaves and quantum yield of PSII were higher in plants
grown under LEDs. Light transmission through leaves was higher
for leaves developed in HPS light and there was a trend for higher
photosynthesis in lower leaves of plants grown in HPS light, indicating better transmission of light into the canopy when using HPS
lamps. Subjecting plants to alternating LED/HPS light resulted in
plants resembling the LED treatment more than the HPS treatment
and, in general, alternating light quality was not beneficial with
respect to plant growth. It was concluded that greater leaf area
(due to lower amount of blue light and higher leaf temperature)
and better penetration of light into the canopy compensated for
the lower specific photosynthetic capacity and lower maximum
quantum yield in plants grown with HPS light.
Acknowledgements
We thank Professor Knut Asbjørn Solhaug for fruitful discussions
and Mrs. Ida Hagen for skilful technical assistance. The project was
funded by the Swedish Research Council Formas (Dnr 225-20131019) and the Norwegian Research Council and Norwegian growers
(project “Veksthusdynamikk”), which are gratefully acknowledged.
Heliospectra AB is also acknowledged for their contribution to the
project.
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