The Effect of Diffuse Light on Crops

Abstract

Light is not evenly distributed in Dutch glass greenhouses, but this can be improved with diffuse light. Modern greenhouse coverings are able to transform most of the light entering the greenhouse into diffuse light. Wageningen UR Greenhouse Horticulture has studied the effect of diffuse light on crops for several years. Modelling and experimental studies showed that crops such as fruit vegetables with a high plant canopy as well as ornamentals with a small plant canopy can utilize diffuse light better than direct light. Diffuse light penetrates the middle layers of a high-grown crop and results in a better horizontal light distribution in the greenhouse. Diffuse light is absorbed to a better degree by the middle leaf layers of cucumber, resulting in a higher photosynthesis. The actual photosynthesis of four pot plant species was found to be increased and crop temperatures were lower during high irradiation. The yield of cucumbers was increased, and the growth rate of several potted plants was increased. These investigations have resulted in a quantitative foundation for the potentials of diffuse light in Dutch horticultural greenhouses and the selection and verification of technological methods to convert direct sunlight into diffuse light.

Full text

The Effect of Diffuse Light on Crops
S. Hemming, T. Dueck, J. Janse and F. van Noort
Wageningen University and Research Centre (Wageningen UR), P.O. Box 16
Bornsesteeg 65, 6700 AA Wageningen
The Netherlands
Keywords: covering material, photosynthesis, light distribution, cucumber, Ficus,
Schefflera, chrysanthemum, kalanchoe
Abstract
Light is not evenly distributed in Dutch glass greenhouses, but this can be
improved with diffuse light. Modern greenhouse coverings are able to transform
most of the light entering the greenhouse into diffuse light. Wageningen UR
Greenhouse Horticulture has studied the effect of diffuse light on crops for several
years. Modelling and experimental studies showed that crops such as fruit
vegetables with a high plant canopy as well as ornamentals with a small plant
canopy can utilize diffuse light better than direct light. Diffuse light penetrates the
middle layers of a high-grown crop and results in a better horizontal light
distribution in the greenhouse. Diffuse light is absorbed to a better degree by the
middle leaf layers of cucumber, resulting in a higher photosynthesis. The actual
photosynthesis of four pot plant species was found to be increased and crop
temperatures were lower during high irradiation. The yield of cucumbers was
increased, and the growth rate of several potted plants was increased. These
investigations have resulted in a quantitative foundation for the potentials of diffuse
light in Dutch horticultural greenhouses and the selection and verification of
technological methods to convert direct sunlight into diffuse light.
INTRODUCTION
Light is not evenly distributed in Dutch glass greenhouses. Fruit vegetables like
cucumber have a high leaf area index and intercept a large quantity of light with the upper
leaves, while the middle and lower leaves receive much less light and contribute very
little to photosynthesis, growth and in the end, production. The crop would benefit if
upper leaves would intercept less incident light and the middle and lower leaves a greater
proportion, in order to realize a more uniform light interception over the foliage. Hovi et
al. (2004) showed that a higher amount of artificial light within a crop achieved by inter-
lighting significantly increased photosynthesis of the lower leaves of cucumber. The same
effect can be realized by diffuse light. From earlier investigations in forests (Farquhar and
Roderick, 2003; Gu et al., 2003), apple trees (Lakso and Mussleman, 1976) and grass
canopies (Sheehy and Chapas, 1976) it is known that diffuse light is able to penetrate
deeper into a plant canopy in comparison to direct light and that photosynthesis in forests
is increased by diffuse light. There are also indications that plants have developed
mechanisms to use diffuse light more efficiently (DeLucia et al., 1996; Vogelmann,
1996). In young plants and small plants like pot plants the horizontal light distribution is
not optimal. Shadows cast from the greenhouse construction have a negative influence on
the plant production. In order to realize a uniform production, the light distribution has to
be uniform over the whole canopy. This can be achieved by diffuse light. Light can be
made diffuse by modern covering materials (Hemming et al., 2004). Such materials
contain pigments, macro- or microstructures, which are able to transform all incoming
direct light into diffuse light. Depending on the design of the structure the incoming light
scatters, the angle of incidence is changed. Efficient structures make the light diffuse
without a significant reduction in light transmission.
During the past four years Wageningen UR has investigated the potential of
diffuse covering materials used in Dutch greenhouses (Hemming et al., 2004). The
suitability of several greenhouse covering materials and their optical properties (PAR
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Proc. IS on Greensys2007
Eds.:S. De Pascale et al.
Acta Hort. 801, ISHS 2008

transmission τdirect and τdiffuse, haze) was investigated in laboratories as well as in practice.
On the basis of light and crop models (Goudriaan, 1988; Marcelis et al., 2000) the effect
of diffuse light on crop photosynthesis was studied (Hemming, 2006). In this paper the
effect of diffuse covering materials on light distribution, plant photosynthesis, plant
growth and development will be elaborated. The results are based on crop experiments
with cucumbers and four different types of potted plants.
MATERIALS AND METHODS
In four greenhouse compartments, each 150 m2, experiments were conducted first
with cucumbers and later with four pot plant species. In two compartments the crops
received mainly diffuse light, in the other two compartments they received natural light.
To change the light conditions inside the greenhouse, roof and side-walls of the glass
greenhouses were covered with either a diffuse plastic film “F-Clean diffuse” or with a
clear plastic film “F-Clean”, both 100 μm from Asahi Glass Europe bv. The optical
properties of both materials are described in Figure 1. The diffuse material had a haze of
50%. Cucumbers ‘Shakira’ were planted on April, 18th 2006. They were grown in 18
rows with 3.5 plants per m2. Rockwool was used as substrate with an average pH of 5.3
and an average EC of 3. On May, 9th, the crop reached the wire, the top was removed, and
two shoots remained. The first flower appeared in the sixth bud after 10 days, and the first
flower in the sixteenth bud appeared after 16 days. First harvest took place on May, 9th
and crop ended on July, 26th 2006. Cuttings of pot chrysanthemum ’Danielson’ and
kalanchoe ‘Kerinci’ and young plants of Ficus benajmina ‘Exotica’ and Schefflera
‘Compacta’ were potted on August, 30th 2006 in a 13 cm pot filled with substrate flush
fine from TrefEgo. Plants were grown in natural short day. Schefflera and Ficus were
fertilized with N-P-K 9-2-4, an EC of 1.7 and a pH of 5.6, chrysanthemum and kalanchoe
were fertilized with NPK 4-2-4, an EC of 2.0 and a pH of 5.6. Plants were grown with 50
plants per m2 and 20 plants per m2 at the end of the growing period. Chrysanthemum tops
were removed after 14 days.
In all compartments greenhouse climate was regulated and monitored: dry and wet
bulb temperature [oC], relative humidity [%], CO2-concentration [ppm], ventilation
opening [%], global radiation [W m-2], PAR [μmol m-2 s
-1]. Crop temperature was
monitored with four IR-camera’s of Growlab Hogendoorn bv, The Netherlands. PAR Lite
sensors and pyranometers CM10 from Kipp & Zonen bv, The Netherlands, were installed
above the crop for permanent measurements. Additionally, light distribution within the
crop, with different heights, on diffuse and clear days, in young and full-grown crops was
measured vertically and horizontally with a Sunscan system from Delta-T Ltd., U.K.
The photosynthetic capacity was measured with an advanced mobile
photosynthesis system (LCpro+, ADC Bioscientific Ltd., U.K.) with a leave chamber of
6.25 cm2. Measurements were carried out in different crop layers of cucumber at two light
levels (465 µmol m-2 s
-1 and 1250 µmol m-2 s
-1) on fully-grown leaves at a CO2-
concentration of 700 ppm, a temperature of 21°C and a relative humidity of 85%.
Moreover full light response curves were measured for the four pot plants. The amount of
chlorophyll was estimated with a SPAD 50 meter from Minolta. For cucumber the
amount of protein content [µg g-1] and the RuBisCo-content [µg g-1] was determined.
Destructive measurements were carried out to examine possible changes in crop
morphology of cucumber every second week. Cucumbers were analyzed in four different
leaf layers, e.g. amount of leaves per layer [-], fresh weight of leaves, stems and fruits per
layer [g], dry weight of leaves, stems and fruits per layer [g], dry matter content, leave
area per layer [m2], LAI per plant [-], SLA per plant [g m-2]. Destructive measurements of
the four pot plants were carried out after six weeks and at the end of the crop growth
period. Next to the parameters mentioned above, the length of the plant [cm], the amount
of lateral shoots [-], dry weight and fresh weight of buds and flowers [g] and the time of
flowering [date] were measured for the flowering pot plants. Leave orientation was
determined with 2D and 3D image analysis techniques.
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RESULTS AND DISCUSSION
To estimate the potential of diffuse greenhouse covering materials, the amount of
natural global radiation has to be known. The Dutch climate is characterised by 3650 MJ
m-2 global radiation per year, of which 1081 MJ m-2 direct light. This amount can be
potentially transformed into diffuse light, the rest is already diffuse. Only 200 MJ m-2 of
the direct light occurs during the winter month, 880 W/m2 during the summer month. It
can be assumed that a diffuse covering material will give the most advantages during
spring, summer and autumn months. However, as long as no light losses appear under the
covering, no disadvantages are to be expected during the winter months.
During the experiments, the greenhouse climate in the different treatments (diffuse
or natural) was comparable (Table 1Error! Reference source not found.).
Measurements in cucumber showed that crop temperature in higher leaf layers in the crop
was 0.2-0.8°C lower in the diffuse treatment, but was 0.4°C higher in the lower layers on
days with high irradiation (data not shown). The amount of PAR light under the diffuse
covering was about 4% less than under the other treatment (Fig. 2). However, the
horizontal light distribution was much more equally under the diffuse covering (Fig. 3).
Measurements of light distribution inside the cucumber crop showed that after three
weeks of growth, more than 85% of the light was being intercepted by the crop and a
difference in light interception between treatments could be observed. More light was
intercepted in the diffuse treatment on clear days, especially by the intermediate leaf
layers (Fig. 4). No difference in light interception between the diffuse and direct light
treatments was observed on cloudy days (data not shown). Leaves at intermediate leaf
layers on the main stem as well as young leaves on the secondary branches had a higher
rate of photosynthesis at normal light conditions (500 µmol m-2 s-1) in diffuse light (Fig.
5). Photosynthesis at light-saturating conditions (1250 µmol m-2 s
-1) was higher under
direct light and in all leaf layers. Upper and middle leaves also contained more
chlorophyll when grown under diffuse light, whereas lower leaves showed lower SPAD
values (Table 2Error! Reference source not found.).
It can be concluded that more light is absorbed by the middle leaf layers and
photosynthesis is increased, thus the assimilation rate was higher due to diffuse light. The
crop temperature probably influenced this process as well, as it was much higher under
direct than under diffuse light conditions. According to theory, the physiologically older
leaves deeper in the crop receive less light, have less RuBisCo and are photosynthetically
less active. RuBisCo was found to be slightly higher in diffuse light and decreased in
lower layers of the crop (Table 3Error! Reference source not found.). This may be due
to a better light absorption in the middle of the crop so that RuBisCo is still able to
actively contribute to the photosynthesis process without being broken down and
reallocated to younger parts of the crop receiving more light.
The proportion harvested cucumbers in relation to plant biomass increased from
June onwards due to diffuse light. Cucumber production in kilo’s increased by 4.3% and
the number of cucumbers increased by 7.8% (data not shown). The fruits were somewhat
smaller on average. However, the light transmission in the diffuse light treatment was ca.
4% lower than under clear covering. Given the same light transmission, the difference
between treatments would have been even greater. With 4% more light, the estimated
difference in production would have been 7.8% in kilo’s and 11% in number of fruits.
This increase in production might have been actually realized if suppliers had been able to
produce greenhouse roof material without the loss of light transmission in the process.
The quality of fruits was judged on a regular basis and was slightly lower in the diffuse
light treatment. However, this did not influence the longevity of the fruits after harvest.
Similar positive effects of diffuse light have also been shown with pot plants. The
growth rate of all pot plants was increased. After six weeks chrysanthemum showed a
higher plant height, more branches, more leaves, a higher leaf area, a higher leaf and stem
dry weight, a higher relative growth rate (RGR) and more flowers. Comparable results
were observed for the other three species of pot plants after six weeks (data not shown).
Similar to the photosynthesis in cucumber, that of the four pot plant species was higher
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under diffuse light than under the clear covering (Fig. 6). Crop experiments with pot
plants were conducted in autumn to analyze the effect diffuse covering materials in
different seasons. The positive effects of diffuse light were clearly visible until the
beginning of November (week 45). After that the light loss of the covering used in the
experiments, about 4%, overruled the positive effects of diffuse light. Since the
experiment with chrysanthemum was finished by then, no negative effects were observed
(Table 4Error! Reference source not found.). The experiment with Ficus, however,
continued until the beginning of December (week 49). From Error! Reference source
not found. it can be clearly seen that the growth rate decreased in December as a result of
lower light levels in the diffuse treatment.
CONCLUSIONS
In conclusion, diffuse light has a positive influence on the production of cucum-
bers, especially during the summer. This positive effect can be explained by a change in
light penetration into the crop and by an increased photosynthesis capacity, so that a crop
like cucumber can utilize diffuse light better than direct light. In addition, diffuse roof
material results a lower crop temperature, especially higher in the crop which likely leads
to a more optimal conditions for photosynthesis.
In our opinion, a diffuse roof material for greenhouses with a minimal loss of light
should be further developed. This means that materials should be used with minimal 50%
haze, a light transmission of at least 90% (perpendicular) and 82% (hemispherical). A
lower light transmission will result in a loss of production, especially in the winter when
light is the limiting factor. Diffuse light in the crop is actually less important in the winter
because most of the natural light is already diffuse due to the predominantly cloudy
weather. The advantage of diffuse light can be realized in late spring, summer and early
autumn when natural light has a more direct character, and when too much (direct) light
in undesirable for many crops. In an earlier study, Hemming et al. (2005) examined the
economic prospects of diffuse roof material and concluded that at a 5% production
increase is possible and a diffuse roof material can be profitable. Diffuse covering
materials have potential advantages for other crops as well, i.e. sweet pepper, as well as
for cut flowers like rose.
ACKNOWLEDGEMENTS
This research is funded by the Dutch Ministry of Agriculture, Nature and Food
quality (LNV) in the scope of the research programme “Energy in protected cultivation”.
Literature Cited
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intercellular reflectance to photosynthesis in shade leaves. Plan, Cell and Environment
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Farquhar, G.D. and Roderick, M.L. 2003. Pinatubo, diffuse light and the carbon cycle.
Science 299: 1997-1998.
Goudriaan, J. 1988. The bare bones of leaf-angle distribution in radiation models for
canopy photosynthesis and energy exchange. Agric. and Forest Meteo. 43: 155-169.
Gu, L., Baldocchi, D.D., Wofsy, S.C., Munger, J.W., Michalsky, J.J., Urbanski, S.P. and
Boden, T.A. 2003. Response of a Deciduous Forest to the Mount Pinatubo Eruption:
Enhanced Photosythesis. Science 299: 2035-2038.
Hemming, S., Dueck, T., Marissen, N., Jongschaap, R., Kempkes, F. and van de Braak,
N. 2005. Diffuus licht – Het effect van lichtverstrooiende kasdekmaterialen op
kasklimaat, lichtdoordringing en gewasgroei. Wageningen UR report 557.
Hemming, S., van der Braak, N., Dueck, T., Elings, A. and Marissen, N. 2005. Filtering
natural light by the greenhouse covering – More production and better plant quality by
diffuse light? Acta Hort. 711: 105-110.
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round cucumber. Scientia Horticulturae 102 (3): 283-294.
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Lakso, A.N. and Musselman, R.C. 1976. Effects of Cloudiness on Interior Diffuse Light
in Apple Trees. J. Amer. Soc. Hort. Sci. 101 (6): 642-644.
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Tables
Table 1. Greenhouse climate during the cucumber experiments using a diffuse and a clear
covering material in 2006.
Clear Diffuse Average
North/South
Day Air temperature [oC] North 23.8 23.9 23.8
South 24.1 24.1 24.1
Average Clear/Diffuse 24.0 24.0
Relative humidity [%] North 68.9 69.7 69.3
South 75.3 76.6 76.0
Average Clear/Diffuse 72.1 73.2
CO2-concentration [ppm] North 430.1 414.0 422.1
South 418.4 436.4 427.4
Average Clear/Diffuse 424.2 425.2
Opening ventilation [%] North 93.3 94.2 93.8
South 99.6 101.1 100.4
Average Clear/Diffuse 96.5 97.6
Table 2. Average SPAD-values (=f([chlorophyl] m-2)) of four different leaf layers of
cucumber, divided in stem and side branches, between 9th of May and 11th of July
2006. grown under a diffuse and clear covering.
Stem Side branches
Leaf layer Crop height Clear Diffuse Clear Diffuse
4 150-200 cm 45,1 48,4 53,5 53,2
3 100-150 cm 40,0 42,6 44,1 43,5
2 50-100 cm 37,6 34,6 - -
1 0-50 cm 32,6 29,6 - -
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Table 3. RuBisCo content (mg g-1 fresh weight) ± standard deviation in four different leaf
layers of cucumber at the 9th of May and 16th of June 2006, grown under a diffuse and
clear covering.
RuBisCo content [mg g-1 fresh weight]
9th of May RuBisCo content [mg g-1 fresh weight]
16th of June
Clear Diffuse Clear Diffuse
Leaf
layer Side
branch Stem Side
branch Stem Side
branch Stem Side
branch Stem
4 - 3,1±2,3 - 3,9±2,2 4,0±2,1 5,4±3,1 5,9±1,9 5,2±3,2
3 - 1,3±1,1 - 1,7±1,6 7,5±3,4 0,9±0,2 6,5±2,2 1,6±1,7
2 - 0,9±0,8 - 1,2±0,8 - - - -
1 - 0,6±0,4 - 0,8±0,6 - - - -
Table 4. Plant growth parameters of chrysanthemum grown under a diffuse or clear
covering. Significances are shown with * at α=0.05, n=10, ns=not significant, -
parameter not measured.
Week 41 Week 45
Clear Diffuse Clear Diffuse
Plant height [cm] 32.15 34.75 * 43.20 44.45 *
Number of branches [-] 4.50 5.50 * 4.90 4.85 ns
Number leaves [-] 71.0 93.2 * 78.2 88.7 *
Leaf area [cm2] 900 1148 * 1175 1347 *
Leaf dry weight [g] 1.96 2.42 * 2.53 2.93 *
Stem dry weight [g] 1.39 1.78 * 4.31 5.00 *
SLA [m2 g-1] - - - 0.047 0.046 ns
RGR [average g g-1 wk-1] 0.56 0.70 0.94 1.06
Number flowers [-] - - - 27.4 30.7 *
Flower dry weight [g] - - - 2.56 2.65 ns
Table 5. Plant growth parameters of Ficus grown under a diffuse or clear covering.
Significances are shown with * at α=0.05 and ** at α=0.10, n=10, ns=not significant, -
parameter not measured.
Week 41 Week 49
Clear Diffuse Clear Diffuse
Plant height [cm] 39.2 41.1 * 64.1 63.0 ns
Number of branches [-] 9.25 9.95 ** 12.8 13.0 ns
Number leaves [-] 31.5 34.0 ns 68.8 65.2 ns
Leaf area [cm2] 496 532 ** 1340 1247 **
Leaf dry weight [g] 2.02 2.11 ns 5.66 5.06 *
Stem dry weight [g] 0.93 0.92 ns 3.38 3.21 ns
RGR [average g g-1 wk-1] 0.49 0.51 0.65 0.59 *
SLA [m2 g-1] - - - 0.024 0.025 ns
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Figures
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
400 450 500 550 600 650 700
PAR transmission [-]
wavelength [nm]
F-Clean clear tdir
F-Clean clear tdiff
F-Clean diffuse tdir
F-Clean diffuse tdiff
τ
dir
τ
diff
τ
dir
τ
diff
Fig. 1. Optical properties of a diffuse (F-Clean diffuse on glass) and a clear (F-Clean on
glass) covering material used in experimental greenhouses.
y = 0.775x
R² = 0.995
y = 0.746x
R² = 0.997
0
10
20
30
40
50
60
0 20406080
inside
outside
PAR [mol m
-2
day
-1
]
Clear South Diffuse South
y = 0.777x
R² = 0.997
y = 0.720x
R² = 0.996
0
10
20
30
40
50
60
0 20406080
inside
outside
PAR [mol m
-2
dag
-1
]
Clear North Diffuse North
Fig. 2. PAR measurements inside and outside the greenhouse in experimental green-
houses covered with a diffuse and a clear covering.
Fig. 3. Horizontal light distribution in greenhouses covered with a diffuse and a clear
covering material on a clear day.
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0
50
100
150
200
250
0 204060801
00
Crop height [ cm]
Light interception [%]
April 19
th
, 2006
0
50
100
150
200
250
0 204060801
Crop height [cm]
Light interception [%]
May 3
rd
, 2006
Clear
Diffuse
00
Clear
Diffuse
0
50
100
150
200
250
0 204060801
00
Cro p height [cm]
Light interception [%]
April 25
th
, 2006
0
50
100
150
200
250
0 204060801
Crop height [cm]
Light interception [%]
May 23
rd
, 2006
Clear
Diffuse
00
Clear
Diffuse
Fig. 4. Vertical light distribution and light interception of a cucumber crop on four
different dates grown under a diffuse and a clear covering on four clear days.
0
5
10
15
20
25
Middle layer
stem
Upper layer
stem
Middle layer
branch
Upper layer
branch
Photosynthesis [µmol m-2 s-1]
June 28
th
, 2006
Diffuse
Clear
0
2
4
6
8
10
12
0 100 200 300 400 500
Photosynthesis [µmol m-2 s-1]
Light intensity [µmol m-2 s-1]
Diffuse
Clear
Fig. 5. Photosynthesis in two leaf layers of
a cucumber crop on June, 28th,
grown under a diffuse and a clear
covering
Fig. 6. Actual photosynthesis in four
different pot plant crops grown
under a diffuse and a clear
covering.
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