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Shinji TAZAWA
Light Source Division, Iwasaki
Electric Co., Ltd.
(Gyoda, Saitama, 361-0021 Japan)
---Abstract-------------
In Part 1 of this report, we introduced fundamental aspects of the use of
artificial light in
horticulture, giving an outline of a number of different artificial light sources and discussing recent
research trends(such as the use of microwave-powered lamps, light-emitting diode
and laser diode devices) in
Japan.
Discipline: Agricultural facilities/Crop production/Horticulture
Additional key words: artificial light
source, supplemental lighting, plant factory
<http://ss.jircas.affrc.go.jp/engpage/jarq/33-3/tazawa/blueball.gif>
1...32):References
(Received for publication, December 18, 1998)
---Introduction-------------
Most terrestrial plants grow by selective absorption of natural light from the sun. In plant
factories and indoor living spaces, artificial light is necessary as a source of light energy. Therefore, it is necessary
to develop technologies to control the light environment and provide effective and economical irradiation
for plants. Part 1 of this report covers basic issues related to plant growth
and light.
---Wavelengths for effective plant growth---------------
Solar radiation is subject to extensive scattering and absorption by the
atmosphere before it reaches the surface of the earth. Direct solar radiation
has wavelengths ranging from 300 to 3,000 nm, and is divided into 3 bands:
ultraviolet radiation, visible radiation and infrared radiation. The
wavelengths of visible radiation for humans are in the range from 380 to 780
nm, and the peak of the visibility curve(photopic vision) is at 555 nm.
Similarly, plants have a range of wavelengths that are physiologically
effective. There are 2 types of effective radiation for plants: physiologically
active radiation and photosynthetically active radiation(PAR). These 2 types of
radiation, ranging from 300 to 800 nm, are physiologically effective in
photosynthesis, pigment biosynthesis, photoperiodism, phototropism and
photomorphogenesis8).
Physiologically active radiation is divided into 5 wavebands: near ultraviolet light(UV)300-400 nm, blue light(B)400-500 nm, green light(G)500-600 nm, red light(R)600-700 nm, and far-red light(FR)700-800 nm (Fig. 1(20KB)) .
Photosynthesis, which uses PAR(waveband 400 to 700 nm), requires an energy
source with high intensity. The units of PAR radiation are expressed as total
photon fluxes in this waveband, since this radiation induces chemical
reactions. The total energy emitted from the light source is designated as photosynthetic photon
flux(PPF). On the other hand, the energy actually received by plants is
designated as photosynthetic photon flux density(PPFD), and its S. I. units are expressed as ?mol?m-2?sec-1 . Although
quantum sensors are preferable for measuring the photon flux, because of their
high cost, radiation is often measured by PPFD with conversion factors for
illuminance.
---Light intensity suitable
for photosynthesis--------------
Light intensity suitable for
photosynthesis depends on the light
adaptation and acclimation properties of the plants, which in turn depend on
the environment of their place of origin. The effect of the light intensity can be estimated to some
extent by changes in morphology. Generally, plants which grow in the shade or
at low light intensities
(shade plants) have large, and thin leaves. Inside their leaves, parenchymatous
cells do not adequately develop, resulting in an increase of the development of
the grana structure and of the chlorophyll content in chloroplasts. The same
morphological changes also occur with exposure to red light. On the other hand, plants which grow at high light intensities (sun plants) have
thick leaves. Inside their leaves, parenchymatous cells are remarkably
developed, resulting in a lower development of the grana structure. However,
many enzymes important for photosynthesis can be observed. The same
morphological changes occur with exposure to blue light. These differences in the morphology can also be
observed in a single plant. Leaves that grow at low light intensities are referred to as shade leaves, and
leaves that grow at high light
intensities are referred to as sun leaves. Accordingly, leaves in the upper and
lower parts of trees have different photosynthetic capabilities9).
Morphological adaptation through changes of the light environment is related to the speed of
photosynthesis. Plants growing at high light intensities (for example, watermelons, tomatoes, cucumbers,
melons and C4 plants)have high saturation points, and they show a maximum
photosynthetic rate at the light
saturation point. Therefore, a large amount of light energy is required to cultivate plants that grow
better at high light
intensities. Fig.2(21KB) was obtained by measuring the absorption and release
of carbon dioxide during photosynthesis, and indicates the light adaption capability for
photosynthesis. When the light
intensity is low, the amount of carbon dioxide released by plant respiration is
higher than the amount of that absorbed for photosynthesis, resulting in a net
release of carbon dioxide. As the light
intensity increases, absorbed and released amounts of carbon dioxide change and
reach an equilibrium at point A where a net release of carbon dioxide is no
longer observed. This point is referred to as the compensation point. If the light intensity increases further,
the amount absorbed reaches point B. This point is the saturation point. A
suitable light intensity can
be determined somewhere between these points A and B according to the
particular requirements. On the other hand, since plants that grow under a low light energy (for example, lettuce,
Cryptotaenia japonica , herbage crops, and most of the indoor ornamental
plants) have low saturation and compensation points, it is relatively easy to
cultivate them, to provide them with supplemental lighting and to maintain
growth with artificial lighting. Table 1(77KB) shows the saturation and
compensation points of major crops, and Table 2(109KB) shows the saturation and
compensation points of ornamental plants. Indoor ornamental plants, most of
which are derived from jungle undergrowth, can maintain growth at a relatively
low light intensity.
In cultivation facilities for plants utilized for salad, and lettuce in
closed-system type plant factories in Japan, a light intensity of about 300 to 400 ?mol?m-2?sec-1 is
used. Factories where a higher light
intensity is needed are hybrid type plant factories where supplemental lighting
of 100 to 150 ?mol?m-2?sec-1 is provided. For indoor ornamental plants,
supplemental lighting of 10 to 50 ?mol?m-2?sec-1, depending on the variety, has
been gradually employed.
---Photosynthesis action spectrum--------------
The efficiency of plant photosynthesis is not the same throughout the 400 to
700 nm waveband. Just as human eyes have visual curves, plants have sensitivity
curves over a wide range. Plants select effective wavelengths from white light and utilize them. Fig. 3(21KB)
shows the photosynthesis action spectra described by Inada(1976)7) . Curve 1
shows the average values for 26 species of herbaceous plants, and curve 2 shows
the average values for arboreous plants. Fig. 4(21KB) shows the photosynthesis
action spectra described by McCree(1972)14) . Curve 1 shows the average values
for 20 species of plants in chambers, and curve 2 shows the average values for
8 species of plants in fields. The sample plants used are listed in Table 3 .
Each of these 4 photosynthesis action spectra has a large peak composed of 2
peaks at about 675 and 625 nm in the red light region, and a small peak between 440 and 450 nm. All 4
photosynthesis action spectra show that red light has a strong action and blue light a weak action. Fig. 5(18KB)
shows the average values for the 4 photosynthesis action spectra, and is used
to evaluate light sources for
plant growth.
Table 3. Plant materials used for the determination of photosynthesis action
spectra
Plants Species
Inada 1. (26 species of
herbaceous plants,
1976)
rice, maize, wheat, barley, oat, soybean, peanut, kidney, bean, pea, cabbage,
turnip, radish, tomato, eggplant, cucumber, squash, lettuce, garland,
chrysanthemum, spinach, onion, sugar beet, sweet potato, perilla, buckwheat,
strawberry
India 2. (7 species of arboreous plants, 1976)
peach, Japanese pear, grape, satsuma mandarin, tea, Japanese black pine, ginkgo
McCree 1. (20 species tested in chamber, 1972)
Maize, sorghum, wheat, oat, barley, secalotricum, sunflower, soybean, tampala,
peanut, lettuce, tomato, radish, cabbage, cucumber, oriental melon, squash,
clover, sugar beet, castor-oil plant
McCree 2. (8 species tested in field, 1972)
Maize, wheat, oat, secalotricum, rice, sunflower, squash, cotton
----Photomorphogenesis----------------
Light acts on plant morphogenesis,
including germination, flowering, stem growth, and leaf opening. Light is also a source of stimuli or
information in different ways depending on the plant species and the stage of
growth. In general, light with
blue, red, and far-red components acts on plants. Table 4(57KB) shows the
action of each range of wavelengths31).
Among these actions, the red and far-red reversible reaction of phytochrome (a
photoreceptor involved in seed germination) is particularly well known. In the
reaction, the promotive effect of germination by red light(660
nm) irradiation is cancelled out by far-red light(730 nm) irradiation. That is, the effect of the
previously irradiated light
appears when red and far-red light
is irradiated alternately. High intensity blue light and low intensity red light induce strong control of internodal growth. It is
well known that with combined irradiation, far-red light is necessary, and that the ratio of red to
far-red light controls
internodal growth12,15,17) . In addition, blue or high energy light promotes the growth of sun
leaves, and red or low energy light
promotes the growth of shade leaves. Daylength controls flowerbud formation
(photoperiod). Plants are generally divided by daylength into 3 groups in which
flowerbud formation is differentiated by specific daytime length: short-day
plants, long-day plants, and intermediate-day plants. In flowerbud formation, light acts as a stimulus, with red light, far-red light or blue light being particularly effective,
depending on the plant species. Besides photomorphogenesis, blue light with a wavelength of 500 nm or
less acts phototropically, and blue light
also acts on stomatal movement.
---Artificial light sources
for plant growth--------------
The artificial light sources
shown in Fig. 6(40KB) can be divided into 2 systems: thermal radiation and
luminescence. Among these light
sources, 6 light sources which
are actually used for plant growth are incandescent lamps, high pressure
mercury fluorescent lamps, self-ballasted mercury lamps, metal halide lamps,
high pressure sodium lamps and fluorescent lamps. Also, xenon lamps and low
pressure sodium lamps are used for research. Fig.7(56KB) shows the energy
spectrum of each lamp, and Table 5(73KB) shows the radiant energy balance and
reduced values of PPFD per 1,000 lx in the 400 to 700 nm waveband.
1) Incandescent lamps (IL)
Incandescent lamps radiate visible light
by thermal radiation generated from tungsten filaments heated to a high
temperature by an electric current. The energy distribution is continuous, but
the intensity of red light is
higher than that of blue light,
which possibly leads to intercalary plant growth. Therefore, these lamps are
not suitable for photosynthesis. Furthermore, since they have a low light conversion efficiency of
around 10 lm/W, as well as high thermal radiation, they are not used for the
cultivation of plants. These lamps are used mainly to control
photomorphogenesis, and for example, in some factories they are used to control
the flowering of chrysanthemums under low light intensities, to prevent dormancy of strawberries and to
promote germination.
2) Fluorescent lamps (FL)
Fluorescent lamps are low-pressure mercury vapor discharge lamps with a hot
cathode. Ultraviolet light
generated by the discharge is transduced to visible light by phosphor coating on the inside of a glass
tube. These lamps easily provide the required radiant energy by use of an
appropriately selected phosphor, but cannot provide sufficiently high energy light on their own for cultivation.
These lamps are often used to grow seedlings in plant factories. Fluorescent
lamps for plants are used not only as supplemental lighting for ornamental
plants in flower shops but also for tissue culture, especially for plant
growth. In addition, a plant factory system has recently been developed, in
which an average value of 650 ?mol?m-2?sec-1 can be achieved by employing a
total lamp system where a 110 W 3-band fluorescent lamp irradiates cultivated
plants at a distance of 30 cm6) . Furthermore, the compact fluorescent lamp has
become popular, and is able to provide local supplemental lighting to indoor
ornamental plants by recessed lights.
3) High pressure mercury fluorescent lamps (HPMVL, phosphor-coated type)
HPMVLs are based on the principle that the luminous efficiency of sources is
enhanced when the vapor pressure of mercury is increased. These lamps are the
most stable lamps, and have been used for many years to grow plants. They
provide light composed mainly
of the radiation line spectrum of mercury, that is, the light lacks the red light component. These lamps therefore enable to
control plant growth. To compensate for the lack of red light, a phosphor lamp which provides red light was developed. The efficiency
of this lamp is around 60 lm/W. It has been used for many years in foreign
countries to provide supplemental lighting and lengthening of daytime. Two
types of lamps are available: a clear bulb type and a phosphor-coated type. The
phosphor-coated type is a type of fluorescent lamp. The phosphor-coated type is
further classified into 2 types: the X type for general use, and the XW type,
which compensates for the missing red light
component. The range of this lamp is from 50 to 2,000 W. Regarding the outer
bulb shape of this lamp, a BT type and an R type are available.
4) Self-ballasted mercury lamps (SBML)
In SBMLs, the arc tube is connected in series to the tungsten filament as a
ballast. These lamps compensate for the red light component, which high pressure mercury vapor
lamps lack. They provide a good spectral distribution, but since the efficiency
is as low as 20 to 27 lm/W, they are used as supplemental lighting for
ornamental plants. For plants, lamps where the input ratio of the arc tube of
mercury lamps to that of the tungsten filament is adjusted are also available.
Two types of outer bulbs are available: a clear bulb type and a fluorescent
type. A BT type and an R type for the outer bulb form of this lamp are
available. The lamps can be selected in the range from 100 to 750 W.
5) Metal halide lamps(MHL)
The structure of metal halide lamps is based on that of mercury lamps, but they
contain various halide additives. There is a wide selection available,
including lamps mainly with line spectra and lamps mainly with continuous
spectra. The efficiency of MHLs is around 100 lm/W, and they provide light with a reduced red light component above 600 nm.
Therefore they are used in plant factories in combination with high pressure
sodium lamps. MHLs on their own are used for supplemental lighting in
greenhouses, and high color rendition type MHLs which provide light with a spectrum distribution
similar to that of natural daylight are used in hybrid type plants factories
and growth chambers. Recently, high color rendering index types(70 to 150 W)
have gradually been used for supplemental lighting and display lighting for
indoor ornamental plants20). Murakami et al.18) carried out research on high
color rendering MHLs containing Dy, Nd, Cs, In, Tl, and Na for use in
horticulture. Two types of these lamps are available: a high efficiency lamp
with a built-in starter and a high color rendition type. BT, T and R types
(only for high color rendition lamps) for outer bulb shapes are available. The
lamps can be selected in the range from 70 W and 2,000 W. Typical additives are
as follows: indium(blue light),
thallium(green light),
sodium(yellow light), and
lithium(red light).
6) High pressure sodium lamps(HPSL)
HPSLs use alumina ceramic for the arc tube, and in the arc tube, sodium and
mercury from an amalgam acting as a buffer gas are enclosed. Neon-argon penning
gas is also sealed in the arc tube to help starting. The efficiency of some of
these lamps exceeds 150 lm/W. Since they have a large red light component which can cause
intercalary growth, they are used with metal halide lamps which provide
compensating blue light. These
lamps are used on their own to cultivate herbage crops with green leaves. These
lamps are used solely in hybrid type plants factories. Three types are
available: a high efficiency type with a built-in starter, an improved color
type with a built-in starter, and a high color rendition type. BT type, T type
and R type for outer bulb shapes are available. The lamps can be selected in
the range from 50 to 940 W. A lamp in which the lack of blue light component is compensated by
the addition of sealed mercury has recently been developed for plants. Inagaki
et al.10)developed a high pressure sodium lamp with an output of 1.2 kW and an
efficiency of 180 lm/W. Xenon, an inert gas for starter assistance, was sealed
in the double-end type lamp at nearly three times the normal pressure.
7) Research trends
(a) Electrodeless discharge lamps(Microwave-powered lamps)
There are several designs of electrodeless discharge lamps depending on the
method of illumination, with the microwave-powered lamp being the most
promising future development for use in horticulture. Until now,
microwave-powered lamps have solely been used for ultraviolet curing in
photoengraving processes. Research is currently being conducted on the
application of high intensities(130 lm/W, 1,000?mol?m-2?sec-1) which could be
achieved by the variation of the sealed gas13). The next challenge facing
microwave-powered lamps would concern the production cost and the life-span of
magnetrons. Fig. 8(31KB) shows the structure of a microwave-powered lamp and
emission spectrum.
(b) Light-emitting diode devices(LED)
LEDs are light-emitting semiconductors with uses ranging from simple indicator
lamps to more complicated bar and numeric displays, where the development of
the blue LED leads to the practical use of full color displays. The LED is a
remarkable evolving technical invention. When current flows through the p-n
junction of compound semiconductors consisting of Gap(gallium phosphide) or
GaAsP(gallium arsenide phosphide), light
is emitted as a result of electrons recombining with holes near the p-n
junction. The characteristics of LEDs are as follows: low voltage operation,
low heat emission, a compact and lightweight design, lack of noise(electron
discharge tubes produce noise) and easy control. Horticultural
applications are being considered for plant cultivation in space32). In this
application, an irradiation source(surface) consisting of a bundle of LED
devices irradiates the plant at a close proximity, moving with the plant as it
grows. At a distance of 1 cm, a 5,000 mcd, 660 nm LED is able to produce an intensity of almost 50,000 lx.
In addition, a combination of red, green and blue devices together with
lighting control can produce a balance that is compatible with photosynthesis.
The next challenge facing LEDs concerns the production cost and the heating
effects resulting from the concentrated use of LED devices. Fig. 9(13KB) shows
the structure of a LED device23) and Fig. 10(( shows the spectral distribution
in composite lighting. Table 6 shows the characteristics of red, green and blue
LED devices21).
c) Laser diode devices(LD)
LDs are light-emitting semiconductors like LEDs.
LDs are mainly used in bar-code readers, writeable compact disks(CD), mini
disks(MD), compact disk read only memory(CDROM), optical communication
transmission, and photocopiers or optical printers. The operation principle of
an LD is equivalent to that of laser oscillation. Light emitted from an LED is reflected by a mirror and
amplified by stimulated emission. The light
is finally emitted through the mirror surface. Fig. 11(24KB) shows a simple LD
structure22). Table 7 shows the wavelengths produced by several kinds of
LDs22). Takatsuji & Yamanaka24) investigated the possibility of using LDs
as light sources in
greenhouses since the photo-electronic transducer efficiency of LDs is very
high. Results showed that irradiation combined with red and blue LD light pulses was a promising future
development in view of the production cost. In addition, Takatsuji &
Mori25) confirmed the growth of lettuce using mixed light irradiation(PPFD: 50?mol?m-2?sec-1)of red LD(660 nm)and blue LED(450 nm).
Tsuchiya et al.28) developed an LD6500 with
a wavelength of 680 nm and output of 200 nW. A 35% photoelectric transducer
efficiency was achieved (theor3200etical maximum 60%). Results of tests carried
out on lettuce at 200?mol?m-2?sec-1 PPFD showed that growth was slow. The
leaves were thin, presumably due to the use of monochromatic and coherent light. In the mixed irradiation test
using red LDs and blue fluorescent lamps(about 6%), plants showed an increased
weight and a normal leaf shape, confirming the effect of blue light29). Mori
& Takatsuji.16) cultivated lettuce by irradiating light from different kinds of LEDs and red LDs(650 nm)
alone or in combination(PPFD: 50?mol?m-2?sec-1 ) and found that the growth was
poor in cases where only red LD irradiation was used. This effect was
considered to be due to the monochromatic characteristic of red LD light. The following problems in the
application of LDs are as follows: sensitivity to electrostatic and current
surges, wavelength increase of about 10 nm as the temperature rises, and need
for development of a blue light
LD.
---Conclusion-----------------
Two main requirements dominate the utilization of artificial light sources in horticulture in
both gardens and commercial greenhouses, the first being efficiency. High
pressure sodium lamps are generally adopted to offer the highest efficiency in
terms of plant growth rate and economy. However, to remain within current
standards of farm products(such as leaf greenness and coloration, internode
length, stem diameter, and leaf thickness), combination with metal halide lamps
is recommended. The second requirement is related to the esthetic improvement
of store or house environments, where the primary concern is not growth but
maintenance of a plant's natural appearance. High efficiency is not a
prerequisite, but the light
quality balance becomes important in order to bring out the essential color
characteristics of plants and flowers as well as maintaining plant health. To meet these
requirements, high color rendering index type MHLs are recommended. Current
horticultural research trends lead to the development of 1.2 kW HPSL, 180 lm/W,
LED and LD devices for use in commercial greenhouses and the application of
microwave and 400 W MHL lamps in growth
chambers.
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