Review Environ. Control in Biol., 41(2), 121-132, 2003

Effects of Light Quality on Growth of Crop Plants under

Artificial Lighting

Eiji GOTO

Department of Biological and Environmental Engineering, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan

(Received March 14, 2003)

Major light factors affecting plant growth are light quality, light intensity, photoper-iod, and the day/night cycle. These parameters can be controlled under greenhouse conditions using artificial light sources. Moreover, application of light pulses and short-term changes of the spectral composition are effective ways to stimulate plants and to

induce desired morphological developments. Controlling spectral qualities of the irradia-tion applied enables faster growth or higher yield at a given radiation energy, and the

production of plants of optimized nutritional value. Recent developments of lighting technology have enabled not only researchers but also farmers to control spectral qualities by combinations of various light sources with different waveband emissions. This review

summarizes previous papers and evaluates significant effects of the quality of artificial light on growth and development of higher plants, especially crops.

Keywords : artificial light, growth, light quality, photomorphogenesis, photosynthetic

photon flux

INTRODUCTION

Plant life depends on light in two ways : light provides the energy for the production of organic matter in photosynthesis, and it is perceived as a morphogenetic stimulus. Photomor-

phogenetic responses include growth effects (such as seed germination, phototropism, and organ elongation) and differentiation (for example flower bud and leaf formation , and the regulation of photosynthetic pigments). Light also induces movements of leaves, stomata, and chloroplasts, which are involved in the regulation of photosynthesis. Major factors affecting plant growth are light quality, light intensity, photoperiod , and the day/night cycle. These parameters can be controlled under greenhouse conditions using artificial light sources . Moreover, application of light pulses and short-term changes of the spectral composition are effective ways to stimulate plants and to induce desired morphological developments . Controlling spectral qualities of the irradiation applied enables faster growth or higher yield at a given radiation energy, and the production of plants of optimized nutritional value . Therefore it is hardly surprising that large-scale plant production under controlled light conditions has become common in industrialized countries .

Effects of light quality on plant growth have been studied for more than 50 years . McCree (1972) measured the action spectrum, absorbance and quantum yield of photosynthe-sis in crop plants and Inada (1976, 1977) determined action spectra for photosynthesis in

Corresponding author: Eiji Goto, fax : +81-3-5841-8175,

e-mail : agoto @ mail.ecc.u-tokyo.ac.jp

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several higher plant species. These studies triggered research into photosynthesis and dry

matter production under irradiation of different spectral qualities. Recent developments of

lighting technology have enabled not only researchers but also farmers to control spectral

qualities by combinations of various light sources with different waveband emissions. This

review summarizes previous papers and evaluates significant effects of the quality of artificial

light on growth and development of higher plants, especially crops.

PLANT RESPONSES TO LIGHT QUALITY

Plants recognize changes in their light environment by sensing light quality using signal-transducing photoreceptors. There are three classes of photoreceptors : phytochromes, crypto-chromes, and phototropin (Smith, 2000). Signals detected by the photoreceptors directly or indirectly affect physiological, morphological, and anatomical features in plants. Since changes in spectral composition will generally evoke responses by more than one receptor, the

growth reaction of the plant will result from an integration of multiple signal-transduction cascades.

In the natural light environment, plants use phytochromes to sense the red : far-red ratio

(R : FR) which mediates the shade-avoidance syndrome (see Smith, 1994, for review). If the ratio is low, as it is in locations shaded by dense plant canopies, plants elongate more rapidly and accelerate flowering to produce seeds early for survival. The shade-avoidance syndrome is easily observed in experiments with controlled R : FR ratio under artificial lighting. However, acceleration of elongation and flowering does not always occur concurrently under low R : FR conditions. Using more than 100 accessions of Arabidopsis thaliana, Botto and Smith (2002) showed that flowering acceleration and hypocotyl elongation were not correlat-ed. These authors suggested that downstream steps of the signal transduction pathway were different in the two shade avoidance responses.

Photosynthetic acclimation is another plant response to a changing light environment. Acclimation results from a combination of leaf- and chloroplast-level reactions to changes in light intensity or spectral quality. In general, chlorophyll content increases while the ratio of

photosystem II to photosystem I (PSII/PSI) decreases under increased R : FR conditions, indicating an involvement of the phytochrome system. However, the light intensity depen-dent increase in transcription level of the gene of small subunit of Rubisco (rbcS), another element of photosynthetic acclimation, was mediated specifically by blue light (Sawbridge et al., 1994). Lopez-Juez and Hughes (1995) showed that the chlorophyll a/b ratio, Rubisco, and cytochrome f proteins of pea seedlings (Pisum sativum L.) increased in response to an increase in blue light and suggested that a blue light photoreceptor controlled the light acclimation responses. It must be cautioned, though, that alternative mechanisms might exist, since the effects of light quality on chlorophyll content and photosynthetic capacity are species-dependent (Murchie and Horton, 1998).

Spectral quality also affects leaf anatomy. Leaf thinning under shade conditions is a

general phenomenon and appears to be mediated by changes in R : FR. Moreover, addition of blue light to a background of pure red light induces substantial changes of anatomical features. In pepper plants (Capsicum annuum L.), the number of chloroplasts per palisade mesophyll cell and the thickness of palisade and spongy mesophyll tissues were low under red or red+far-red illumination as compared to blue enriched light (Schuerger et al., 1997) ; the differences observed were correlated with the amount of blue light.

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LIGHT SOURCES FOR RESEARCH AND PLANT PRODUCTION

Electric lamps such as fluorescent (FL) lamps, metal halide (MH) lamps, high and low

pressure sodium (HPS and LPS) lamps, and incandescent (INC) lamps have been used for growing plants under artificial light or supplemental lighting. The control of light quality usually is achieved by combining different kinds of lamps and regulating their relative inten-sities. In greenhouses, photoselective glasses and plastic films are employed to remove specific wavelengths from the natural irradiation, or to modify the R : FR ratio (Murakami et al ., 1993 ; Murakami et al., 1997 ; Yoshimura et al., 2002). In particular, CuSO4 filters absorb red and far-red, reducing R : FR (Rajapakse and Kelly, 1992, 1993, 1995).

LEDs (Light emitting diodes) have attracted attention since the late 1980s and are used today in research and plant production. LEDs have a pronounced peak and a narrow-bandwidth wavelength emissions. They are variable in intensity, and thus suitable for research into light quality effects, for example in photosynthesis (Tennessen et al., 1994). LEDs can be switched on and off at short intervals, facilitating the application of short light

pulses. Mori et al. (2002) examined growth of lettuce (Lactuca sativa L.) under pulsed LED illumination varying pulse durations (21us to 10 ms) and duty ratios (10 to 100%). Far-red and red LEDs were used to elucidate mechanisms of gravitropism and photomorphogenesis

(Johnson et al., 1996), and to study low temperature storage capacities of postharvest chervil (Anthriscus cerefolium L.; Fujiwara et al., 1997). Wilson et al. (1998) employed red LEDs to determine responses of broccoli seedlings (Brassica oleracea L.) to light quality during low-temperature storage. Nhut et al. (2002) developed a plant culture system with red and blue LEDs for micropropagation.

Okamoto et al. (1996) devised several plant growth apparatus using red and blue LEDs , and conducted growth experiments changing the blue to red ratio. Goto et al. (2000) developed a high-power LED lighting system in which the maximum photosynthetic photon flux (PPF) reached 600 jcmolm-2 s-1(480 jumolm-2 s-1 for red and 1201amolm-2 s-1 for blue) . LEDs are promising light sources for plant growth units in space stations (Barta et al., 1992), and a variety of growth chambers with LEDs have been developed by NASA's groups , universities, and companies supported by NASA. Recently, NASDA constructed a small

plant growth unit with blue and red LEDs for the Japanese Experimental Module in the International Space Station, in which a seed to seed experiment of Arabidopsis thaliana will be conducted under microgravity (Aizawa-Yano et al., 2002).

Laser diodes (LDs) with narrow wavebands are a recent development. They were used in the study of growth in lettuce (Mori and Takatsuji, 1999; Yamazaki et al., 2000). Yamazaki et al. (2002) grew rice plants (Oryza sativa L.) until seed production under red LDs supplemented with blue fluorescent lamps.

SPECTRAL PARAMETERS

Several spectral parameters are widely used to describe experimental conditions , evaluate light environments, and compare light sources. Each parameter is significant ; in most cases, considering one of them alone is insufficient for a sound interpretation of plant responses to varying light environments.

PPF is the most frequently employed estimate of photosynthetically active radiation

(PAR) (Dougher and Bugbee, 2001 a). PPF weighs each photon between 400 and 700 nm equally and quantum sensors to measure PPF are widely used. However, since photosynth-etic efficiency is not homogenous across this range (McCree , 1972; Inada, 1976), yield photon

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flux (YPF) was proposed as an alternative measure of PAR. YPF weighs different wave-lengths according to the average leaf photosynthetic efficiency curve originally developed by

McCree (1972). YPF can be calculated from photon flux data and wavelength-specific

photosynthetic efficiency of which digital file is available in the literature (Sager et al., 1988). In fact, many studies conducted in the U.S. used YPF as well as PPF. Although quantum sensors designed to measure YPF are commercially available, Barnes et al. (1993) questioned the accuracy of the sensors. Since YPF incorporates the average photosynthetic efficiency, the measure does not characterize a specific spectral condition. Under the present circumstances, PPF is a reasonable parameter to describe photosynthetically relevant spectral properties. It must be cautioned, though, that the values measured by a PPF sensor contain errors compared with ideal values measured by a spectroradiometer (Tibbitts et al., 1986; Barnes et al., 1993).

The ratio of the photon fluxes in the red and in the far-red is directly related to the spectral

properties of the phytochrome system. Although definitions of red and far-red differ between authors, there has been a tendency to use the following standardized formula : R/FR=

(photons between 655 and 665 nm)/(photons between 725 and 735 nm) (Smith, 1994). The phytochrome photoequilibrium is the equilibrium of the FR-absorbing form (Pfr) and the R-absorbing form (Pr) ; it is represented as the ratio of Pfr and total (P: Pfr+ Pr). The

photoequilibrium (Pfr/P or gyp) can be estimated by the relationship between R : FR and the phytochrome photoequilibrium (Smith and Holmes, 1977; Smith, 1982). Hanyu et al. (1996) proposed a calculation method of the photoequilibrium using photon flux data measured by a spectroradiometer.

Dougher and Bugbee (2001b) compared growth of lettuce under either HPS or MH lamps and attempted to identify parameters responsible for the differences observed. In a variety of factors tested, only the spectral properties in the yellow wavelength range (580-600 nm) could explain the findings. In contrast to MH, HPS has an emission peak in this range. According to Rajapakse and Kelly (1994), considering R : FR alone as a parameter of light quality might lead to erroneous conclusions in experiments using CuSO4 filters. To account for the influences of varying light sources and general experimental conditions, these authors suggest-ed to use R : FR, B : R, and B : FR ratios, as well as the estimated photoequilibrium to characterize spectral conditions sufficiently.

GROWTH EXPERIMENTS USING VARIOUS LIGHT SOURCES

Growth experiments using conventional light sources such as MH, HPS, LPS, FL, and INC lamps and/or photoselective or CuSO4 filters have been conducted in numerous crop species since the 1970's. These include cereals (wheat [Triticum aestivum L.] : Tibbitts et al., 1983; Barnes and Bugbee, 1991, 1992; Dougher and Bugbee, 2001a; rice : Inada, 1973; sorghum [Sorghum bicolor L.] : Warrington et al., 1976a,1976b ; Britz and Sager, 1990), pulse crops (soybean [Glycine max L. Merr.] : Warrington et al., 1976a, 1976b; Cathey and Campbell, 1977; Britz and Sager, 1990; Wheeler et al., 1991 ; Britz and Cavins, 1993; Dougher and Bugbee, 2001a; kidney bean [Phaseolus vulgaris L.] : Barreiro et al., 1992; Maas et al., 1995; Hanyu et al., 1996; Hanyu and Shoji, 2000a, 2000b; peanut [Arachis hypogaea L.] : Montley et al., 2001), and vegetables (lettuce : Cathey and Campbell, 1979; Krizek and Ormrod, 1980; Tibbitts et al., 1983; Knight and Mitchell, 1988a, 1988b; Koontz et al., 1987; Mortensen and StrOmme, 1987; Inada and Yabumoto, 1989; Murakami et al., 1992; Eskins et al., 1997; Tomita et al., 1998; Dougher and Bugbee, 2001 a, 2001 b ; spinach : Tibbitts et al., 1983; Hanyu and Shoji, 2002; eggplant [Solanum melongena L.] : Murage et al., 1997; tomato [Lycopersicon esculentum L.] : Mortensen and StrOmme, 1987; cucumber

[Cucumis sativus L.] : Cosgrove, 1981; Sung and Takano, 1997; Sung et al., 1997, 1998;

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radish [Raphanus sativus L.] : Inada and Yabumoto, 1989; Tomita et al., 1998; white mustard [Sinapis alba L.] : Tibbitts et al., 1983).

LEDs emitting a variety of defined wavelengths are available now, and became more common recently as a light source in plant growth studies. These include reports on the effects of light quality on plants such as wheat (Tripathy and Brown, 1995 ; Goins et al., 1997),

pepper plant (Brown et al., 1995; Schuerger et al., 1997), lettuce (Hoenecke et al., 1992; Yanagi et al., 1996; Okamoto et al., 1997; Yorio et al., 2001), spinach (Yorio et al., 2001; Stryjewski et al., 2001), and radish (Yorio et al., 2001).

RED AND FAR-RED LIGHT EFFECTS

Changes in R : FR induce morphological modifications under natural light conditions, and similar effects are observed under artificial light. However, several morphological responses have been reported to be specific to artificial illumination. Krizek and Ormrod

(1980) characterized growth of lettuce under different doses of far-red light and found that higher far-red levels increased stem elongation, node number, and fresh and dry weight of shoots. Knight and Mitchell (1988a) reported that lower R : FR with equal PPF increased leaf area and leaf dry weight in lettuce. Far-red light has no direct effect on photosynthesis and seems not to promote dry matter production. Murakami et al. (1992) studied growth in lettuce and explained the observed increase in dry matter of plants grown under lower R : FR as a side-effect of low R : FR-induced stem extension and leaf expansion. As a result of this

primary effect, larger leaves came closer to the light sources and received higher doses of photosynthetically active radiation. Consequently, photosynthesis of the whole plant in-creased. This example shows that effects of light quality on the photosynthetic performance of the whole plant or canopy should be carefully evaluated in relation to the light reception status of the plant.

Examining combined effects of blue light and supplemental far-red light on growth in kidney bean, Hanyu and Shoji (2000b) noted that enhanced red light at constant far-red light did not accelerate leaf expansion or stem extension. They concluded that increases in red light and decreases in far-red light had different effects. This demonstrates the difficulties in assessing red and far-red effects using the R : FR parameter alone. Barnes and Bugbee (1991)

grew wheat plants under the same PPF (200pmolm-2 s-1) at varying R : FR. Low R : FR conditions inhibited tillering and slightly accelerated main culm development. On the contrary, additional blue light enhanced tillering. The authors concluded that wheat was more responsive to reductions of R : FR than to changing blue light levels. Brown et al.

(1995) examined growth and morphogenesis of pepper plants under different blue light dosages and R : FR at constant PPF (300,umolm-2 s-1). They compared MH lamp (20% blue and 10% far-red), red LEDs alone (0%/l%), red LEDs with supplemental blue fluorescent light

(l%/ 1%), and red plus far-red LEDs (0%/20%). Biomass was reduced under red LEDs alone as compared with red LEDs plus blue or MH lamps. Addition of far-red light enhanced stem

growth, resulting in a reduced leaf/stem dry matter ratio. Thus, small changes in blue light caused significant alterations of stem growth. The authors suggested that R : FR responses must be interpreted in context with the absolute irradiance in each wavelength, and that blue light or the interaction of blue light with other wavelengths are critical in determining

photomorphogenic responses.

BLUE LIGHT EFFECTS

Blue light reduces cell expansion and thus inhibits leaf growth and stem elongation

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(Cosgrove, 1981). Growth inhibitions by blue light are mediated by blue light photorecep-tors and are considered distinct from inhibitory phytochrome effects. Some blue light responses are independent of total PPF. Growth responses to blue light proceed faster than

similar red light-dependent effects in many plants. Specific blue light responses appear to require higher radiation intensity than responses mediated by phytochrome (Cosgrove, 1994).

Maas et al. (1995) showed that internode elongation in kidney bean was stimulated by decreasing fractions of blue light at constant PPF. Hanyu and Shoji (2000a) demonstrated that increased fractions of blue or red light at constant PPF and R : FR enhanced total dry matter production in this species. However, while blue light decreased stem length without affecting leaf area, red light accelerated both leaf expansion and stem extension. It was concluded that blue light-dependent increase in total dry matter was due to increased dry matter production per unit leaf area. On the other hand, the red light-induced rise in total dry matter was brought about by an increased leaf area. Wheeler et al. (1991) examined stem

growth in soybean at blue light levels ranging from 23 to 39,umolm-2 s-1 under HPS lamps supplemented with blue fluorescent lamps. Stem and internode lengths of soybean were found reduced with increasing blue light up to 30pmolm-2 s-1. The authors suggested that blue-deficit light sources caused abnormal stem elongation which could be prevented by supplemental blue light.

Barnes and Bugbee (1992) grew wheat plants under 2 and 50,umolm-2 s-1 of blue photons at the same PPF (200 jumolm-2 s-1) and R : FR. At higher blue light levels the plants

produced more tillers and showed accelerated main culm development. Leaf length was reduced, while plant dry matter was not affected. Thus, tillering and shoot and leaf develop-ment are partly controlled by a blue light photoreceptor. Tripathy and Brown (1995) measured chlorophyll biosynthesis of wheat seedlings under 400 pmolm-2 s-1 red light sup-

plemented with 0, 10, or 25 pmolm-2 s-1, respectively, of blue light. Blue-deficient conditions (0 and l01umolm-2 s-1) reduced the amount of chlorophyll in leaves. Goins et al. (1997) examined morphogenesis, photosynthesis, and seed yield of wheat plants under different blue

photon levels at constant PPF (350 pmolm-2 s-1). They compared daylight fluorescent lamps, red LEDs alone, red LEDs plus 1% supplemental blue fluorescent light, and red LEDs

plus 10% blue light. Plants grown under red LEDs alone had shorter main culms at the beginning of anthesis, longer flag leaves, and greater main culm length at final harvest than

plants grown under daylight fluorescent lamps. Under red LEDs alone, plants produced fewer subtillers and a lower seed yield. Shoot dry matter and net photosynthetic rate in-

creased at higher blue light dosages. Wheat grown under red LEDs plus 10% blue light had almost the same shoot dry matter and seed yield as under daylight fluorescent lamps. The authors concluded that wheat could produce seeds under red LED alone but required sup-

plemental blue light (30 pmolm-2 s-1) for normal seed production.Schuerger et al. (1997) grew pepper plants under different conditions of blue, red, and

far-red irradiation at constant PPF (300pmolm-2 s-1). They compared MH lamps and three types of red-based LED treatments (0% blue, 99% red, l % far-red ; 0%/83%/l7%; l%/1%/98%). The result indicated that a low level of blue light (41umolm-2 s-1) was sufficient for normal morphogenesis. Moreover, anatomical characteristics of pepper plants seemed to depend more strictly on blue light than on R : FR. Hoenecke et al. (1992) grew lettuce under different blue light levels (0 to 60 pmolm-2 s-1) at two PPFs (150 and 300pmolm-2 s-1) using red LEDs and blue fluorescent lamps. Red LEDs alone induced extended hypocotyls and elongated cotyledons, which could be prevented by additional blue light at more than 15pmolm-2 s-1. Thus, blue light effects correlated with absolute blue irradiance rather than with the blue

photon percentage of total PPF. Noteworthily, the effects were independent of the photon level of red light. Yorio et al. (2001) compared growth of lettuce, spinach, and radish under

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different blue photon levels (0, 30, 60 pmolm-2 s-1) at a PPF of 300 umolm-2 s-1, using white FL lamps or red LEDs with or without blue fluorescent lamps. Shoot dry weight of lettuce was low at red LEDs alone but almost constant between 30 and 60,umolm-2 s-1 of blue light. Root dry weight increased as blue photons increased. However, shoot dry weights of spinach and radish were considerably lower at 30 1umolm-2 s-1 than at 60,umolm-2 s-1 blue light. Evidently, the minimum level of blue photons required for normal dry matter production varied among these crops.

In summary, numerous studies have shown that the addition of small doses of blue

photons to blue-deficient light sources greatly changes morphological and anatomical features of leaves and stems. Although the critical level of blue photons differs between plant species, it is commonly observed that blue light effects are independent of total PPF and R : FR. In this context, the B : R ratio and the interactions and antagonisms between blue and red light-dependent signalling pathways are of highest interest.

SHORT-TERM EXPOSURE TO BLUE OR RED LIGHT AT THE BEGINNING OR END OF THE DARK PERIOD

Hanyu and Shoji (2002) exposed spinach to 50,umolm-2 s-1 of supplemental light for 30 minutes at either the beginning or the end of the 14 h-dark period. Blue light at the end of the dark period induced an acceleration of growth leading to a 20% increase of total dry matter. Exposure to red light at the beginning of the dark period had similar effects. However, neither blue light at the beginning of the dark period nor red light at the end of the dark period was effective. In cucumber seedlings, short-term exposure to supplementary blue light at the end of the 10 h-dark period increased dry weight, leaf area, and stem diameter compared with the control (Sung and Takano, 1997; Sung et al., 1997, 1998). These authors tested different PPF levels and exposure periods and concluded that illumination with 30 jcmolm-2 s-1 of blue light for 5 minutes caused the greatest acceleration of growth. Under these conditions, stomatal conductance, transpiration rate, and photosynthetic rate were maintained at higher levels than in control plants without supplemental lighting.

In the natural light environment, R : FR values at sunset and sunrise are lower by approximately 0.2 than at midday (approximately 1.0; Smith, 1982). Since R : FR declines at the end of day, Pfr/P decreases concomitantly. Thus, active phytochrome decreases relative to its inactive form at the beginning of the dark period. This process induces various physiolog-ical responses in the dark period, which are collectively referred to as the End-of-Day FR effect. The short-term exposure to red light at the beginning of the dark period probably delays the End-of-Day FR effect by increasing Pfr/P, leading to accelerated growth.

The enhancement of growth that is induced by low intensities of blue light applied at the end of the dark period cannot be due to increased photosynthesis. Since the blue fraction of natural PPF is higher at sunrise than at midday (Ishii and Yamazaki, 2002), plants may recognize the onset of the light period by sensing blue light. Sawbridge et al. (1994) and Lopez-Juez and Hughes (1995) showed that an increase in blue light enhanced transcription

levels of the gene of rbcS, Rubisco, and cytochrome f proteins. The opening of stomata is

promoted more strongly by blue than by red light. The short-term exposure to blue light apparently stimulates processes that optimize physiological conditions for photosynthetic activity and consequently increase photosynthetic efficiency during the light period.

OTHER LIGHT QUALITY EFFECTS

A limited number of studies has been devoted to effects of light quality on chemical

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components, quality, and taste of food produced under artificial illumination. Britz and Cavins (1993) examined growth and development of soybean during vegetative and reproduc-tive stages. The absence of blue light had a large influence on vegetative morphology, but the initiation of the reproductive stage was not influenced by spectral quality. Total seed yield

per plant, dry matter per seed, and the protein and oil percentage in the seeds were similar for all treatments. However, levels of oleic acid (18: 1) and linoleic acid (18:2) differed between the blue-deficient and the other treatments, suggesting that a blue light photoreceptor as well as phytochromes regulate fatty acid metabolism during seed maturation. Britz and Sager

(1990) studied growth, dry matter partitioning, and the contents of starch and soluble sugars in soybean and sorghum under different blue light irradiation levels at constant PPF. Blue-deficient conditions decreased the root : shoot ratio and increased leaf area ratios. The authors suggested that starch accumulation in leaves, leaf export, and shoot and leaf growth were influenced by spectral quality. Eskins et al. (1996) examined the intensity of bitter taste in mature lettuce leaves and showed that it was controlled by spectral composition. Tomita et al. (1998) reported that addition of UV-A affected bitterness of lettuce and radish leaves. Because blue light photoreceptors detect not only blue light but also UV-A, the photoreceptor

responsible for this UV-A effect might be identical to the blue light receptor that mediates the blue light responses mentioned above.

DIFFERENCES BETWEEN SPECIES REGARDING SPECTRAL QUALITY

Mortensen and StrOmme (1987) grew tomato and lettuce under varying B : R ratio and R : FR, but at constant PAR. Plant height of tomato was significantly decreased under blue-enriched conditions as compared with other treatments. Total leaf dry weight was slightly decreased by blue light, whereas the decrease in stem dry weight was dramatic. In contrast to tomato, total dry weight of lettuce showed little differences between treatments, probably because lettuce stems are small and do not influence the total plant weight significantly. Dougher and Bugbee (2001a) examined growth and development of soybean, wheat, and lettuce plants under different spectral conditions. They used HPS and MH lamps with or without yellow filters and compared different fractions of blue wavelengths at two PPF levels

(0.2 to 52,umolm-2 s-1 of blue at 200 pmolm-2 s-1 PPF, and 0.5 to 130,umolm-2 s-1 blue at 500 pmolm-2 s-1 PPF). Blue light influenced stem length, leaf area, and total dry mass in lettuce but did not affect total dry mass in soybean. Wheat did not respond to blue light. Regarding stem elongation, soybean responded to the ratio of blue photons and total PPF, whereas lettuce responded to the absolute amount of blue photons. Thus, blue light effects were species dependent.

Dicotyledonous plants seem more sensitive to spectral changes than monocots (Schuerger et al., 1997). Leaf morphology differs between erectophile and planophile types. In wheat, a graminean monocot, leaf and stem meristems are sheltered from direct irradiation by older leaves, and show little responsiveness to blue light. Lettuce and soybean are dicots with

planophile morphology. Their meristems in expanding leaves and stems are exposed to the light. In general, previous studies seem to support the idea of a connection between blue light

sensitivity and meristem morphology in dicots. However, responses of monocots to blue light vary widely among species. In fact, several studies on wheat have demonstrated that light

quality influenced morphogenesis, photosynthesis, and seed yield (Barnes and Bugbee, 1992; Goins et al., 1997). Leaf morphology in rice is fairly sensitive to blue light (Inada,1973), and so is chlorophyll biosynthesis, leaf emergence rate, and age at heading time (Goto, unpub-lished). Further research on the differences between species regarding responses to the spectral composition are necessary, especially with respect to R : FR and blue light.

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CONCLUSIONS

Far-red light has no direct effect on photosynthesis. However, changes in morphological

features of leaves or stems induced by far-red light or altered R : FR might enhance photosynth-

etic activity and eventually result in an increase of dry matter production. Since blue light is

less efficient photosynthetically than red light, the fraction of blue light tends to be minimized

when designing artificial lighting systems. However, blue light deficient illumination induces

abnormal morphology and ultimately inhibits growth. Addition of small dosages of blue

light prevents these undesirable reactions. Adequate doses should be determined not only in

terms of absolute photon flux densities, but should also take interactions between red and blue

light into account. Pulsed illumination or short-term supplemental lighting with different

spectral qualities can be useful to modify physiological and morphological responses and to

produce more yield at a given radiation energy. Producing higher yield per radiation energy,

minimizing energy to produce the same yield, and producing higher quality without reduction

of growth rate are desired targets in agricultural production. Control of light quality is the

most readily available method to achieve these aims. To date, physiological and molecular

biological research efforts in the field of photomorphogenesis appear mostly unrelated to

studies of plant production under artificial lighting. Combining agricultural research with

the physiological and molecular biological approaches is the key to improved techniques of

crop growth control by lighting conditions in the future.

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