- Email: [email protected]
The effect of light quality on growth of lettuce
The effect of light quality on growth of lettuce Yuta SAITO*, Hiroshi SHIMIZU** Hiroshi NAKASHIMA***, Juro MIYASAKA****, Katsuaki OHDOI*****
* Graduate student, Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan ( e-mail: [email protected]u.ac.jp, Tel:81-075-753-6165) ** Professor, Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan (e-mail: [email protected]u.ac.jp.) *** Associate Professor, Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan (e-mail: [email protected]u.ac.jp) **** Assistant Professor, Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan (e-mail: [email protected]u.ac.jp) ***** Assistant Professor, Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan (e-mail: [email protected]u.ac.jp) Abstract: Plant factory is a plant production system with controlled temperature, carbon dioxide concentration and light environment to enhance plant growth efficiently. Fluorescent lamp is popularly used in a plant factory, however, the spectrum of fluorescent light is broad and some portion of them is not necessary for photosynthesis. On the other hand, LED (light emitting diode) has narrow band width and is getting easy to use these days. The final goal of this project is to develop a new light apparatus with LEDs for plant factory. The specific objective is to investigate the effective light quality for growing plant more rapidly. The effect of light generated by monochromic and mixed radiation from LEDs (blue, green, red) on the response of lettuce (Lactuca sativa L.) were evaluated and compared with the effect of lettuce irradiated by fluorescent lamps. Photosynthesis rate of lettuce increased in monochromic red light and the light mixed with red and blue light. Number of leaves decreased in monochromic blue light. Fresh weight of lettuce was increased in monochromic red and fluorescent light, but monochromic red light significantly decreased dry weight rate. In the view of energy saving, LED is much better than fluorescent lamp. Especially, red LED is good. Keywords: plant factory, light quality, carbon dioxide concentration, light intensity, morphogenesis, fresh weight, LED
1. INTRODUCTION Agriculture and food issues are getting known widely. For example, residual agricultural chemicals, poor harvest by abnormal weather conditions are often heard all over the world. Especially in Japan, there are other problems such as the increase of imported foods and the decrease of farmers. Plant factory is expected to be one of the ways to solve the issues. The plant factory is controlled inside environment such as lighting, temperature, carbon dioxide concentration and cleanliness to enhance plant growth effectively. That is why plant factory can produce a lot of safe vegetables in spite of season or weather. However, the initial and running costs for environmental control are very expensive. These problems prevent a plant factory from spreading widely. In running cost, the cost of lighting for plant growth is one of the large portions. Recently a light emitting diode (LED) is spreading as a new light source. A LED has good life performance, energy- saving ability and various and narrow bandwidth of wavelength. Combining several LEDs with different bandwidth enables to produce many kind of light quality. The effects of light qualities on the growth and development of plant have been studied for a long time. Red light promotes
germination and photosynthesis (Flint and McAllister 1937;; Balegh and Biddulph 1970). Blue light effects stomatal opening and chloroplast development (Takemiya et al. 2007;; Akoyunoglou and Anni 1984). Green light slows down or stop the plant development, but it brings positive effects in special conditions (Folta 2004, 2007;; Terashima et al. 2009). In many plants, it has been studied that the absorbance of leaves and the quantum yield of photosynthesis concerning various wavelength. We may produce an optimum light quality for plant growth by combining the LED¶s characteristic of light quality with researches of plant growth. In fact, photosynthesis rate and growth rate of plant grown under LED were reported (Mori and Takatsuji 2000, Mori et al. 2002). The final goal of our study is to improve lighting apparatus for plant factories to spread them widely. So the specific objective of this research is to investigate the best light qualities for plant growth by examining photosynthesis rate and growth response of lettuce (Lactuca sativa L.) radiated by LEDs and fluorescent lamps.
h/d light period and 25 °C in light period and 18 °C in dark period. Table 1. Light intensity rates of blue, green and red LEDs for five light qualities using LED systems (%)
Blue LED
Green LED
Red LED
R
0.0
0.0
100.0
RB
16.7
0.0
83.3
B
100.0
0.0
0.0
SL-L
33.4
33.9
32.7
FL-L
29.8
39.6
30.7
Fig. 1. Developed LED system 2.3 Hydroponics Nutrient solution pools of hydroponics were consisted of culture bed (730 wide mm × 665 length mm × 170 height mm) covered tarpaulin and culture panel. The quantity of nutrient solution in the pools was maintained even by pump and overflows. Nutrient solution was kept pH 5.9±0.2, EC 2.00±0.10 by fertilizer controller (CEMCO) with acidity regulator (Otsuka) and fertilizers (Otsuka). Fig. 2. Spectrums of LEDs: Blue, Green and Red were used in this research. 2. MATERIALS and METHODS 2.1 Light Source We used two light sources. One was a fluorescent lamp (FLR20S・W/M, TOSHIBA), the other was a LED system. The LED system was consisted of blue, green and red LEDs (LUXEON Rebel, Philips Lumileds) and condenser lenses as shown in Figure 1. Each spectrum of LEDs is shown in Figure 2. Photosynthetic photon flux density (PPFD) from LEDs system can be controlled by adjusting currents from a DC power supply. 2.2 Plant Materials and Growth Condition Lettuce (Lactuca sativa L. cv´Greenwave´, Takii Seed Co., Ltd.) plants were grown in urethane sponge with water under fluorescent lamps for one week. PPF of fluorescent lamps was set at 200 µmolm-2s-1. After one week, lettuce plants were transplanted to hydroponic system. We conducted two experiments namely photosynthesis rate measurement and growth experiment. In photosynthesis rate measurements, transplanted plants were grown under fluorescent lamps for a month (11/26-12/27, 2009). In growth experiment, they were grown under six light qualities from fluorescent lamps and LED systems for four weeks. All of the plants were grown in a growth chamber (KCLP-1000ⅠCT, NKsystem) set at a 16
2.4 Light Quality In photosynthesis measurement and growth experiment, six light qualities were used. One was fluorescent light (FL) set by fluorescent lamps and the others were set by LED systems. The light qualities set by LED systems were monochromic red light (R), monochromic blue light (B), the light mixed with red and blue light (RB), the light imitating sunlight (SL- L), the light imitating fluorescent lamp light (FL-L). Table 1 shows light intensity rates of blue, green and red LEDs about these five light qualities. We referred to the spectrum data of sunlight (Air Mass 1.5: ASTM G173, NREL) and a fluorescent lamp (FLR20S・W/M, TOSHIBA) to set SL-L and FL-L. This paragraph explains the reasons why we used these six light qualities. Fluorescent lamps are popularly used in plant factories, so FL was used to reproduce plant growth in plant factories. R was used because it was reported that monochromic red light irradiation showed the highest photosynthesis rate in many monochromic light irradiations (Balegh and Biddulph 1970;; Björkman 1973). Previous research reported dry weight of plant grown increased in blue light (Hirai et al. 2006). That is why B was used. For RB, it was reported that the light mixed with red and blue light promoted plant growth and photosynthesis. To reproduce plant growth in sunlight which plants have adapted, SL-L was used. FL-L was used to research the effect of the light which removing ultraviolet and infrared rays from fluorescent light.
Photosynthesis rate (µmol m-2 s-1)
treatment was lower than that of RB treatment (Mori and Takatsuji 2000). But our result was opposite of previous result. It could be due to the difference plant specie, experiment environment and growth condition. The photosynthesis rates under SL-L, FL-L and FL were not so different. This could be due to little difference among their light qualities. In spite of light intensity or CO2 concentration, the result that the highest photosynthesis rate was in R, followed in RB and in the others in this order was not changed.
9 8 7 6 5 4 3 2 1 0 R
RB
B
SL-L
FL-L
FL
Fig. 3. Relation between light qualities and photosynthesis rates: This data was measured in the case when light intensity was set at 200 ρିଶ ିଵ and CO2 concentration was set at 600 ppm. 2.5 Photosynthesis Rate Measurement LI-6400 (LI-COR) was used for photosynthesis rate measurement. Plant leaf was fastened with a leaf chamber of LI-6400 whose CO2 concentration set at 400 ppm and applied one light quality whose light intensity was set at 300 µmolm- 2 -1 s for an hour. After confirming photosynthesis rate was stable, the measurement was started. The light intensity was set at 300 (250 in FL) µmolm-2s-1 at first, followed 200, 150, 100, 80, 60, 40, 20, 10 µmolm-2s-1. In each light intensity, CO2 concentration was set from 0 ppm to 1500 ppm per 100 ppm and photosynthesis rate was measured and stored at every condition. After measurement was completed to 10 µmolm-2s-1, light quality was changed to the next and the same measurement procedure was repeated. In all measurement, the same area of the same leaf of the same plant was used. 2.6 Growth Experiment
3.2 Growth Experiment 3.2.1 Number of leaves Figure 4 shows the relation between light quality and number of leaves. Large standard errors can be seen in R and FL treatments. Plants grown under R treatment had long leaves and petioles, so that overlapping of leaves happened. The overlapping makes difference to the light quantity supplying plants and plant growth like number of leaves. The light intensity of the FL treatment is different among transplanted spots because the light intensity of fluorescent lamp decreases near edge of the lamp tube. It could cause the difference of individual growth under FL. Other light qualities happened less overlapping of leaves than R and had the same light environment among individuals because light sources set right above. So standard errors in their treatment were significantly low. Number of leaves was increased in SL-L and significantly decreased in B. Plants grown in SL-L treatment had more new and small leaves than others. Decreasing leaves of lettuces grown under blue light has been reported previously (Hirai et al. 2006). This research reported that the effect of blue light on decreasing leaves has the significant difference among monochromic blue, blue-green, green and red light treatment. And in our research, this effect of blue light has the significant difference among B, R, RB, SL-L, FL-L and FL treatment. From these researches, it is considered that blue light could decrease number of leaves and green and red lights could suppress this decreased effect of blue light.
Number of leaves (leaves/plant)
This experiment had six treatments with six light qualities. One treatment with nine plants was considered as a replication. But one plant in R treatment and two plants in FL treatment were not grown well, so they were removed in replications. Growth responses were determined to measure fresh weight, dry weight and number of leaves per plant. To compare the costs, we measured electric power consumptions. The number of leaves was measured all leaves contained dead or very small new leaves. The dry weight was measured after drying the samples in a hot air oven at 70 °C for 4 days. 3. RESULTS and DISCUSSIONS 3.1 Photosynthesis Rate Measurement Figure 3 shows the relation between light quality and photosynthesis rate. Photosynthesis rate increased in monochromic red light (R) and RB. These light qualities contained more red light than others, so it was considered that red light could promote photosynthesis. This response has been reported previously in many species (Emerson and Lewis 1943;; Balegh and Biddulph 1970;; Björkman 1973). Another research reported that photosynthesis rate of R
25.0 20.0 15.0 10.0 5.0 0.0 R
RB
B
SL-L
FL-L
FL
Fig. 4. Relation between light quality and number of leaves: Each value is the mean ± standard errors of replications.
80 60 40 20
0 R
RB
B
SL-L FL-L
FL
2.5
140 Production efficiency (g/W)
2
120
Power consumption (W)
100
1.5
80
1
60 40
0.5
20
0
Power consumption (W)
Production efficiency (g/W)
Fresh weight (g)
100
0 R
B
FL
Fig. 5. Relation between light quality and fresh weight: Each value is the mean ± standard errors of replications.
Fig. 7. Relation among light quality, production efficiency and electric power consumption
3.2.2 Fresh weight and Dry weight
It is considered R treatment could suppress dry matter production for fresh weight. We think the reason why dry weight rate decreased in R could be a bad plant posture of light absorption caused by overlapping of leaves and hanging down leaves. Figure 6 also shows that the higher rate of red light a light quality had, the less dry weight of plant grown under the light quality became.
We measured fresh and dry weight of plant parts above a culture panel. Figure 5 shows the relation between light quality and fresh weight. Large standard errors of fresh weight can be seen in R and FL treatments. It was caused by the difference of competition or light environment among individuals as seen in number of leaves. Since plant photosynthesizes to grow up, it is considered the light quality which shows high photosynthesis rate produces heavy fresh weight of plant. In fact, photosynthesis rate and fresh weight under five light qualities set by LED system were highly correlated (R2 = 0.80ሻ. Fluorescent lamps had wider radiation range than LED systems had. It could occur that plants under FL treatment had big fresh weight for photosynthesis rate.
Dry weight rate (%)
Dry weight had big difference among individuals. Figure 6 shows the relation between light quality and dry weight rate which means dividing dry weight by fresh weight. Dry weight rate had small difference among individuals. Small amount of dry weight rate increased in FL and B and reduced in R. It was reported that lettuce grown under red light had the smallest dry weight among that under monochromic blue, blue-green, green and red light (Hirai et al. 2006). In our research, plant in R treatment had as big dry weight as that in B treatment had. This result was different from previous research, but dry weight rate was much lower in R.
Figure 7 shows the relation among light quality, production efficiency and electric power consumption. Production efficiency means dividing average fresh weight by electric power consumption. R and B treatment using LED systems had much lower power consumption and higher production efficiency compared to FL. Production efficiency of B, which was the smallest in that of other light qualities using LED systems, was about 1.8 times higher than that of FL. One of the most important reasons of this result could be the difference of energy saving ability between LEDs and fluorescent lamps. On the other hand, the large difference among light qualities using LEDs was also observed. Production efficiency of R treatment was about 1.7 times higher than that of B treatment. This difference was because red light has lower power consumption than blue light. However, power consumption differs from light sources, so this data is one of examples.
5
4. CONCLUSIONS
4
2
In conclusion, monochromic red light might be the most effective for photosynthesis and growth. However, monochromic light also makes plant shape bad and the plant is worthless as a product. To improve plant shape, plant must be radiated mixture light.
1
REFERENCES
0
Akoyunoglou, G. and Anni, H. (1984). Blue light effect on chloroplast development in higher plants. In Senger H (ed.), Blue light effects in biological systems, 397-406. Springer-Verlag, Berlin. Balegh SE. and Biddulph O. (1970). The Photosynthetic Action Spectrum of the Bean Plant. Plant Physiol., 46, 1- 5 Björkman O. (1973). Comparative studies on photosynthesis in higher plants. Photophysiology, 8, 1-63
3
R
RB
SL-L
FL-L
B
FL
Fig. 6. Relation between light quality and dry weight rate: Light quality contains more red light in order from left. Each value is the mean ± standard errors of replications.
3.2.3 Comparison of electric power consumption
Flint and McAllister. (1937). Wavelengths of radiation in the visible spectrum promoting the germination of light- sensitive lettuce seed. Smithsonian Inst. Publs., MIisc. Collections, 96, 1-8. Folta. (2004). Green Light Stimulates Early Stem Elongation, Antagonizing Light-Mediated Growth Inhibition. Plant Physiology, 135, 1407-1416. Folta. and Maruhnich. (2007) Green light: a signal to slow down or stop. Journal of Experimental Botany, 58 (12) 3099-3111. Hirai T., Amaki W., Watanabe H. (2006). Effects of Monochromatic Light Irradiation by LED on the Internodal Stem Elongation of Seedlings in Eggplant, Leaf Lettuce and Sun flower. J.SHITA, 18 (2), 160-166.
Mori Y. and Takatsuji M. (2000). Influence of laser and LED Lights on the growth of lettuce. The Illuminating Engineering Institute of Japan, 33, 283-284. Mori Y., Takatsuji M. and Yasuoka T. (2002). Effects of Pulsed White LED Light on the Growth of Lettuce. J.SHITA, 14 (3), 136-140. Takemiya A., Takahashi Y. and Shimazaki K. (2007). Leaf Temperature Reduction by Blue Light-dependent Stomatal Opening. Cryobiology and Cryotechnology, 53 (1), 1-5. Terashima I., Fujita T., Inoue T., Wah S.C. and Oguchi R. (2009). Green Light Drives Leaf Photosynthesis More Efficiently than Red Light in Strong White Light: Revisiting the Enigmatic Question of Why Leaves are Green. Plant Cell Physiol., 50 (4), 684-697
* Graduate student, Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan ( e-mail: [email protected]u.ac.jp, Tel:81-075-753-6165) ** Professor, Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan (e-mail: [email protected]u.ac.jp.) *** Associate Professor, Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan (e-mail: [email protected]u.ac.jp) **** Assistant Professor, Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan (e-mail: [email protected]u.ac.jp) ***** Assistant Professor, Graduate School of Agriculture, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan (e-mail: [email protected]u.ac.jp) Abstract: Plant factory is a plant production system with controlled temperature, carbon dioxide concentration and light environment to enhance plant growth efficiently. Fluorescent lamp is popularly used in a plant factory, however, the spectrum of fluorescent light is broad and some portion of them is not necessary for photosynthesis. On the other hand, LED (light emitting diode) has narrow band width and is getting easy to use these days. The final goal of this project is to develop a new light apparatus with LEDs for plant factory. The specific objective is to investigate the effective light quality for growing plant more rapidly. The effect of light generated by monochromic and mixed radiation from LEDs (blue, green, red) on the response of lettuce (Lactuca sativa L.) were evaluated and compared with the effect of lettuce irradiated by fluorescent lamps. Photosynthesis rate of lettuce increased in monochromic red light and the light mixed with red and blue light. Number of leaves decreased in monochromic blue light. Fresh weight of lettuce was increased in monochromic red and fluorescent light, but monochromic red light significantly decreased dry weight rate. In the view of energy saving, LED is much better than fluorescent lamp. Especially, red LED is good. Keywords: plant factory, light quality, carbon dioxide concentration, light intensity, morphogenesis, fresh weight, LED
1. INTRODUCTION Agriculture and food issues are getting known widely. For example, residual agricultural chemicals, poor harvest by abnormal weather conditions are often heard all over the world. Especially in Japan, there are other problems such as the increase of imported foods and the decrease of farmers. Plant factory is expected to be one of the ways to solve the issues. The plant factory is controlled inside environment such as lighting, temperature, carbon dioxide concentration and cleanliness to enhance plant growth effectively. That is why plant factory can produce a lot of safe vegetables in spite of season or weather. However, the initial and running costs for environmental control are very expensive. These problems prevent a plant factory from spreading widely. In running cost, the cost of lighting for plant growth is one of the large portions. Recently a light emitting diode (LED) is spreading as a new light source. A LED has good life performance, energy- saving ability and various and narrow bandwidth of wavelength. Combining several LEDs with different bandwidth enables to produce many kind of light quality. The effects of light qualities on the growth and development of plant have been studied for a long time. Red light promotes
germination and photosynthesis (Flint and McAllister 1937;; Balegh and Biddulph 1970). Blue light effects stomatal opening and chloroplast development (Takemiya et al. 2007;; Akoyunoglou and Anni 1984). Green light slows down or stop the plant development, but it brings positive effects in special conditions (Folta 2004, 2007;; Terashima et al. 2009). In many plants, it has been studied that the absorbance of leaves and the quantum yield of photosynthesis concerning various wavelength. We may produce an optimum light quality for plant growth by combining the LED¶s characteristic of light quality with researches of plant growth. In fact, photosynthesis rate and growth rate of plant grown under LED were reported (Mori and Takatsuji 2000, Mori et al. 2002). The final goal of our study is to improve lighting apparatus for plant factories to spread them widely. So the specific objective of this research is to investigate the best light qualities for plant growth by examining photosynthesis rate and growth response of lettuce (Lactuca sativa L.) radiated by LEDs and fluorescent lamps.
h/d light period and 25 °C in light period and 18 °C in dark period. Table 1. Light intensity rates of blue, green and red LEDs for five light qualities using LED systems (%)
Blue LED
Green LED
Red LED
R
0.0
0.0
100.0
RB
16.7
0.0
83.3
B
100.0
0.0
0.0
SL-L
33.4
33.9
32.7
FL-L
29.8
39.6
30.7
Fig. 1. Developed LED system 2.3 Hydroponics Nutrient solution pools of hydroponics were consisted of culture bed (730 wide mm × 665 length mm × 170 height mm) covered tarpaulin and culture panel. The quantity of nutrient solution in the pools was maintained even by pump and overflows. Nutrient solution was kept pH 5.9±0.2, EC 2.00±0.10 by fertilizer controller (CEMCO) with acidity regulator (Otsuka) and fertilizers (Otsuka). Fig. 2. Spectrums of LEDs: Blue, Green and Red were used in this research. 2. MATERIALS and METHODS 2.1 Light Source We used two light sources. One was a fluorescent lamp (FLR20S・W/M, TOSHIBA), the other was a LED system. The LED system was consisted of blue, green and red LEDs (LUXEON Rebel, Philips Lumileds) and condenser lenses as shown in Figure 1. Each spectrum of LEDs is shown in Figure 2. Photosynthetic photon flux density (PPFD) from LEDs system can be controlled by adjusting currents from a DC power supply. 2.2 Plant Materials and Growth Condition Lettuce (Lactuca sativa L. cv´Greenwave´, Takii Seed Co., Ltd.) plants were grown in urethane sponge with water under fluorescent lamps for one week. PPF of fluorescent lamps was set at 200 µmolm-2s-1. After one week, lettuce plants were transplanted to hydroponic system. We conducted two experiments namely photosynthesis rate measurement and growth experiment. In photosynthesis rate measurements, transplanted plants were grown under fluorescent lamps for a month (11/26-12/27, 2009). In growth experiment, they were grown under six light qualities from fluorescent lamps and LED systems for four weeks. All of the plants were grown in a growth chamber (KCLP-1000ⅠCT, NKsystem) set at a 16
2.4 Light Quality In photosynthesis measurement and growth experiment, six light qualities were used. One was fluorescent light (FL) set by fluorescent lamps and the others were set by LED systems. The light qualities set by LED systems were monochromic red light (R), monochromic blue light (B), the light mixed with red and blue light (RB), the light imitating sunlight (SL- L), the light imitating fluorescent lamp light (FL-L). Table 1 shows light intensity rates of blue, green and red LEDs about these five light qualities. We referred to the spectrum data of sunlight (Air Mass 1.5: ASTM G173, NREL) and a fluorescent lamp (FLR20S・W/M, TOSHIBA) to set SL-L and FL-L. This paragraph explains the reasons why we used these six light qualities. Fluorescent lamps are popularly used in plant factories, so FL was used to reproduce plant growth in plant factories. R was used because it was reported that monochromic red light irradiation showed the highest photosynthesis rate in many monochromic light irradiations (Balegh and Biddulph 1970;; Björkman 1973). Previous research reported dry weight of plant grown increased in blue light (Hirai et al. 2006). That is why B was used. For RB, it was reported that the light mixed with red and blue light promoted plant growth and photosynthesis. To reproduce plant growth in sunlight which plants have adapted, SL-L was used. FL-L was used to research the effect of the light which removing ultraviolet and infrared rays from fluorescent light.
Photosynthesis rate (µmol m-2 s-1)
treatment was lower than that of RB treatment (Mori and Takatsuji 2000). But our result was opposite of previous result. It could be due to the difference plant specie, experiment environment and growth condition. The photosynthesis rates under SL-L, FL-L and FL were not so different. This could be due to little difference among their light qualities. In spite of light intensity or CO2 concentration, the result that the highest photosynthesis rate was in R, followed in RB and in the others in this order was not changed.
9 8 7 6 5 4 3 2 1 0 R
RB
B
SL-L
FL-L
FL
Fig. 3. Relation between light qualities and photosynthesis rates: This data was measured in the case when light intensity was set at 200 ρିଶ ିଵ and CO2 concentration was set at 600 ppm. 2.5 Photosynthesis Rate Measurement LI-6400 (LI-COR) was used for photosynthesis rate measurement. Plant leaf was fastened with a leaf chamber of LI-6400 whose CO2 concentration set at 400 ppm and applied one light quality whose light intensity was set at 300 µmolm- 2 -1 s for an hour. After confirming photosynthesis rate was stable, the measurement was started. The light intensity was set at 300 (250 in FL) µmolm-2s-1 at first, followed 200, 150, 100, 80, 60, 40, 20, 10 µmolm-2s-1. In each light intensity, CO2 concentration was set from 0 ppm to 1500 ppm per 100 ppm and photosynthesis rate was measured and stored at every condition. After measurement was completed to 10 µmolm-2s-1, light quality was changed to the next and the same measurement procedure was repeated. In all measurement, the same area of the same leaf of the same plant was used. 2.6 Growth Experiment
3.2 Growth Experiment 3.2.1 Number of leaves Figure 4 shows the relation between light quality and number of leaves. Large standard errors can be seen in R and FL treatments. Plants grown under R treatment had long leaves and petioles, so that overlapping of leaves happened. The overlapping makes difference to the light quantity supplying plants and plant growth like number of leaves. The light intensity of the FL treatment is different among transplanted spots because the light intensity of fluorescent lamp decreases near edge of the lamp tube. It could cause the difference of individual growth under FL. Other light qualities happened less overlapping of leaves than R and had the same light environment among individuals because light sources set right above. So standard errors in their treatment were significantly low. Number of leaves was increased in SL-L and significantly decreased in B. Plants grown in SL-L treatment had more new and small leaves than others. Decreasing leaves of lettuces grown under blue light has been reported previously (Hirai et al. 2006). This research reported that the effect of blue light on decreasing leaves has the significant difference among monochromic blue, blue-green, green and red light treatment. And in our research, this effect of blue light has the significant difference among B, R, RB, SL-L, FL-L and FL treatment. From these researches, it is considered that blue light could decrease number of leaves and green and red lights could suppress this decreased effect of blue light.
Number of leaves (leaves/plant)
This experiment had six treatments with six light qualities. One treatment with nine plants was considered as a replication. But one plant in R treatment and two plants in FL treatment were not grown well, so they were removed in replications. Growth responses were determined to measure fresh weight, dry weight and number of leaves per plant. To compare the costs, we measured electric power consumptions. The number of leaves was measured all leaves contained dead or very small new leaves. The dry weight was measured after drying the samples in a hot air oven at 70 °C for 4 days. 3. RESULTS and DISCUSSIONS 3.1 Photosynthesis Rate Measurement Figure 3 shows the relation between light quality and photosynthesis rate. Photosynthesis rate increased in monochromic red light (R) and RB. These light qualities contained more red light than others, so it was considered that red light could promote photosynthesis. This response has been reported previously in many species (Emerson and Lewis 1943;; Balegh and Biddulph 1970;; Björkman 1973). Another research reported that photosynthesis rate of R
25.0 20.0 15.0 10.0 5.0 0.0 R
RB
B
SL-L
FL-L
FL
Fig. 4. Relation between light quality and number of leaves: Each value is the mean ± standard errors of replications.
80 60 40 20
0 R
RB
B
SL-L FL-L
FL
2.5
140 Production efficiency (g/W)
2
120
Power consumption (W)
100
1.5
80
1
60 40
0.5
20
0
Power consumption (W)
Production efficiency (g/W)
Fresh weight (g)
100
0 R
B
FL
Fig. 5. Relation between light quality and fresh weight: Each value is the mean ± standard errors of replications.
Fig. 7. Relation among light quality, production efficiency and electric power consumption
3.2.2 Fresh weight and Dry weight
It is considered R treatment could suppress dry matter production for fresh weight. We think the reason why dry weight rate decreased in R could be a bad plant posture of light absorption caused by overlapping of leaves and hanging down leaves. Figure 6 also shows that the higher rate of red light a light quality had, the less dry weight of plant grown under the light quality became.
We measured fresh and dry weight of plant parts above a culture panel. Figure 5 shows the relation between light quality and fresh weight. Large standard errors of fresh weight can be seen in R and FL treatments. It was caused by the difference of competition or light environment among individuals as seen in number of leaves. Since plant photosynthesizes to grow up, it is considered the light quality which shows high photosynthesis rate produces heavy fresh weight of plant. In fact, photosynthesis rate and fresh weight under five light qualities set by LED system were highly correlated (R2 = 0.80ሻ. Fluorescent lamps had wider radiation range than LED systems had. It could occur that plants under FL treatment had big fresh weight for photosynthesis rate.
Dry weight rate (%)
Dry weight had big difference among individuals. Figure 6 shows the relation between light quality and dry weight rate which means dividing dry weight by fresh weight. Dry weight rate had small difference among individuals. Small amount of dry weight rate increased in FL and B and reduced in R. It was reported that lettuce grown under red light had the smallest dry weight among that under monochromic blue, blue-green, green and red light (Hirai et al. 2006). In our research, plant in R treatment had as big dry weight as that in B treatment had. This result was different from previous research, but dry weight rate was much lower in R.
Figure 7 shows the relation among light quality, production efficiency and electric power consumption. Production efficiency means dividing average fresh weight by electric power consumption. R and B treatment using LED systems had much lower power consumption and higher production efficiency compared to FL. Production efficiency of B, which was the smallest in that of other light qualities using LED systems, was about 1.8 times higher than that of FL. One of the most important reasons of this result could be the difference of energy saving ability between LEDs and fluorescent lamps. On the other hand, the large difference among light qualities using LEDs was also observed. Production efficiency of R treatment was about 1.7 times higher than that of B treatment. This difference was because red light has lower power consumption than blue light. However, power consumption differs from light sources, so this data is one of examples.
5
4. CONCLUSIONS
4
2
In conclusion, monochromic red light might be the most effective for photosynthesis and growth. However, monochromic light also makes plant shape bad and the plant is worthless as a product. To improve plant shape, plant must be radiated mixture light.
1
REFERENCES
0
Akoyunoglou, G. and Anni, H. (1984). Blue light effect on chloroplast development in higher plants. In Senger H (ed.), Blue light effects in biological systems, 397-406. Springer-Verlag, Berlin. Balegh SE. and Biddulph O. (1970). The Photosynthetic Action Spectrum of the Bean Plant. Plant Physiol., 46, 1- 5 Björkman O. (1973). Comparative studies on photosynthesis in higher plants. Photophysiology, 8, 1-63
3
R
RB
SL-L
FL-L
B
FL
Fig. 6. Relation between light quality and dry weight rate: Light quality contains more red light in order from left. Each value is the mean ± standard errors of replications.
3.2.3 Comparison of electric power consumption
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