Solid-State Lamps (LEDs) for the Short-Wavelength Supplementary Lighting in Greenhouses: Experimental Results with Cucumber

Solid-State Lamps (LEDs) for the Short-Wavelength Supplementary Lighting in Greenhouses: Experimental Results with Cucumber A. Novičkovas, A. Brazaitytė, P. Duchovskis, Z. Bliznikas and A. Žukauskas J. Jankauskienė, G. Samuolienė, A. Viršilė Institute of Applied Research and R. Sirtautas Vilnius University Institute of Horticulture Vilnius Research Centre for Agriculture and Forestry Lithuania Babtai, Kaunas District Lithuania Keywords: cucumber, growth, photosynthesis pigments, saccharides, solid-state illumination, LEDs Abstract The solid-state lighting technology based on light-emitting diodes (LEDs) offers vast possibilities in horticultural lighting. Here we present the prototypes of shortwavelength single-monochromatic solid-state lamps developed for the supplementation of high-pressure sodium (HPS) lamps used in greenhouses. Each lamp features high-power AlInGaN LEDs, a stabilized-current switch-mode AC/DC power supply, an optimized heat sink with lateral reflectors, and a protective lid with lenses which allows for operation in humid environments. Four types of lamps with peak emissions at 455, 470, 505, and 530 nm were made. Lamps were installed in a phytotron greenhouse. Transplants of cucumber hybrid Mandy were grown in a phytotron greenhouse under natural daylight with supplemental illumination of HPS lamps and our solid-state lamps. The biometrical measurements, assessment photosynthetic pigments and saccharides content were performed at the end of the experiment. Our investigations revealed that the supplemental 470, 505 and 530 nm LEDs illumination with high pressure sodium lamps increased leaf area, fresh and dry weight of cucumber transplants, decreased hypocotyls elongation and enhanced their development. while, the supplemental 455 nm LED illumination caused slower growth and development of cucumber transplants. The synthesis of photosynthetic pigments mostly enhanced 455, 470 and 530 nm supplemental illumination, but supplemental green (505 and 530 nm) light slightly decreased chlorophyll a and b ratio in leaves of cucumber hybrid Mandy. INTRODUCTION The vegetable transplants in greenhouses are grown during autumn winter period, when the natural light level is low and, consequently, supplemental lighting is needed. High-pressure sodium lamps are mostly used for the supplemental illumination. These lamps emit light in the visible (400-700 nm) and the invisible (700-850 nm) ranges, but the peak emission is in the yellow light (~589 nm) region. High amount of yellow light causes the stem elongation of plants (Wheeler, 1991; Spaargaren, 2001; Wheeler, 2008). The solid-state lighting using light-emitting diodes (LEDs) represents a fundamentally different technology from the gaseous discharge-type lamps currently used in horticulture and has more advantages than the traditional forms of lighting (Massa et al., 2008; Morrow, 2008). Their small size, durability, long lifetime, fast switching, simple control of the generated flux, low thermal radiation directed towards plants, and the option to select specific wavelengths for the targeted plant response make LEDs more suitable for plant-based uses than many other light sources (Massa et al., 2008). The first attempts to design LED-based lighting systems for the plant illumination were targeted to space applications (Barta et al., 1992). LEDs illumination has been studied for the tissue culture applications (Kim et al., 2004; Seabrook, 2005; Kurilčik et al., 2008) and for lighting of various plants in the controlled environments and supplemental lighting in the Proc. XXVIII th IHC IS on Greenhouse 2010 and Soilless Cultivation Ed.: N. Castilla Acta Hort. 927, ISHS 2012 723

greenhouses (Tennessen et al., 1995; Yorio et al., 2001; Morrow, 2008; Brazaitytė et al., 2009). There is some data in the literature about cucumber as well as tomato growth and harvest using blue LEDs as supplement for the HPS lamps during the whole growth period (Menard et al., 2006). Blue light responses are found on the inhibition of hypocotyls elongation, internode and petiole elongation, initiation of flowering time, phototropism and other processes in plants (Ahmad et al., 2002). Therefore using blue light sources for the supplementation of HPS lamps could improve vegetable transplants quality. It has been shown that green light affects plant processes via cryptochromedepended and cryptochrome-independed means (Folta and Maruhnich, 2007). The addition of green light may increase plant growth, since green light can penetrate the plant canopy better than red or blue lights. Leaves in the lower canopy would be able to use the transmitted green light for photosynthesis (Kim et al., 2006). In the literature we did not find any data about vegetable seedlings growth using green LEDs as a supplement to the HPS lamps. The aim of this study was to present the prototypes of short-wavelength singlemonochromatic solid-state lamps developed for the supplementation of high-pressure sodium (HPS) lamps used in the greenhouses. MATERIALS AND METHODS Short-wavelength single-monochromatic lamps were designed using the four types of high-power (Luxeon III series (3 W), Philips Lumileds Lighting Company, USA) AlInGaN LEDs: 455 nm (LXHL-LR3C), 470 nm (LXHL-LB3C), 505 nm (LXHL-LE3C) and 530 nm (LXHL-LM3C). Each lamp features 24 units of LEDs mounted on an aluminum rectangular tube type heatsink with lateral reflectors and a stabilized-current switch-mode AC/DC power supply. The LEDs are protected against humid environment by a translucent polycarbonate lid with hemispherical lenses. The gap between the lenses and LED domes is filled with translucent silicon rubber in order to maximize optical coupling. Such a design makes cleaning from dust and insect contamination easy. Four lighting modules, each of which contains four solid-state lamps of the same wavelength described above, were installed in a phytotron greenhouse of the Institute of Horticulture, Lithuanian Research Centre for Agriculture and Forestry. The cucumber hybrid Mandy was seeded in the peat substrate (ph 6.0-6.5) enriched with fertilizes PG MIX (NPK 14:16:18 1.3 kg m -3 ) within a phytotron greenhouse. Plants were watered when necessary. During the transplants cultivation, the day/night temperature was 17-22/14-17 C and the relative air humidity was 50-60%. The daylight (January-February, Lithuania, lat. 55 N, the photosynthetic photon flux density (PPFD) averaging about 100-200 µmol m -2 s -1 ) was supplemented by high-pressure sodium lamps (Son-T Agro, Philips) at a PPFD of 90 μmol m -2 s -1 (18 h photoperiod) and solid-state lighting modules (18 h photoperiod). The generated PPFD of each type of solid-state modules was 15 μmol m -2 s -1. The reference transplants were grown under daylight and supplemental illumination of HPS lamps. Plants were harvested on the 30 th day after sowing and oven-dried at +105 C for 24 h to determine the dry weight. Leaf area of cucumber plants was measured by WinDias leaf area meter (Delta-T Devices Ltd., UK). Plant height was measured up to the top of cucumber transplants. Inflorescence length was measured at the third leaf from the bottom. These measurements were performed in ten replicates (n=10). Photosynthetic pigments content per one gram of fresh foliage weight was measured in 100% acetone extract according to D. Wettstein method (Wettstein, 1957) using Genesys 6 spectrophotometer (ThermoSpectronic, USA). Measurements were performed in four replicates (n=4) during seedlings transplantation. One to two grams of fresh cucumber transplants leaf per sample for saccharides analysis was grounded, diluted with 4 ml of double distilled water, extracted for 24 h and filtered, and before analysis purified using 0.2 µm syringe filters. Fructose, glucose and sucrose were determined using HPLC system (model 10A, Shimadzu, Japan) equipped with refractive index detector (RID 10A, Shimadzu), column oven (CTO-10AS VP, 724

Shimadzu), degasser (DGU-14A, Shimadzu) and pump (LC-10AT VP, Shimadzu). The separations were performed on an Adsorbosil column with NH 2 groups (150 4.6 mm). The mobile phase of 75% acetonitrile in double distilled water was used. Three analytical samples of sugars (n=3) were measured for each. Statistical analyses were conducted using STATISTICA 7.0 for Windows. Statistical differences between measurements on the different illumination were also analysed following the Student s t-test. Differences were considered significant at a probability level of P<0.05. RESULTS AND DISCUSSION The height of cucumber transplants was significantly increased by the supplementary green 505 and 530 nm light of short-wavelength single-monochromatic solid-state lamps (Fig. 1), however hypocotyls length was significantly decreased when the illumination by the high pressure sodium (HPS) lamps were combined with 530 nm light (Fig. 1). Plant response to the supplemental blue 455 and 470 nm light differed. Cucumber height was not significantly affected by additional blue light from the blue LEDs (Fig. 1), however hypocotyls length significantly decreased by supplementary 470 nm light (Fig. 1). Transplants fresh weight seemed to be significantly increased by supplementary 470 nm, however, significant differences were not observed under the effect of supplemental 455 nm light (Fig. 1). while, shoot fresh weight of lastmentioned plants were significantly lower than in plants affected by supplemental blue 470 nm and green 505 and 530 nm LEDs lamps. Similar trends were observed in shoot dry weight, though no significant differences were determined from the HPS lamps alone (Fig. 1). Supplemental 455 nm solid-state lamps led to a reduced leaf area of cucumber transplants (Fig. 2). while, supplemental blue light from 470 nm LEDs and green light from 505 and 530 nm LEDs positively affected leaf area formation, but significant differences from the HPS lamps alone were observed in plants under supplemental 470 and 505 nm light. Solid-state lamps developed for the supplementation of high-pressure sodium (HPS) lamps also affected the cucumber transplants development. According to our obtained results blue light from 455 nm LEDs significantly suppressed inflorescence development. The length of mostly developed flower at the third leaf of cucumber transplants under this illumination was about two times less then under illumination with supplemental green 530 nm light (Fig. 2). Generally, additional green light from green LEDs significantly enhanced the cucumber inflorescence development. while, different illumination had no significant effect on leaf development. All cucumber transplants had four fully expanded leaves (data not shown). The presented data showed clearly that spectra quality had substantial effect on the growth and development of cucumber. A contrary effect of supplemental blue light from 455 and 470 nm LEDs was revealed. The supplemental 455 nm light suppressed cucumber transplants growth and development. Plants under supplemental 455 nm light were shorter, however, their hypocotyls were longer. Especially evident differences were between shoot and dry weight, leaf area and inflorescence length. Blue light stimulates stomata opening, chlorophyll formation, increase in photosynthesis rate and above-ground biomass per surface area (Wheeler et al., 1991; Menard et al., 2006). Blue light through the blue light photoreceptors participates in the control of stem and hypocotyl elongation (Ahmad et al., 2002; Bouly et al., 2007). Because the HPS lamps produce very little radiation in the blue region, their spectrum is supplemented using other light sources. Some authors suggest that the use of high-pressure sodium or other blue-deficient sources for lighting may cause abnormal stem elongation, but this can be prevented by adding a small amount of supplemental blue light provided from the blue (400-500 nm) fluorescent lamps (Wheeler et al., 1991). As our obtained data showed, it is important to select the best wavelengths of blue light for plant cultivation. In 1990 the light-emitting diodes (LEDs) tested for plant growth allowed to realize these purposes (Bula et al., 1991). In agreement with Menard and others (Menard et al., 2006), 455 nm supplemental LEDs to the HPS lamps decreased 725

internodes length and increased shoot dry weight of cucumber and tomato. Our results revealed that the blue light from blue 470 nm LEDs had more positive effect on the cucumber transplant growth and development than from 455 nm LEDs. According to our obtained data, green light from green 505 and 530 nm LEDs significantly enhanced cucumber transplants growth and development, and decreased hypocotyls elongation. Both of these supplemental illuminations had a similar effect on these indices. Our earlier experiments also revealed that additional green light from 520 nm LEDs in the high-power solid-state lighting modules with the blue, red and far-red LEDs positively affected cucumber transplants growth and development in the phytotron chambers (Brazaitytė et al., 2009). Contrary, tomato transplants cultivated under the modules with the main blue, red and far-red LEDs and supplemented by green 520 nm LEDs were not elongated, but leaf area, and fresh and dry weight, especially roots, were small (Brazaitytė et al., 2010). Data in the literature about the effect of green light is controversial. Some authors obtained reductions in growth, leaf number, internodes length, and a delay in flower induction of marigold, carnation and lettuce, when white light was supplemented with the green wavelengths (Klein et al., 1965). Kim and others noticed that green light (the addition of 24% green light to red and blue LEDs) enhanced lettuce growth. However, light sources with a higher fraction of green light (>50% of total PPF) were found to reduce plant growth (Kim et al., 2006). Photosynthesis system responds to light most sensitively. Photosynthetic pigment content in higher plants is an important indicator for determining plants physiological state. Chlorophyll loss is associated to environmental stress (Netto et al., 2005). Generally, supplemental solid-state lamps to HPS lamps more or less increased photosynthesis pigments content in leaves of cucumber transplants (Fig. 3). The chlorophyll a level, the main photosynthesis pigment, was found to be significantly increased under the illumination with supplemental green 530 nm light (Fig. 3). Similar effect was noticed in leaves of cucumber transplants under the illumination with supplemental blue 470 nm LEDs, but chlorophyll a content in this case differed insignificantly from HPS lamps alone. Supplemental blue 470 nm and green 530 nm significantly increased chlorophyll b content in the leaves of cucumber transplants (Fig. 3). while, carotenoids content significantly increased in cucumber transplants cultivated under the illumination with supplemental blue light from 455 nm LEDs (Fig. 3). As it is known, the peak of carotenoids absorption spectrum is near this wavelength and that could cause the greater carotenoids synthesis in cucumber leaves. The obtained data showed a contrary effect of supplemental blue and green light of different wavelength on chlorophylls content. The illumination with supplemental blue light from 455 nm LEDs slightly decreased chlorophyll a and chlorophyll b content in the leaves of cucumber transplants as compared with plants under 470 nm light (Fig. 3). Supplemental green 505 nm light significantly decreased chlorophyll a content, but chlorophyll b content decreased insignificantly comparing with plants under 530 nm light. The chlorophylls ratio (chlorophyll a/chlorophyll b) is considered the best indicator upon the photosynthesis process efficiency (Walters et al., 2003). Our investigation revealed that the supplemental green 505 nm and 530 nm light slightly decreased chlorophyll a/b ratio in the leaves of cucumber transplants (Fig. 3). Plants depending on chlorophyll content in their leaves absorb from 43 to 87% of the green light (Nishio, 2000). Green light can penetrate into the plant canopy better than red or blue. Leaves in the lower canopy would be able to use the transmitted green light in photosynthesis. The addition of green light may improve plant growth and photosynthesis, but the findings were inconclusive at the 5 percent green light level (Kim et al., 2006). As showed less chlorophyll a/b ratio in leaves of plants under supplemental green light, caused photosynthesis efficiency to be a little lower in cucumber transplants under supplemental green light comparing with blue, but this light enhanced the plant growth and development. Metabolism reactions of hexoses are one of the examples of reaction of plants metabolic flexibility during the adaptation to unfavorable conditions (Plaxton, 1996). 726

Glucose content in plants decreased when wavelengths of supplemental illumination increased (Fig. 4). Different supplemental illumination had no effect on fructose content in the leaves of cucumber transplants. According to our data, supplemental blue light from blue 455 and 470 nm LEDs had no significant effect on sucrose content (Fig. 4). while, the supplemental green 505 nm light significantly increased and 530 nm decreased sucrose content in the cucumber transplants. Low sugar status enhances photosynthesis, reserve mobilization, and export, whereas the abundant presence of sugars promotes growth and carbohydrate storage (Koch, 1996; Rolland et al., 2002). So, a low sucrose content in the cucumber under supplemental 530 nm green light enhanced photosynthesis in their leaves as the high chlorophyll content (Fig. 3) showed and promoted their growth and development (Figs. 1-2). CONCLUSIONS Our investigations revealed that supplemental 470, 505 and 530 nm LEDs illumination with high pressure sodium lamps increased leaf area, fresh and dry weight of cucumber transplants, decreased hypocotyls elongation and enhanced their development. while, the supplemental 455 nm LED illumination caused slower growth and development of cucumber transplants. The synthesis of photosynthetic pigments was mostly enhanced by 455, 470 and 530 nm supplemental illumination, but the supplemental green (505 and 530 nm) light slightly decreased chlorophyll a/b ratio in the leaves of cucumber hybrid Mandy. Literature Cited Ahmad, M., Grancher, N., Heil, M., Black, R.C., Giovani, B., Galland, P. and Lardemer, D. 2002. Action spectrum for cryptochrome-dependent hypocotyl growth inhibition in arabidopsis. Plant Physiology 129:774-785. Bouly, J.-P., Schleicher, E., Dionisio-Sese, M., Vandenbussche, F., Van Der Straeten, D., Bakrim, N., Meier, S., Batschauer, A., Galland, P., Bittl, R. and Ahmad, M. 2007. Cryptochrome blue light photoreceptors are activated through interconversion of flavin redox states. J. of Biological Chemistry 282(13):9383-9391. Brazaitytė, A., Duchovskis, P., Urbonavičiūtė, A. Samuolienė, G., Jankauskienė, J., Kasiulevičiūtė-Bonakėrė, A., Bliznikas, Z., Novičkovas, A., Breivė, K. and Žukauskas, A. 2009. The effect of light-emitting diodes lighting on cucumber transplants and after-effect on yield. Žemdirbystė Agriculture 96(3):102-118. Brazaitytė, A., Duchovskis, P., Urbonavičiūtė, A., Samuolienė, G., Jankauskienė, J., Sakalauskaitė, J., Šabajevienė, G., Sirtautas, R. and Novičkovas, A. 2010. The effect of light-emitting diodes lighting on the growth of tomato transplants. Žemdirbyste Agriculture 97(2):89-98. Bula, R.J., Morrow, R.C., Tibbitts, T.W., Barta, D.J., Ignatius, R.W. and Martin, T.S. 1991. Light-emitting diodes as a radiation source for plants. HortScience 26(2):203-205. Barta D.J., Tibbitts T.W., Bulla R.J. and Morrow R.C. 1992. Evaluation of light emitting diode characteristics for a space-based plant irradiation source. Advances in Space Res. 12:141-150. Folta, K.M. and Maruhnich, S.A. 2007. Green light: a signal to slow down or stop. J.l of Experimental Botany 58(12):3099-3111. Yorio, N.C., Goins, G.D., Kagie, H.R., Wheeler, R.M. and Sager, J.C. 2001. Improving spinach, radish, and lettuce growth under red light-emitting diodes (LEDs) with blue light supplementation. HortScience 36:380-383. Kim, H.H., Wheeler, R.M., Sager, J.C., Goins, G.D. and Norikane, J.H. 2006. Evaluation of lettuce growth using supplemental green light with red and blue light-emitting diodes in a controlled environment a review of research at Kennedy Space Center. Acta Hort. 711:111-119. Kim, S.J., Hahn, E.J., Heo, J.W. and Paek, K.-Y. 2004. Effects of LEDs on net photosynthetic rate, growth and leaf stomata of chrysanthemum plantlets in vitro. 727

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Figurese cm 24 22 20 18 16 14 4.4 4.0 3.6 3.2 plant height and hypocotyl length height length g 27 24 21 18 15 3.2 2.8 2.4 fresh dry shoot weight 2.8 ± SD 2.0 ± SD Fig. 1. Height, hypocotyl length and shoot weight of cucumber transplants grown under different illuminations. 1 SON T Agro, 2 +455 nm, 3 +470 nm, 4 +505 nm, 5 +530 nm. significantly different from control (1) plants as determined by paired t-test(p<0.05). mm 16 14 12 10 8 6 4 inflorescence length ± SD cm 2 900 850 800 750 700 650 600 550 leaf area ± SD Fig. 2. Inflorescence length and leaf area of cucumber transplants grown under different illuminations. 1 SON T Agro, 2 +455 nm, 3 +470 nm, 4 +505 nm, 5 +530 nm. significantly different from control (1) plants as determined by paired t-test (P<0.05). 729

mg g -1 FW 1.4 1.3 1.2 1.1 1.0 0.9 0.8 0.52 0.48 chlorophyll a carotenoids ± SD mg g -1 FW 0.52 0.48 0.44 0.40 0.36 0.32 0.28 0.24 3.30 3.15 chlorophyll b chlorophyll a and chlorophyll b ratio ± SD mg g -1 FW 0.44 0.40 0.36 0.32 3.00 2.85 2.70 2.55 0.28 ± SD 2.40 ± SD Fig. 3. Photosynthesis pigment content and chlorophyll a/b ratio of cucumber transplants grown under different illuminations. 1 SON T Agro, 2 +455 nm, 3 +470 nm, 4 +505 nm, 5 +530 nm. significantly different from control (1) plants as determined by paired t-test (P<0.05). mg g -1 FW 9 8 7 6 5 4 3 2 1 sucrose glucose fructose 0 Fig. 4. Saccharides content of cucumber transplants grown under different illuminations. 1 SON T Agro, 2 +455 nm, 3 +470 nm, 4 +505 nm, 5 +530 nm. significantly different from control (1) plants as determined by paired t-test (P<0.05). 730