Effects of light intensity and amount of supplemental LED lighting on photosynthesis and fruit growth of tomato plants under artificial conditions

Transcription

1 Full Paper J. Agric. Meteorol. 69 (2): , 2013 Effects of light intensity and amount of supplemental LED lighting on photosynthesis and fruit growth of tomato plants under artificial conditions Shoko HIKOSAKA, Soushi IYOKI, Mariko HAYAKUMO, and Eiji GOTO (Graduate School of Horticulture, Chiba University, Matsudo, Chiba, , Japan) Abstract We assessed the effects of light intensity (photosynthetic photon flux [PPF in µmol m -2 s -1 ]) and number of irradiated leaves on photosynthesis and the fruit growth of individual tomato plants to develop supplemental LED lighting techniques for greenhouse tomato production. In Experiment (Exp.) 1, three PPF levels (P200, P500, and P1000) were applied to a post-anthesis tomato plant for three weeks, each plant pruned to have one leaf and one truss with three flowers. The fruit set and leaf and fruit dry-weight increased with increasing PPF; however, after P500 and P1000 treatments, the leaves showed signs of stress and accompanying disorders. Thus, to increase the fruit set ratio and growth rate of tomato fruits and plants, the total amount of irradiation received by each plant should be increased by increasing the number of irradiated leaves, rather than raising the PPF per leaf. For prolonged cultivation, P200 was the optimal PPF per leaf under the tested treatments. Exp. 2 used standard tomato plants with no leaves or trusses removed. We used an assimilation chamber to examine the effect of the number of leaves receiving P200 irradiation on the photosynthetic rate (Pn) per plant (above ground part). The Pn per plant in treatments where one and two leaves were irradiated by supplemental LED lighting were, respectively, 12 and 28% higher than that in the control (only top lighting). Therefore, fruit growth and yields in tomato cultivation may be raised via acceleration of photosynthesis by increasing the number of leaves that receive P200 irradiation rather than by increasing PPF. Key Words: Artificial light, Assimilation chamber, Fruit set, Leaf disorder, PPF. 1. Introduction Tomatoes are one of the most widely cultivated crops under protected cultivation in the world, and are a highly popular food in Japan. Recently, the number of greenhouses used for tomato cultivation, which have an area of more than 1 ha each, has increased in Japan, such as those in the U.S., Canada, the Netherlands, and E.U. countries. However, the productivity of tomato plants in Japan is kg m -2 per year, which is half that of produced in the Netherlands (Hisaeda et al., 2007). To improve the productivity of tomatoes in Japan, it is necessary to optimize the plant growth environment of greenhouses. Received; September 18, Accepted; January 22, Corresponding Author: s-hikosaka@faculty.chiba-u.jp Light is one of the most important environmental factors for plant growth and yields in winter greenhouses because light directly affects photosynthesis. However, it is not easy to keep the optimal light environment in winter greenhouses, because natural light intensity and the daily light integral (DLI) are not stable and sufficient. Furthermore, most greenhouses in snowfall areas and mountainous regions in Japan do not have enough DLI between autumn and spring for plant growth. In contrast, the air temperature in winter greenhouses is a relatively controllable environmental factor by using heating facilities. Recently in Northern Europe and Canada, supplemental lighting techniques have been developed for many plants, illuminating them plants from the top or the side by using high-pressure sodium lamps, fluorescent lamps, and light emitting diodes (LEDs) for in tomato (Gosselin et al., 1996; Gunnlaugsson and

2 J. Agric. Meteorol. 69 (2), 2013 Adalsteinsson, 2006; Heuvelink et al., 2006), cucumber (Hovi et al., 2004; Hovi-Pekkanen and Tahvonen, 2008; Pettersen et al., 2010; Trouwborst et al., 2010), and sweet pepper production (Hovi-Pekkanen et al., 2006). These countries have insufficient DLI from natural light between the autumn and spring to grow such crops. Supplemental lighting techniques aim to improve the light distribution within the plant canopy, and to promote photosynthesis and fruit growth in the lower plant layers. Many studies have shown the positive effects of supplemental lighting on fruit growth and yield (Gosselin et al., 1996; Hovi et al., 2004; Gunnlaugsson and Adalsteinsson, 2006; Hovi-Pekkanen and Tahvonen, 2008; Pettersen et al., 2010). Some of these studies showed the seasonal effect of supplemental lighting, i.e., that increases yields of tomato (Heuvelink et al., 2006) and sweet pepper (Hovi-Pekkanen et al., 2006) throughout the year, except from June to August. Furthermore, other reports have found no effect or a negative effect (Gunnlaugsson and Adalsteinsson, 2006; Trouwborst et al., 2010). These seasonal or negative results suggest the DLI from natural and supplemental lighting per plant, light source and/or cultivar are important to determine fruit growth rates and yield. In addition, light intensity (photosynthetic photon flux [PPF in µmol m -2 s -1 ]) should be optimized depending on the crop species and several growth factors (temperature, CO 2, air humidity, etc.) to provide sufficient supplemental lighting without causing leaf stress and associated leaf disorders (Moe et al., 2006). The photosynthetic rate per a certain leaf area of tomato leaves gradually decreases with age; however, the maximum amount of photosynthates per leaf is present just before the full expansion of each leaf (Shishido et al., 1991). Shishido and Hori (1991) reported that the accumulation of 14 C-assimilates in an inflorescence (truss) of the tomato plant is probably affected by the proximity of the truss to the source leaf, the developmental stage of the truss, and the connection of the vascular system to the stem. Hence, it is hypothesized that the PPF, age and position of leaves receiving irradiation should be optimized to distribute photoassimilates efficiently to the fruits of tomato plants. However, little research focuses on the PPF to individual leaf, and the measurement of photosynthetic rate per leaf or plant under supplemental lighting conditions. It is difficult to determine the optimal lighting conditions of a tomato canopy or an individual tomato plant bearing many leaves and fruits under unstable greenhouse conditions. Therefore, the simplified sourcesink model, in which a single leaf and a single truss are used to represent each plant, seemed appropriate for clarifying the relationship between an individual leaf and fruits under artificial conditions. For instance, Fukushima et al. (2012) used this model to prioritize candidate genes for functional genomic studies of tomato metabolic pathways. Our final objective is to determine the optimal supplemental lighting conditions for greenhouse tomato production. As a first step, we examined the effects of PPF on an individual leaf (Experiment [Exp.] 1) and the effects of the number of leaves receiving supplemental lighting on the photosynthetic rate of a standard tomato plant under artificial conditions (Exp. 2). 2. Materials and Methods 2.1 Effects of LED light intensity (PPF) on fruit set and growth of tomato plants with one leaf and one truss (Exp. 1) Tomato plants (Solanum lycopersicum L., Reiyo ; Sakata Seed Co., Yokohama, Japan) were sown in a 144-cell tray filled with commercial soil substrate. One week after sowing, all plants with fully expanded cotyledon leaves were transplanted individually into plastic pots filled with granulated rockwool (Grodan B.V., Roermond, The Netherlands). A month after sowing, all plants with six expanded leaves were transplanted individually into plastic containers (2 L) filled with granulated rockwool. The seedlings were grown in a controlledenvironment room with air temperatures of 23 /18 (light/dark period), in a 16 h day -1 light period provided by three-band white fluorescent lamps (FHF32EX-N-H; Panasonic Co., Osaka, Japan) at PPF 300 µmol m -2 s -1 on the top of the plant canopy, relative humidity of 70%, and CO 2 concentration of 1000 µmol mol -1. Plants were irrigated twice a day with half-strength Otsuka nutrient solution A (1-1.5 L per day per plant; 8 mm NO 3 -, 2 mm PO 4 3-, 2 mm Ca 2+, 1 mm Mg 2+, 4 mm K +, and 0.65 mm NH 4 + ) and a micro-nutrient solution (Otsuka AgriTechno Co., Ltd., Tokyo, Japan). Flower (fruit) trusses were numbered acropetally. Commercial plant hormones of auxin (Tomato-tone, ISK Biosciences K. K, Tokyo, Japan)

3 S. Hikosaka et al.:effects of supplemental LED lighting on photosynthesis and fruit growth of tomato plants were applied to each flower truss (1 st and 2 nd truss in this experiment) for the fruit set on the same day that anthesis occurred. In addition, fruits and leaves were sprayed daily with 1% CaCl 2 to inhibit the occurrence of blossom end rot during the fruit growth stage. At the anthesis date of the 2 nd truss (seven weeks after sowing), all the leaves and trusses were removed, except for the three flowers on which plant hormone was sprayed at the 2 nd truss, a leaf just below this truss, and apical portions of the main shoot (Fig. 1). LEDs were the sole source of light after this leaf removal treatment. LED irradiation was provided to the leaf for three weeks by a red LED panel (23 cm 12 cm, 18 W, Shibasaki, Inc., Saitama, Japan), with a peak wavelength of 660 nm (Showa Denko K. K., Tokyo, Japan) at one of three PPF levels (200, 500, and 1000 µmol m -2 s -1, P200, P500, and P1000). There were three plants per treatment. The period of irradiation was the same as the light period during the seedling stage (16 h day -1 ). In all of the treatments, the plants were subjected to the same environmental conditions as those during the seedling stage, except for the light source and PPF. We measured the fruit set ratio of the 2 nd truss, in addition to the dry weight and percent dry mass of the Red LED Panel Fig. 1. Schematic representation of red LED lighting in Experiment 1. At the anthesis stage of the 2 nd truss, all the leaves and trusses were removed, except for the flowers of the 2 nd truss, the leaf just below the 2 nd truss, and the apical portions of the main shoot. The average photosynthetic photon flux (PPF) of the leaf surface was set at 200, 500, and 1000 μmol m -2 s-1. leaves and fruits at three weeks after initiating the lighting treatment. The fruit set ratio was calculated as a percentage of the number of fruit that grew (diameter was over 5 mm) to the three flowers that remained at the 2 nd truss. The net photosynthetic rate and photosynthetic ability (light response curve) were measured at one and three weeks after initiating the treatment. The net photosynthetic rate of the center leaflet was measured using a portable photosynthesis system (LI-6400, LI-COR, Lincoln, NB, USA) equipped with a blue-red LED unit (measurement) chamber. The conditions in the unit (measurement) chamber were set to air temperature of 25, relative humidity of 60%, and CO 2 concentration of 1000 µmol mol -1, and the PPFs for the light response curve were recorded at 0, 200, 400, 600, 800, and 1000 µmol m -2 s Effects of the amount of LED light irradiation on the photosynthetic rate of tomato plants (Exp. 2) Tomato seedlings of the same cultivar as Exp. 1 were grown under artificial conditions until anthesis of the 2 nd truss. Seven to 10 days after anthesis of the 2 nd truss (about eight weeks after sowing), a whole plant (i.e., with all leaves intact) and an LED panel (described below) were placed in an acrylic semi-closed assimilation chamber (W 0.45 m D 0.7 m H 1.0 m; 315 L) set in a walk-in type growth chamber (W 1.36 m D 1.7 m H 1.9 m; KG150MLA, KI Holdings Co., Ltd., Yokohama, Japan). The environmental conditions in the walk-in type growth chamber were set to air temperature of 25, relative humidity of 60-70%, and CO 2 concentration of 1000 µmol mol -1. The main light source was metal halide lamps (MT150FCEH-WW/S; Iwasaki Electric Co., Ltd., Tokyo, Japan) on the ceiling of the walk-in type growth chamber; the PPF at the top of the plants inside the semi-closed assimilation chamber was set at 300 μmol m -2 s -1. The supplemental lighting source was a blue and red (blue-red) LED panel (B/R photon ratio: 0.1; 23 cm 12 cm; 16 W; Shibasaki Inc., Saitama, Japan; peak wavelengths of 450 nm and 660 nm; Showa Denko K. K., Tokyo, Japan). Based on preliminary experimental results, there was no difference in the shape or damage of irradiated leaves between the red LED and the blue-red LED panels from PPF 200 to 700 μmol m -2 s -1 (maximum intensity of the blue-red LED panel). The amount of supplemental LED lighting per plant was altered by changing the number of

4 J. Agric. Meteorol. 69 (2), 2013 Fig. 2. Schematic representation of the control and supplemental lighting treatments in Experiment 2. The average photosynthetic photon flux (PPF) of the leaf surface irradiated by LED was set at 200 μmol m -2 s-1. In Exp. 2, a whole plant and a blue-red LED panel were placed in a semi-closed assimilation chamber (315 L); the irradiated leaves were the nearest leaves one just below and one just above the 2 nd truss. irradiated leaves. Shishido and Hori (1991) reported that the leaf just below an inflorescence (truss) and the leaves nearest the truss were the main contributors of photoassimilates to the truss. Based on this report, three treatments were designed to apply supplemental lighting to a single leaf just below the 2 nd truss and to two leaves, one just below and one just above the 2 nd truss, and not to apply at all (control) (Fig. 2). The irradiated leaves were fixed in position with a soft wire, and the average PPF of each leaf surface was set at 200 μmol m -2 s -1. The airflow rate to the assimilation chamber was 20 L min -1. The net photosynthetic rate per plant was calculated as the difference in the CO 2 concentration between the inlet and the outlet of the assimilation chamber, as measured by two CO 2 analyzers (ZFP9, Fuji Electric Inc., Tokyo, Japan). After the measurement of the net photosynthetic rate per plant, including the underground part (root and substrates), we removed the above ground part of plant and measured the respiratory CO 2 efflux from the roots and substrates. The net photosynthetic rate per above ground part was calculated from the net photosynthetic rate per plant including underground part and the respiratory CO 2 efflux from the underground part. Measurements were repeated three times for 40 min per plant, with three plants being measured in each treatment. 3. Results and Discussion 3.1 Effects of LED light intensity (PPF) on fruit set and growth of tomato plants with one leaf and one truss (Exp. 1) After three weeks of exposing a single tomato leaf and fruit truss (i.e., a simplified source-sink model ) to LED irradiation at one of the three PPF levels, the fruit set ratio, leaf and fruit dry weights, and percent dry mass of the leaf increased with increasing PPF (equivalent to the total amount of irradiation received by the plant) (Table 1). The percent dry mass of the fruits remained similar regardless of PPF. It seemed that high PPF, which created a high amount of irradiation per plant, caused accelerated fruit set formation and fruit growth via increased distribution of photoassimilates to the fruits. Grimstad (1987) reported that the irradiation of leaves in high plant layers (which also represents young leaves) increased the number and size of tomato fruits. This result was explained by the fact that young leaves have higher photosynthetic rates than older leaves (Shishido et al., 1991). Generally, the cell number determines the final fruit size (weight) at the mature stage (Bünger-Kibler and Bangerth, 1982; Higashi et al., 1999; Bertin et al., 2002); hence, a sufficient supply of photoassimilates during anthesis

5 S. Hikosaka et al.:effects of supplemental LED lighting on photosynthesis and fruit growth of tomato plants Table 1. Effects of LED light intensity (PPF) on the fruit set, dry weight, and dry mass ratio of a tomato plant with one leaf and one fruit truss (Experiment 1). Treatment The ratio of Dry weight (g) Dry mass ratio(%) fruit set z (%) leaf y fruit x leaf y fruit x P200 P500 P b w 3.4 a 4.5 a 0.4 c 1.4 b 2.7 a 8.4 b 9.4 b 11.1 a z Average of three plants (three fruits per plant). y Average of three plants (three leaves). x Average of 8-11 fruits. w Different letters indicate significant differences (P < 0.05) among the treatment means as obtained by Tukey-Kramer s test. (when cell proliferation starts) may increase the number of cells in tomatoes (Bertin et al., 2002) and cucumbers (Marcelis, 1993; Marcelis and Baan Hofman- Eijer, 1993). Thus, in the current study, it appears that the high amount of irradiation per plant may have increased the supply of photoassimilates to flowers at the anthesis stage, and therefore, increased the fruit set ratio, fruit growth rate, and fruit dry weight. After one week, the net photosynthetic rate of irradiated leaves was similar regardless of PPF (Fig. 3). After three weeks of treatment, leaves in the P200 and P1000 treatments had lower net photosynthetic rates than those in the P500 treatment measured after oneand three-weeks of treatment. Greenhouse tomato leaves grown under low PPF (PPF µmol m -2 s -1 ) showed a low net photosynthetic rate when a high PPF ( µmol m -2 s -1 ) was applied, because the leaves had acclimated under low PPF and could not respond to higher PPF due to photoinhibition (Takayama et al., 2006; Nishizawa et al., 2009). In comparison, tomato leaves grown at a medium PPF of 380 µmol m -2 s -1 showed higher photosynthetic rates in response to high PPF than those grown at low PPF (Nishizawa et al.,2009). Based on these results, the optimal range of PPF to maintain the photosynthetic rate of each leaf was between PPF 380 and 500 µmol m -2 s -1. However, some symptoms of leaf damage, including chlorosis, leaf curling, and drying, were apparent under the P500 treatment and especially severe under the P1000 treatment in this study. The water content of leaves decreased with increasing PPF (Table 1). These results suggest that, while the fruit growth rate in- Net phtosynthetic rate (μmol CO 2 m -2 s -1 ) week after anthesis 3 weeks after anthesis P200 P500 P Light intensity (μmol m -2 s -1 ) Fig. 3. Effect of light intensity (PPF) of the red LED on the net photosynthetic rate (light response curve) of tomato plants with one leaf and one fruit truss (Experiment 1). The net photosynthetic rate of a center leaflet was measured using a portable photosynthesis system (LI-6400, LI-COR, Inc.). Vertical bars indicate standard error (SE; n=2-4). Arrows indicate irradiation at a PPF matching the treatment condition (P200, P500, and P1000)

6 J. Agric. Meteorol. 69 (2), 2013 creased with increasing irradiation, maintaining this rate for a long period may cause signs of leaf stress. Hence, a PPF greater than 500 µmol m -2 s -1 for three weeks is too high to maintain the high photosynthetic rate of an irradiated leaf until harvest (ca. seven to eight weeks after anthesis). Several possible reasons for leaf damage include; (1) light quality, (2) excessive heat of irradiated leaves from LED lighting, or (3) photoinhibition due to excessive accumulation of photoassimilates in leaves from excessive light exposure. Trouwborst et al. (2010) hypothesized that supplemental LED lighting at PPF 221 µmol m -2 s -1 (80% red and 20% blue at peak wavelengths of 667 and 465 nm, respectively) may cause leaf curling because of LED light quality under low natural irradiance (winter) conditions. Although we used 100% red (660 nm) LEDs at 200 µmol m -2 s -1, few signs of leaf stress were visible in the P200 treatment. It is possible that the light quality was not the primary cause of leaf curling, with differences in the irradiation period, CO 2 concentration, and plant shape being present between the experiment results of Trouwborst et al. (2010) and this study. Furthermore, it is unlikely that a rise in leaf temperature caused leaf stress in this study, because the difference in leaf temperature between P200 and P1000 was only about 1 in this experiment (data not shown). Although the factors that inhibit photosynthesis under higher PPF were not clear in this study, because we did not measure the starch content of leaves, the results indicate that photoinhibition was caused by the excessive accumulation of photoassimilates in leaves. The simplified source-sink model used in this experiment is a plant model for easy-translocation of photoassimilates because it has a higher ratio of sink (three fruits) to source (one leaf) than that in a standard plant structure. Therefore, standard tomato plants with many leaves (low ratio of sink to source) under P500 or P1000 treatments may present leaf disorders earlier or severer than that recorded in the current experiment. Furthermore, the irradiation period of 12 h day -1 under P500 treatment indicated the similar leaf disorders with the current light period (data not shown). Based on the current experiment, it was appropriate to increase the number of leaves being irradiated under a lower PPF (i.e., 200 µmol m -2 s -1 ) to increase the total amount of irradiation received by each plant to maintain a high fruit set ratio and fruit growth rate for a long period without injuring the leaves. Therefore, the safest level of PPF for increasing fruit growth in -1 long-term tomato cultivation is PPF 200 µmol m-2 s based on the tested treatment conditions in this study. 3.2 Effects of the amount of LED light irradiation on the photosynthetic rate of tomato plants (Exp. 2) According to the results of Exp. 1, a high PPF, which created a high amount of irradiation per plant, promoted fruit growth; however, symptoms of leaf damage were apparent under P500 and P1000 treatments. Therefore, we increased the number of leaves irradiated at a lower PPF (i.e., 200 µmol m -2 s-1 ) to increase the total amount of irradiation per plant to maintain a higher photosynthetic rate in Exp. 2. The net photosynthetic rate per plant (above ground part) increased with an increase in the number of leaves irradiated by supplemental LEDs (Fig. 4). In particular, the photosynthetic rate in the two-leaf irradiation treatment was significantly higher than that of the control. The amount of irradiation of the irradiated leaf (or leaves) by supplemental LEDs, calculated from the irradiated leaf area and the PPF on the leaf Net phtosynthetic rate (µmol CO 2 per plant) b Control One-leaf Fig. 4. Effects of the number of leaves irradiated with blue-red LEDs (PPF 200 μmol m -2 s-1 ) on the net photosynthetic rate per tomato plant (above ground part) (Experiment 2). Control, One-leaf, and Twoleaf, respectively, indicate plants without supplemental lighting, with one irradiated leaf just below the 2 nd truss, and two irradiated leaves one just below and one just above the 2 nd truss. Vertical bars represent the standard error (n=3). Different letters indicate significant differences (P<0.05) obtained by Tukey-Kramer s test among the treatment means. b a Two-leaf

7 S. Hikosaka et al.:effects of supplemental LED lighting on photosynthesis and fruit growth of tomato plants surface, was 4.69 μmol s -1 per leaf and 8.62 μmol s -1 per two leaves in the one-leaf and two-leaf treatments, respectively (data not shown). The net photosynthetic rates in the one-leaf and two-leaf treatments were, respectively, 12% and 28% greater than the control. Although we could not measure the fruit set ratio and fruit growth rate in this experiment, because the measurement period for the net photosynthetic rate was approximately 40 min for each treatment, it is suggested that increasing the number of leaves exposed to low levels of PPF irradiation is an effective way to increase the total amount of irradiation per plant and increase the photoassimilates of each plant without incurring leaf disorders. Further practical experiments are required to confirm that fruit growth and yield may be increased with supplemental LED lighting and to observe any symptoms of leaf stress caused by high amounts of sunlight and long irradiation periods in low air temperature and low CO 2 concentration ( µmol mol -1 ) under winter greenhouse conditions. Additionally, it is necessary to examine whether the extra photoassimilates resulting from supplemental lighting are distributed to fruits or other organs in a plant. 4. Conclusion The increased total amount of irradiation per plant caused an increase in the fruit set ratio and fruit growth of tomato plants grown under artificial conditions; however, high PPF (more than PPF 500 μmol m -2 s -1 ) per leaf caused leaf stress, which led to visible leaf stress and associated disorders within a short period (ca. three weeks in this experiment). Therefore, irradiating a few leaves at a low PPF (PPF 200 μmol m -2 s -1 ) during and after the anthesis stage of the target truss is recommended to increase photosynthesis, fruit set ratio, and fruit growth. Acknowledgment This research project was supported by the Ministry of Agriculture, Forestry and Fisheries (MAFF) under the title of Elucidation of biological mechanisms of photoresponse and development of advanced technologies utilizing light ( ). References Bertin, N., Gautier, H., and Roche, C., 2002: Number of cells in tomato fruit depending on fruit position and source-sink balance during plant development. Plant Growth Regul., 36, Bünger-Kibler, S., and Bangerth, F., 1982: Relationship between cell number, cell size and fruit size of seeded fruits of tomato (Lycopersicon esculentum Mill.), and those induced parthenocarpically by the application of plant growth regulators. Plant Growth Regul., 1, Fukushima, A., Nishizawa, T., Hayakumo, M., Hikosaka, S., Saito, K., Goto, E., and Kusano, M., 2012: Exploring tomato gene functions based on coexpression modules using graph clustering and differential coexpression approaches. Plant Physiol., 158, Gosselin, A., Xu, H., and Dafiri, M., 1996: Effects of supplemental lighting and fruit thinning on fruit yield and source-sink relations of greenhouse tomato plants. J. Jpn. Soc. Hortic. Sci., 65, Grimstad, S. O., 1987: Supplementary lighting of early tomatoes after planting out in glass and acrylic greenhouses. Sci. Hortic., 33, Gunnlaugsson B., and Adalsteinsson, S., 2006: Interlight and plant density in year-round production of tomato at northern latitudes. Acta Hortic., 711, Heuvelink, E., Bakker, M. J., Hogendonk, L., Janse, J., Kaarsemaker, R. C., and Maaswinkel, R. H. M., 2006: Horticultural lighting in the Netherlands: new developments. Acta Hortic., 711, Higashi, K., Hosoya, K., and Ezura, H., 1999: Histological analysis of fruit development between two melon (Cucumis melo L. reticulatus) genotypes setting a different size of fruit. J. Exp. Bot., 50, Hisaeda, K., Takayama, K., Nishina, H., Azuma, K., and Arima, S., 2007: Studies on improvement of tomato productivity in large-scale greenhouse- Analysis of vertical distribution of light intensity and net CO 2 fixation in tomato plant canopy. J. SHITA., 19, (in Japanese with English abstract). Hovi, T., Nakkila, J., and Tahvonen, R., 2004: Interlighting improves production of year-round cucumber. Sci. Hortic., 102, Hovi-Pekkanen, T., and Tahvonen, R., 2008: Effects of interlighting on yield and external fruit quality in year-round cultivated cucumber. Sci. Hortic., 116, Hovi-Pekkanen, T., Näkkilä, J., and Tahvonen, R.,

8 J. Agric. Meteorol. 69 (2), : Increasing productivity of sweet pepper with interlighting. Acta Hortic., 711, Marcelis, L. F. M., 1993: Effect of assimilate supply on the growth of individual cucumber fruits. Physiol. Plant., 87, Marcelis, L. F. M., and Baan Hofman-Eijer, L. R., 1993: Cell division and expansion in the cucumber fruit. J. Hortic. Sci., 68, Moe, R., Grimstad, S. O., and Gislerod, H. R., 2006: The use of artificial light in year round production of greenhouse crops in Norway. Acta Hortic., 711, Nishizawa, T., Shishido, Y., and Murakami, H., 2009: Effect of temporary changes in light intensity on carbon transport, partitioning and respiratory loss in young tomato seedlings raised under different light intensities. Physiol. Plant., 136, Pettersen, R. I., Torre, S., and Gislerød, H. R., 2010: Effects of intracanopy lighting on photosynthetic characteristics in cucumber. Sci. Hortic., 125, Shishido, Y., and Hori, Y., 1991: The role of leaf as affected by phyllotaxis and leaf histology on the de velopment of the fruit in tomato. J. Jpn. Soc. Hortic. Sci., 60, (in Japanese with English abstract). Shishido, Y., Yun, C. J., Yuhashi, T., Seyama, N., and Imada, S., 1991: Changes in photosynthesis, translocation and distribution of 14 C-assimilates during leaf development and the rate of contribution of each leaf to fruit growth in tomato. J. Jpn. Soc. Hortic. Sci., 59, (in Japanese with English abstract). Takayama, K., Ishigami, Y., Goto, E., Hisaeda, K., and Nishina, H., 2006: Analysis of vertical distributions of chlorophyll fluorescence parameter (Fv/Fm), SPAD value and chlorophyll concentrations within tomato plant canopy in a large-scale greenhouse. J. SHITA., 18, (in Japanese with English abstract). Trouwborst, G., Oosterkamp, J., Hogewoning, S. W., Harbinson, J., and Van, L. W., 2010: The responses of light interception, photosynthesis and fruit yield of cucumber to LED-lighting within the canopy. Physiol. Plant., 138,