Environ. Control Biol., 51 (1), 41 47, 2013
Original Paper
Effect of Supplemental Lighting from Different Light Sources on Growth and Yield of Strawberry Kota HIDAKA1, Kazuhiro DAN1, Hitoshi IMAMURA1, Yuta MIYOSHI2, Tomohiko TAKAYAMA1, Kunichika SAMESHIMA1, Masaharu KITANO2 and Makoto OKIMURA1 1
NARO Kyushu Okinawa Agricultural Research Center, National Agriculture and Food Research Organization (NARO), 1823 1 Miimachi, Kurume, Fukuoka 839 8503, Japan 2 Faculty of Agriculture, Kyushu University, 610 1 Hakozaki, Higashi-ku, Fukuoka 8128581, Japan (Received November 20, 2012; Accepted February 3, 2013)
Although supplemental lighting has been successfully used to boost greenhouse vegetable production, it has not found wide application in forced strawberry cultivation. In this study, we examined the effect of supplemental lighting from two different light sources on strawberry growth and yield. Strawberry plants were exposed to LED or fluorescent lamp illumination for 12 h (6:0018:00) daily from January to April. Under LED illumination, PPFD values greater than 400 mol m−2 s−1 were recorded at leaf heights of 1030 cm, and leaf photosynthetic rates in plants exposed to LED supplemental lighting were much higher than in controls and plants exposed to fluorescent light. This accelerated photosynthesis promoted plant growth, as manifested by increases in leaf dry matter production, leaf area, and specific leaf weight, leading in turn to significant increases in average fruit weight, number of fruits, and marketable yield. The higher yields observed in LEDexposed plants compared with those under fluorescent lamp illumination were due to comparatively higher LED light intensities. Fruit soluble solids content, an index of sweetness, also increased under LED lighting. These results suggest that supplemental lighting using higher irradiance LED is an effective method for high yield production during forced strawberry cultivation. Keywords : high yield, LED, photosynthesis, strawberry, supplemental lighting
INTRODUCTION
Commercial strawberry production in Japan is seriously plagued by many problems, including lack of farm operation successors, sluggish commodity prices, and rising crude oil expenses. As a consequence, production areas and levels are continuously declining. For a solution of these problems, the spread of large-scale industrial facilities and the development of consistently high-yield strawberry production technique for large-scale greenhouse are required, and further this developed technique is also expected to be applicable to the small-scale family-managed greenhouse. For high yield production, the control of environmental factors (e.g., light, air temperature, CO2 concentration, humidity, and wind velocity) is essential to allow plants to realize their full photosynthetic potential. Light is one of the most important environmental factors affecting plant growth, and directly influences leaf photosynthesis and fruit yield in strawberry (Hidaka et al., 2012). Because of variable light environments dependent on factors such as cropping season and cultivation location, inadequate light levels frequently lead to declining productivity during greenhouse vegetable production in Japan. Consequently, the development of a supplementary lighting technique, not dependent on cropping season or cultivation location, is
needed for consistently high strawberry production. Although supplemental lighting has been used to achieve high yields of some greenhouse vegetables, such as tomatoes and cucumbers (Demers et al., 1998; Hovi et al., 2004; Gunnlaugsson and Adalsteinsson, 2006; Trouwborst et al., 2010), there are currently few examples of supplemental lighting applied to strawberry production (Ceulemans et al., 1986). On the other hand, there have been many recent advances in LED light source technology, and the agricultural use of commercial LED lighting is expected to become more widespread (Goto, 2009; 2011). To aid in the development of supplemental lighting techniques in forced strawberry cultivation, we examined the effects of supplemental lighting on plant growth and yield using two different types of commercially-available light sources: LEDs and fluorescent lamps. MATERIALS AND METHODS
Plant materials and growth conditions Strawberry plants (Fragaria × ananassa Duch. ‘Fukuoka S6’) were grown by bench cultivation in a 37 m long×9 m wide section of a large-scale greenhouse (37 m long × 27 m wide × 4.5 m high) located at the NARO Kyushu Okinawa Agricultural Research Center, Japan. At the beginning of June 2011, seedlings selected from mother
Corresponding author : Kota Hidaka, fax: ＋8194243 7014, e-mail :
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stocks were planted in plastic pots (0.15 m diameter; 2.6 L volume) filled with mixed substrate (peat moss: coconut shells: charcoal 3:5:2 [v/v/v]) and were grown on a seedling bench. OK-F-1 nutrient solution (Otsuka Chemical Co., Ltd., Japan) with an electrical conductivity of 0.6 dS m−1 was supplied at a rate of 300 mL d−1 /plant at 9:00, 11:00, 13:00, 15:00, and 17:00 daily. In the summer season, nutrient supplementation was halted to induce anthesis, with only water supplied. Seedlings were transplanted to substrate-filled cultivation beds (30 m long×30 cm wide × 80 cm high), with 20 cm intervals between plants and inter-row spacing of 15 cm, on October 13, 2011, and were then grown under nutrient solution supplementation until April 24, 2012. Substrates and nutrient solutions were the same as those used for seedling cultivation, with approximately 3 L of substrate used per plant. Supplemental lighting system and illumination conditions Two different light sources—LED and fluorescent lamp—were used for supplemental lighting. The LED lighting system consisted of an LED lamp unit (LLM0312A, Stanley Electric Co., Ltd., Japan) coupled to an exclusive power supply (LLP0019A, Stanley Electric Co., Ltd., Japan). Three LED lighting systems were placed 30 cm apart directly over each cultivation bed at a height of 50 cm above plant bases as shown in Fig. 1a. The fluorescent lighting system consisted of a fluorescent lamp (FL40SBR-A, NEC Lighting Co., Ltd., Japan) and a power supply unit (PL1-40T, Hasegawa Co., Ltd., Japan). Two fluorescent lighting systems, with adjacent fluorescent lamps spaced 15 cm apart, were set 30 cm above plant bases as shown in Fig. 1b. Light source power consumption was 26 W for each LED lighting system and 40 W for each fluorescent lighting system. Three different supplemental lighting treatments— LED (LED), fluorescent lamp (FL), and natural light conditions (control)—were applied to plants. Treatments were carried out from January 19 to April 24, 2012, with LED and FL supplemental lighting applied 12 h daily (6:00 to 18:00) under the control of timer switches (TB22101, Panasonic Co., Ltd., Japan). LED, FL, and control spectral properties were measured in the experimental greenhouse on a cloudy day in January 2012. Measurements were obtained using a multichannel spectrometer (S-2431, Soma Optics Co., Ltd., Japan) at heights of 0, 10, 20, and 30 cm above the bases of plants. Spectral distributions of LED, FL, and natural light intensities at these four different heights are shown in Fig. 2. For the LED light source, peaks in light intensity were observed at wavelengths of 450 and 550 nm. For FL, emission lines were detected at 405, 435, and 545 nm, with smaller peaks recorded at 450 and 610 nm. As measured light intensity is inversely proportional to distance from light source, the highest values for LED and FL were recorded at a height of 30 cm. In a similar fashion, we measured the vertical profile of photosynthetic photon flux density (PPFD) for the different treatments using a quantum sensor (LI-190SL, LI-COR Inc., USA). ( )
Fig. 1
Photograph of supplemental lighting under the different light sources of LED (a) and fluorescent lamp (b).
Fig. 2
Spectral distributions of light intensities under the supplemental lighting with different light sources. Light intensities were measured at different four heights of 0, 10, 20 and 30 cm from plant bases. Measurement was conducted in a greenhouse on cloudy daytime. LED, supplemental lighting with LED; FL, supplemental lighting with fluorescent lamp; Control, non-lighting.
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Measurement of environmental conditions During the experiment, we monitored air and soil temperatures in the middle of beds situated along the central axis of the greenhouse. Copper-constant thermocouples were set 10 cm above and below plant bases to measure air and soil temperatures, respectively. Leaf temperature was also measured in plants subjected to the different treatments by placing thermocouples on abaxial leaf surfaces. Air, soil, and leaf temperature data were automatically recorded every 10 min using a data logger (GL200A, Graphtec Co., Ltd., Japan). Measurement of leaf photosynthesis For best effect, supplemental lighting should be adjusted based on leaf photosynthetic rate measurements to maximize leaf photosynthesis. To detect the light saturation point of strawberry leaves, the relationship between PPFD and photosynthetic rate was determined in October 2011 using a portable photosynthesis and fluorescence system (LI-6400XT, LI-COR). Photosynthetic rates were measured under nine different light intensities, i.e., 0, 20, 50, 100, 200, 400, 600, 800, and 1200 mol m−2 s−1 PPFD, under conditions of 25°C, 50% relative humidity, and 380 mol mol−1 CO2 in a leaf chamber equipped with an artificial light source. In addition, leaf photosynthetic rates of plants grown under the different supplemental lighting treatments were measured in the leaf chamber. Measurements were carried out on a cloudy day on fully-expanded third leaflets of greenhouse-grown strawberry plants, with four plants measured per treatment. PPFD, leaf temperature, and photosynthetic rate in the leaf chamber were all recorded simultaneously. Analyses of growth and yield To analyze the effect of supplemental lighting on growth, we measured plant height and leaf area of fully expanded third leaves under different treatment conditions on January 30, March 9, March 22, and April 12, 2012. Leaf area was estimated by using the measured values of length (LL ) and width (WL ) of leaflets and the equation obtained from relationships between the product values of LL multiplied by WL and leaf areas of individual leaves (Fig. 3) as
follows: Leaf area＝1.743×(LL×WL)＋5.992
For an investigation of the relationships between LL ×WL values and leaf areas, 40 leaves from small to large size were sampled. Thereafter, length and width of leaflets were measured, and leaf areas were also measured by using a leaf area meter (AAC-410, Hayashi Denko Co., Ltd., Japan). On April 24, the final day of the experiment, plants were harvested and their leaf areas measured. Plants were then separated into fruits, leaves, crowns, and roots, dried for 48 h at 80°C using a circulation drier, cooled to room temperature, and weighed. In addition, specific leaf weight (SLW), frequently used as an index of leaf thickness (Chiariello et al., 1989; Yamamoto et al., 2002), was calculated from the ratio of leaf dry weight to leaf area. Measurements were mean values of eight plants. To analyze the effect of supplemental lighting on yield, marketable fruit (fruit fresh weight＞6 g) was harvested from eight plants of each treatment. After weighing, harvested fruits were used to determine fruit quality parameters such as soluble solids content (SSC), titratable acidity (TA), and flesh firmness (FF). SSC of fruit juice was measured using a digital refractometer (PAL-1, Atago Co., Ltd., Japan). Fruit juice TA was measured with a coulometric acidity meter (CAM-500, Kyoto Electronics Manufacturing Co., Ltd., Japan), and fruit FF was determined using a penetrometer (RX-2, Aikoh Engineering Co., Ltd., Japan). For measurements of fruit quality, eight fruits were used from each treatment. Leaf photosynthesis, plant growth, marketable yield, and fruit quality data were analyzed by analysis of variance (ANOVA). Differences among means were tested for significance using the Tukey-Kramer test at P＜0.05. RESULTS AND DISCUSSION
The relationship between PPFD and photosynthetic rate of fully expanded third strawberry leaflets is shown in Fig. 4. Photosynthetic rate increased rapidly as light inten-
Fig. 4 Fig. 3
Relationships between the product values of leaflet length (LL ) multiplied by leaflet width (WL ) and leaf area of individual leaves of strawberry plants (Fragaria ×ananassa Duch. ‘Fukuoka S6’). The equation of the linear regression line and the coefficient of determination (R2) are shown in the panel. These data were obtained from measurements of 40 leaves.
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Relationships between photosynthetic photon flux density (PPFD) and photosynthetic rates of fully expanded third leaflet. Data are mean ± S.E. of four plants. Photosynthetic rate was measured at the controlled environment of air temperature 25°C, relative humidity 50%, CO2 concentration 380 mol mol−1, and PPFD of 0, 20, 50, 100, 200, 400, 600, 800, 1200 mol m−2 s−1 with a portable photosynthesis and fluorescence system (LI-6400XT, LI-COR Inc., Lincoln, NE, USA).
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sity increased from 0 to 100 mol m−2 s−1 , with a more gradual increase above 200 mol m−2 s−1. Photosynthetic rate was close to the light saturation point when PPFD reached 400 mol m−2 s−1, with its value about 82% of that at 1200 mol m−2 s−1 PPFD. We could thus conclude that at CO2 concentrations of 380 mol mol−1 , supplemental lighting is more effective at PPFD values above 400 mol m−2 s−1. Figure 5 shows the vertical profile of PPFD under supplemental lighting. In the controls, PPFD levels were nearly constant: approximately 60 mol m−2 s−1 at heights of 0, 10, 20, and 30 cm. PPFD levels measured in plants under FL conditions increased with plant height; 146 mol m−2 s−1, twice the intensity experienced at the base of the plant, was recorded at 20 cm, while 260 mol m−2 s−1 , more than three times that of basal levels, was measured at 30 cm. Under LED exposure, PPFD was much greater at every height, with a reading of 320 mol m−2 s−1 at 0 cm, four-fold higher than the amount detected under FL and control lighting at that position. At normal leaf heights— 10, 20, and 30 cm—PPFD was 400, 690, and 1200 mol m−2 s−1, respectively. Consequently, it could be assumed that under LED supplemental lighting, leaf photosynthesis was close to the light saturation point, about 400 mol m−2 s−1 as shown in Fig. 4. PPFD, leaf temperature, and photosynthetic rate of fully expanded third leaflets under supplemental lighting on a cloudy day are shown in Fig. 6. Using supplemental lighting, fully expanded third leaflets were exposed to increased PPFD, with values due to LED lighting more than seven times higher than in controls and FL. Higher PPFD values were recorded under FL compared with controls, but the differences were not significant. Elevated leaf temperatures about 2°C higher than in FL and controls were observed under LED. Photosynthetic rate was proportional to PPFD, with the highest value detected under LED. LED and FL photosynthetic rates were 20- and 2-fold higher, respectively, than in control plants. Terashima et al. (2009) reported that in sunflower leaves, irradiation of green light under the moderate strong white light drives leaf photosynthesis more effectively than does red light. The LED lighting system used in this study produced a higher proportion of green light (Fig. 2) of greater intensity at typical leaf height positions (Fig. 5); the green light component may
thus be partly responsible for the observed acceleration of leaf photosynthesis under LED. From these results, it can be seen that in the absence of adequate solar radiation, wintertime LED supplemental lighting remarkably enhanced leaf light reception and temperature, which led to an acceleration of leaf photosynthesis in the exposed plants. However, this positive effect of supplemental lighting may not be always found under the adequate solar radiation. Changes over time in plant height and leaf area of fully expanded third leaves under supplemental lighting are illustrated in Fig. 7. While plant heights under different treatments were nearly identical on January 30, values thereafter were markedly higher in LED compared with FL and controls. At the end of the experiment, average plant height under LED was about 1.2 times higher than under FL and 1.6 times higher than under control conditions. Leaf area was also increased by supplemental lighting, with final values under LED about 1.5 and 2.7 times greater than under FL and control conditions on April 12. Plant height and leaf area increased more rapidly under LED than FL as a result of the acceleration of leaf photosynthesis owing to higher light intensity on the leaf surface. Leaf area expansion was delayed under FL compared with LED; this may be because of the delayed increase in plant height, which affects distance between leaf surface and light source, and thus compounds the effect of relatively lower photosynthetic activity arising from lower light intensity on the leaf
Fig. 6
Fig. 5
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Vertical profile of PPFD under the supplemental lighting. LED, supplemental lighting with LED; FL, supplemental lighting with fluorescent lamp; Control, nonlighting.
PPFD (a), leaf temperature (b) and photosynthetic rate (c) of fully expanded third leaflet under the supplemental lighting. Data are mean ± S.E. of four plants. Different letters indicate the significant difference at P ＜0.05 by Tukey-Kramer test. Photosynthetic rate was measured at the controlled environment of air temperature 25°C, relative humidity 50% and CO2 concentration 380 mol mol−1 with a portable photosynthesis and fluorescence system (LI-6400XT, LI-COR). LED, supplemental lighting with LED; FL, supplemental lighting with fluorescent lamp; Control, non-lighting. Environ. Control Biol.
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Fig. 7
Time courses of plant height (a) and leaf area (b) of fully expanded third leaf under the supplemental lighting. Data are mean±S.E. of eight plants. LED, supplemental lighting with LED; FL, supplemental lighting with fluorescent lamp; Control, non-lighting.
surface. Leaf dry weight, total leaf area, and specific leaf weight (SLW) of plants under supplemental lighting are shown in Fig. 8. Supplemental lighting caused a marked increase in leaf dry weight per plant, with values under LED and FL more than 2 and 1.5 times higher, respectively, than in controls. Leaf area per plant was also increased by supplemental lighting; values under LED and FL were about 1.6 and 1.5 times higher, respectively, than those of controls. Significant differences in leaf area per plant were not observed between LED and FL. SLW was also significantly increased by LED supplemental lighting, but not by FL. These observations are consistent with the results of another study, in which strawberry plants exposed to 11 h of light from high-intensity discharge (HID) lamps experienced increases in leaf area (Jurik et al., 1982; Ceulemans et al., 1986). The increased total leaf areas observed under LED and FL were presumably caused by the acceleration in leaf photosynthesis. Furthermore, the increase in leaf dry weight is also a direct reflection of increased leaf photosynthetic rate (Fig. 6). Many studies have shown that increases in light generally produce thicker leaves (Wilson and Cooper, 1969; Willmot and Moore, 1973; Nobel et al., 1975). Chabot and Chabot (1977) detected increases in leaf thickness and SLW under high light conditions, and suggested that the observed leaf thickening was related to increases in mesophyll cell size and amounts of mesophyll tissue. Furthermore, a positive correlation between SLW and net photosynthesis was observed in apple and strawberry (Chabot and Chabot, 1977; Marini and Barden, 1981). We thus conclude that the higher SLW values observed in our study under LED were a consequence of leaf thickening, which was due to the increased accumulation of assimilation products occurring during accelerated leaf photosynthesis. Table 1 lists SSC, TA, and FF of harvested fruits from supplemental lighting treatments. SSC values in LED were higher than those in controls, whereas values in FL were almost identical to controls. TA was unaffected by supplemental lighting. FF was decreased under LED compared Vol. 51, No. 1 (2013)
Fig. 8
Effect of supplemental lighting on leaf dry weight (a), leaf area (b) and specific leaf weight (SLW, c). Data are mean±S.E. of eight plants. Different letters indicate the significant difference at P＜0.05 by TukeyKramer test. LED, supplemental lighting with LED; FL, supplemental lighting with fluorescent lamp; Control, non-lighting.
Table 1
Effect of supplemental lighting on fruit quality. Different letters indicate the significant difference at P＜0.05 by Tukey-Kramer test (n＝8). LED, supplemental lighting with LED; FL, supplemental lighting with fluorescent lamp; Control, non-lighting; SSC, soluble solids content; TA, titratable acidity; FF, flesh firmness.
Treatment LED FL Control
SSC (°Brix)
TA (%)
FF (N)
9.25a 8.65ab 8.43b
0.58 0.64 0.55
1.47a 1.65b 1.73b
with FL and controls, perhaps because of increased fruit temperature caused by the higher light intensity of the LED source. Figure 9 shows average fruit weight, number of fruits, and marketable yield under the different supplemental lighting treatments. Average weight per fruit increased under LED, with a value about 1.3 and 1.5 times higher than in FL and controls, respectively. The number of harvested fruits per plant followed the same pattern. Differences between FL and controls with respect to average fruit weight and number of fruits were not significant. The yield of marketable fruit yield was 1.8 and 2.4 times higher, respectively, in LED compared with FL and controls. Although marketable yield in FL was 1.3 times higher than in controls, this difference was not significant. In our study, LED supplemental lighting increased average fruit weight and number of fruits, resulting in ( )
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Fig. 10 Effect of supplemental lighting on total dry matter and dry matter of respective organs of fruit, leaf, crown and root. Data are mean of eight plants. Different letters indicate the significant difference at P＜0.05 by Tukey-Kramer test. LED, supplemental lighting with LED; FL, supplemental lighting with fluorescent lamp; Control, non-lighting.
Fig. 9
Effect of supplemental lighting on average fruit weight (a), number of fruit (b) and marketable yield (c). Data are mean±S.E. of eight plants. Different letters indicate the significant difference at P＜0.05 by TukeyKramer test. LED, supplemental lighting with LED; FL, supplemental lighting with fluorescent lamp; Control, non-lighting.
remarkable increases in marketable yield. Roussos et al. (2009) reported that fruit size can be enhanced by increasing assimilate supply to the fruit. We can therefore attribute the observed increase in average fruit weight under LED to higher photosynthate allocation to the fruit. Observed increases in SSC, which is related to fruit sweetness (Azodanlou et al., 2003), may also be due to higher photosynthate accumulation under LED. For achievement of high fruit yield, high carbohydrate allocation to the fruit is important (Heuvelink, 1997). According to Sung and Chen (1991), acceleration of leaf photosynthesis increases fruit set per plant, leading to increased yields in strawberry. Flower bud differentiation and development require high levels of carbohydrates (Peng and Iwahori, 1994), and soluble carbohydrate content in shoot tips, leaves, and roots of June-bearing strawberries may play a decisive role in flower bud differentiation (Eshghi et al., 2007). In addition, Nishizawa and Shishido (1998) have suggested that carbohydrate reserves in strawberry roots are used to support growth of inflorescences and developing leaves. In our study, plant dry matter production was increased by supplemental lighting (Fig. 10), with total dry matter production under LED more than two times that of controls. Furthermore, marked increases in dry matter were not limited to fruit, but were also observed in leaves, crowns, and roots under LED: leaf, crown and root dry matter values were 2.5, 2, and 1.6 times higher, respectively, in LED vs. controls. Consequently, we conclude that the striking increase in fruit set per plant observed under LED conditions ( )
(Fig. 9b) was caused by higher allocation of carbohydrates, which were produced abundantly with accelerating leaf photosynthesis, to bud primordia. Our results demonstrate that supplemental lighting with a higher irradiance LED lighting system significantly enhances leaf photosynthesis, leading to plant growth acceleration, fruit quality improvement, and significant increase in marketable yield of strawberries in forced cultivation. To develop a more effective lighting method with using higher irradiance LED systems, future studies are planned to examine the effect of factors such as timing, intensity, and duration on leaf photosynthesis, growth and fruit yield of strawberry. This study was supported by Grants-in-Aid for Scientific Research (No.23380150) from the Japan Society for the Promotion of Science. The authors express their gratitude to Messrs. Kenji Kuroiwa, Hideo Harada (NARO Kyushu Okinawa Agricultural Research Center) for their technical assistance.
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