Plant lighting system with five wavelength-band light-emitting diodes providing photon flux density and mixing ratio control
© Yano and Fujiwara; licensee BioMed Central Ltd. 2012
Received: 26 July 2012
Accepted: 21 November 2012
Published: 22 November 2012
Plant growth and development depend on the availability of light. Lighting systems therefore play crucial roles in plant studies. Recent advancements of light-emitting diode (LED) technologies provide abundant opportunities to study various plant light responses. The LED merits include solidity, longevity, small element volume, radiant flux controllability, and monochromaticity. To apply these merits in plant light response studies, a lighting system must provide precisely controlled light spectra that are useful for inducing various plant responses.
We have developed a plant lighting system that irradiated a 0.18 m2 area with a highly uniform distribution of photon flux density (PFD). The average photosynthetic PFD (PPFD) in the irradiated area was 438 micro-mol m–2 s–1 (coefficient of variation 9.6%), which is appropriate for growing leafy vegetables. The irradiated light includes violet, blue, orange-red, red, and far-red wavelength bands created by LEDs of five types. The PFD and mixing ratio of the five wavelength-band lights are controllable using a computer and drive circuits. The phototropic response of oat coleoptiles was investigated to evaluate plant sensitivity to the light control quality of the lighting system. Oat coleoptiles irradiated for 23 h with a uniformly distributed spectral PFD (SPFD) of 1 micro-mol m–2 s–1 nm–1 at every peak wavelength (405, 460, 630, 660, and 735 nm) grew almost straight upwards. When they were irradiated with an SPFD gradient of blue light (460 nm peak wavelength), the coleoptiles showed a phototropic curvature in the direction of the greater SPFD of blue light. The greater SPFD gradient induced the greater curvature of coleoptiles. The relation between the phototropic curvature (deg) and the blue-light SPFD gradient (micro-mol m–2 s–1 nm–1 m–1) was 2 deg per 1 micro-mol m–2 s–1 nm–1 m–1.
The plant lighting system, with a computer with a graphical user interface program, can control the PFD and mixing ratios of five wavelength-band lights. A highly uniform PFD distribution was achieved, although an intentionally distorted PFD gradient was also created. Phototropic responses of oat coleoptiles to the blue light gradient demonstrated the merit of fine controllability of this plant lighting system.
Light-emitting diodes (LEDs) offer many benefits for applications in plant studies and potentially in commercial cultivation. Their solidity and longevity enable easier installation and manipulation compared to conventional lighting devices such as incandescent and fluorescent lamps, which have fragile glass sheaths [1–3]. Their mechanical reliability makes LED light sources movable , even with some speed and vibration, above plant canopies. Another feature of LEDs is that their chip volume is generally much smaller than that of whole plants. This beneficial feature enables manifold designs of light sources from irradiation inside a single tissue culture vessel, using only a few LEDs  to irradiation of greenhouse crops using large LED arrays  according to the cultivation scale. The small volume of LEDs also widens the variety of available plant irradiation methods such as irradiation of specific organs  and unrestricted directional irradiation [6, 8]. Close proximity irradiation  is also possible using visible-spectrum LEDs that do not emit collateral infrared radiation, which would result in potentially undesirable increase in plant temperature. Radiant flux is controllable by regulating the electric power input to LEDs. Because of this merit, excessive electricity dissipation can be avoided and desirable plant responses can be derived by feeding minimal electric power to LEDs. An LED dim lighting for seedling storage  is a demonstration of plant quality improvement using minimal electric power input. Dynamic control of lighting for beneficial cultivation can be realized by regulating the LED input power temporally in response to plant condition feedback . Not only is day–night periodic irradiation control possible; high-frequency on-off cycling can also be done using LEDs. Thereby, rapidly occurring photochemical reactions can be investigated . At 50% peak wavelength the band of light emitted from an LED is generally narrow, except for white LEDs with a fluorescent material, which enables users to select a specific light wavelength range or combinations of ranges using various LEDs. Moreover, LED lighting with some plant-photoreceptor-activating wavelengths in addition to necessary background light is anticipated for modification of specific plant functions . Optimum spectrum lighting is also desirable for efficient energy usage of plant lighting .
Room exists for engineering efforts to improve LED characteristics. The conversion efficiency for electric energy into light energy is reported as around 20%–30% [1, 12]. The remaining input electrical energy is transformed into heat. That heat must be removed from LEDs to avoid damage to LED chips and to provide stable light emissions [3, 11, 12]. Another challenge is to reduce the initial LED cost, which is still higher than that of fluorescent lamps, although it is decreasing rapidly [3, 12]. Notwithstanding these hurdles, the numerous and important merits of LEDs described above underpin their new lighting value for plant researchers and plant growers. To apply these merits in plant studies and cultivation, light spectra that are valid for inducing various plant responses should be clarified precisely and exhaustively. For this reason, a multi-peak-wavelength plant lighting system that irradiates a wide area with a highly uniform distribution of photon flux density (PFD) is necessary. Such a plant lighting system is expected to provide an appropriate photosynthetic PFD (PPFD; wavelengths of 400–700 nm) for growing numerous seedlings at once and for cultivating some mature leafy vegetables. Such systems are also expected to provide controllability of PFDs and of the mixing ratio of the respective light spectra emitted from all LED types used in the lighting system.
We developed a plant lighting system that irradiates light including violet, blue, orange-red, red, and far-red wavelength bands using five LED types. The lighting system was designed for indoor applications where no sunlight is available. For this reason, blue, orange-red, and red light were necessary for driving plant photosynthesis. In addition, violet and far-red types of LEDs were included in the lighting system to extend its usability for applications such as secondary metabolite synthesis and photomorphogenesis studies. These wavelength bands are known to be important independently or complementarily for plant photosynthesis, pigment synthesis, growth, and development [13–22]. The lighting system can produce high PPFD sufficient for growing vegetables from seedlings to mature plants. The irradiated area of 30 cm × 60 cm is suitable for growing many seedlings concurrently using a conventional cell tray. Furthermore, the mixing ratio of PFDs of five wavelength bands and the distribution of PFDs are controllable using a computer and drive circuits to extend the usefulness of this system for diverse plant studies. The phototropic responses of oat coleoptiles [23–28] induced using a blue light gradient demonstrated the fine lighting controllability of this developed plant lighting system.
Description of LEDs used for the LED panel
PFD control system
Below the LED panel at z = 17.3 cm, SPFDs were measured at 91 points across x = ±30 cm and y = ±15 cm at 5 cm intervals for wavelengths of 350–800 nm using a spectroradiometer (MS-720; Eko Instruments Co., Ltd., Tokyo, Japan). Although the LED panel irradiated 40 cm × 70 cm area, the marginal 5 cm of the irradiated area was excluded from light measurements because light sharply changes such a marginal area and plants are unlikely to be positioned there. The position z = 17.3 cm corresponds with the height of the spectroradiometer sensor position from its bottom, which was placed on the floor. Independent PFDs emitted by each LED type alone were estimated from the measured single SPFD curve with five peaks when IFss were fed to every LED. A Gaussian function was used to separate the five independent PFDs from the measured SPFD curve. The optimum Gaussian function was ascertained using Mathematica 7 (Wolfram Research, Inc., Illinois, USA) with a coefficient of determination r2 of greater than 0.97.
Plant irradiation using the lighting system
To assay plant sensitivities to the light control quality of the plant lighting system, the phototropic response of oat coleoptiles to an SPFD gradient of blue light was investigated. Oat seeds (Avena sativa L. cv. Super-hayate) were purchased from Snow Brand Seed Co. Ltd. (Sapporo, Japan). The plant lighting system was placed in a dark room where the room air temperature was maintained at 24.5 ± 1.5°C. A cell tray with 2.5 cm cell intervals was positioned below the LED panel. Vermiculite was put in each cell so that the vermiculite surface was positioned at z = 17 cm. The vermiculite was watered with tap water before sowing. Thirty-six oat seeds were sown in the tray cells as one seed per single cell. The 36 seeds were positioned at x = ±1.3, ±3.8, ±6.3, ±8.8, ±11.3, and ±13.8 cm at y = 0 and ±2.5 cm. After 66 h, the LEDs started to irradiate the seeds with the SPFD value of 1 μmol m–2 s–1 nm–1 at all five λps at z = 17.3 cm. After 23 h irradiation, every germinated seedling was photographed using a digital camera (E-P1; Olympus Corp., Tokyo, Japan) from the -y direction. The phototropic curvature of coleoptiles was measured from digital images as an angle formed by the z axis and central axis of each coleoptile. No-bending vertical growth of coleoptiles was defined as 0 deg curvature. The curvature of coleoptiles to the positive x directions was defined as the positive curvature. This series of experiments was repeated four times. The next series of experiments was conducted five times similarly to the description presented above, but only the blue light (λp = 460 nm) SPFD had a gradient in the x direction by gradually differentiating the IF values of each module’s blue LEDs. The SPFD gradient was defined as the change of the SPFD value (μmol m–2 s–1 nm–1) per displacement (m) along the x direction. Therefore, the SPFD gradient unit becomes μmol m–2 s–1 nm–1 m–1. Consequently, the unit which represents the relation between the phototropic curvature (deg) and the blue-light SPFD gradient (μmol m–2 s–1 nm–1 m–1) becomes deg per μmol m–2 s–1 nm–1 m–1. The blue light spectrum with a peak at around 460 nm wavelength is known to induce phototropic responses in higher plants [23, 37, 48].
PFD and mixing ratio control
Phototropic curvature of oat coleoptiles
Light is among the most difficult physical environmental factors to regulate uniformly in terms of its distribution and temporal control of intensity throughout plant experiments. For field or greenhouse experiments, temporal consistency of natural light is impossible to achieve. In a growth chamber experiment, uniform distribution of light is difficult to achieve because of the chamber limitation and light source size. As this experiment demonstrated, using numerous concentrated small lighting elements enables more uniform distribution of lighting than that obtained using a few large light sources such as incandescent and fluorescent lamps. This advantage ensures uniform plant response when many seedlings are grown below the LED panel. The lighting system produced average PPFD of 438 μmol m–2 s–1 at z = 17.3 cm, where plants are likely to be positioned. This value was sufficiently high to grow leafy vegetables . PFD values are continuously controllable from zero to the maximum value by regulating electric current fed to respective LED types. Transistor circuits with a computer signal control were effective for this purpose (Figures 4 and 5). The computer program with the graphical user interface assists manifold variations of PFD and mixing ratio designs for plant irradiation. This usability enables the lighting system to emit either a fairly even distribution (Figures 8 and 9A) or an intentionally distorted distribution (Figure 9D) of light to a plant canopy below the LED panel through control of PFD mixing ratio.
Results of the phototropism experiments suggest the importance of attentive adjustment of light characteristics in a plant experiment environment. Uniform lighting can induce uniform and reproducible plant responses, thereby delivering increased stringency for investigation of complex plant functions. The mixing ratio controllability of PFDs also enables us to study plant light responses to inhomogeneous light distribution. This study demonstrated a linear relationship, under our experimental conditions, between a blue-light SPFD gradient and the oat coleoptile curvature (Figure 9E). The PFD controllability of this lighting system is not limited to blue light. For example, a mixing ratio gradient of red/far-red, which often occurs in natural and cultivation environments, has attracted the attention of plant scientists [40, 43, 44]. The present lighting system is suitable for such studies as well.
Although the present lighting system provides five peak wavelengths, an apparent limitation is the wavelength coverage of emitted light. Physiologically important green light [50–55] should be included in future versions of plant lighting systems. Furthermore, as plant light response studies advance, light of more varied spectra will be anticipated for emission by artificial lighting systems. Plants have evolved under sunlight. For that reason, they may use a full range of the ground level sunlight spectrum. Hogewoning et al.  reported striking growth enhancement of cucumber plants irradiated with an artificial quasi-solar-spectrum light compared to cucumber growth when irradiated with fluorescent or high-pressure sodium lamps. In principle, quasi-ground-level-sunlight spectra are producible using various combinations of LEDs [57–59]. The LED lighting systems are expected to contribute substantially to a better understanding of the nature of plant light responses.
For control of both the PFD and mixing ratio of illumination, we developed a five-wavelength-band plant lighting system using 2800 LEDs of five types. The SPFD values were controlled uniformly at the irradiated area of 30 cm × 60 cm. Alternatively, an intentionally distorted SPFD gradient could be created. A computer graphical user interface facilitated the adjustment of these lighting parameters. The SPFD control performance was tested through the phototropic response of oat coleoptiles. The oat coleoptiles grew straight upward under a uniform SPFD distribution below the LED panel. On the other hand, phototropic curvature was induced by a blue light (λp = 460 nm) gradient, suggesting the merit of PFD and the mixing ratio controllability of the LED plant lighting system.
We thank Mr. Yukinari Doi of Shimane University for his assistance in fabricating the LED drive circuits. We also thank Mrs. Haruko Maeyama of The University of Tokyo for numerous data assembling. This work was conducted as a sub-project of the Committed Research Project “Elucidation of biological mechanisms of photo-response and development of advanced technologies using light” by the Ministry of Agriculture, Forestry and Fisheries of Japan.
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