BACKGROUND
light drives the processes of photosynthesis and plant development, and ultimately affects crop yield.
the culmination of over a century of plant photobiology research shows that plants possess complicated photosensory networks that monitor and respond to a wide spectrum of ambient light energies.
the spectral sensitivity of the plant light-sensors greatly exceeds the range of human vision, as light effects on physiology have been observed from energies arising from the uv-b wavebands  <cit>  into the near infra-red  <cit> .
this broad range of environmental information is processed by integrated signaling networks that tailor growth and development to best fit ambient light conditions.
light sensing pathways have been dissected through use of narrow-bandwidth light sources.
since individual photoreceptors are generally tuned to sense specific regions of the spectrum, narrow-bandwidth irradiation allows isolation of effects associated with a particular waveband.
for instance, the phytochromes mediate responses to red and far-red light, with partial activity extended through the green, blue and near-uv wavebands.
cryptochromes are required for maximal response to blue and uv-a  <cit> , while the phototropins exhibit autophosphorylation when stimulated with light qualities from the blue-green interface  to uv-c  <cit> .
other sensors share sensory overlap with the phototropins, and at least several other receptors remain to be characterized  <cit> .
narrow-bandwidth, research-quality light is typically generated using a broad spectrum source  filtered through an infra-red heat sink and several layers of acetate theatre filters or colored plastic.
fluorescent lamps also are used, as they emit three principle light qualities that can be readily filtered to obtain narrow-bandwidth light.
led light has been used principally in studies of phytochrome reversibility, as switchable red and far-red led arrays are commercially available.
many reports have demonstrated the utility of red/far-red led sources in modulating phytochrome responses during de-etiolation  <cit> , modulation of root growth  <cit> , root greening  <cit>  and senescence  <cit> .
led technology has been incorporated into lighting regimes to modulate plant growth and development for decades as acute supplementation of sunlight  or as the basis of plant growth in commercial chambers .
additionally, led-generated light is well-suited for small growth chambers and other applications where a significant fluence rate is required, but little physical space is available for conventional lamps.
the practical aspects of led lighting make them particularly well-suited for space applications where light treatments need to be precise and reliable with little heat radiation and low weight.
leds may also be especially useful in retrofitting incandescent or fluorescent growth chambers for terrestrial applications.
there is a need for a light source that may facilitate plant growth in a chamber environment, yet still allow for dynamic variation in light quality and quantity for experimental studies.
such devices may reveal interesting interactions between light sensing systems.
recent findings demonstrate that even "benign" wavebands  have significant influence in plant physiology in concert with red and blue light  <cit> .
in this report we detail the design and construction of a compact led light array for use in plant research.
these light sources utilize norlux hex-led arrays and are capable of delivering  <dig> –  <dig> μmol m- <dig> s- <dig> of combined red , green , and/or blue  light.
the design allows precise fluence-rate control of individual wavebands, allowing growth of plants under different combinations of light energies.
these designs are the same as those used to generate plant-growth data in a number of recent studies of red, blue and green light interaction  <cit> .
two designs are presented in figures  <dig>   <dig>   <dig>   <dig>  the first is a plan that may be suitable for use in the laboratory or classroom for fewer than one thousand dollars.
the second depicts an expandable system that may be appropriate for large growth chambers.
together the two plans presented represent tested and proven designs to introduce efficient lighting to chambers where light quality, quantity, duration and mixture can be readily controlled.
RESULTS
the advent of new semiconductor technologies has inspired a marked decrease in the price of led-based devices.
an increased number of consumer-grade products have become available to the researcher, and now these new tools may be integrated into various light-research applications.
the goal of this work is to provide an interface between research needs and new technology.
with this, the best-available research tools may be implemented by researchers without a significant investment in development.
the plans presented herein offer two options for led light source construction, based on the need of an experimental illumination tool or requirement for large-area irradiation.
led-based lighting regimes are being adopted by municipalities and medical facilities for their consistent, low-power, low-maintenance output.
however, one of the most important practical applications of this technology is in the design for lighting regimes to support plant growth.
it is of great interest to not only to foster plant growth, but to control plant growth.
basic plant research has demonstrated that specific light wavebands may affect discrete aspects of plant physiology, such as germination  <cit> , stem growth  <cit> , biomass  <cit>  and the transition to flowering  <cit> .
the supplementation of specific wavebands or skewing of overall spectrum may help modulate the progression of these developmental events.
the possibility that combinatorial light regimes may help to optimize growth and control developmental transitions makes the implementation of led technology particularly attractive to the design of controlled environments targeted to plant production for aesthetic applications, or applications relevant to human nutrition.
if spectral quality alone can delay or hasten the floral transition it may have profound effects on modulating the delivery of nursery goods or perhaps affect the availability of consumable produce in a finite, controlled environment.
this attribute alone makes led lighting a compelling platform for specific plant growth routines, such as those proposed for long-term inhabitation of space.
since humans rely specifically on vegetative parts  or reproductive parts  of plants for nutrition, it is critical to develop systems which impart control of the progression of plant development to affect plant output toward the particular needs of humans.
the implementation of narrow-wavelength led technology may benefit plant growth schema through supplementation or complete retrofitting of existing chambers.
its compact design may replace existing infrastructure with long-life and consistent output.
here, antiquated lamp systems, replete with toxic, inefficient fluors, may be refitted with efficient light sources that require little to no maintenance with comparable light output.
although previously unattainable without substantial engineering, the geometry of the systems provided in this report brings led technology to the average plant biology laboratory.
despite their vast advantages over conventional lighting systems, the led arrays described in this report offer opportunities for improvement and expansion.
larger installations  require close attention to array density, as the fluence rate of rgb hex led lights decays significantly toward the edges of the irradiation area.
careful arrangement modified to the application lessens the frequency of "hotspots" or other gradients of light intensities under the light fixture.
it is impossible to eliminate all variability under the arrays under all fluence rates and light combinations.
the spacing of hex units in individual systems needs to be carefully tailored for the specific application.
another potential improvement would be to integrate par sensors into the system to provide irradiance feedback and adjust light intensity through a computer-aided regulatory circuit.
this would allow the user to enter a specific irradiance value for the desired wavelength and would compensate for changes in led output that occur over time and with temperature changes in the ambient environment.
this system has been developed using leds emitting three principal wavebands.
the clear extrapolation is to add additional led types to generate additional spectrum coverage.
leds currently manufactured include uv, far-red and infra-red light.
from the plans presented within this report it may be possible to develop lighting systems that roughly approximate solar output by compounding the effects of multiple leds.
such a system may prove especially valuable in optimizing plant physiology and may have applications to human physiology as well.
the arrays described were tested for support of normal plant developmental responses.
these are most conspicuous in early seedling development, as initial responses to light are rapid, robust, and have been well characterized  <cit> .
three responses to light, namely inhibition of stem elongation , stem growth promotion  and phototropism , have been extremely well characterized and may be used to test and verify the utility of these led arrays on early plant developmental responses.
three assays were conducted.
first, end-point stem growth was measured in plants grown under three fluence rates of red or blue light.
red and blue light strongly inhibit early stem elongation through the phytochrome and cryptochrome systems, respectively  <cit> .
the results of two independent fluence-rate/response experiments using over  <dig> seedlings are shown in figure 5a and 5b.
figure 5a shows the height of seedlings grown for  <dig> h under constant blue light and figure 5b shows the effect of the same treatment with red light.
the cry <dig> and phyb mutants are presented as controls.
the data show that constant blue or red light inhibit seedling elongation in a manner roughly proportional to fluence rate.
inhibition is detectable even at low fluence rate , leading to a strong inhibition of stem growth elongation.
the results of these trials mirror the previously published results  <cit> , suggesting that the led arrays described herein act in a manner similar to those used previously.
while phytochromes and cryptochrome effects are salient as stem growth inhibition after days of growth in constant light, other rapid responses involve other light sensors and can be measured on the order of minutes rather than days.
contrary to the effects of red and blue, a short single pulse of green light stimulates rapid elongation of the hypocotyl in the dark-grown seedling  <cit> .
the response persists in all photomorphogenic mutants, it occurs when plants are grown in constant dim red light, and growth promotion is the opposite of what occurs when seedlings are irradiated with red or blue light.
this evidence renders it difficult to conveniently ascribe this response to any of the known light sensors, and it is likely being mediated by a separate green-sensitive transduction pathway.
figure 5c, shows the results of  <dig> independent seedlings treated with green light from the norlux hex arrays, compared to previously published data  <cit> .
seedlings irradiated with a short, single pulse of green light begin to grow rapidly within  <dig> min, attaining 140% of their dark rate before growth rate declines to dark levels after an hour.
the results are highly similar to published findings, again indicating that norlux hex arrays are a suitable alternative to other led or fluorescent light sources.
phototropism is the rapid curvature of the hypocotyl toward a unilateral light source.
in arabidopsis phototropism is blue light induced and is mediated by the phototropins, autophosphorylating serine-threonine kinases associated with the plasma membrane  <cit> .
the response is exquisitely sensitive to blue light.
here wild-type seedlings were irradiated from the side while being imaged in 5-minute intervals with an infra-red ccd camera .
the degree of curvature was monitored over two hours and compared to previous results .
the results indicate that the rapid response of phototropism is very similar between the arrays designed in this study and those produced commercially.
CONCLUSIONS
as the availability of new technologies increases, research programs are challenged with the need to retool their capabilities to exploit their potential.
since many of these new technologies are electronic and/or computer related, a certain degree of technical prowess is required to move ideas from the drawing board to application.
the development costs of these technologies may be significant.
this report offers two clearly applicable models of led light source development that may be implemented in the study of plant growth and output, helping to narrow the long-term challenges of illumination to support plant growth.
these designs allow the fine control of specific wavebands shown to influence plant growth and development.
such designs represent the first step in defining conditions that will optimize, or perhaps even control, plant growth and development.
the light sources used in this report sustain normal plant early developmental responses, suggesting that they are an appropriate substitute for, or complement to, other plant illumination solutions.

 light drives the processes of photosynthesis and plant development, and ultimately affects crop yield. the culmination of over a century of plant photobiology research shows that plants possess complicated photosensory networks that monitor and respond to a wide spectrum of ambient light energies. the spectral sensitivity of the plant light-sensors greatly exceeds the range of human vision, as light effects on physiology have been observed from energies arising from the uv-b wavebands into the near infra-red. this broad range of environmental information is processed by integrated signaling networks that tailor growth and development to best fit ambient light conditions. light sensing pathways have been dissected through use of narrow-bandwidth light sources. since individual photoreceptors are generally tuned to sense specific regions of the spectrum, narrow-bandwidth irradiation allows isolation of effects associated with a particular waveband. for instance, the phytochromes mediate responses to red and far-red light, with partial activity extended through the green, blue and near-uv wavebands. cryptochromes are required for maximal response to blue and uv-a, while the phototropins exhibit autophosphorylation when stimulated with light qualities from the blue-green interface to uv-c. other sensors share sensory overlap with the phototropins, and at least several other receptors remain to be characterized. narrow-bandwidth, research-quality light is typically generated using a broad spectrum source filtered through an infra-red heat sink and several layers of acetate theatre filters or colored plastic. fluorescent lamps also are used, as they emit three principle light qualities that can be readily filtered to obtain narrow-bandwidth light. led light has been used principally in studies of phytochrome reversibility, as switchable red and far-red led arrays are commercially available. many reports have demonstrated the utility of red/far-red led sources in modulating phytochrome responses during de-etiolation, modulation of root growth, root greening and senescence. led technology has been incorporated into lighting regimes to modulate plant growth and development for decades as acute supplementation of sunlight or as the basis of plant growth in commercial chambers. additionally, led-generated light is well-suited for small growth chambers and other applications where a significant fluence rate is required, but little physical space is available for conventional lamps. the practical aspects of led lighting make them particularly well-suited for space applications where light treatments need to be precise and reliable with little heat radiation and low weight. leds may also be especially useful in retrofitting incandescent or fluorescent growth chambers for terrestrial applications. there is a need for a light source that may facilitate plant growth in a chamber environment, yet still allow for dynamic variation in light quality and quantity for experimental studies. such devices may reveal interesting interactions between light sensing systems. recent findings demonstrate that even "benign" wavebands have significant influence in plant physiology in concert with red and blue light. in this report we detail the design and construction of a compact led light array for use in plant research. these light sources utilize norlux hex-led arrays and are capable of delivering – μmol m- s- of combined red, green, and/or blue light. the design allows precise fluence-rate control of individual wavebands, allowing growth of plants under different combinations of light energies. these designs are the same as those used to generate plant-growth data in a number of recent studies of red, blue and green light interaction. two designs are presented in figures the first is a plan that may be suitable for use in the laboratory or classroom for fewer than one thousand dollars. the second depicts an expandable system that may be appropriate for large growth chambers. together the two plans presented represent tested and proven designs to introduce efficient lighting to chambers where light quality, quantity, duration and mixture can be readily controlled.  the advent of new semiconductor technologies has inspired a marked decrease in the price of led-based devices. an increased number of consumer-grade products have become available to the researcher, and now these new tools may be integrated into various light-research applications. the goal of this work is to provide an interface between research needs and new technology. with this, the best-available research tools may be implemented by researchers without a significant investment in development. the plans presented herein offer two options for led light source construction, based on the need of an experimental illumination tool or requirement for large-area irradiation. led-based lighting regimes are being adopted by municipalities and medical facilities for their consistent, low-power, low-maintenance output. however, one of the most important practical applications of this technology is in the design for lighting regimes to support plant growth. it is of great interest to not only to foster plant growth, but to control plant growth. basic plant research has demonstrated that specific light wavebands may affect discrete aspects of plant physiology, such as germination, stem growth, biomass and the transition to flowering. the supplementation of specific wavebands or skewing of overall spectrum may help modulate the progression of these developmental events. the possibility that combinatorial light regimes may help to optimize growth and control developmental transitions makes the implementation of led technology particularly attractive to the design of controlled environments targeted to plant production for aesthetic applications, or applications relevant to human nutrition. if spectral quality alone can delay or hasten the floral transition it may have profound effects on modulating the delivery of nursery goods or perhaps affect the availability of consumable produce in a finite, controlled environment. this attribute alone makes led lighting a compelling platform for specific plant growth routines, such as those proposed for long-term inhabitation of space. since humans rely specifically on vegetative parts or reproductive parts of plants for nutrition, it is critical to develop systems which impart control of the progression of plant development to affect plant output toward the particular needs of humans. the implementation of narrow-wavelength led technology may benefit plant growth schema through supplementation or complete retrofitting of existing chambers. its compact design may replace existing infrastructure with long-life and consistent output. here, antiquated lamp systems, replete with toxic, inefficient fluors, may be refitted with efficient light sources that require little to no maintenance with comparable light output. although previously unattainable without substantial engineering, the geometry of the systems provided in this report brings led technology to the average plant biology laboratory. despite their vast advantages over conventional lighting systems, the led arrays described in this report offer opportunities for improvement and expansion. larger installations require close attention to array density, as the fluence rate of rgb hex led lights decays significantly toward the edges of the irradiation area. careful arrangement modified to the application lessens the frequency of "hotspots" or other gradients of light intensities under the light fixture. it is impossible to eliminate all variability under the arrays under all fluence rates and light combinations. the spacing of hex units in individual systems needs to be carefully tailored for the specific application. another potential improvement would be to integrate par sensors into the system to provide irradiance feedback and adjust light intensity through a computer-aided regulatory circuit. this would allow the user to enter a specific irradiance value for the desired wavelength and would compensate for changes in led output that occur over time and with temperature changes in the ambient environment. this system has been developed using leds emitting three principal wavebands. the clear extrapolation is to add additional led types to generate additional spectrum coverage. leds currently manufactured include uv, far-red and infra-red light. from the plans presented within this report it may be possible to develop lighting systems that roughly approximate solar output by compounding the effects of multiple leds. such a system may prove especially valuable in optimizing plant physiology and may have applications to human physiology as well. the arrays described were tested for support of normal plant developmental responses. these are most conspicuous in early seedling development, as initial responses to light are rapid, robust, and have been well characterized. three responses to light, namely inhibition of stem elongation, stem growth promotion and phototropism, have been extremely well characterized and may be used to test and verify the utility of these led arrays on early plant developmental responses. three assays were conducted. first, end-point stem growth was measured in plants grown under three fluence rates of red or blue light. red and blue light strongly inhibit early stem elongation through the phytochrome and cryptochrome systems, respectively. the results of two independent fluence-rate/response experiments using over seedlings are shown in figure 5a and 5b. figure 5a shows the height of seedlings grown for h under constant blue light and figure 5b shows the effect of the same treatment with red light. the cry and phyb mutants are presented as controls. the data show that constant blue or red light inhibit seedling elongation in a manner roughly proportional to fluence rate. inhibition is detectable even at low fluence rate, leading to a strong inhibition of stem growth elongation. the results of these trials mirror the previously published results, suggesting that the led arrays described herein act in a manner similar to those used previously. while phytochromes and cryptochrome effects are salient as stem growth inhibition after days of growth in constant light, other rapid responses involve other light sensors and can be measured on the order of minutes rather than days. contrary to the effects of red and blue, a short single pulse of green light stimulates rapid elongation of the hypocotyl in the dark-grown seedling. the response persists in all photomorphogenic mutants, it occurs when plants are grown in constant dim red light, and growth promotion is the opposite of what occurs when seedlings are irradiated with red or blue light. this evidence renders it difficult to conveniently ascribe this response to any of the known light sensors, and it is likely being mediated by a separate green-sensitive transduction pathway. figure 5c, shows the results of independent seedlings treated with green light from the norlux hex arrays, compared to previously published data. seedlings irradiated with a short, single pulse of green light begin to grow rapidly within min, attaining 140% of their dark rate before growth rate declines to dark levels after an hour. the results are highly similar to published findings, again indicating that norlux hex arrays are a suitable alternative to other led or fluorescent light sources. phototropism is the rapid curvature of the hypocotyl toward a unilateral light source. in arabidopsis phototropism is blue light induced and is mediated by the phototropins, autophosphorylating serine-threonine kinases associated with the plasma membrane. the response is exquisitely sensitive to blue light. here wild-type seedlings were irradiated from the side while being imaged in 5-minute intervals with an infra-red ccd camera. the degree of curvature was monitored over two hours and compared to previous results. the results indicate that the rapid response of phototropism is very similar between the arrays designed in this study and those produced commercially.  as the availability of new technologies increases, research programs are challenged with the need to retool their capabilities to exploit their potential. since many of these new technologies are electronic and/or computer related, a certain degree of technical prowess is required to move ideas from the drawing board to application. the development costs of these technologies may be significant. this report offers two clearly applicable models of led light source development that may be implemented in the study of plant growth and output, helping to narrow the long-term challenges of illumination to support plant growth. these designs allow the fine control of specific wavebands shown to influence plant growth and development. such designs represent the first step in defining conditions that will optimize, or perhaps even control, plant growth and development. the light sources used in this report sustain normal plant early developmental responses, suggesting that they are an appropriate substitute for, or complement to, other plant illumination solutions. 
however, there is a need for a versatile and inexpensive led array platform where individual wavebands can be specifically tuned to produce a series of light combinations consisting of various quantities and qualities of individual wavelengths. the presentation of these proven plans for led array construction allows the teacher, researcher or electronics aficionado a means to inexpensively build efficient, adjustable lighting modules for plant research. although specific light attributes, such as color and fluence rate, influence plant growth and development, researchers generally cannot control the fine spectral conditions of artificial plant-growth environments. these factors render led-based light strategies particularly appropriate for space-biology as well as terrestrial applications. these arrays represent a means to modulate precise quality and quantity in experimental settings to test the effect of specific light combinations in regulating plant growth, development and plant-product yield. plant growth chambers are typically outfitted with fluorescent and/or incandescent fixtures that provide a general spectrum that is accommodating to the human eye and not necessarily supportive to plant development. leds are particularly well suited for plant growth chambers, as they have an extraordinary life, require little maintenance, and use negligible energy. in this technical report we describe the practical construction of tunable red-green-blue led arrays to support research in plant growth and development. many studies over the last several decades, primarily in arabidopsis thaliana, have clearly shown that variation in light quantity, quality and photoperiod can be manipulated to affect growth and control developmental transitions.
 although specific light attributes, such as color and fluence rate, influence plant growth and development, researchers generally cannot control the fine spectral conditions of artificial plant-growth environments. plant growth chambers are typically outfitted with fluorescent and/or incandescent fixtures that provide a general spectrum that is accommodating to the human eye and not necessarily supportive to plant development. many studies over the last several decades, primarily in arabidopsis thaliana, have clearly shown that variation in light quantity, quality and photoperiod can be manipulated to affect growth and control developmental transitions. light emitting diodes has been used for decades to test plant responses to narrow-bandwidth light. leds are particularly well suited for plant growth chambers, as they have an extraordinary life, require little maintenance, and use negligible energy. these factors render led-based light strategies particularly appropriate for space-biology as well as terrestrial applications. however, there is a need for a versatile and inexpensive led array platform where individual wavebands can be specifically tuned to produce a series of light combinations consisting of various quantities and qualities of individual wavelengths. two plans are presented in this report.  in this technical report we describe the practical construction of tunable red-green-blue led arrays to support research in plant growth and development. two light fixture designs and corresponding circuitry are presented. the first is well suited for a laboratory environment for use in a finite area with small plants, such as arabidopsis. the second is expandable and appropriate for growth chambers. the application of these arrays to early plant developmental studies has been validated with assays of hypocotyl growth inhibition/promotion and phototropic curvature in arabidopsis seedlings.  the presentation of these proven plans for led array construction allows the teacher, researcher or electronics aficionado a means to inexpensively build efficient, adjustable lighting modules for plant research. these simple and effective designs permit the construction of useful tools by programs short on electronics expertise. these arrays represent a means to modulate precise quality and quantity in experimental settings to test the effect of specific light combinations in regulating plant growth, development and plant-product yield. 