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LED Technology Paves the Way for a Spectral Revolution

(An abridged version is found in the July issue of Maximums Yield Indoor Gardening magazine.)

The introduction of LEDs is bringing a lot of excitement to the indoor grower's world. Most people are ecstatic about energy–savings and are looking forward to big returns on electricity and air conditioning bills. Others are elated by the longevity of LEDs and the prospect of never having to change light bulbs. However, most people don't realize that with LEDs there is a much greater impact than reduced operating costs and extended lifetimes, at least for the horticulture industry. Possessing the ability to shape spectrums, LEDs will fundamentally alter how grow lights are used, in the same way that fertilizers changed farming.

Light and Nutrition

There are many factors that contribute to a plant's well being, but it goes without saying that two important aspects that plants need are light and nutrition. For centuries, humans have understood that most plants thrive in fertile soil and under bright sunlight. Great farmlands are found in areas where both are in abundance. Over time, humans have accumulated knowledge on how to provide plants with better and more nutrients. The science of developing nutritional additives for plants has become a huge and very specialized field, covering anywhere from the traditional approach of making compost to creating complex chemical compounds to formulating premium additives that are injected through hydroponics. The advancements in nutrient development continue to provide better plant growth and higher yields.

In contrast, progress in lighting has been very limited. Farmers still rely heavily on the sun in an open space, which is unreliable as weather is unpredictable and capricious. As people learned to adapt to unstable weather conditions and move indoors, they applied their knowledge of how plants respond to light. For example, most growers know that plants are more responsive to redder light in the flowering stages of growth, and bluish light is optimal for vegetating phases. However, artificial sunlight options for indoor growers are pretty much restricted to high pressure sodium (HPS), metal halide (MH), and fluorescent lamps. These are all broadband sources, meaning they emit a full spectrum of light, from the usual red, orange, yellow, green, blue, indigo, violet to more harmful rays, like UV and IR. Some lights, such as HPS, contain more red, while others, like MH emit more blue. These limited choices mean that growers have to be satisfied with the spectrum offering of these lights, because even if they identify better wavelengths that are more suited to help a particular plant develop, there are no means to obtain those wavelengths.

Better Spectrum Selection with LEDs

Advancements in lighting, especially in LED technology, have proven that this is no longer the case. Artificial sunlight is not restricted to standard broadband sources (HPS, MH, T5, etc.) anymore. With LEDs, better spectrums can be developed. LEDs have been around since the early 60's, used mostly in indicator lights, but only recently has the technology progressed to the point where it can be used as general illumination. LEDs are made with different semi-conductors, and when electricity is applied to the compound, emit a particular color, depending on the chip. Already, LEDs have permeated through our everyday lives, from lighting traffic lights to our cell phone screens. It can as easily be applied to the indoor growing industry. This ability to produce particular wavelengths is the biggest advantage of using LEDs. Because it can emit a specific wavelength, growers can now optimize lights for plant growth. By mixing various LED chips, a complex light spectrum can be created as unique formulas for different growth conditions. From creating just blue or red lights to combining different mixes, LEDs are opening up a brand new dimension for growers to have new ways to accelerate or slow down growth, improve yields, or morph the shape of plants.

Less Heat, More Light

LEDs actually allow more useful light to be projected onto plants. Broadband sources are generally larger and require bigger reflectors to make efficient use of the light. On the other hand, the light from LEDs is directional, so it only takes a simple reflector to direct the light to where you need it to go. Broadband sources, including the sun also emit more than what is required for photosynthesis. Much of this light results in heat, which is crippling to plant performance if the temperature of the environment is elevated beyond what plants can tolerate. Another added nuisance to the indoor grower is that artificial sunlight sources also create heat on their own, due to inefficiency in converting energy from electricity to light.

Although plants could benefit from having more light, the heat from these broadband sources has long limited the amount of light supplied to plants. Place them too close and the plants will burn, whether from convection or radiation. With LEDs, users can now pick out specific red and blue wavelengths ideal for photosynthesis, thereby eliminating the excess light that produces unwanted heat. This new prospect combined with cleverly designed optics will allow growers to project more light onto plants than ever before, while still keeping the environment cool.

Plants and Light

The portion of the light spectrum utilized by plants falls within the range of 400-700nm, referred to as Photosynthetically Active Radiation. Aside from chlorophyll and the photopigments that drive photosynthesis, plants contain a variety of photoreceptors to sense and utilize light. These include crytochromes, phytochromes, and phototropins. Each react to different wavelengths and the ratio of their active and inactive forms cause different reactions in plants. At various stages of plant development different light conditions are required for different lengths of time. By studying the effects of particular wavelengths and combinations of wavelengths on the various cycles of plant development we can combine them in such a way that we promote maximum vegetative growth and crop yield

Experiment 1: Single Color Spectrum Test

We at Kessil Research have experimented with the effects of isolated red and blue wavelengths on plants, primarily tomatoes, using our innovative high power LED grow light (H150, H350) with a custom spectrum formula as the source. Research in the field of artificial light for the promotion of plant growth and production has traditionally used tomatoes as an indicator of light performance because they are day neutral, meaning the length of photoperiod has little effect on their growth and development, and they reach the fruiting stage relatively quickly. Tomatoes also serve as an ideal candidate crop due to their indeterminate growth, which exaggerates, and facilitates the analysis of qualitative results. For our experiments, we grew tomatoes for four weeks from when they were small plants with few leaves.

Red light promotes flowering in plants. It is commonly associated with the elongation of internodal lengths, a function driven by phytochormes. This elongation was dramatic in both the stems and the petioles of the tomatoes grown under the Red LED treatment. The tomatoes under this treatment produced fewer leaves, and as a result were less photosynthetically capable. Flowering was expedited under this light treatment, but the number of flowers produced by each tomato was relatively few. These results coincide with expectations for tomatoes grown under a primarily red light treatment.

This initial experiment focused on the wavelengths associated with photosynthesis, one red light spectrum (625-650nm), R1, and two different blue light spectrums (425-470nm), B1 and B2, centering on vegetative growth, without which crop yield would be limited. We grew a variety of tomatoes under isolated blue and red LED treatments and tracked light performance on the parameters of plant height, internode length, leaf number, leaf area/plant, and days to flowering. Without disturbing the plants, daily measurements were taken from the date of transplant, at 4-5 leaves, to early fruit development.

Blue light encourages vegetative growth in plants. It often increases leaf production and vegetative density. For this reason plants are grown under lights high in blue wavelengths during the initial stages of development. A high leaf number count and low internodal lengths are expected in plants grown under blue light treatments. This was not the case in the tomatoes under the first blue light, B1. Instead we observed rapid stem elongation and a relatively low leaf number. However, the second blue light, B2, with only a 15nm difference in peak wavelengths as demonstrated in the spectrum graph, showed a shorter stem and higher leaf count.

The slight variation in spectrum resulted in drastically different performances. The plant under B1 continued to stretch as though it were straining to find more light. B2 on the other hand, had a stronger vegetative growth. To the eye, the two blues looked the same, but the 15 nm difference caused a huge disparity between the performances of the two plants. Just like how two liquid fertilizer solutions may look identical, but their separate chemical contents will result in variations in plant performance, two LED lights with the same color may seem no different than the other, the actual "spectral content" of the LED will determine plant performance.

Experiment 2: Red Blue Mix Test

In response to the above experiment, we created 2 red-blue mixed spectrum lights, M1 and M2 and analyzed the performance of combinations of red and blue light. M1 treatment contained a higher degree of blue light than M2.

The experiment tested the performance of M1 and M2 in promoting vegetative growth. Pictured are three plants grown under B2 (from the previous experiment), M1, and M2, from the left to right. As stated before, B2 demonstrated strong stems and closely set leaves. The tomatoes grown under M1 treatment also displayed a denser pattern of vegetative growth with higher leaf area and some stem and petiole elongation. With the added red spectrum, M1 had better growth overall when compared with B2. M2 demonstrated taller stems, but more scattered leaf development than the other two. The progression from blue to red spectrum is clearly observed in this side-by-side comparison.

The Spectral Revolution

We have presented a small sample of the experiments we are doing at Kessil Research. We hope we have demonstrated that with the ability to tune spectrums, a whole new dimension of possibilities has been opened. Though there is still much knowledge to pursue, we anticipate a huge lighting revolution for the growing industry. We envision countless groups searching for the optimal spectrum recipe. Indoor gardening will quickly leap forward at lightning speeds with advancements that have never been seen before. It's a spectral revolution!

About the Authors

Brian Chiang is a seasoned veteran in the photonics industry. For the last 13 years, he has been with DiCon Fiberoptics, Inc., an advanced technology company based in California. Brian received his Bachelor's degree in physics from UC Berkeley and Master's degree in physics from UC Davis. He is currently the managing director for Kessil Lighting, a DiCon business division.

Josh Puckett graduated from Sonoma State University with a Bachelors degree in biology and an emphasis in plant biology and is currently working at the UC Davis Foundation Plant Services. He has years of hands on working experience in the horticulture and agriculture industries. He also serves as an advisor for the Kessil Research team.