Enriching Horticultural Lighting for Faster Growth and Better Crops

LED lighting technology presents a strong case for commercial growers to upgrade from traditional horticultural lighting, often utilizing high-pressure sodium (HPS) lamps. LED-based alternatives consume less power, and so can significantly reduce utility costs. Radiated heat is greatly reduced, which gives growers more control over the climate inside greenhouses and prevents burning and drying of the plants. This also allows lamps to be placed nearer the plants, which not only enhances utilization of greenhouse space but also gives flexibility to use extra lights for filling-in shaded areas.

As if these advantages were not enough to have growers of any types of crops, from flowers to food, chomping at the bit to join the LED-lighting revolution, there is the added advantage of the superior controllability that allows LED light to be tuned for maximum emissions at Photosynthetically Active Radiation (PAR) wavelengths. Moreover, as LED technologies continue to move forward and our understanding of plant biology and responses to light to various wavelengths continues to evolve, the characteristics of the emitted flux can be engineered even more precisely to potentially optimize the appearance, market value, and growth rate of plants and further boost returns for growers.

Improving Spectral Content

Red and blue wavelengths are known to encourage photosynthetic activity, and are easy to generate using LEDs. Early LED light engines for horticulture have tended to concentrate on these wavelengths. However, several other wavelengths are known to encourage plant growth. Future generations of lighting products that include the right quantities of these additional wavelengths could further accelerate growth and improve plant quality.

To understand how, we will first look at the multiple aspects of plant growth that are connected with exposure to light.

Then we will assess how individual wavelengths stimulate these responses, to understand how a wider range of wavelengths besides plain red and blue are needed to maximize plant growth, quality, and health.

Then finally we will analyze the other important factor that constitutes the light recipe: the intensity of each wavelength incident on the plant's surface. The intensity must be optimized to ensure the desired qualities in the end product: ensuring the best possible appearance among plants for decorative purposes; better taste, texture and nutritional content of food crops; and higher-yielding plants such as Aloe Vera, as cultivated for the production of latex.

Understanding Plants to Create Better Lighting

Photosynthesis is the process that converts water and carbon dioxide into complex carbohydrates (i.e. sugars) and oxygen using energy from light. However, although the energy radiated by the sun that reaches the earth's surface consists of the entire spectrum of visible light (and more), plants utilize light within a limited range of wavelengths for photosynthesis.

Figure 1: Characteristic absorption spectra of different pigments used for photosynthesis [1]

The wavelengths used are related to the absorption characteristics of the different pigments present within organelles called chloroplasts that are responsible for different functions of photosynthesis. Most of these pigments absorb light in the wavelengths that correspond to the colors blue and red. This explains why most leaves appear green, as these wavelengths are not absorbed, and why carrots appear orange, as they contain very little chlorophyll. The most common pigments are chlorophyll A, chlorophyll B and the carotenoids.

The range of wavelengths absorbed by plants is referred to as photosynthetically active radiation (PAR). It corresponds to wavelengths in the 400-700 nm range. Chlorophyll A is the primary photo-pigment accounting for around 75% of photosynthetic activity and has absorption peaks at approximately 435 nm and about 675 nm. Chlorophyll B, once thought to be an accessory photo-pigment, extends the range of wavelengths that can be used for photosynthesis with absorption peaks in the region of 460 nm and about 640 nm. Energy from these wavelengths is captured by chlorophyll B and then passed to chlorophyll A through electron spin resonance. All higher plants have these two pigments, which accounts for their green color.

Carotenoids have a comparatively much wider wavelength absorption range than the chlorophylls with an absorption range from about 400-510 nm. In addition to their accessory light-harvesting function, more recent research has revealed further roles of carotenoids: for example, their absorption wavelengths overlap with those of the chlorophylls as they protect the chlorophylls from photooxidation when light intensity is high in the short-wavelength high-energy ranges [2].

In addition, plants have a variety of photoreceptors that are critical to plant development yet have absorbance wavelengths mostly outside the PAR region. These include phytochromes, which regulate processes like chlorophyll synthesis.

Several other responses are linked to the intensity and spectral content of light reaching a plant's surface. These include responses to growing in the shade, circadian rhythm, circannual rhythm and weather variations, and can have a strong influence on photosynthesis rate, photomorphogenesis (plant anatomy), phototropism (direction of growth) and photonasty (non-directional changes such as flower opening). All are dependent on a wide variety of photoreceptors and can be particular to specific orders, families and genus of plants [3].

Improving our understanding of the numerous ways in which light influences plant development can highlight opportunities to further improve artificial lighting, aiming to orchestrate and optimize growth for bigger and better crops and greater commercial performance.

Richer Spectral Palette

To identify ways to improve horticultural LED lighting, let us first acknowledge that red light in the 630–660 nm range is the main driver of photosynthesis. It is not only essential for the growth of stems but also regulates flowering, dormancy and seed germination. Blue light from 400-520nm is another major driver of photosynthesis. It is also linked to the regulation of chlorophyll concentration, lateral bud growth and leaf thickness. However, over exposure can inhibit growth, so blue light must be carefully controlled and mixed with other wavelengths.

In addition to the red and blue wavelengths, it is becoming apparent that green, far-red, deep blue, and ultraviolet wavelengths are also needed to stimulate the wider variety of beneficial responses.

Green light (500–600 nm) was once disregarded as being unimportant to plant development, but recent investigations have revealed that plants in the shade of others are influenced by green wavelengths as part of the "shade-avoidance response" that encourages faster growth. Artificially inducing this response has obvious value for commercial growers.

Introducing far-red, which is in the infrared range of the spectrum at 720–740 nm, also enhances the shade-avoidance response resulting in greater stem length. In addition, it is known to enhance germination and can reduce the flowering time of plants.

Ultraviolet in the 280–400 nm range is still highly experimental in the cultivation of plants. Studies suggest it can provide protection against fungal growth, for plants such as lettuce and tomato that are resistant to its mutagenic properties. In addition, UV may encourage the generation of certain protective secondary metabolites, molecules like anti-oxidants and phenols, which are important for human nutrition.

Superimposing photosynthetic responses on LED-emitter wavelengths as a unified image shows how artificial lighting can be tuned to provide optimized wavelengths for plant growth. Many articles have presented simplified diagrams linking red and blue wavelengths with a subset of photosynthetic responses. Figure 2 presents a more complete overview, showing how additional wavelengths can drive other essential responses.

Figure 2: Plant responses to illumination within and outside the PAR range

With a richer spectral palette available, luminaire designers can pay extra attention to the differing needs of various types of plants. This includes optimizing the light recipe not only for specific species and even cultivars (sub-species) of plants, but also to better meet the needs of growers cultivating certain types of crops, such as flowers, decorative pot plants, culinary plants, or plants grown to yield natural products for use in industrial processes.

In addition to faster growing, which is always desired to reduce the time to revenue, plant characteristics can be adjusted to satisfy differing market expectations. For example, the most desirable salad vegetables have thin, light leaves for improved texture when eating. On the other hand, one of the most desirable qualities in aloe vera plants is to have thick leaves to produce a high quantity of latex. Regarding flowering, decorative plants need to maintain their flowers for as long a duration as possible. Conversely, when growing pineapples, inhibiting the flowering process enhances control of the harvest period.

To help designers create more sophisticated horticultural lighting products, especially wavelengths of 450 nm (deep blue), 660 nm (hyper red) and 730 nm (far red) are required. These wavelengths enable a diverse range of combinations that can be tailored to the needs of the target cultivar.

Completing the Light Recipe

Of course, spectral content is just one aspect of the light recipe. The intensity of each usable wavelength present is also critical.

Unlike LED lighting for streets or buildings, which is tuned to the response of the human eye and can be measured in terms of the luminous flux in lumens (lm), horticultural lighting is quantified in terms of photosynthetic photon flux (PPF) expressed in μmol/s.

A Reference for Presentation of Research Results in the Plant Sciences defines PPF as the total amount of photosynthetically active photons that are produced by a light source each second [4]. This is the most appropriate metric, because photosynthesis is a biochemical process quantified by the number of sugar molecules generated per number of photons, even though photons of different wavelengths have different energy levels. The conversion from electrical power to PPF is performed using the Plank-Einstein relation and Avogadro's number and is the sum of all photons generated in the wavelength range.

From the PPF, two further metrics can be derived that are important when designing for horticultural lighting applications. Photon efficacy, expressed in μmol/J, quantifies how efficient the LED is at creating PPF per joule of electrical energy used. This is often quoted in datasheets for horticultural lighting products as a figure of merit.

In addition, photosynthetic photon flux density (PPFD, μmol/m²/s) quantifies the total amount of photosynthetically active photons that reach the target area per second. This parameter is highly reliant upon the distance and angle from the source, and is usually measured using a quantum meter selective to PAR wavelengths. Figure 3 illustrates how the PPF for a given light source relates to PPFD in a lighting application. Note that the PPF and PPFD metrics are based on PAR – that is, the overall quantity of radiation present in the photosynthetically active region, from 400 nm to 700 nm. Although this has provided a sound basis to guide the early development of LED horticultural lighting, it is not adequate as a sole index for assessing future generations of products.

Figure 3: PPF is a productrelated parameter that influences the PPFD in the target application

Firstly, it gives equal weight to all wavelengths within the PAR range, whereas all wavelengths are not equally important for photosynthesis. Moreover, as this article has observed, wavelengths outside the PAR region are now being found to have an appreciable impact on plant growth and development.

Optimizing the Implementation

Clearly there is scope to refine the metrics used to describe horticultural lighting products. This is needed now, due to the far more precise control over the lighting recipes made possible by ongoing advancements in LED technology. Work by bodies such as the American Society of Agricultural and Biological Engineering (ASABE) is aiming to standardize methods of measuring and testing energy consumption and performance characteristics of horticultural lighting. The ASABE has already published several standards and guidelines, which should help identify and coordinate the use of LEDs for plant growth.

The development of marketable LED horticultural luminaires is in its infancy, although some pilot schemes are beginning to yield guidance about product selection, return on investment, and effects on crop performance.

Guidelines are emerging to help prospective buyers evaluate lighting systems, and astute installers and product designers can learn from the findings to deliver better results for customers. A report by the Lighting Research Center (LRC) at Rensselaer Polytechnic Institute [5], for Lighting Energy Alliance members and Natural Resources Canada found that up to four times as many LED luminaires were needed to deliver the same Photosynthetic Photon Flux Density (PPFD) as traditional HPS lamps. So LED product designers need to be careful to ensure their luminaires indeed consume less power across an entire installation. In addition, when a large number of luminaires are installed to maintain PPFD, greater shading occurs reducing the amount of natural daylight reaching the plant's surface, necessitating a greater output from the luminaire. This can destroy the greater efficacy of LED luminaires. However, this issue will be mitigated and eventually eliminated as the performance of LEDs continues to improve.

In addition, different types of plants are known to respond differently to differing light recipes. This could enable designers to offer easily selectable presets – say in a menu of a smartphone or tablet app - that are optimized for specific crops, although the performance benefits of this are debatable.

It is very early to cite reliable data on the ultimate effects on yields. One LED manufacturer has suggested that crop yields could increase nearly 10 % by "toppingup" existing HPS lighting with tuned LED lighting, based on trials with a small sample of fruit and flower growers.

Conclusion

Although the energy savings and improved climate control made possible by LED-lighting technology are valuable benefits for growers, the opportunity to accelerate plant growth and enhance product quality to meet specific market needs is perhaps the most exciting opportunity. Acquiring the best possible understanding of plant responses to various wavelengths, and then leveraging high-quality LEDs covering all the valuable wavelength ranges, enables lighting designers to bring new products to market that will deliver even bigger and better returns for growers.

References:
[1]    Jigang, L., Gang, L., et al, 2011, Phytochrome Signaling Mechanisms,
        The Arabidopsis Book / American Society of Plant Biologists, 9, e0148
[2]    Yamamoto, H.Y., Bassi, R., 2006, Chapter 30: Carotenoids:
        Localisation and function. Oxygenic Photosynthesis:
        The Light Reactions, P539-563, Springer Science & Business Media
[3]    Hogewoning, S.W., Douwstra, P., et al, 2010, An artificial solar
        spectrum substantially alters plant development compared with climate
        room irradiance spectra, Journal of Experimental Botany, 61 (5),    
        P1267-1276
[4]    Salisbury, F.B., 1996, Chapter 9: Electromagnetic Radiation, Units,
        Symbols, and Terminology for Plant Physiology: A Reference for
        Presentation of Research Results in the Plant Sciences, P75-80,
        Oxford University Press
[5]    Leora C. Radetsky, Lighting Research Center, Rensselaer Polytechnic
        Institute, LED and HID Horticultural Luminaire Testing Report;
        www.lrc.rpi.edu/programs/energy/pdf/HorticulturalLightingReport-Final.pdf