http://www.ledsmagazine.com/articles/print/volume-13/issue-7/features/horticulture/science-advances-in-matching-led-lighting-to-horticultural-needs.html?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed:+LedsMagazineRss+(LEDS+-+LEDs+Magazine+RSS)
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Still, the growers across the horticulture and floriculture space need both better metrics that are pertinent to the application and access to data on light recipes.
As we covered in an article last year, the American Society of Agricultural and Biological Engineers (ASABE) Agricultural Lighting Committee began in 2015 to work on the standardized metrics issue. The work is considering metrics related to the PAR (Photosynthetically Active Radiation) spectrum among others. The PAR range is generally defined as the 400-700-nm spectral band where photons actively drive photosynthesis. Common metrics related to PAR include Photosynthetic Photon Flux (PPF) measured in micromoles per second (μmoles/sec) and Photosynthetic Photon Flux Density (PPFD) measured in μmoles/m2/sec where a mole can be directly converted mathematically to photons.
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Others have made more forceful statements. Neil Yorio, vice president of Biological Innovation and Optimization Systems (BIOS), commented on the topic; his company is working on LED luminaires for cannabis and other sectors. "The use of only red and blue LEDs is pretty outdated, and when you see products with that spectrum, it is based on older science and is frequently misinterpreted," said Yorio. "The reason people have chosen blue and red is because these wavelength peaks align with the absorption profile of chlorophyll a and b isolated in test tubes. That is not what is happening in intact leaves. We know today that all wavelengths of light in the PAR range are useful in driving photosynthesis. No doubt spectrum has importance, but it is associated with plant morphology such as size and shape rather than growth and yield or biomass."
Yorio's point is that research shows we can impact plant height and flowering by changing the spectrum. And as we will discuss a bit later, some growers continuously modulate light intensity and SPD mix to plants as the plants do have something akin to a circadian rhythm, although most every plant species has a unique rhythm and recipe requirement for optimal yield.
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White light can be important for reasons beyond baseline production in leafy greens. Several people we interviewed for this article said lettuce may not mature to look green without some light from the green spectral band. On the other hand, sometimes growers may control the spectrum to create new colors in produce. Gus van der Feltz, global director of city farming at Philips Lighting, for example, said you may want to grow a specialty lettuce with a red coloration. Also,
white light is required at times for workers as we mentioned in a prior horticultural lighting feature, and as stated earlier the blue energy peak in white LEDs is a plus.
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The lighting and LED manufacturers, meanwhile, are working diligently to deliver products that enable growers to succeed. Cree, for example, recently announced what it called a Photo Red LED in the far-red spectrum that might be useful in encouraging flowering. Spectrum King is apparently using that LED in its fixtures.
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Ultimately, LED lighting is poised to revolutionize all types of horticulture. Consider tomatoes. In one of the earlier linked feature articles, we cover a North Carolina tomato farmer called Patterson Greenhouses that was able to significantly extend the growing season even in a relatively warm region of the globe.
We also covered a Philips Lighting tomato project in France where the fruit is apparently tastier under LEDs (Fig. 7). That project used the aforementioned interlighting with LED luminaires from Philips that you can see in the photo running horizontally between plants.
Kitt Peak Solar Spectrum with Fraunhofer lines