Maximum Yield Industry News October/November 2020 - Page 35

To rephrase the issue , the current definition of PAR assumes that the photosynthesis rate of higher plants is linear with respect to incident radiation within the spectral range of 400nm to 700nm . There are obviously minimum and maximum irradiance limits to where this assumption applies , but it is necessary in order for the concept of PAR and hence PPE to have any meaning . The Emerson effect violates this assumption by making the photosynthesis rate nonlinear — add far-red radiation beyond 700nm and you will change the rate in the manner that depends on the spectral power distribution of the PAR radiation . This issue does not concern just horticultural luminaires with 725nm far-red LEDs . Most red-emitting phosphors used in white-light LEDs and red-emitting phosphor-coated LEDs ( e . g ., Fig . 2 ) have significant emissions in the far-red , and so may invoke the Emerson effect . Regardless , it remains that the definition of PAR cannot be redefined . It is not a matter of penalizing horticultural luminaires with far-red emissions , but of simply having a metric that makes sense .
Fig . 2

The
goal should
be to
educate the customer that , despite a possibly lower PPE value for the product , the far-red radiation represents a valueadded feature .”
PHOTOMORPHOLOGICAL CONSIDERATIONS
There is an additional complication with far-red radiation . Horticultural luminaires including far-red LEDs typically employ 660nm red and 725nm far-red LEDs . These wavelengths correspond nicely with the peak absorptances of the Pr and Pfr isoforms of phytochrome , a plant photoreceptor that is responsible for plant morphology , including everything from seed germination to flowering , leaf senescence , shade avoidance , circadian rhythms , and secondary metabolite production . By offering luminaires with fixed red to far-red ( R : FR ) ratios , luminaire manufacturers have only begun to explore the horticultural possibilities of far-red radiation ( e . g ., Ashdown 2018 ). Does the current definition of Photosynthetically Active Radiation and hence Photosynthetic Photon Efficacy disadvantage horticultural luminaire manufacturers who include far-red LEDs in their products ? In one sense , the answer is yes . However , this is a very narrow view of the issue that focuses on a single metric . The goal should be to educate the customer that , despite a possibly lower PPE value for the product , the far-red radiation represents a value-added feature .
Ian Ashdown is a research engineer with nearly 40 years ’ experience in lighting design , research and development , and software engineering . He is currently Senior Scientist and President of SunTracker Technologies ( Victoria , BC ), and President of byHeart Consultants ( West Vancouver , BC ). Ian also serves as an advisory board member for Maximum Yield .
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