Post on 02-Aug-2020
transcript
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Common Misconceptions about Artificial Lighting
Cary A. Mitchell
Department of Horticulture & Landscape ArchitecturePurdue University
Indiana Horticultural Congress & Trade Show
Indianapolis Marriott East
February 12, 2019
Definition: What is “Artificial” Lighting?
• Figuratively, it means light that is “not real”, or “not really light”.
• In the USA, lighting engineers tend to refer to “electric lighting”.
• Elsewhere in the world it is referred to as “artificial lighting”.
• Either way, the intent is to refer to light that is “not solar” or “not natural”.
• The important thing is to understand what is being referred to.
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Grower Reluctance about Supplemental Artificial LightingReasons given that small greenhouse growers may not use supplemental lighting (SL):
‐‐ cost of lighting equipment too expensive (capital investment)?
‐‐ electrical cost too high (too big a part of operational cost)?
‐‐ price can get for product not consistently high enough to justify SL?
‐‐ local market size not large enough to justify SL?
‐‐ current profit margin doesn’t seem to justify investment in SL?
‐‐ RoI or profitability not presently there for using SL?
‐‐ business too small to benefit from investment in SL?
‐‐Other reasons?
Suggestion to Reluctant Growers: Try it in a small section of your GH; see if it works!
Could adoption of SL help make small GH growers more competitive?
• ‐‐ might depend whether grower wants to be competitive year‐round
‐‐ likely will depend on what will be grown and when during the year
‐‐ may be related to business size/growth and desire to meet contractsor market demands
‐‐ could be affected by present business location or re‐location
‐‐ need to be willing to spend ($$) to make ($$$$)!
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What could adoption of SL do for GH growers who want to be more competitive and profitable?
• Will extend ability to grow crop of choice productively year‐round
• Will make crop‐production cycle more predictable and controllable in terms of meeting contracts or market expectations
• Should improve the quality of product (texture, color, flavor) if other environmental factors besides light are well managed/controlled
• May shorten the production time of the crop
• Could enhance the size and mass of the product
• Could improve the potential market value of the crop
Technical terms relevant to GH SL
• Photosynthetically active radiation (PAR): 400 – 700 nm (wavelength range)‐‐ broadband blue light: 400 – 500 nm‐‐ broadband green light: 500 – 600 nm‐‐ broadband red light: 600 – 700 nm
• Photosynthetic photon flux density (PPFD): μmol/m2/s (instantaneous intensity)
• Daily light integral (DLI): mol/m2/d of PAR (intensity integrated over 24 hours)
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Technical reasons to consider seasonal greenhouse SL in a temperate continental climate
• Photoperiod is limiting to solar daily light integral fall to spring
• Weather can be limiting to solar daily light integral, especially in the cloud/green belt, at any time of year
• Enhanced solar light reflection off of flat greenhouse panes by more oblique angles of incidence fall to spring
• Absorbance of solar light by translucent double‐poly GH covering structures
• Self/mutual shading of high‐wire fruiting crops during daily sun tracking in GH at any time of year
PhotoperiodAffects DLI At winter
solstice, photoperiodincreasesNorth to South
December 21
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Avg. greenhouse DLI across the contiguous USDecember June
2.5 ‐ 55 ‐ 7.57.5 ‐ 1010 ‐ 12.5
12.5 ‐ 1515 – 17.517.5 ‐ 20
20 – 22.522.5 – 2525 – 27.527.5 ‐ 30
After Korczynski et al., 2002
Weather as well as climate affects DLI, and light is always lost going into a greenhouse
Lettuce needs a DLI of 15 for
good production
Tomato needs a DLI of 25
Sunlight alone is not sufficient to
produce greenhouse crops year‐
round
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What kinds of SL do growers use for GH production?
• Some growers use small (low wattage) light bulbs instead of HIDs‐‐ savings on electrical power won’t matter if can’t hit target DLI‐‐ leaving such lamps on for extended photoperiods may induce flowering
• Some growers may use photoperiod lamps instead of HIDs‐‐ will have insignificant effects on DLI‐‐ may cause undesirable photomorphogenic effects, such as bolting
• High‐intensity discharge lamps are the current standard for greenhouse SL‐‐ high‐pressure sodium (HPS) are orange‐biased and dominate‐‐ metal halide are blue‐biased and slightly less efficient
• Light‐emitting diodes (LEDs) are emerging and consume much less power‐‐ available in different colors and are much cooler than HIDs‐‐ are more expensive but last much longer
In order to know how much light you are adding, you have to measure it!
• Early light meters measured units of “brightness” or luminosity
• They used units such as “lumens”, “foot‐candles”, or “lux”
• Those sensors are calibrated for human vision
• Plants respond to different parts of the light spectrum than people do
• Light sensors for human vision and plants use different units:‐‐ photometric measurement at solar noon on a sunny day = 10,000 ft‐c‐‐ quantum measurement at solar noon on a sunny day = 2000 μmol/m2/s‐‐ one cannot substitute for or approximate the other
• Unfortunately, some photometric light meters are still being used by growers
• Quantum sensors/meters are useful for measuring broad‐band PAR
• Spectroradiometers are needed when measuring output from narrow‐band LEDs
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Foot‐candle & Lux meters respond to the absorbance spectrum of the human eye, not that of plants!
Human eye response555 nm ℷmax
A lumen is the brightness of a birthday candle about a foot away from you!
Extracted chlorophyllsStronly absorb blue andred Light, hardly at allin the green
Plants have thereputation ofreflecting green Light , henceappear greento humans
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Relative quantum efficiency of different light sources for photosynthesis
In fact, intact Leaves absorbrather stronglyin the green, soit is useful PAR
In fact, accesory pigments absorbin the green, dispelling the myth About green light not being effective In photosynthesis
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Spectral response of quantum sensor
Quantum sensorsMeasure numbersof photons hittinga sensor per unit time. They are notsensitive to the effectiveness ofphotons of different wavelengths
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Spectral Comparison of Light Sources Used for Plant Growth
A spectrophotometer is required not only to scan different wavelenghtsin a light source, but can calculate theeffectiveness of each wavelength to drive photosynthesis.
Current Situation for Light‐Emitting Diodes (LEDs)
• Probably best used in GHs at low intensity for interlighting of high‐wire crops
• Higher‐output arrays being developed for overhead/side SL in GHs
• Probably the superior choice for sole‐source indoor lighting of leafy greens
• Limiting light intensities can be used for micro‐, baby, and leafy greens
• LEDs can be selected or “tuned” to provide different spectra during growth cycle
• Photon emitters are cool enough to locate LEDs near plant tissue (save power)
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Spectroradiometer integrates area under curves for 90% red: 5% blue LED lighting
The “Cardinal Factors” of Plant Growth
• Light
• Temperature
• Nutrients
• Water
• Atmosphere
★ These variables are absolutely required for plant growth and development, are dose responsive, and highly interactive.
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Dose Response
Maximum
Dose Response
Maximum
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3‐way Environmental Interactions
Light intensity
Photosynthesis
Cucumber
Can’t predict interactions among light, CO2, and temperature
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Growth Curve: Leaf Area
Day: 15
Regression curveTreatment 1: 10% blue lightTreatment 2: 5% blue light
Leaf lettuce
Plants are not equally responsive to growth Optimization at all stages of their growth cycle
Growth Curve: Shoot Dry Weight
LAG PHASE: 7.1 kWh/g
Regression curveTreatment 1: 10% blue lightTreatment 2: 5% blue light
Lettuce not very responsive to optimizing conditions oflight enhancement and CO2 enrichment during lag phase
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Marketing claims for LEDs, presumably for sole‐source lighting
• Claims that plants prefer blue light for vegetative growth‐‐ in general, blue light inhibits leaf expansion and stem elongation‐‐ may enhance synthesis of anthocyanins, phytonutrients‐‐ For leafy greens, only a small amount of blue light (e.g., 5%) may beneeded for best productivity
• Claims that plants prefer red light for reproductive development‐‐ in general, red light promotes leaf expansion and stem elongation‐‐ growth of leafy greens is powerfully stimulated by red light‐‐ Actually, far‐red light (700 – 800 nm) may be needed to induce floweringin long‐day plants.
White LEDs
• Actually are blue LEDs with phosphor coating on lens creating broad‐band white light
• Cool‐white, medium‐white, warm‐white, depending on blue ratio
• White arrays made in China are flooding US LED market‐ these arrays are cheap but not long‐lasting according to reputation
• Current trend in indoor agriculture (sole‐source lighting) is white + red LEDs
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A role for White LEDs in plant‐growth regulation
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For SL or SSL with artificial lighting
• Stay in the linear portion of the photosynthetic light‐saturation curve for both SL and SSL; otherwise, light and $$$ are wasted!
• Opportunity to develop “smart” SL systems depending what the weather and sun are doing at the moment
• Cannot afford to allow other Cardinal factors to become more limiting than light‐ e.g., nutrient levels such as N in the greenhouse‐e.g., CO2 levels in the warehouse
• A technology learning curve may be the key to improved CEA grower success!
CAM Contact Informationcmitchel@purdue.edu
Dr. Cary A. MitchellDepartment of Horticulture & Landscape Architecture
Purdue University625 Agriculture Mall Drive
West Lafayette, Indiana 47907‐2010 USA(765) 494‐1347