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Horticultural Lighting

Selecting the best horticultural lighting for indoor grows, greenhouses and plant research can seem daunting, especially when you’re just starting out. While it’s important to choose the most practical horticultural lighting, there are tonnes of options, and several factors you need to consider, including color temperature, lumens, foot-candles, PAR, and PPFD.  Although all of these terms are useful for describing lighting, they are not all useful for growers!  Here, we will walk you through each of these considerations, to explain which ones are important for your plants – and which ones are NOT!  Let’s get started by talking about how humans and plants perceive light.

Did you know that birds are incapable of tasting capsaicin, the chemical that gives hot peppers their heat? Their tongues have no receptors for capsaicin, so no “spicy” signals are transmitted to their brain.  In contrast, we humans have special receptors on our tongues called nociceptors, which are plenty capable of sensing spice and sending the appropriate “this is spicy!” signal to our brain.  A similar difference exists between the light receptors of human eyes and plant leaves.  The human eye has different light receptors than a plant leaf, so we see light differently than a plant.  As a result, several of the ways that we describe light are biased towards the type of light that humans are able to see.  While plants “see” photosynthetic photon flux, humans see lumens.

Light : A Human Perspective

Lumen (lm) is a unit describing the amount of light (visible to the human eye) emitted by a source per second.  Figure 1 shows the wavelength range of light that a human eye can see.  Human eyes are most sensitive to light in the yellow and green regions of the spectrum and less sensitive to colors like deep blue and red.  Meanwhile, human eyes have a very difficult time seeing infrared and ultraviolet wavelengths of light.  Since lumens are a human-centric measurement, we should not use this measure for describing horticultural lighting.

Figure 2: Comparison between foot-candle and lux. Foot-candle is measured over an area of 1 ft^2 while lux is measured over an area of 1 m^2

Color Rendering Index, or CRI, describes the ability of a light source to show an object’s color accurately in comparison with a natural light source (such as the sun on a cloudless day).  The highest value a light can achieve is a CRI of 100; lower CRI values result in objects appearing unnatural or discolored.  Figure 3 shows an example of how a human eye may see an apple illuminated by lights with varying CRI values.  Under a light with a CRI of 100, the apple appears bright red; under a light with a CRI of 70, the apple appears dark and blueish.  This measure is dependent on how the human eye sees light, and so is not a useful parameter for choosing horticultural lighting.

Correlated Color Temperature, or CCT, describes the color of a light source and is measured in degrees Kelvin (°K).  The higher the CCT of a light source, the cooler the light’s color. For example, a very red light achieves a CCT of about 1000 K while a very blue light can achieve a CCT of about 10,000 K.  Warm white lights will have a CCT around 2700 K, neutral white will be around 4000 K, and cool white around 5000 K.  Similarly to CRI, this measure is dependent on light perception by the human eye, and once again, is not useful for describing or choosing horticultural lighting.

Figure 1: Wavelengths of light perceived by the human eye. Our eyes are most sensitive to light in the yellow and green regions of the spectrum and less sensitive to colors like deep blue and red

Lux (lx) is a unit that describes the number of lumens visible in a square meter.  100 lumens spread out over an area of 1 m2 will have an illuminance of 100 lx.  The same 100 lumens spread out over 10 m2 produces a dimmer illuminance of only 10 lx.  For our friends in America, we might use the term foot-candle to talk about light in units of measurement that concern distance in feet and inches.  A foot-candle describes the number of lumen per square foot.  Thus, one foot-candle is equal to approximately 10.764 lx (Figure 2).  As before, this measure is only relevant for how we perceive light, and is irrelevant for plant growth.  Like lumenslux is similarly poor for describing horticultural lighting.

Figure 3: Effect of CRI on how a human eye perceives the color of an apple.

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Part 4: Which Wavelengths do Photoreceptors Absorb?

In order to choose the best light for growing your plants, it’s essential to understand which wavelengths of light are required for normal plant growth.  Plants are experts at capturing light energy and converting it into sugars through the process of photosynthesis.  The first step of photosynthesis is the absorption of light by specialized molecules called pigments that are found in plant cells.  In addition to pigments, plants have a number of other light receptor molecules known as photoreceptors.  We will explore the range and function of key plant pigments and photoreceptors and identify the wavelengths of light they absorb and respond to.  This article, which will cover photoreceptors is the last in a 4-part series.

 

Words of caution: a complex network of factors control plant growth and development.  This article focuses on just one of these factors: light spectrum.  When deciding which wavelengths of light will be best for your plants, consider how all factors (light intensity, temperature, soil, etc.) interact together.  It’s also important to remember that most of what we know about pigments and photoreceptors is derived from studies with the model plant Arabidopsis (the plant equivalent of the lab mouse) and much remains to be learned about other species.  Different plant species have variations in the chemical composition of their pigments and photoreceptors.  For this reason, pigments and photoreceptors from different species can have slightly different absorption peaks than the values listed here.

Light Wavelengths for: Photoreceptors

Photoreceptors are non-pigment molecules that respond to changes in light intensity, quality, direction, and duration. Photoreceptors are not found in the LHC, but rather in other areas of the cell like the nucleus and mitochondria (Figure 1).  Photoreceptors allow the plant to sense its environment and modify plant growth accordingly (Figure 2). For example, a plant can tell that a neighboring plant is shading it because the neighbor’s leaves block certain wavelengths of light, causing an altered light spectrum to reach the plant (Figure 2).  Photoreceptorsfall into five broad categories, and we will give a short description of each along with the wavelengths of light it responds to:

1.  Phytochromes convert between red-absorbing (Pr; 600–700 nm) and far-red absorbing (Pfr; 700– 750 nm) forms (Figure 3). The change is reversible: with sufficient far-red light, Pfr converts back to Pr (Figure 3). The ratio of red:far-red light is more important than the absolute amounts of each type of light. Phytochromes control seed germination, chlorophyll synthesis, stem elongation, the size, shape, and number of leaves, as well as the timing of flowering1. Generally, a low red:far-red (i.e., a high amount of far-red light) causes stems to elongate, leaves to grow longer and wider, and chlorophyll content to increase. A low red:far-red can also cause flowering to happen earlier2.

2.  UV Resistance locus 8 (UVR8) is a photoreceptor that responds to UV-B wavelengths (280–315 nm).  UVR8 regulates flavonoid biosynthesis and the circadian clock. Flavonoids have diverse functions: they can transmit chemical signals, regulate physiological responses, and modulate the cell cycle.  UVR8 also regulates epidermal cell expansion (expansion of the plant “skin”), number of stomata (air pores), and it improves tolerance to UV-B light3. Relatively little information is known about the interaction between UVR8, wavelength, and plant development. So far, we know that increased UV-B light causes flavonoid content to increases, epidermal cells to expand, and stomata number to increase.

3.  Chryptochromes respond to blue light (390–500 nm) and control many aspects of plant growth and development. Chryptochromes control stem elongation (etiolation), flowering time, the circadian clock, stomatal opening, and anthocyanin production. Generally, increased amounts of blue light reduce stem elongation, open stomata, and increase anthocyanin content. Blue light can promote flowering of long-day plants and inhibit flowering of short-day plants.

4.  Phototropins respond to blue (390–500 nm) and UV-A (320–390 nm) light. They control many of the same responses as chryptochromes. Phototropins control stem elongation, stomatal opening, phototropism (bending of the plant towards the light), leaf solar tracking, and chloroplast migration (movement of chloroplasts to prevent mutual shading). Generally, increased amounts of blue light promote phototropism, leaf solar tracking, and chloroplast migration.

5.  Zeitlupe family photoreceptors respond to blue light (390–500 nm) and regulate the circadian clock and flowering. Very little is known about how these photoreceptors function. We know that during long days, zeitlupe photoreceptors can induce flowering.

Figure 1: While chlorophylls and carotenoids are usually found in chloroplasts, phytochromes are typically found in the nucleus or mitochondria.

Figure 2: Plants can sense their environment and modify their growth accordingly. A tall plant receives the full spectrum of light from the sun. A shorter plant is shaded by its neighbor and so does not get the full spectrum of light. Instead it gets a spectrum that is low is blue and red light. Photoreceptors in the leaves of the short plant receive this altered light spectrum. The photoreceptors respond accordingly and signal to the short plant to grow a taller stem.

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Part 3: Which Wavelengths do Anthocyanins and Betalains Absorb?

In order to choose the best light for growing your plants, it’s essential to understand which wavelengths of light are required for normal plant growth.  Plants are experts at capturing light energy and converting it into sugars through the process of photosynthesis.  The first step of photosynthesis is the absorption of light by specialized molecules called pigments that are found in plant cells.  In addition to pigments, plants have a number of other light receptor molecules known as photoreceptors.  We will explore the range and function of key plant pigments and photoreceptors and identify the wavelengths of light they absorb and respond to.  This article, which will cover anthocyanins and betalains is the third in a 4-part series.  Click here to read about chlorophylls.  Here to read about xanthophylls and carotenes.  And here to read about photoreceptors.

Words of caution: a complex network of factors control plant growth and development.  This article focuses on just one of these factors: light spectrum.  When deciding which wavelengths of light will be best for your plants, consider how all factors (light intensity, temperature, soil, etc.) interact together.  It’s also important to remember that most of what we know about pigments and photoreceptors is derived from studies with the model plant Arabidopsis (the plant equivalent of the lab mouse) and much remains to be learned about other species.  Different plant species have variations in the chemical composition of their pigments and photoreceptors.  For this reason, pigments and photoreceptors from different species can have slightly different absorption peaks than the values listed here.

Light Wavelengths for: Anthocyanins and Betalains

Anthocyanins and betalains are pigments that range in color from orange to red to purple to blue.  It is these pigments that give berries, beets, and autumn leaves their colours.  Anthocyanins are found in the vacuole of plant cells, which is where most water is stored (Figure 1 and 2).  Anthocyanins and betaliains never occur in the same plant – it’s either one or the other (but usually it’s anthocyanin). Anthocyanins protect plant cells from various environmental stresses, including excess light, nutrient deficiencies, and salt stress.  Anthocyanins may also act as a deterrent to herbivores such as aphids1.

Which wavelengths of light do anthocyanins and betalians absorb? Many of these pigments absorb in the yellow, green, UV-A, and UV-B wavelengths (280–400 and 500 – 550 nm; Figure 3).  Providing plants with excess light at these wavelengths typically will not enhance photosynthesis because anthocyanins and betalains mainly function in a protective capacity.

However, like carotenes, anthocyanins and betalains can positively contribute to the appearance and health benefits of plant products.  For this reason, some growers may wish to enhance anthocyanin content to give a plant a more purple or red colour.  Modifying light spectrum is one method of altering anthocyanin content in a plant.  Growers can increase anthocyanin content by increasing total light levels, as well as increasing levels of green and UV light.  Inducing a stress on the plant (such as drought, temperature, or nutrient stress) can also stimulate anthocyanin and betalain production.

Figure 1: Each plant cell contains a large vacuole that function in storing water, minerals, and other water-soluble compounds. The vacuole also functions in keeping the plant cell from collapsing in on itself.

Figure 2: Red anthocyanins are stored in the vacuoles of these petal cells from a geranium flower petal. The vacuole typically occupies most of the space within a plant cell.

Figure 3: Light absorption for various plant pigments and photoreceptors. Chlorophyll A is the most abundant pigment and chlorophyll B is the second most abundant. Xanthophylls (lutein and vioxanthan) and carotenes (beta-carotene) are the next most abundant pigments. Anthocyanins (malvidin, pelargonidin, and chrysanthemin) and photoreceptors (phytochrome) are also essential for how a plant senses its environment.

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Part 2: Which Wavelengths do Xanthophylls and Carotenes Absorb?

In order to choose the best light for growing your plants, it’s essential to understand which wavelengths of light are required for normal plant growth.  Plants are experts at capturing light energy and converting it into sugars through the process of photosynthesis.  The first step of photosynthesis is the absorption of light by specialized molecules called pigments that are found in plant cells.  In addition to pigments, plants have a number of other light receptor molecules known as photoreceptors.  We will explore the range and function of key plant pigments and photoreceptors and identify the wavelengths of light they absorb and respond to.  This article, which will cover xanthophylls and carotenes is the second in a 4-part series.

Plant cells have chloroplasts that convert light energy into sugar.  Each chloroplast has many light harvesting complexes (LHC) that absorb this light energy.  LHC have two main parts: the reaction center and the antenna.  The reaction center is a single chlorophyll A molecule (Figure 1).  The antenna is a mix of many pigments such as chlorophylls, xanthophylls, and carotenes (Figure 1).  A packet of light energy (photon) is captured by the pigments in the antenna and transferred to the reaction center where it is converted into two electrons.  These electrons are essential for photosynthesis.  Most pigments in the LHC are chlorophylls (about 65% of them)!  There are also xanthophylls (about 29%) and carotenes (about 6%).

Words of caution: a complex network of factors control plant growth and development.  This article focuses on just one of these factors: light spectrum.  When deciding which wavelengths of light will be best for your plants, consider how all factors (light intensity, temperature, soil, etc.) interact together.  It’s also important to remember that most of what we know about pigments and photoreceptors is derived from studies with the model plant Arabidopsis (the plant equivalent of the lab mouse) and much remains to be learned about other species.  Different plant species have variations in the chemical composition of their pigments and photoreceptors.  For this reason, pigments and photoreceptors from different species can have slightly different absorption peaks than the values listed here.

Figure 1: Diagram of a light harvesting complex (LHC) within the chloroplast of a plant cell. The LHC is made up of the reaction centre (a single chlA molecule) and the antenna (a mix of chlorophylls, xanthophylls, and carotenes).

Light Wavelengths for: Xanthophylls and Carotenes

In addition to chlorophylls, antenna complexes also contain xanthophylls and carotenes, which are two classes of pigments within the carotenoid group.  Typically, xanthophylls are yellow while carotenes are orange.  It is these pigments that give carrots, yellow peppers, and pumpkins their color.  Xanthophylls and carotenes absorb wavelengths of light that chlorophylls cannot absorb.  They also function as a sunscreen for the plant by protecting it from the potentially harmful effects of high light.  Lastly, some carotenes are also important for the long-distance flow of electrons within the LHC.

Which wavelengths of light do xanthophylls and carotenes absorb? Many of the carotenes and xanthophylls absorb in the wavelength range of 425 to 475 nm.  Beta-carotene has the highest absorption at 450 nm while the xanthophylls lutein and vioxanthan absorb the most at approximately 435 nm (Figure 2).  To optimize photosynthesis, your plants should be provided with light that satisfies the wavelengths requirements of these pigments.  Although these pigments absorb in the UV region of the spectrum, growers must be very cautious when growing with UV lights.  These wavelengths can cause oxidative damage and modify DNA of plants and humans.

Some xanthophylls and carotenes contribute to the smell and flavor of a plant and have positive effects on human health.  For this reason, some growers may wish to increase the concentration of xanthophylls and carotenes in their plants.  Synthesis of these pigments depends on several factors, to include light.  In particular, blue and UV light can is known to increase carotenoid content in some species.  Our most popular grow light, the Optilux, has a spectrum designed to satisfy the absorption requirements of xanthophylls and carotenes.

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Part 1: Which Wavelengths do Chlorophylls Absorb?

In order to choose the best light for growing your plants, it’s essential to understand which wavelengths of light are required for normal plant growth.  Plants are experts at capturing light energy and converting it into sugars through the process of photosynthesis.  The first step of photosynthesis is the absorption of light by specialized molecules called pigments that are found in plant cells.  In addition to pigments, plants have a number of other light receptor molecules known as photoreceptors.  We will explore the range and function of key plant pigments and photoreceptors and identify the wavelengths of light they absorb and respond to.  This article, which will cover chlorophyll pigments is the first in a 4-part series.

Words of caution: a complex network of factors control plant growth and development.  This article focuses on just one of these factors: light spectrum.  When deciding which wavelengths of light will be best for your plants, consider how all factors (light intensity, temperature, soil, etc.) interact together.  It’s also important to remember that most of what we know about pigments and photoreceptors is derived from studies with the model plant Arabidopsis (the plant equivalent of the lab mouse) and much remains to be learned about other species.  Different plant species have variations in the chemical composition of their pigments and photoreceptors.  For this reason, pigments and photoreceptors from different species can have slightly different absorption peaks than the values listed here.

Figure 1: Diagram of a light harvesting complex (LHC) within the chloroplast of a plant cell. The LHC is made up of the reaction centre (a single chlA molecule) and the antenna (a mix of chlorophylls, xanthophylls, and carotenes).

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A PAR Apart – the “REAL PAR”

Since its creation, the PAR has created much discussion. The latest developments in specific spectrum lighting create a clear-cut necessity for some explanations.

The PAR (or PPFD) is a system of measurement of energy available to plants, in the form of light.

The creators, at the time, decreed that a chlorophyllous green plant has a need for light of frequencies between 400 nm and 700nm (nanometre). Simply explained: from a light blue through to a deep red. Green, yellow and orange are found between the two.

It is now known that these plants need light that is situated beyond these limits. However the PAR remains the standard measure in the horticultural world!

The different frequencies of light stimulate specific functions in chlorophyllous green plants, varying depending on the phase of development.

The frequencies from 500nm to 600nm (the green spectrum) are virtually unused, particularly during the flowering phase. If the great majority of the surface of these plants is green, it is not a hazard. The green part of the light is reflected not absorbed. This is why our eyes see these plants as green.

When plants grow under sunlight there is not strictly speaking a “waste of energy”. However, when the same plant is cultivated under artificial light, all light produced and not absorbed is a waste of energy, natural resources and money.

For the moment, the most efficient way of producing light is the LED (Light-Emitting diode) and on a bigger scale, a COB of LEDs.

The O-Magma team has developed their flowering lamps on these facts:

  1. Our “Light motor” is a unique, COB of LEDs, specifically designed to supply the exact frequencies of light (not the useless ones), necessary during the flowering phase of chlorophyllous green plants. The blue and red peaks are perfectly aligned, with the plant’s needs.
  2. Our COB is based on our privileged access to the latest technological breakthroughs, so giving us an unbeatable output, efficiency and yield.
  3. Our COB produces more than 1.4* times more light per watt than the best of the HPS (High Pressure Sodium) light sources. (ref: Dr. Caroline Horst, Horticultural Lighting Conference, Eindhoven 2017).
  4. Our COB does not produce light in the “useless frequencies”, 500nm to 600nm, which represents 33%** of the PAR measurement. So when compared with a “full spectrum” light source, such as an HPS or white LEDs, which produce this unused light and is mechanically counted in their PAR reading, it is necessary to increase PAR reading of the specific spectrum light by 33% to be able to compare the “REAL PAR” readings.

LED efficiency vs. HPS (1.4) x useless light not produced (1.33) =  1.4 x 1.33 = 1.86

 Conclusion

Compared with the best of the HPS tubes, the O-Magma technology is 86% more efficient … Furthermore, 90% of the heat generated is extracted from the grow area, unlike all of other lamps.

 

Peter J. Thornhill

25 November 2016