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710nm - 730nm, up to 850nm
Although the 730nm wavelength is outside the photosynthetically active range, it has the strongest action on the far-red absorbing form of phytochrome, converting it back to the red-absorbing form. It becomes necessary for plants requiring relatively low values of the phytochrome photoequilibrium to flower. Can be used at the end of each light cycle to promote flowering in short-day plants such as Peppers.
Also, a higher ratio of far-red to red than found in sunlight can trigger the shade stretch response- where a plant when sensing it is shaded based on an elevated ratio of far-red to red- will stretch to try to elevate its canopy above its competitors. This is why too much far-red is not advised if compact plants are desired, or in general. But small amounts or FR as provided by California LightWorks for example, in the R/FR channel is very beneficial, and for this reason the ratio or R to FR is fixed on one channel in the 550 series.
Figure 1: California Lightworks Far Red Module
Far-Red can be manipulated through the amount of light given to the plant. This receptor keeps the plant in the vegetative stage. Light in the far-red spectrum will signal this receptor to pass the chemical signal to veg. During times of light, both receptors are balanced in number, but in darkness, Far-Red receptors slowly change into Red receptors.
With longer dark periods, the number of Far-Red receptors reduces until there aren’t enough to counter the signal from the Red receptors, which tell the plant to flower.
Want to speed up your switch from veg to flower? Give plants an uninterrupted 24-36 hours of darkness before going to a 12-12 cycle. It will make more of the Far-Reds change into Reds, giving a more powerful signal to your plant that it is time to bloom. Normally, the transition can take a couple of weeks to be seen on your plants. By giving them a really long night, you can speed this up dramatically.
It’s intimidating to choose the best lighting for growing cannabis, especially when there are so many factors to consider, like spectrum, lumens, PPFD, CRI, CCT, and more! Don’t know what those terms mean? We’ve already covered the basics of horticultural lighting, so read that first if you haven’t already. We are going to dive deeper into one of those factors – spectrum. Plants use photosynthetically active radiation (PAR; 400 – 700 nm) to drive photosynthesis. But plants use wavelengths outside of the PAR range – mainly UV and far-red light to understand and respond to their environment. This article will explore how far-red light impacts plant growth and development. This article is part of a series covering all the wavelengths of light, including UV, violet, blue, green, yellow, orange, red, and far-red light! Far-red light is radiation with wavelengths between 750 and 850 nm and it falls between the red and infrared regions of the spectrum.
Plants sense signals from their environment, such as light, that cause them to change their growth. Light is sensed using photoreceptors, like phytochrome. Phytochrome has an inactive (red light absorbing) and active form (far-red light absorbing), and it switches between the two forms based on the light conditions. The light conditions are often quantified by looking at the ratio of red light to far-red light (usually shortened to “R:FR”). How much red (R) and far-red (FR) light is needed to make phytochrome switch back and forth? Well, it depends on many different factors, such as the species, growing conditions, and light intensity. In one often-used lab species, Arabidopsis, a R:FR of ~2 will switch phytochrome from inactive to active1. A R:FR of ~0.25 will switch it back. Via phytochrome, far-red light controls seed germination, stem elongation, and flowering time.
Far-Red Light and Seed Germination
Far-red light mediates seed germination in some species. It’s important that a seed germinates in a good growing environment – one that has lots of light and isn’t too shaded. If a seed germinates in a shady spot, it can be detrimental to the plant because it will not get enough light to grow. Shade environments are enriched in far-red wavelengths compared to other colors of light, so high amounts of far-red radiation can prevent seeds from germinating. Germination rate drops by about 30% when R:FR changes from 1.1 to 0.6. For this reason, seeds should be germinated under bright lights that have high amounts of red light and low amounts of far-red light. Far-red wavelenghts can trick a seed into thinking that it is in a shaded environment, and it is less likely to germinate.
Figure 2: When full-spectrum light hits the leaves of a plant, the photosynthetic pigments absorb much of the red and blue light. Therefore, the light reaching the lower leaves of the canopy is enriched in green and far-red light. Photoreceptors in the lower leaves receive this altered spectrum and signal to the lower leaves that they are being shaded.
Far-Red Light and Vegetative Growth
The R:FR also impacts vegetative growth in plants. High amounts of far-red light can cause stems to elongate and leaves to get longer and wider. This is because the plant is trying to stretch up in hopes of reaching more sunlight. As a result, a plant can look “stretched out” and these long skinny stems are sometimes too weak to hold up heavy cannabis flowers. High amounts of far-red light can also decrease the amount of chlorophyll, anthocyanins, and antioxidants in the plant6. Chlorophylls and anthocyanins are pigments that make a plant colorful, which can factor into the novelty and value of the harvest. Antioxidants protect against harmful free radicals – both for the plant and the humans consuming it! Ideally, a grower wants a plant with vibrant color and high antioxidant levels. To produce plants with strong stems and a vibrant color, they should be given high amounts of red light and low amounts of far-red light. This is especially true if the plants are being grown at high-density.
Far-Red Light and Flowering
By this point, you might be starting to notice a trend: plants associate far-red light with shade. Thus, if you give a plant too much far-red light, they will think they are in a shady environment. Too much shade can be stressful to a plant, so it takes precautionary measures to avoid these conditions. Seeds will avoid germinating and stems will stretch out to reach more light. In response to too much shade, a plant will often start flowering. Flowers are the reproductive tissues of a plant and if a plant thinks there is a risk of death (from too much shade), it begins reproducing ASAP so that it may pass its genetics to the offspring. High amounts of far-red light accelerate flowering in many species – tomato, potato, cucumber, beans, wheat, mustard, and many ornamental flowers. In some species, far-red light also increases the number of flowers produced.
As growers, we can use this knowledge to our advantage. If we wish a plant to begin flowering (such as a stubborn cannabis plant that refuses to bud out), we can give it high amounts of far-red light. Far-red light should be applied for a short period to induce flowering, and stopped once buds begin to appear. Small amounts of far-red light applied at nighttime (~2 μmol s-1 m-2) is also effective at accelerating flowering and increasing flower number.
For most stages of plant growth, a grower should maintain a high R:FR ratio. In other words, plants should be provided with high amounts of red light (and other colors of light, like blue and green light) and low amounts of far-red light. If a grower wishes to induce flowering, they can provide a plant with high amounts of far-red light (either during the day or night) for a short period. Once flowering starts, the plants should be returned to their regular lighting conditions. When choosing a light for growing cannabis, look for a horticultural light that has high amounts of red and blue light, moderate amounts of other colors (green, yellow, and orange), and low amounts of far-red and UV light. When used for an extended period, far-red light can be detrimental to plant growth, so it should be avoided when purchasing a grow light. Far-red light causes plants to stretch out and reduces the amount of chlorophyll (which is essential for plant growth) in leaves. When used for a short period, far-red light can stimulate flowering, which can be advantageous if you have a stubborn plant that refuses to flower.
Figure 3: Far Red in the electromagnetic spectrum
The electromagnetic spectrum is categorized by wavelength. The longer the wave, the less energy it holds. So blue light, which has a short wavelength of 475 nanometers, has more energy than red light, which has a wavelength of 660 nm. Infrared light is just beyond the range of human vision at 730nm. Although we cannot see it, we can feel infrared radiation as warmth. For instance, a glowing charcoal emits red light in the visible spectrum and infrared light that we sense as heat.
The plants use red and infrared light to regulate stem growth and flowering response. Plant cells produce a chemical called a phytochrome, which has two versions. One version, PR, is sensitive to red light (660 nm). Red light converts PR into PFR. PFR signals the plant to grow short stocky stems and also helps the plant grow into specific shapes. The plants also use red and infrared light to measure uninterrupted darkness. As far as plants are concerned in terms of flowering, if there’s no red light, it’s dark.
PFR is sensitive to infrared light (730 nm), which converts it into PR. When PR levels build to a critical amount, scientists hypothesize that a hormone called floragen becomes active and induces the plant to flower. The reason floragen is called hypothetical is that researchers can see its effects, but they haven’t found it yet.
PFR reverts to PR naturally. For PFR to be present, it must be renewed continuously by the presence of red light. When plants are shaded, they get less of the needed red light. In the absence of red light, the PR version predominates and the stem stretches to reach the light. Lower side branches shaded by leaves from above have PR and grow longer until they reach the light. Then they modify their growth in the presence of PFR.
Outdoors during the day, there is more red light than infrared. However, at dawn and dusk the first and last light from the sun isn’t the visible red of the rising or setting sun, but infrared, which is at the far end of the electromagnetic spectrum. The infrared converts the PFR to PR and the critical dark-time begins or ends its countdown.
This has too many implications for them all to be discussed here. For instance, it explains why plants grown under incandescent lamps stretch (more infrared than red light). The effects of the two spectrums can also be used in innovative indoor lighting programs.
Sleep Initiation using Far Red | Radiation – Potent and Effective
Far Red radiation – or Far Red light – designates the far end of the red light spectrum that is visible on the edge between red and infrared light. Usually referring to a wavelength between 710 and 850nm, Far Red radiation can be perceived by various organisms as dimmed light. Plants make use of an absorbance spectrum and read Far Red light as an indicator for nightfall. This mechanism can be utilized in artificial lighting scenarios.
Plants posses a natural photoreceptor called phytochrome, a protein that perceives Far Red light levels in the plant’s surroundings. Phytochrome regulates various aspects of the plant’s growth such as germination, flower formation and photoperiodism. Photoperiodism designates the regulation of the plant’s day and night cycles. Plants display two essential types or conformations of phytochromes: the pr-type (r=red), which has a maximum absorption level of ca. 660nm and the pfr-type (fr=far red) with a maximum absorption level of 730nm.
So what is the connection between phytochromes and grow lights?
It’s simple: by utilizing the Far Red spectrum we can influence processes like photoperiodism to optimize the plant’s regeneration phase during the night and effect better harvests by optimizing its capacity for photosynthesis. This process is as simple as it is efficient: by leaving Far Red light turned on for 10 to 15 minutes after turning off the regular lights in the evening, the Far Red spectrum will act as an initiator for the natural sleep phase of the plant.
As such the natural day/night cycle of the plant is influenced by the Far Red spectrum in artificially speeding up the process of nightfall. Making the plant “think” night was falling very quickly effects the plant to enter its regeneration phase quicker than it would when using a regular light setup. As a result you can either increase the regeneration time available for the plant without extending the night phase or shorten the plant’s night phase without diminishing its regenerative effect. This method is beneficial for artificial grow light setups because it allows to switch from the conventional 12h/12h rhythm of the plant’s day/night cycle to a more productive 13,5h/10,5h rhythm. As a result the plant gains additional 1,5 hours for photosynthesis.
Source: URSA | Excite | California Lightworks | Pro-Emit