We all know how important light is to the production of cannabis. In order to optimize the photosynthetic activity of cannabis plants, one must uderstand how the plant captures and uses light energy to create plant tissues and compounds, such as glucose (for food) and cannabinoids like THC (for us). Light intensity and light quality—i.e. wavelength—both play an extremely significant role in photosynthesis and cannabis growth. In this article, we will examine how this occurs and which components of light play the biggest parts.
What is Light?
Light has the characteristics of both a wave and a particle. Figure 1 shows the wavelengths of red and blue light. The distance between the peaks of the wave is measured in nanometers (nm). As that distance or frequency changes, so does the color of the light. The color red, for example, resides at one end of the visible spectrum and is the result of light with a wavelength of 620 to 750 nm. Blue light resides at the opposite end and has a shorter wavelength of 650 to 675 nm. Light is also produced in wavelengths that are out of the visible range of our eyes, such as ultraviolet (UV) light, and this light also factors into plant processes, especially at the end of the flowering period.
Anything that has color or pigments, such as plant leaves, reflects or absorbs light. The light that is reflected off an object hits our eyes, causing us to see that color. The primary wavelength of light reflected by cannabis is green, due in large part to pigments such as chlorophyll a and b in the leaves; what we don’t see as reflected light is mostly absorbed by the leaves. Cannabis leaves absorb most forms of visible light except green and yellow. The majority of the reflected light is green, so the reflected yellow light is not noticeable.
However, leaves can begin to turn yellow if plants are not healthy. This occurs because the unhealthy leaves don’t contain enough chlorophyll molecules. Due to the lack of chlorophyll, green light isn’t reflected by leaves and yellow becomes the dominant reflected wavelength. The same phenomenon happens every fall when the leaves of deciduous trees begin to break down chlorophyll; the trees then pull the energy-rich breakdown products out of the leaves before they drop for winter. The result of the lost chlorophyll molecules are the bright reds, oranges and yellows of the fall season.
As stated, light also has the characteristics of a particle. These particles of light are called photons. When the sun is shining or there’s a light on, photons of light are pouring down like raindrops. The higher the intensity (strength) of the light, the greater the quantity of photons emitted; this is why keeping artificial light close to plant canopies indoors is so vital. Each type of photon has a specific wavelength that it travels along. So a bulb that is said to be abundant in blue light would emit photons with wavelengths from 650 to 675 nm.
Photosynthetic Pigments in Cannabis
The amazing thing about plants, including cannabis, is how they take the energy from photons of light and convert it into molecules of chemical energy. This process is called photosynthesis, which simply means “synthesis using light.” Figure 3 shows the basic chemical equation for photosynthesis. Light is responsible for driving this reaction, which is actually far more complicated than the equation shown here. In fact, there are at least 50 known intermediate steps for the conversion of CO2 into sugar. These 50 intermediate steps are only the ones that have been discovered thus far by scientists; there are likely to be more in the future.
During photosynthesis, cannabis captures photons of light using several different molecules. The primary molecule responsible for this is chlorophyll, which occurs in two main forms: chlorophyll an and b. Chlorophyll a has maximum photosynthetic activity when photons of light are at wavelengths of approximately 630 to 660 nm, while chlorophyll b has maximum activity at wavelengths of 650 to 660 nm (Figure 6). Notice that in Figure 6, when the curves for both chlorophyll a and bare in the green-light portion of the graph, their photosynthetic activity decreases greatly. Again, this is because chlorophyll a and b do not absorb, but rather reflect, green light. It should also be noted that Figure 2 depicts the sun’s natural spectrum on Earth, which shows that wavelengths around 650 nm (blues) and 650 nm (reds) are the least abundant in nature. This has led some scientists to conclude that the reason there’s higher absorption activity in leaves at these wavelengths is because the plants have evolved to become more efficient at processing what is least available to them.
The second most abundant light-absorbing compounds in cannabis are carotenoids, which are also shown in Figure 6. Carotenoids are important for photosynthesis in all plants. Notice they have very low photosynthetic activity in the areas of yellow, orange and red light. In addition to playing an important role in photosynthesis, these compounds are also responsible for the color of yellow flowers, orange carrots and red tomatoes.
The Light-Harvesting Complex
The pigments discussed make up a highly complex structure called the light-harvesting or antennae complex, which exists in all plants. This complex is composed of about 200 to 300 chlorophyll molecules, numerous carotenoids, and several other light-sensitive molecules and important proteins. All of these components are arranged around a central chlorophyll molecule called the reaction center. The reaction center is responsible for the conversion of light energy into chemical energy through the transfer of a single electron.
This process works as follows: When one of the light-sensitive molecules, such as chlorophyll, is struck by a photon of light, it enters into an excited state. As it returns back to its normal state, the energy is transferred toward the reaction center. It takes many photons striking many molecules in the light-harvesting complex to reach the goal of transferring a single electron to the reaction center. Picture one person trying to push a boulder that will not move; if the number of people pushing that boulder increases, together they will create enough force to roll it. The same action occurs during photosynthesis, whereby energy from a number of light photons is required to create enough energy for the transfer of a single electron.
This electron transfer is the very first chemical step in the creation of sugar (glucose) from carbon dioxide (CO2) and water (H2O) during photosynthesis. Furthermore, this sugar is the primary source of energy for the reactions and biochemical processes that will lead to the glistening, resinous THC glands that top your cannabis buds.
Now we understand a little bit more about how cannabis plants capture light photons and convert their energy into chemical energy and biomass. How then, as a grower of cannabis, can you maximize plant efficiency and yields with commercially available lights? When it comes to light intensity (i.e. quantity of light), the answer is pretty simple: more light. It is difficult to overdo the quantity of light for cannabis production (always assuming that you have proper temperature controls in place). Generally, the more lights you have and the more powerful those lights, the better your light intensity (though not so much better for your electric bill).
But adding more lamps or using higher wattages isn’t the only way to increase light intensity. Keeping lamps close to the garden canopy will increase the intensity of light reaching your plants and make more efficient use of the electricity you’re paying for. The inverse-square law dictates that a specified physical quantity or strength is inversely proportional to the square of the distance from its source. In the case of light, this basically means that the strength of light decreases exponentially with every foot of distance between the lamp and your plants.
High Light & CO2
As shown in the photosynthetic equation (Figure 3), carbon dioxide is required for growth and photosynthesis in plants. In many grow setups, light intensity is typically very high and the production occurs indoors. With high (intense) light and/or contained conditions, it is likely that carbon dioxide will quickly become a limiting factor and put a bottleneck on yields. Figure 6 shows the effect of light and carbon dioxide on the rate of photosynthesis. In a normal atmosphere, carbon dioxide is at 400 parts per million. If your grow operation is in an enclosed area that contains many cannabis plants, all of them constantly consuming CO2 for photosynthesis, there’s a very high possibility that the concentration of CO2 is actually going to be lower than 400 ppm. When this occurs, you’ll be limiting the productivity of your plants. For this reason, it’s recommended in these situations that you implement a CO2 supplementation system to maximize photosynthesis and yields.
Effects of Light Quality
The spectral quality or wavelength of light is another important factor when it comes to the productivity and quality of cannabis—and it’s also one of the most complicated. In many different types of plants, the quality of light can affect such things as disease resistance, plant anatomy and morphology, nutrient uptake and the formation of secondary compounds (such as THC).
A number of scientific studies have shown that blue light has an influence on the number of chloroplasts (which contain the light-harvesting complex and chlorophyll) and the stomatal opening. Stomata are the part of the leaf that controls gas exchange and therefore how much carbon dioxide is available to the leaf for photosynthesis. Many other studies have shown that plants under white LED light (which contains all the colors of the spectrum) grow better than plants under red LED light alone, blue LED light alone, or even red and blue LED lights in combination. These studies re-emphasize the importance of full-spectrum light in our gardens.
Another study—this one performed at Indiana University by two researchers named Mahlberg and Hemphill—tested THC content in the leaves of cannabis plants grown under sunlight, red light, blue light, green light or complete darkness. The experiment revealed that the plants grown under sunlight had the highest THC content. The THC concentration decreased successively in the plants grown under red light, blue light and green light; the plants grown in darkness had the lowest THC content. Additionally, under every light treatment, the researchers found that the leaves receiving the highest quantity of light had the highest concentration of THC. Although the researchers weren’t specifically looking to evaluate THC content, they did discover that wavelength, or the spectral quality of the light source (i.e. full spectrum versus a single or limited spectrum), is an important factor for THC production, and also that shading can also lower THC concentrations.
With all of this information at hand, choosing a light or combination of lights for growing can be a daunting task. Figure 7 shows most of the possibilities for light sources, as well as the distribution of spectral wavelengths produced by each.
Notice that sunlight has by far the broadest possible spectrum of light available—for horticultural purposes, it’s the very definition of full-spectrum light. This characteristic, along with the high intensity and raw power of sunlight, is why this source of light is the best for cannabis production… not to mention that it’s free! But as we all know, this isn’t always an option, and plants must often be grown indoors under artificially lit conditions. Therefore, choosing a combination of lights that matches the spectral ratio of the sun is the best option for your plants.
All of the light sources shown in Figure 7 have been around for some time, with the exception of plasma lights. These are the latest and greatest light source for indoor cannabis production and produce a spectrum similar to the sun (bear in mind that sunlight is also plasma light). These lights are also highly energy-efficient. Currently, the main problem with plasma lights is their price, which can reach several thousand dollars. With increased demand and manufacturing, however, expect these prices to drop over time.
Light-Emitting Diodes (LEDs)
LEDs are one of the most recent lighting products available for cannabis cultivation. They are unique in that they can be designed to produce a narrow spectrum or a specific quality of light. The two most common wavelengths available are red and blue. If you refer back to Figure 4, you can see that the red and blue areas of the graph are where the highest rate of photosynthetic activity occurs. We’ve already discussed how these two color spectrums are the least abundant in nature, which forces plants to be most efficient at processing them. But there’s another factor to be considered, at least with the red wavelengths, and this is the fact that red light carries with it the most photons out of any of the spectral wavelengths. LED manufacturers cite this as one of the reasons that LED lamps are made with these two types of light and sold for cannabis and indoor plant production.
Still, feeding plants only one or two spectral wavelengths would be like feeding a child only oranges and eggs. For instance, giving plants only red or blue light would throw off biological processes on a molecular level within the plant. Red light alone would cause severe stretching; blue light alone would slow the photosynthetic mechanism to a crawl. No matter how you slice it, plants need full-spectrum light, which they’ve evolved under for millions of years, to achieve their natural and maximum potential.
Another problem with the limited range of LEDs is the altered look of the plant under these wavelengths. Diseases and nutritional problems that are easily distinguished under other lights can be hard or impossible to distinguish due to the lack of a complete visible-light spectrum when growing with LEDs. Other problems include the fact that LEDs are mono-directional and do not emanate light like HID bulbs. This is why reflectors can’t be used with them to help disperse light throughout a garden, and why hundreds and sometimes even thousands of diodes are needed to adequately cover gardens.
Still, there are pluses to LED lamps, such as their low power draw and the fact that they radiate very little heat—both attractive characteristics for indoor growers. They also contain no hazardous materials such as mercury and have a longer lifetime than incandescent, fluorescent, metal halide (MH) and high-pressure sodium (HPS) lights. LEDs may have a place in our gardens, but they are best used as supplemental lighting along with standard high-intensity discharge (HID) bulbs. Big agricultural greenhouses in Europe, India and North America have started to incorporate lines of red LED lights throughout garden canopies to supplement overhead HID lighting and increase photon quantities. But with the coming of true plasma light, it is likely that both the LED fad and our HID bulbs may soon be a thing of the past.
So now you might be asking, “How can I mimic the quality of the sun and efficiently—at low cost—produce the best cannabis possible?” Unfortunately, there is no simple answer to this. There’s a good deal of anecdotal information all over the Internet on which wavelengths are best for cannabis and THC production, but most of this information is not proven and was likely developed under less-than-scientific conditions. Think back to the light-harvesting complex: If you’re using only red light to grow your plants, there’s a lot of light-absorbing molecules that respond to many different wavelengths just sitting around doing nothing. This decreases the efficiency of your plants’ photosynthetic system and production.
This is why, as a grower, you need a system of lights that best mimics the quality of sunlight (while also staying within your budget). Remember to keep your lamps close to the garden canopy and utilize proper ventilation for atmosphere and temperature control. Putting lights on light movers— especially when using a combination of different bulbs—is also a great idea. And supplementing HID lighting with broader-spectrum lighting will go a long way toward ensuring that your plants are happy, healthy and, most importantly, productive.
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