In the first part of our Horticultural Lighting Guide, we looked at the basics of lighting for plants, we learned what a photon is, what PAR, PPF & PPFD are, and how PAR maps help us determine the number and layout of the lighting fixtures needed for the best illumination.
In the part two, we are building on top of the basics from part one and we are going to be talking about the importance of the color spectrum and even the specific effects of individual colors on the plants.
Different colors have different effects on plants
We are going to start with the McCree Curve. Plants see light differently from the humans, they are more sensitive to some colors than others. The McCree curve shows the photosynthetic efficiency of different wavelengths of light. As you can see, blue and red are the most photosynthetically efficient, but other colors such as green and yellow are not at zero, so they are also useful.
This was a result of the research performed by Dr. Keith J. McCree at the Texas A&M University in the 1970s and the curve as well as the PAR range have been the gold standard up until very recently. PBAR, mentioned in part 1 extends the PAR coverage to 280nm on the low end and 800nm on the upper end of the spectrum as now we know that plants can also use these wavelengths, although to a lesser extent.
Now that we know that plants can use the entire visible spectrum of colors and even the invisible ones such as UV or Far Red, let’s break them down individually and see what effects they have on the plants. We will start with the most photosynthetically active ones – blue and red.
Blue light is one of the most important colors for photosynthesis. For high-quality vegetative growth, blue light is essential. Blue light reduces plants stretch and it promotes tight internodal spacing and strong plant stems. It also helps produce more attractive vegetation because blue light promotes anthocyanin production, compounds that contribute to leaf coloration. Cryptochromes in the plant are sensitive to blue light and they affect plant morphology because they tell the plant which way to grow up towards the light. In addition to that, cryptochromes help maintain the circadian rhythms of the plant. Blue light also regulates the opening of the leaf stomata, which are tiny pores found on leaves necessary for both controlling water loss through transpiration, and carbon dioxide uptake.
Red light is the most efficient color for photosynthesis. Red light is at the peak of the McCree curve, and both Chlorophyll a and b have absorbance peaks in the red part of the light spectrum. Phytochromes in plants are sensitive to light in the red and far-red regions of the spectrum, and are used by the plant for a variety of tasks. Plants use them to determine the length of the day in photoperiodic plants, as temperature sensors for the plant, and for photomorphogenesis in seed germination. Long wavelength light such as red penetrate soil best, and are used by the seedling to know which way is “up,” and thus what direction to grow to break out of the soil.
Some will tell you that plants don’t use green light because plants are green and that means they reflect all of the green light. In fact only a small percentage of green light is reflected back at your eyes. Looking at the McCree curve we can see that green light, though not as photosynthetically efficient as blue or red, is still fairly efficient for photosynthesis. Green light penetrates the plant canopy better than any other color, which helps mitigate senescence of the lower leaves. Green light also penetrates further into the leaf mesophyll, exciting chloroplasts which blue and red light cannot reach, further enhancing photosynthesis. Two other mechanisms, described in greater detail in this research paper by Terashima et al., named the detour effect and the sieve effect, contribute to the usefulness of green light for photosynthesis. Lastly, as the same paper illustrates, the quantum yield of green light is higher than blue light, because some fraction of blue light is absorbed by flavonoids and/or carotenoids and the energy transfer to reaction centers is much less than 100%. For example, lutein transfers its energy to chlorophyll with an efficiency of just 70%, and neoxanthin has an energy transfer efficiency around 9%.
Ultra-Violet (UV) Light
UV light is defined as the part of the electromagnetic spectrum beginning at about 400nm, down to 10nm. The atmosphere of the Earth filters out UV light with a wavelength smaller than 280nm, which is the cusp between UVB and UVC, so for the purposes of this article, we will only be dealing with UVA and UVB.
UVA is defined as UV radiation with a wavelength between 315-400nm. However, UVA LEDs are typically only offered between 365nm to 405nm. Regular optical materials such as glass and acrylic will not transmit UV radiation with a wavelength smaller than 360nm efficiently. Chlorophyll A has an absorption peak around 415nm, and so 405nm UVA LEDs are speculated to be useful for photosynthesis, with some manufacturers marketing them as part of a spectrum intended for vegetative growth. More research is needed on all the benefits UVA can bring to horticulture.
UVB, particularly UVB centered around 285nm is receiving a lot of attention these days. In particular, the work of Dr. Peter Barber has demonstrated numerous positive benefits associated with UVB light treatments. Plants have a photoreceptor called UVR8 which is most sensitive to UVB light, centered at 285nm. UVR8 is responsible for triggering the stress response in plants. In a way similar to exercise for humans, where the stress of working out without over-exerting ourselves actually helps us get stronger, the stress response in plants can be manipulated for positive outcomes if done carefully. Dr. Barber’s research has demonstrated that UVB light can be used to treat food crops in cold storage, extending their shelf life. He has also shown that low doses of UVB can increase THC levels in cannabis, along with other secondary metabolites such as terpenes.
UVB is difficult and expensive to work with, requiring special glass to transmit it efficiently, and special care taken to avoid damage to people and plants near them. Consideration of the benefits and challenges of UVB are necessary for anyone considering experimenting with UVB for horticulture.
Far Red Light
There are a number of claims regarding the effect of far red light, but it should be noted that most of it is unverified by experimental science. LEDs are now enabling new experiments on far red, so expect more science literature involving far red in the future. Here is what we know for sure about far red.
Far red is generally defined as the part of the electromagnetic spectrum with a wavelength between 700 to 780 nm. It appears dim to the human eye as it mostly lies outside the visible spectrum. Far red LEDs typically shine within a narrow-band centered on 720-730 nm.
Photoreceptors in plants called phytochromes are sensitive to red and far-red light, and are used by plants to evaluate the environment in which they are growing. In full sunlight, the ratio of red light is much higher than far red. Plants typically respond well to this type of growing environment. In the shade, the opposite is true. There is a much higher ratio of far red to red in the shade. This initiates what is known as the shade avoidance response in plants. The plants will focus all of their energy on growing tall, neglecting vegetative growth, in order to try and get above the canopy and into the full sun. This type of tall, spindly plant is typically undesirable.
The ratio of red to far red is also used by the photoperiodic plants to determine what time of the year it is, and whether or not to begin flowering. As the sun sets and the day turns to night, the ratio of far red to red goes up. Photoperiodic plants use this information to gauge the length of the day. Short-day and long-day plants will only flower once the plant determines the length of day is appropriate.
There is some speculation that the combination of red and far red LEDs in the right ratio can initiate the Emerson effect, which enhances the efficiency of photosynthesis. Such an effect would be highly desirable, but experimental science confirming this is still lacking.
LEDs are a fairly new technology in horticulture, and they allow plant scientists to study the effects of light on plants that has not been possible before. Previously when using bulb technology, in order to change the spectrum you would have to use special filters that would modify the color of the light source with very low efficacy, making research on light intensive crops cost prohibitive. GVA Lighting Inc., the parent company of Linnaeus Lighting, is well known for its world-class color-changing LED fixtures. We have leveraged this experience to design a new research-grade horticultural lighting fixture, which is more of a scientific instrument than a grow light. The result of that effort is HEDERA APEX, a 6-channel spectrum-adjustable fixture designed for research applications. It’s all in our slogan “growing science” which has a double meaning – while we apply the most recent scientific research into our products, we also want to grow and contribute to the research itself. Now, plant scientists can design their own mix of 6 LED channels and experiment with almost infinite range of light recipes, to study their effects on whatever plant species is of interest to them.
In the images below, you can see the spectrums of the typical lighting sources including the High Pressure Sodium bulbs. Staying with the theme, we are going to be digging deeper into HPS lighting vs LED lighting in the next part of the Horticultural Lighting Guide so please make sure you are subscribed to our social media channels so that you don’t miss that.
HPS vs LED is a very recent topic especially in the cannabis growing community and we will be talking about advantages and disadvantages of both as well as upfront cost, return-on-investment, hybrid (LED & HPS setups) and when it is simply better to used HPS in the next part of the Horticultural Lighting Guide.