Part II of the series takes a look at how the new era of LED lighting technology has allowed researchers to investigate the impact of individual wavelengths of light on plants.
These findings have changed our understanding of the value of different light and inspired cultivators to experiment with manipulating different qualities of light in order to steer crop growth.
In this article, we take a look at the effects of light between 100nm and 600nm wavelengths.
Glossary of Essential Terms
Colour Rendering Index (CRI)
A measure of a light source’s ability to show object colours realistically or naturally.
Measured between 1-100, the higher the value the more a light fixture can render all frequencies of its colour spectrum when compared to a perfect reference light e.g sunlight.
Luminous Efficiency
A measure of how well a light source produces light that is visible to humans.
Photobiology
The science of how light impacts living organisms.
Photosynthesis
A light-driven chemical reaction in plants that converts carbon dioxide and water into glucose and oxygen. Glucose is a simple sugar that is used as a store of chemical energy to drive essential processes like respiration, building cell walls and making proteins.
Photoreceptors
Proteins in plants that sense changes in the spectrum, intensity and direction of light and allow plants to adjust their growth accordingly. The major groups are cryptochromes, phototropins and phytochromes.
PAR (Photosynthetic Active Radiation)
The wavelength of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis.
All wavelengths between 400 and 700nm contribute to photosynthesis and also transmit information about the plant’s surroundings.
PBAR (Photobiologically Active Radiation)
Describes the wavelengths between 280-800nm but beyond the PAR region but which also transmit critical information to plants. These occur in the UV range below 400nm and in the far-red range above 700nm and strongly affect different aspects of plant growth.
PPFD (Photosynthetic Photon Flux Density)
The amount of light (amount of photons) within the PAR range that land on a given surface per second. Measured in µMol/m2/S.
This is the most accurate measure for growers to determine the light intensity that is reaching their plant canopy.
Pigments
Chlorophyll (a and b) is the green pigment in plants that harvest light energy. Accessory pigments like carotenoids and xanthophylls also increase the useable wavelengths of light for photosynthesis.
Phenolics
A group of compounds in plants that are involved in various roles including a protective role in preventing high-light stress by acting as “sunscreens” and absorbing harmful UV radiation.
Phenolics are also involved in the taste and flavour of plants. Flavonoids are one of the largest groups of phenolics and anthocyanins are responsible for the coloured pigments in flowers and food.
Secondary Metabolites
Non-essential bioactive compounds which give plants aroma, colour, and taste and provide resistance against external biotic and abiotic stress.
Cannabis contains over 500 secondary metabolites with the most well-known comprising cannabinoids, terpenes and flavonoids.
White Light
A combination of all the colours in the spectrum and the light naturally provided by the sun.
Individual Wavelengths of Light and Plant Photoreceptors
Photoreceptors are the proteins in plants that sense changes in the spectrum, direction and intensity of light. Based on this information plants are able to adjust their growth accordingly, a process known as plasticity (Landi et al 2020).
The main groups of photoreceptors that absorb light in the PAR (400nm-700nm) range are phototropins, cryptochromes and phytochromes. UV-B resistance 8 (UVR8) photoreceptors absorb light in the PBAR range and are responsible for UV light absorption (Fig.1).
Fig.1: The most well-known photoreceptors are UV-B resistance 8 (UVR8), phototropins (PHOT), cryptochromes (CRY) and phytochromes (PHY).
Phytochromes exist in two forms, the Pr inactive form and the Pfr active form. The transformation between the two forms is most efficiently stimulated by red light however it also occurs at lower efficiencies under UV and blue. Source: Huché-Thélier et al 2016.
Manipulating the light spectrum and exposing various species of plants to specific wavelengths of light (monochromatic light) has allowed researchers to identify various distinct physiological and physical effects of different types of light.
Recently, the rise of controlled environment commercial Cannabis cultivation has resulted in more Cannabis specific studies (Babaei et al 2022).
UV Light (100–400nm)
There are three types of UV radiation, UV-A (315-400nm), UV-B (290-315nm) and UV-C (100-280mm). UVC is the most energetic of these and therefore the most dangerous for people and plants. Luckily, the most abundant form of UV radiation that reaches us from the sun is UVA and some UVB, with most UVC and more energetic UVB being largely absorbed by the atmosphere (Loconsole and Santamaria 2021).
Given the energetic demands of UV wavelengths, their inclusion in a fixture tends to reduce its overall efficacy and lifespan. However, the benefits of UVA and UVB radiation are potentially substantial and are increasingly included in newer broad-spectrum LED lights.
You can read about one future collaboration between LED industry leaders Fluence Bioengineering and Biolumic, a plant biotechnology company, here.
Safety considerations are important when working with fixtures that emit UV since regardless of the exact wavelength, all UV light can be dangerous if the intensity and duration of exposure are excessive. The use of protective gear is necessary, especially eye protection and clothing designed to minimise UV exposure.
UV Light and Plant Health
Although not within the PAR range, UV is biologically active and can regulate plant growth in a species-specific way. UV-B light (280-315nm) has been shown to increase leaf area, fresh and dry biomass, branching and even secondary metabolite production in some plants (Huche et al 2016, Namdar et al 2019). Other aspects of plant health and physiology have also been shown to be impacted by UV-B light exposure (Fig.2).
Fig.2: A review on the potential uses of UV-B light to reduce pest and disease incidence in crops highlights the various aspects of plant health and physiology that have so far been shown to be affected by UV-B irradiation. Source: Meyer et. al 2021
However, like its effects on humans, exposure to UV radiation in plants can be detrimental to their health. Depending on the intensity, duration and source of exposure, UV radiation can cause damage to DNA, chloroplasts and photosynthetic pigments (Hideg et al 2013).
Some studies have shown that UV exposure can lead to stronger plants since it stimulates leaf thickness and waxiness and can reduce insect pressure and vulnerability to pathogens (Meyer et al 2021, Loconsole et al 2021). UVC irradiation can be used to increase resistance to pathogens and has been used to reduce the incidence of powdery mildew in grapevine (Keller et al 2003 ) and cucumbers (Suthaparan et al 2014) and Botrytis cinerea in post-harvest treatments of cut roses (Vega et al 2020) and strawberry fruits and leaves (Janisiewicz et al 2016).
Facilities that use beneficial insect predators should be aware that many insects need a mix of UV, blue and green light to guide their efficient behaviour. Fluence has an interesting webinar on this topic, available here.
Although dosing plants with UV could increase their overall resistance to environmental and biotic stress, this must be balanced with the potential for delivering an excessive dose and the resulting damaging effects.
UV Light and Secondary Metabolite Production
The first study to suggest UV-B radiation affected cannabinoid accumulation in flowers of drug-type cannabis was done in the 80s (Lydon et al 1987). When radiation increased from 0-12.4KJ/m2 Δ9 THC content increased from 25-32%. This work was some of the earliest to suggest THC was a photoprotectant acting as a type of plant “sunscreen,” helping plants prevent light-induced molecular damage.
A more recent trial tested UV-B and UV-C exposure on two strains at higher dosages of 0.16-13kJ/m2 using wavelengths that peaked at 287nm. It found gross plant morphology (Fig.3a) and dry flower yield from apical flowers (Fig.3b) decreased in both tested cultivars with increasing UV exposure (Rodriguez et al 2021b). Δ9 THC and CBD also decreased in one strain whilst total terpene decreased in both. The authours concluded UV application did not result in any significant benefits to the yields or the secondary metabolite production of either strain.
Fig.3a. From left to right, the effects of minimum, low, moderate and high UV-B exposure on the morphology of two strains of Cannabis. Symptoms including lower leaf chlorosis increased with higher UV exposure. The height of both strains was reduced by between 26-31% in plants with minimum exposure compared to those with the most. Source: Rodriguez et al 2021b.
Fig.3b. From left to right, the effects of minimum, low, moderate and high UV-B on the morphology of apical flowers from two strains of Cannabis. The size of apical flowers was reduced linearly in both strains by 78% and 69% in plants with the most exposure compared to those with the minimum. Source: Rodriguez et al 2021b.
The researchers suggest some interesting reasons for the results, mainly that THC concentration in current strains is around 10x higher than those used in older studies so the capacity for UV to increase their potency beyond this saturation point could be limited. They also questioned whether increased THC production following exposure to UV radiation was an acute adaptive response that could take place during a single production cycle or if it was an adaptive response that happens in plants over successive generations of UV exposure.
Before generalised conclusions can be drawn about the impact of UV on cannabis, studies need to expose plants to wavelengths independently to prevent responses that are a result of the interactive effects of UV-A and UV-B.
Since then, more research suggests UV light affects plants distinctly depending on the wavelength they are exposed to.
One trial reported UV-A increased cannabinoid levels other than Δ9 THC (Maganini et al 2018). Another trial by leading lighting manufacturer Valoya found intense UVA exposure caused increased CBD and terpenes by 7,7% and 10% respectively when compared to a generic white LED without UV. You can watch that webinar (Latest Developments In Research On LED for Cannabis 2021) here.
Blue Light (400-500nm) – Compaction and Colouration
Blue light is the dominant wavelength of light when the sun is at its highest point in the sky and during the summer months. Blue wavelengths are absorbed very efficiently by chlorophyll making it very beneficial for plant growth and yields.
Although both red and blue light regulate stomatal responses, the opening of stomata is more rapid and strong under blue light (Babaei et al 2022). Therefore, optimal photosynthesis requires at least some level of blue light.
Blue Light and Compaction
Blue light is associated with decreased stretching behaviour in plants (dwarfing) and can be thought of as the counter to the extension in shoots and stem behaviour that is stimulated by red light (Fig.4).
One experiment in cannabis compared the growth of plants under a full-spectrum white light and an LED light with a balanced ratio of blue-red radiation (1:1). It confirmed the effects of blue light on compaction as plants grown under white lights were 23% taller and had a 20% greater leaf area with no differences in the number or length of lateral branches (Lalge et al 2017).
In trials comparing the effect of several LED lights on the growth of hemp, the ratio of blue-red light differentially affected results (Wei et al 2021). Overall, LED lights have been shown to increase the dry weight of stems and branches compared to high-intensity discharge (HID) lighting due to their ability to increase the growth of side branches.
Blue light's ability to cause compaction and reduce spindling has also been shown in wheat (Dong et al 2014) whilst adding 25% or more blue light to a red spectrum was also shown to reduce the height, leaf area and fresh weight of tomatoes, salvias and petunias (Wallaeger and Runkle 2014).
Fig.4: In experiments with Salvia blue light suppressed extension growth. Generally, blue light has a dwarfing effect and results in plants that are shorter and smaller with thicker and darker green leaves compared to those grown without blue light. Source: Runkle et al 2017.
Compact plants are better suited to support heavy apical flowers and are easier to maintain and inspect than more stretched and spindly plants. Growers favour this spectrum of light during the vegetative stage to increase the lateral growth of plants to make them bushier. They can also use lights with a higher proportion of blue light to keep mother plants compact when space is at a premium.
Blue Light and Secondary Metabolite Production
Blue light can be used to increase the medical value of cannabis by increasing cannabinoids and terpenes. In one of the first published trials comparing the effects of HPS lighting with two LED lights, scientists found plants grown under HPS lights had up to 26-38% less THC and 29-35% less CBD than those grown under LEDs (Magagnini et al 2018).
Of the two LED lights the fixture containing UVA light and a greater proportion of blue light gave the best overall results as it also increased CBG accumulations in the flowers the most. This was significant since CBG is the precursor of the acid form of other cannabinoids (Magagnini et al 2018).
Aside from confirming blue light encouraged horizontal branching and reduced flower mass, another study also showed that flowering under blue-enriched LEDs led to a greater accumulation of CBGA (Namdar et al 2018). Although plants flowered under HPS lights had a greater mass of dried flower (+40%) they produced the lowest amounts of secondary metabolites on a per g basis (-66%) compared to LED-grown plants.
This highlights again the importance of balancing the ratio of blue: red light to increase secondary metabolite production without compromising the negative effects on final yields.
Blue Light and Pigment Synthesis
Trials on the impact of the light spectrum in lettuce have also confirmed the role of shorter wavelengths of blue light in pigment synthesis and enhancing colouration. Lettuce plants treated with blue light had greater chlorophyll and b-carotene (Kong et al 2021). This suggests the levels of other anthocyanins rise with decreasing leaf area and vice versa. Although blue light can enhance certain plant quality factors it is important to note these often happen at the expense of yields.
In another trial, researchers compared the effects of three different LED spectrum variants, blue, green and red on the growth of hemp seedlings compared to sunlight. They found polyphenols and flavonoids were increased in blue light LEDs (Livadario et al 2018). Blue light is therefore useful for enhancing crop quality traits linked to visual aesthetics and the nutritional profile of some crops.
Some growers harness the benefits of blue light for enhancing crop quality traits related to colouration by adding blue light in the last week of flower once the risk of affecting crop yields is absent.
Green Light (500-600nm) The Under-Valued Wavelength
Looking at the PAR curve it would be easy to conclude that green wavelengths represented the least efficient wavelengths of light since they are the most poorly absorbed by chlorophyll a and chlorophyll b (Fig.5).
Fig.5: Green light has some of the highest luminous efficiency of any wavelength meaning it is the easiest to perceive for humans. Source: Runkle 2017b.
However, an overreliance on McCree’s PAR curve has been used to overinterpret the efficiencies of some wavelengths of light for driving photosynthesis and has caused the role of green light in photosynthesis to be undervalued.
McCree’s experiments actually found green light absorption was only 15% less than the average of the other PAR wavelengths and that plants utilised it at only 9% less than the average use of other wavelengths (McCree 1971).
Despite this green light remains an undervalued part of the light spectrum with many people in the dark about its fundamental role in plant growth.
Green Light and Canopy Penetration
One of the first breakthroughs in the understanding of the value of green light found it had an important role in driving photosynthesis in leaves beneath the plant canopy (Fig.6). In studies with spinach, scientists found green light penetrated the leaf surface deeper than red or blue wavelengths (Sun et al 1998).
Due to its ability to penetrate deeper within the leaf surface, in experiments with sunflowers under strong white light, green light stimulated photosynthesis more than additional blue or red light (Terashima et al 2009). This confirmed earlier work that found adding 24% of green light to red-blue LED fixtures increased lettuce growth significantly under the same light intensity (Kim et al 2004).
However, other work cautions against attributing the same properties to all wavelengths of green light. Wavelengths between 580-600 or "yellower" wavelengths actually decreased lettuce yields possibly by suppressing chlorophyll and chloroplast formation (Doughter and Bugbee 2001).
Not all studies report the exact wavelengths of green light tested making the generalisation of data across plant species less reliable.
Fig.6: Blue light is absorbed by various pigments in the top layers of leaves so penetrates the least of the three wavelengths compared. Red light is mainly absorbed by chlorophyll deeper within the leaf. Greenlight can reach the deepest due to its low absorbance by chlorophyll and its scattering within the leaf. Source: Smith et al 2017
Depending on the plant species a minority of green photons are reflected and most are actually absorbed with an additional 5-10% transmitted through the leaves onto lower leaves. By penetrating more deeply beneath the leaf surface than blue and red wavelengths and driving greater rates of photosynthesis in the lower canopy green light can prevent the senescence of lower leaves and conserve overall plant health (Runkle 2017b).
Green Light and Plant Health
Green light’s role in maintaining healthy plants has also been linked to its ability to stimulate antioxidant activity. This has been shown in sprouting hemp, lentil and wheat seeds (Livadario et 2018, Samuolienė et al 2011).
A trial comparing the growth of sub-canopy lighting with blue-red (BR) and blue-green-red (BGR) spectrums found including green light had a positive effect on terpene accumulation (Hawley et al 2018).
Although both spectrums increased the content of THC and CBGA levels the BGR lighting had a higher impact on terpene accumulation than BR lighting. This result was the same on the upper and lower canopies with limonene, linalool and myrcene levels all rising (Hawley et al 2018).
Green Light and Plant Inspection
White light is a mixture of blue, green and red light. A huge benefit of adding green light to a spectrum is its role in facilitating plant inspection since this green light has the highest luminous efficiency meaning it is the easiest for humans to perceive.
Anyone who has worked under blue-red LEDs will know the difficulty in scouting crops under these lights and the challenges with identifying nutrient deficiencies and spotting pests and pathogens.
A lack of green light can cause eye strain making it more difficult for staff to work for long periods of time. Although red-green LEDs are an economic choice of fixture often used in greenhouses to supplement the sun's natural white light, they do present these challenges which should be carefully considered when sourcing equipment.
Summary
A number of exciting discoveries have been made regarding the impact of UV, blue and green wavelengths of light on plans. This knowledge base has great potential for the development of new lighting strategies designed to maximise specific crop qualities and traits.
Species-specific studies remain important however as the behaviour of plants to different wavelengths is not always strictly conserved between crops.
Another challenge is the need to manipulate narrower wavelengths of light in order to identify if changes in plant behaviour are altered depending on the specific wavelengths of a particular light. For example, the evidence showing that not all green wavelengths trigger the same responses in plants has set the precedent for the value of experiments using narrower wavelengths of a particular light colour.
Keep your eyes peeled for Part III of our lighting series where we take a look at the impacts of red and far-red light on Cannabis and other plants.
If you'd like more information on lighting strategies for your project including target environmental conditions for each of the growing phases or advice on lighting equipment or R&D trials designed to optimise your current practices, get in touch with us today.
References
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ISO 690
Livadariu, O., Raiciu, D.A.N.I.E.L.A., Maximilian, C.A.R.M.E.N. and Capitanu, E., 2019. Studies regarding treatments of LED-s emitted light on sprouting hemp (Cannabis sativa L.). Rom Biotechnol Lett, 24(3), pp.485-490.
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