If you’re looking to optimize your indoor cannabis cultivation practices, then understanding the role of lighting is key. As light is the primary source of energy for plant photosynthesis, it plays a vital role in the growth, development, and overall health of cannabis plants. By understanding the importance of lighting and its impact on plant morphology (shape), flowering, and cannabinoid production, growers can optimize their cultivation practices to achieve maximum yield and quality. 

In this guide, you will learn the key aspects of lighting in indoor cannabis cultivation, including light intensity, light spectrum, photoperiod, and how to create an ideal lighting environment for successful and thriving cannabis plants.

Role of light in cannabis plant growth and development

Light plays a key role in the growth and development of cannabis plants. It is an essential energy source for photosynthesis, influences plant morphology (shape) and structure, and helps regulate various physiological processes. 

Here are the key concepts and roles of light in cannabis cultivation:

Photosynthesis

Let’s revise how “plants make their own food”, also known as “photosynthesis”. Photosynthesis is the process by which plants convert light energy into chemical energy to fuel their growth and development. Cannabis plants, like other green plants, rely on photosynthesis to produce the sugars and carbohydrates they need for survival. 

In a few words, light energy is captured by the pigments in the leaves, primarily chlorophyll, and is used to convert carbon dioxide (CO2) from the air and water (H2O) into glucose and carbohydrates + oxygen (O2) through photosynthesis. Amazing, right? 

So Carbon (C) is captured from the CO2 in the air and converted into glucose (sugars), which is the primary source of energy for plant growth, while oxygen (O2) is released as a byproduct. This happens inside the leaves and the plants feed with those sugars.

What is PAR/PPFD in Cannabis lighting

It’s important to understand that the measurements in watts for LED light fixtures don’t give us information on light intensity as it happens with HPS light fixtures. For every type of light fixture and even sunlight, we measure light intensity with PAR/PPF/PPFD.

Later in this guide, I explain these concepts in more detail, but here’s a quick summary for starters:

PAR (Photosynthetically active radiation) is the spectral range of light from 400 to 700 nanometers that plants can use in photosynthesis. This includes all the visible colors in the rainbow and others that we cannot see. There’s a new definition called ePAR, which includes far-red light and it goes from 400-750 nm. We will see it in a minute.

PPFD (Photosynthetic Photon Flux Density) measures how many photons (or how much light) from a light source reach a certain area at a given time, or how much light arrives at the plant in this case.

In other words, PPFD measures the amount of PAR light your plant canopy receives per second and square meter.

Units: PPFD is measured in micromoles per square meter per second (μmol/m2.s).

Later in this guide, I explain how much PPFD plants need for each stage.

Stretching and plant height

Light intensity and spectrum influence the elongation of cannabis plants. When plants are exposed to far-red light and low light intensity they can stretch too much, causing plants to grow taller with increased internode spacing. Controlling light intensity and spectrum can help manage plant height and minimize excessive stretching. These concepts are also explained later in this guide.

Leaf temperature and transpiration

Light affects leaf temperature and the transpiration process. Intense light can increase leaf temperature, which influences the transpiration rate, and the release of water vapor through leaf stomata. The stomata are microscopic pores or gas valves on the surfaces of leaves and stems that allow plants to take in carbon dioxide (CO2) and release oxygen (O2) during photosynthesis and regulate the release of water vapor through transpiration. Too much transpiration can negatively affect plants and no transpiration means no water and nutrient uptake.

Proper light management helps keep appropriate leaf temperature and regulates transpiration, ensuring optimal water uptake and nutrient absorption.

Terpene and cannabinoid production

Light intensity can impact the production of cannabinoids and terpenes, the chemical compounds responsible for the unique aroma, flavor, and therapeutic properties of cannabis. Certain light spectrum wavelengths and intensities may influence the expression of specific terpenes and cannabinoids, allowing growers to manipulate the plant’s chemical profile, but recent studies showed that they have a minor role compared with light intensity.

By providing the appropriate light conditions, cannabis growers can optimize plant growth, maximize yield, and enhance the quality of the final product.

Determining light intensity and coverage

Understanding light intensity measurements (lux, foot-candles, PPFD, DLI)

Light intensity for cannabis cultivation is often measured in lux, foot-candles, and Photosynthetic Photon Flux Density (PPFD). As said before, watts don’t give us the necessary information about light intensity.

Lux and foot-candles measure visible light, with lux being lumens per square meter (lm/m2) and foot-candles being lumens per square foot, but they are not useful measurements for plant growth, so we rather not use them to avoid confusion. You can use a luxometer if you can’t get a PAR meter as they are cheaper and easier to find. There are some calculators to convert lux into PPFD taking into account the type of light you are using, but they are not the most accurate.

On the other hand, PPFD, measured in micromoles per square meter per second (μmol/m²/s), is the most relevant for growers as it quantifies the light plants can use for photosynthesis. Understanding these measurements helps growers ensure their plants receive the right amount and quality of light for optimal growth.

The Daily Light Integral (DLI) measures the total amount of photosynthetically active radiation (PAR) that a plant receives over a 24-hour period. DLI is expressed in moles of light (mol) per square meter (m²) per day (d), or mol/m²/day. Both the intensity of the light (measured in µmol/m²/s) and the duration of exposure (hours per day) determine the DLI.

How to calculate optimal light intensity for cannabis plants

The units of measurement for usable light for plants to do photosynthesis (PAR) are PPF and PPFD. To answer the question of “How much light do your plants receive and use”, we talk about PAR and PPFD.

Photosynthetic Photon Flux Density (PPFD) measures the number of photons (how much light) from a light source at a given time that reaches a specific area. 

To calculate the optimal light intensity for cannabis plants, growers need to focus on PPFD.

A PPFD of 90 μmol/m²/s is only slightly above the light compensation point, which means is too low for cannabis growing. Seedlings and cuttings thrive at 100-300 μmol/m²/s, while vegetative stages require 400-600 μmol/m²/s, and flowering stages benefit from 700-900 μmol/m²/s. For the flowering stage, we recommend a maximum PPFD of 1000 μmol/m2.s (if you are not supplementing CO2).

Cannabis has an increasing rate of photosynthesis up to a PPFD of 1500 μmol/m2.s when CO2 is supplemented. In commercial facilities and big grow spaces, the supplementation of CO2 is needed to achieve these values.

For most home growers’ setups, these are the recommended values (without CO2 added):

Now with these values, let’s calculate the DLI (Daily Light Integral).

An increase in DLI means an increase in flower yield so it’s a really important measurement for cannabis growing.

For example, let’s say we give our plants 1000 μmol/m2.S for 12 hours a day. 1 hour has 3600 seconds. So, here’s the math:

1000 μmol/m2.S x 3600 s/h = 3.6 x 10⁶ (to convert μmol into mol)

= 3.6 mol/m2.h 

x

Cannabis flowering stage 12 h /day =

43.2 mol/m2.day – DLI 

So for 1000 μmol/m2.s x 12 hours, you get 43.2 mol/m²/day which is optimum for the flowering stage.

Plants can receive 60 mol/m2.day in the right conditions and still increase their yield.

Using a PAR meter, PPFD meter, or quantum sensor to measure PPFD at the canopy level ensures accurate readings. If you don’t have a PAR meter, there’s an app called PPFD Meter app that is quite accurate at measuring PPFD when it’s properly calibrated. 

Adjusting your light fixture height and intensity helps maintain these PPFD levels, promoting healthy growth and maximizing yield. Balancing light intensity prevents issues like light burn or insufficient light, ensuring plants receive the optimal energy for each growth stage.

Signs that your light fixture is not at the right distance from the canopy

Here are some signs or symptoms that plants show when the light distance or intensity is not right for their growth stage.

If plants show elongated stems and spaced internodes, it means the light intensity received is not enough. When this is the case, you need to adjust the distance from the canopy by placing your light closer to the plants or adjusting the light intensity with the dimmer if possible. 

On the other hand, when plants show leaves with borders pointing up with the shape of a “canoe” or “taco”, top leaves pointing straight up like “praying” or top leaves with a very light green or even whitish color, it means the light is too intense or the fixture is too close to the plants and needs to be moved up or the light intensity lowered with the dimmer. 

A light too close to the plant also can cause stress by high temperatures, and in some cases, there’s anecdotal evidence of hermaphroditism in the sites that were burned by being in contact with the light fixture so be extra careful! 

Buds growing too close to the light fixture can become stressed by high temperatures and develop “foxtail” flowers.

As the plants grow towards the light fixture, you need to adjust the distance and/or intensity to keep the PPFD levels according to the plant stage needs.

How to achieve proper light coverage in the grow space

Achieving proper light coverage in a grow space involves ensuring uniform distribution of light across the entire canopy. This can be accomplished by strategically positioning multiple light sources to avoid hotspots and shadows (especially in big tents or grow rooms) or by using light fixtures with good light distribution.

Reflective materials like mylar or white paint on walls and floors can enhance light spread, maximizing efficiency. Using adjustable light fixtures allows fine-tuning of light angles and height, ensuring all plants receive consistent illumination. Proper coverage ensures each plant gets adequate light, promoting even growth and preventing some plants from outcompeting others for light resources.

Most modern LED light fixtures come with clear indications about light coverage area and some of them include recommendations about hanging distance. They look like these.

For HPS (High-Pressure Sodium) fixtures, there are some general recommendations as they are all similar in light intensity and coverage.

Lights

150W – covers 2′ x 2′ (0.6m x 0.6m) area

250W – covers 2′ x 2′ (0.6m x 0.6m) area up to 2.5′ x 2.5′ (0.8m x 0.8m)

400W – covers 3′ x 3′ (0.9m x 0.9m) area up to 3.5′ x 3.5′ (1m x 1m)

600W – covers 3.5′ x 3.5′ (1m x 1m) area up to 4′ x 4′ (1.2m x 1.2m)

1000W – covers 4′ x 4′ (1.2m x 1.2m) area  up to 5′ x 5′ (1.5m x 1.5m)

Remember, these are general guidelines. It is recommended to follow the manufacturer’s instructions as each fixture is different but also watch how plants react to light and if they show some of the previously mentioned symptoms.

Understanding Light Spectrum

Overview of the electromagnetic spectrum

The electromagnetic spectrum includes a wide range of electromagnetic radiation, including visible light, which is crucial for cannabis plant cultivation. Visible light ranges from approximately 400 to 700 nanometers (nm) and is divided into different colors: violet, blue, green, yellow, orange, and red. Each color corresponds to a specific wavelength and energy Understanding the electromagnetic spectrum allows growers to harness the power of different light wavelengths to optimize plant growth, development, and overall productivity.

The role of different light wavelengths for cannabis plants

Light is composed of various wavelengths, each corresponding to a specific color. Cannabis plants use different light wavelengths for various purposes in their growth cycle.

Blue Light (400-500 nm):

Blue light is essential during the whole cycle and especially during the vegetative stage of cannabis growth. It is absorbed by chlorophyll and plays a crucial role in the regulation of phototropism (plant movement towards light) and the development of strong, healthy leaves. Blue light promotes compact, bushy growth as it inhibits cell expansion, so it is important for controlling internode spacing.

The enhanced Blue Spectrum (in the white light) promotes and accelerates vegetative growth. Blue photons of light control elongation and leaf expansion. The right fraction of blue light keeps plants compact and avoids unwanted elongation. Blue light also helps grow more tight buds as opposed to spaced-out flowers. 

Red Light (600-700 nm):

Red light is crucial during the whole plant cycle and especially during the flowering stage of cannabis growth. It is absorbed by chlorophyll and triggers the production of phytochrome, a photoreceptor that helps regulate the plant’s flowering response. 

Red light promotes bud formation, flowering, and overall reproductive growth. It also plays a role in influencing plant height and stretching.

The enhanced Red spectrum boosts crop yield and quality as it promotes efficient photosynthesis. Red LEDs are very efficient when measured on a photon flux so choosing a LED fixture with white LEDs that includes some red LEDs is a wise decision. 

A very high red fraction can cause photobleaching in cannabis buds, also known as white top buds, but recent studies showed that it does not affect yield or quality. 

Green Light (500-600 nm):

Green light is not as efficiently absorbed by chlorophyll and is reflected back, giving plants their green appearance. Until recently, it was thought that green light didn’t contribute to photosynthesis, but now we know this is not true. Chlorophyll does not absorb green photons as efficiently as others but they penetrate deep into the leaf, in a way that the others do not. Its role is relatively minimal compared to blue and red light but it contributes in some way.

Green light can penetrate the leaf surface and go deeper into the canopy, reaching lower leaves and promoting overall plant health and development. This is one of the reasons why white light LEDs (that has all the colors) are better than red+blue LEDs light for cannabis growth.
It’s a myth that green light can’t cause light pollution during the dark hours. It can, but only if used in excess.

A key role of green light in cannabis cultivation is facilitating human vision so any minor deficiency, pest or fungal infection can be detected earlier. When you grow with white light (that includes green photons) is easier to spot those than when you grow with purple light or with “yellow” light from HPS bulbs.

Full-Spectrum Light:

Full-spectrum light (white LEDs) includes a range of wavelengths across the visible light spectrum, that includes blue, green, and red light. Full-spectrum light closely mimics natural sunlight and provides a balanced and comprehensive lighting environment for cannabis plants during their entire growth cycle. It offers the advantages of both vegetative and flowering growth stages and supports overall plant health, growth, and yield.

Humans see “Full spectrum light” as white light, so don’t confuse this term with the older “blurple” fixtures or “Red + blue” (purple light) fixtures which are sometimes also called “full spectrum” for marketing purposes. Of course, you can grow perfectly healthy cannabis plants with these red + blue fixtures, but they are not the most efficient ones nowadays.

So which is the most efficient fixture nowadays?

The best light fixtures are modern LED grow lights that have a red + white LED combination, which makes the most efficient LED panels for cannabis growing. They can also include far-red light for better results. Here’s why:

Infrared light and Far-red Light (700-750 nm):

Infrared and far-red light, with wavelengths between 700-750 nm, play a key role in the growth and development of cannabis plants as they enhance cell expansion. These wavelengths are just beyond the visible spectrum (for humans) and significantly influence the plant’s process that influences its form and structure (photomorphogenesis). Far-red photons can alter plant morphology by increasing stem elongation and leaf expansion, which typically increases radiation capture and yield.

Recent studies by the scientists Dr. Shuyang Zhen and Dr. Bruce Bugbee have shown that far red photons cause photosynthesis. This raised a new definition for PAR light, the new definition is called ePAR and includes the photons in far-red light, within the range of 400-750 nm. The acronym ePAR stands for Extended Range Photosynthetic Reactive Radiation.

Those studies show that ePAR is a better indicator of photosynthesis. The renowned instrument company Apogee Instruments is making ePAR sensors now to replace the old PAR sensors.

Why is it important? Infrared light can penetrate deeper into the plant canopy, as it passes through the leaves, promoting more uniform growth and aiding in the development of lower branches. In nature, plants interpret far-red light as a signal that upper leaves or bigger plants are casting a shadow over lower leaves, so the plant reacts by making their branches longer and elongating their stems to reach the light that is up.

Far-red light impacts the phytochrome system in plants, which regulates some developmental processes such as flowering, stem elongation, and leaf expansion. 

Using the right balance of these light spectrums can enhance photosynthesis efficiency and optimize growth conditions, leading to healthier plants and potentially higher yields. Incorporating infrared and far-red light in your cannabis grow setup can thus be a game-changer for maximizing plant performance.

Far-red photons from sunlight in a greenhouse also help plants with photosynthesis. In indoor cultivation, far red lights can be used as they penetrate the canopy and improve photosynthesis.

UV (UVA UVB UVC) Light: 

UV photons are known to induce photoprotective compounds, some of them could be cannabinoids. Many studies are being carried out nowadays to learn how UV light, comprising UVA, UVB, and UVC, can impact cannabis growth and potency. This is still a controversial field of study so I will list the claims made by light fixture companies (and some growers with anecdotal evidence) and also the most recent scientific studies on the field.

It is said that UVA (320-400 nm) enhances the production of resin and terpenes, which are crucial for the plant’s aroma and flavor profile. 

UVB (280-320 nm) is supposed to stimulate the production of THC, the primary psychoactive compound in cannabis, thereby potentially increasing the plant’s potency. 

UVC (100-280 nm), although less commonly used due to its potential to damage plant cells, can serve as a sterilizing agent to reduce the presence of harmful pathogens. In recent studies, UVC light was successfully used to control botrytis and powdery mildew.

UV light exposure is experimentally used to achieve more robust plants with higher resin and cannabinoid content. However, striking a balance is important, as excessive UV exposure can stress plants and inhibit growth. For now, recent studies haven’t confirmed these claims and only show a slight difference from the control plants regarding potency.

One of the cited studies concluded that “Long-term exposure of various intensities of relatively short-wavelength UV radiation had generally negative impacts on cannabis growth, yield, and inflorescence quality. UV radiation provoked substantially reduced yield in one cultivar, reduced inflorescence quality in both cultivars, and had no commercially relevant benefits to inflorescence secondary metabolite composition.” (Rodríguez Morrison)

There’s a publicly known (but rather old) paper by J. Lydon about UV light that many people use as a claim, but if you read it carefully, you will learn that the differences in cannabinoid production were almost insignificant and not enough evidence for such claims. “ (…) the authors to conclude that the effect of UV-B on cannabinoid synthesis is “equivocal”, which means uncertain”.* You can find more information on the citations below.

This doesn’t mean it cannot happen, but scientific evidence still cannot confirm these claims and most other evidence is anecdotal. You can find those studies at the end of this guide so you can read and interpret them.

As new studies and reliable information come up, I will update this guide with it.

Summary of the main spectral effects

• Blue photons: inhibit cell expansion
• Red photons: efficient photosynthesis
• Green photons: facilitate human vision
• Far-red photons: enhance cell expansion
• UV photons: induce photo-protective compounds/pigments (some could be cannabinoids)

How important is spectral distribution for increasing yield

Recent studies suggest “that fixture efficacy and the initial cost of the fixture are more important for return on investment than spectral distribution at high photon flux”. Also, “Red LEDs have a higher efficacy than blue (and by proxy white) LEDs because red photons have less energy than green and blue” (Westmoreland-Kusuma-Bugbee, 2021). This indicates that LED fixture manufacturers and growers should consider white+red fixtures that have a high portion of red.

So if you have to choose between a high photon flux fixture with high PPFD or a fixture with a more interesting spectral distribution, you may find that high PPFD is more important to achieve higher yields and there are no significant changes in potency (CBD and THC concentrations in flower). Also, in matters of spectral distribution, white+red fixtures that have a high portion of red are the best choice nowadays.

Another study suggests that “Increasing LI (Light Intensity) also increased harvest index and the size and density of the apical inflorescence; both markers for increasing quality. However, there were no and minor LI treatment effects on potency of cannabinoids and terpenes, respectively. This means that growers may be able to vastly increase yields by increasing LI but maintain a relatively consistent secondary metabolite profile in their marketable products.” (Rodríguez Morrison, 2021)

Lighting schedule and photoperiod

Overview of the vegetative and flowering photoperiods

Cannabis plants need distinct photoperiods for vegetative and flowering stages. During the vegetative stage, a longer light period of 18-24 hours daily promotes robust growth and development. This extended light exposure allows the plant to focus on producing leaves and stems. In contrast, the flowering stage commonly demands a shift to a 12 hours light and 12 hours dark cycle.

This change mimics natural seasonal shifts, signaling the plant to begin flowering and producing buds. Managing these photoperiods accurately is crucial for the plant’s transition between growth stages and maximizing yield.

Read this guide to learn more about switching your photoperiod into the flowering stage.

Recommended lighting schedules for each growth stage

For the vegetative stage, a common lighting schedule is 18 hours of light and 6 hours of darkness, which supports vigorous growth. Some growers opt for a 24-hour light cycle to maximize vegetative growth, but this can negatively affect some cultivars depending on the growing conditions. 

Once the plant reaches the desired size, transitioning to the flowering stage involves switching to a 12/12 light/dark cycle. This schedule is crucial for inducing flowering and ensuring bud development. Maintaining consistent lighting schedules helps plants thrive and progress through their growth stages efficiently.

Some Cannabis cultivars or “strains” can flower with 13 or even 14 and 14.30 hs of sunlight but 12/12 is the most commonly used photoperiod. If you are growing outdoors or you want to experiment with longer photoperiods, you will find that some cultivars respond by flowering and others don’t. If you already know the flowering time of your cultivar, you can manage the germination time with the hours of sunlight for your area in a given time and thus maximize the plant’s growth.

The importance of darkness and light-tight environments during the flowering stage

During the flowering stage, uninterrupted darkness is vital for cannabis plants. Light leaks or “light pollution” during the dark period can confuse the plant’s natural hormonal signals, potentially causing stress, re-vegetation, or in some cases hermaphroditism (anecdotal evidence).

Ensuring a light-tight environment prevents these issues, allowing the plant to focus on bud production. Properly managing the dark cycle helps maintain the plant’s health and maximizes resin and cannabinoid production. Growers must ensure that the grow space remains completely dark during the designated dark periods to achieve optimal flowering results.

Moonlight is not enough light to cause problems, even under a full moon. Plants are adapted to grow under moonlight so no problem with that! The sensitivity threshold PPFD for light pollution for Cannabis is around 0.006 μmol/m2.s and full moon is around 0.002 μmol/m2.s so don’t worry if your plants are outside.

On the other hand, street lights can negatively affect photoperiod and short streaks of light in a grow tent can cause this negative effect as well. Try keeping the dark hours as dark as possible without any interruptions.

For more information on these and other matters regarding lighting, look for Dr. Bruce Bugbee’s YouTube videos as they have clear explanations and scientific-based data for Cannabis growing.

Light distance and hanging height

How to determine the optimal distance between lights and plants

Determining the optimal distance between lights and plants is very important for healthy growth and preventing damage. This distance varies based on the type of light used. It’s better to check the PPFD chart on the manufacturer’s or seller’s site where you bought your light fixture.

Most LED grow lights have their own measurements and optimal distance recommendations. If the website only gives you a PPFD chart with some hanging distances, you can interpret it by comparing the values in the chart with the values recommended for each plant’s stage previously mentioned in this guide.

High-Intensity Discharge (HID) lights are easier to interpret as they mostly require a greater distance due to their higher heat output. It’s important to refer to the manufacturer’s guidelines for specific recommendations. Maintaining the correct light distance ensures plants receive adequate light without risking burn or stress.

Here’s a quick guide for the most common HPS (High-Pressure Sodium) fixtures with their recommended values and distances from the canopy. 

Adjusting hanging height as plants grow

Regularly monitoring plant height and adjusting light fixtures as the plant grows helps to keep plants healthy and to maximize light efficiency. Using adjustable hangers or pulleys makes this process easier and more precise.

How to prevent light burn and light stress

Preventing light burn and light stress involves managing light intensity and distance effectively. Light burn occurs when lights are too close, causing leaf discoloration, curling, or necrosis (leaf death). To avoid this, keep the recommended distances and monitor plants for signs of stress. 

Gradually acclimating plants to higher light intensities can prevent shock. Ensuring proper airflow and using fans can also dissipate heat, reducing the risk of light burn. Regularly checking and adjusting light setups ensures plants receive optimal lighting without negative effects.

Takeaway: the importance of lighting in indoor cannabis cultivation, PPFD and light spectrum

Lighting is key for successful indoor cannabis cultivation, directly influencing plant health, growth, and yield. Understanding PPFD (Photosynthetic Photon Flux Density) is essential, as it measures the light usable by plants for photosynthesis. 

The light spectrum is also important, with blue light promoting vegetative growth and red light enhancing flowering. Proper lighting ensures plants receive the right energy at each growth stage, maximizing productivity and quality.

Explore and experiment with different lighting setups for optimal results

Exploring and experimenting with different lighting setups can lead to optimal results in cannabis cultivation. Each grow environment is unique, and what works best can vary. Trying different light intensities, spectrums, and configurations can reveal the ideal setup for your specific conditions. 

Stay informed about new technologies and techniques, and don’t hesitate to adjust your approach based on observations and results. This willingness to experiment and adapt is key to mastering indoor cannabis cultivation and achieving the best possible yields.

Which grow lights do you prefer for growing Cannabis? Tell me in the comments! Happy growing!

Citation of sources:

Citation: Westmoreland FM, Kusuma P, Bugbee B (2021) Cannabis lighting: Decreasing blue photon fraction increases yield but efficacy is more important for cost effective production of cannabinoids. PLoS ONE 16(3): e0248988. https://doi.org/10.1371/journal.pone.0248988

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0248988

Citation: Rodriguez-Morrison V, Llewellyn D and Zheng Y (2021) Cannabis Inflorescence Yield and Cannabinoid Concentration Are Not Increased With Exposure to Short-Wavelength Ultraviolet-B Radiation. Front. Plant Sci. 12:725078. doi: 10.3389/fpls.2021.725078

https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2021.725078/full

Citation: Lydon, J., Teramura, A.H. and Coffman, C.B. (1987) UV-B Radiation Effects on Photosynthesis, Growth and Cannabinoid Production of Two Cannabis sativa Chemotypes. Photochemistry and Photobiology, 46, 201-206.

https://doi.org/10.1111/j.1751-1097.1987.tb04757.x

Citation: Westmoreland FM, Kusuma P and Bugbee B (2023) Elevated UV photon fluxes minimally affected cannabinoid concentration in a high-CBD cultivar. Front. Plant Sci. 14:1220585. doi: 10.3389/fpls.2023.1220585

https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2023.1220585/full

* In a highly cited paper, Lydon et al. (1987) reported that THC in flowers of a drug-type variety increased from 2.5 to 3.1% as biologically effective UV-B increased from 0 to 13.4 kJ m^-2 d^-1. Notably, there was no effect in a fiber-type variety, which led the authors to conclude that the effect of UV-B on cannabinoid synthesis is “equivocal”, which means uncertain (Lydon et al., 1987). 

Citation: Rodriguez-Morrison V, Llewellyn D and Zheng Y (2021) Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment. Front. Plant Sci. 12:646020. doi: 10.3389/fpls.2021.646020

https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2021.646020/full

Citation: Zhen S, Bugbee B. Far-red photons have equivalent efficiency to traditional photosynthetic photons: Implications for redefining photosynthetically active radiation. Plant Cell Environ. 2020;43:1259–1272. https://doi.org/10.1111/pce.13730

https://onlinelibrary.wiley.com/doi/10.1111/pce.13730

Citation: Zhen S and Bugbee B (2020) Substituting Far-Red for Traditionally Defined Photosynthetic Photons Results in Equal Canopy Quantum Yield for CO₂ Fixation and Increased Photon Capture During Long-Term Studies: Implications for Re-Defining PAR. Front. Plant Sci. 11:581156. doi: 10.3389/fpls.2020.581156

https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2020.581156/full

Citation: Punja ZK. Emerging diseases of Cannabis sativa and sustainable management. Pest Manag Sci. 2021 Sep;77(9):3857-3870. doi: 10.1002/ps.6307. Epub 2021 Feb 27. PMID: 33527549; PMCID: PMC8451794.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8451794/

Citation: The action spectrum, absorptance and quantum yield of photosynthesis in crop plants. K.J. McCree (1973). Institute of Life Science and Biology Department, Texas A and M University, College Station, Texas U.S.A.

https://www.sciencedirect.com/science/article/abs/pii/0002157171900227?via%3Dihub

Photos:

https://en.wikipedia.org/wiki/Photosynthetically_active_radiation

https://en.wikipedia.org/wiki/Far-red_light

https://en.wikipedia.org/wiki/Photosynthesis

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