Cannabis and Photosynthesis

Updated: Jun 9, 2020

You probably remember the term "photosynthesis" in your natural science class. Most people, however, lose the need to remember the process it translates, finally, until they start growing cannabis! In this weed blog, we will delve into the process of photosynthesis, how the quality of light can affect this process, and how it affects the production of cannabinoids.


  1. What is photosynthesis?

  2. Lighting quality: wattage, lumens, PAR and PPFD

  3. Optimal conditions for photosynthesis

  4. How photosynthesis rate could affect cannabinoid rate

  5. Is there a difference between photoperiod and auto-flowering varieties in terms of photosynthesis?

Cannabis requires multiple external resources to develop properly. It requires nutrients for protein production, building cell walls, and facilitating biochemical processes. It needs water to dissolve and transport these molecules. All of these factors contribute to the healthy growth and development of the plant. But the main factor responsible for growth is neither in a bottle nor at the bottom of a compost. Instead, it comes from the sun (or a powerful set of bulbs). Let us delve into the details of this essential process.


The meaning of the word photosynthesis resides in the word itself. "Photo" means light and "synthesis" means the creation of an organic compound. Plants convert light into biochemical energy in order to survive. But how do they do it? Well, they are equipped with impressive biochemical machinery! To understand this process properly, we will have to delve into the cellular level.

Photosynthesis mainly takes place within the leaves. More specifically, it takes place in specialized cells known as mesophyll cells. These cells form a layer just below the surface of the leaves where they work to capture light. They contain small organelles called chloroplasts, which contain a lot of the pigment chlorophyll, the chemical compound that gives plants their green appearance. As a pigment, chlorophyll has the ability to absorb light. Plants store this molecule in a pillar structure called thylakoids. The space between these structures is called the stroma.

Photosynthesis occurs in two main phases: light-dependent and light-independent reactions. The first stage of light-dependent reactions involves carbon dioxide (CO₂) and water (H₂O) which penetrate the leaves. CO₂ enters through small openings called stomata and perspiration attracts water through tunnels called xylem.

Then, the photons generated by the sun (or your culture lamps) crash into the chlorophyll molecules. The electrons absorb energy and become "excited". A series of light-dependent reactions occur, which ultimately results in energy storage in the form of ATP (the cellular value of energy) and NADPH (an electron carrier). All this action takes place in the thylakoid membrane.

These molecules are often used in what is called the Calvin cycle (also called light-dependent reactions) which takes place within the stomata. They are used to "fix" carbon dioxide and produce three-carbon sugar molecules. They are then combined to form our sweet and sweet friend, glucose. These simple molecules are used by the plant for energy and to produce larger carbohydrate molecules.


So, we approached the why of the need for plants in light to conduct photosynthesis. But are some lamps better than others? The answer is yes. In most parts of the world, the photons from the sun are more than enough to drive photosynthesis. However, indoor growers will need a lighting system that will provide enough energy for the plants to flourish.

There are many different types of indoor grow lights: LED, HID and fluorescent are just a few examples. Each type has its own advantages and disadvantages, but the general quality of the lighting is the main essential factor.


You will need to pay attention to the wattage (the amount of electrical power) when purchasing lamps. If the wattage is too low, the light source will not emit optimal light for plant development. The lights that produce between 400 and 600 W / m² are sufficient to provide satisfactory crops. The lamps that produce 1000 W + per m² will push the production of heads to its limit.


Growers can determine the quality of their lighting based on several measurements. First and foremost, a lux meter can be used to analyze the amount of light received in a particular area of ​​the culture space.

Lux is a measure of lumens (the amount of light emitted by a device) over a given area. Lux meters measure the type of light that can be detected by the human eye, therefore, they do not offer a specific measure regarding the amount of light available to a plant. But they are enough for the home grower who wants to have an idea of ​​the amount of light received by his crop.

Be sure to provide your plants with nearly 40,000 lux during the growth phase and 60,000 during the flowering phase.


The lux seems useful, but what if you want to know the exact cultivation power of a particular lamp? This is where PAR (photosynthetically available radiation) comes into play. PAR means light between 400 and 700 nm which plants use for photosynthesis. The PAR measurement unit is the micromole per second (μ / mol / s), it indicates to growers the number of photons included in the range which strike the leaves of the plant every second.

This is called PPFD or density of the magnetic flux of photosynthetic photons.

PAR can be measured using a PAR meter. These devices use sensors that detect light between 400 and 700 nm. An average PPFD can be obtained by taking measurements at various places in the canopy while maintaining the same height. Aim for close to 350 μ / mol / s during the growth phase and 850 μ / mol / s during flowering.

Lamp manufacturers should provide this information to you. For real PPFD data, make sure the company provides you with the distance between the canopy and the light source, several measurements, an average, and a min: max ratio.


The intensity of the light is not the only condition for optimizing photosynthesis. Research has found that temperature and carbon dioxide can also improve this process.

Photosynthesis depends on many enzymes to carry out a biochemical reaction. These proteins cannot function effectively during cold temperatures (0 to -10 ° C) which decreases the rate of photosynthesis and ends up blocking growth. In the same way, too high temperatures (above 20 ° C) also limit these capital enzymes. They work best between 10 and 20 ° C.

It is interesting to see that CO₂ allows cannabis plants to thrive at slightly higher temperatures. Higher levels of this gas can also help give photosynthesis a boost in conjunction with powerful lights. The more the leaves are exposed to light, the more carbon it requires to convert energy into sugar. If you rotate 600 W lamps in a relatively small space, you will have enough power to guarantee the increase in CO₂ levels. Growers can push CO₂ levels to the optimum value of 1,500 to 2,000 ppm using the CO₂ tank. However, a simpler option remains the dissolution of the tablet in the substrate.


It is logical that a better rate of photosynthesis allows plants to produce more energy and to maximize their production of cannabinoids. There is not a lot of research on this subject and it seems very nuanced. For example, one study has shown that several cannabis ecotypes have demonstrated an increase in the rate of photosynthesis in warmer climates, but an increase in the rate of cannabinoids when grown in colder temperatures. More research is clearly needed to clarify things.

It is also known that plants produce different cannabinoid profiles under different lights at the same intensity. Research has shown that HPS (high-pressure sodium) lamps produce more flowers on a dry basis, while LEDs produce higher levels of cannabinoids CBG, THC, and CBD.


The varieties photoperiodic and autoflower respond differently to lighting. The two varieties photosynthesize in exactly the same way, but the photoperiod varieties require a change of cycle to flower. Cultivators generally keep their photoperiodic plants under a light cycle of 18 hours of the day for 6 hours of the night during the growth phase. A change in cycle 12 hours day and 12 hours night forces them to go into flowering. They will remain in the growth phase indefinitely if they receive more light than that.

In contrast, auto-flowering varieties bloom regardless of environmental factors. They can tolerate a 24 hour light cycle for their entire life and still produce buds. This means that auto-flowering varieties will have more opportunities to conduct photosynthesis during their flowering phase. However, they will still need a night period to breathe. An optimal program for indoor auto-flowering varieties is 20 hours of the day for 4 hours of the night during the entire growing cycle.


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