There are many biological processes that occur, mostly simultaneously, in a living plant. Some processes continue even after the cultivar is harvested like in the case of the curing process.


The Cannabis plant photosynthesizes in the presence of light like most plants, it is categorized as a C3 plant.

C3 carbon fixation in photosynthesis leads to several characteristics:

  • Thrives where CO2 concentration is > 200ppm (cannabis specifically thrives in >400ppm CO2)
  • Cannot survive in very hot environments

Cannabis stands out from other plants for its tolerance to high light intensity. see Ideal light conditions


Most plants are constantly respiring and cannabis is no exception. In the process of respiration, plants uptake CO2 and expel Oxygen. CO2 is essential to the process of photosynthesis. If a plant is exposed to less than 200ppm it will be unable to photosynthesize at all and will most likely die. Airflow or "wind speed" has a strong effect on the rates of respiration and transpiration.[1] The rate of respiration is directly correlated with the stomata density on growing leaves.[2]

Supplimenting additional CO2

In a city typical Co2 levels are around 400-450ppm. This is acceptable for a growing plant but raising levels above 1000ppm can increase growth rates by up to 30%[3] [4] when combined with sufficient light intensity by enabling higher rates of photosynthesis. For more on the limitation of low Co2 at high levels of light intensity see: Upper limit of light intensity.

For more on how co2 is applied in a grow setting see: Adding extra Co2.


The cultivar constantly moves water around internally via capillary action and releases water vapor into the environment through the stomata, this is the process of transpiration. The density of stomata and how open they are will determine the rate of transpiration. The VPD of a plant's environment will affect its stomal response and in turn, its rate of transpiration. For higher VPD values rate of transpiration is generally higher up until a maximum threshold[5]. For Cannabis the maximum VPD is around 1.6kPa (Seedlings should be exposed to 0.4-0.8kPa until they are established). The rate of transpiration is also correlated to the light intensity the plant is being exposed to i.e. High light intensity equals a high rate of transpiration. Of course, the inverse correlation between humidity and rate of transpiration has been known since the 1930s.[6] Wind speed and CO2 levels also cause a plant response that will affect the rate of transpiration.

The rate of transpiration also affects the rate of water uptake through roots, this is because 97% of water Cannabis absorbs is lost via transpiration (this is a typical characteristic of a C3 plant). A higher water intake means that nutrients can be delivered more quickly.

Circadian rhythm

Like animals, plants also have a Circadian rhythm, a sort of internal biological clock that is programmed by external stimuli. It has been shown that matching a plant's environment to its natural circadian rhythm can increase photosynthesis[7][8][9]

More on circadian rhythm in plants

Plant Circadian Rhythms - C. Robertson McClung April 2006

Plant responses to circadian rhythm


  1. - Effects of air current speed on gas exchange in plant leaves and plant canopies (2003)
  2. John W. Kimball, 2020. Gas Exchange in Plants. Available at: [Accessed March 11, 2022].
  3. B. A. Kimball, 1983, Carbon Dioxide and Agricultural Yield: An Assemblage and Analysis of 430 Prior Observations -
  4. Hendrik Poorter, 1993, Interspecific Variation in the Growth-Response of Plants to An Elevated Ambient CO2 Concentration -
  5. Plant responses to rising vapor pressure deficit, Grossiord 2020 -
  7. - Plant Circadian Clocks Increase Photosynthesis, Growth, Survival, and Competitive Advantage (2005)
  8. - Interactions between circadian clocks and photosynthesis for the temporal and spatial coordination of metabolism (2015)
  9. - Leaf photosynthesis, plant growth, and carbohydrate accumulation of tomato under different photoperiods and diurnal temperature differences