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.
Like any plant, Cannabis will photosynthesize in the presence of light. It is categorized as a C3 carbon fixation plant (because of the way it captures and processes CO2), this gives it several characteristics:
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 intake 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]
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 (movement of stomata) and in turn, its rate of transpiration.[5] For higher VPD values rate of transpiration will increase until a maximum threshold[6]. The overall stomal response is also dependant on other factors such as plant health and age. For Cannabis the maximum VPD it can tolerate is around 1.6kPa.
The rate of photosynthesis is also correlated to transpiration. Higher rates of photosynthesis are enabled by higher rates 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.
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