Carbon dioxide (CO2) is a gas which comprises about 0.038% or 380 parts per million (ppm) of the earth’s atmosphere.
CO2 is one of the two raw materials required for plant photosynthesis. (Water is the other.)
Cannabis uses CO2 only in the presence of light. Photosynthesis occurs immediately after the plant receives light. The plant starts mining CO2 from the air by opening its stomata, tiny organs found on the leaf surface, primarily on the underside. They function much like pores in the skin. They regulate the absorption of water, gas, oxygen, (O2), and CO2 into the plant, as well as the evacuation of water and O2 from the plant.
Once CO2 is absorbed into the plant, it is directed to the chloroplasts—the plant organelles that contain light-absorbing chlorophylls—where photosynthesis takes place.
Photosynthesis consists of a complex series of reactions in which light energy is used to convert carbon dioxide and water to sugar, releasing oxygen as a byproduct.
The amount of CO2 in the air has a profound effect on the rate of photosynthesis and plant growth. Photosynthesis speeds up as the amount of CO2 in the air increases, as long as there is enough light to power it. Conversely, as the CO2 content of the air falls, photosynthesis slows to a crawl and virtually stops at a CO2 concentration of around 200 ppm, no matter what the other conditions. Lacking CO2, plants continue respiration and growth for a short time, until their sugars are used up; then they slow down their metabolism to conserve energy. Only when more CO2 is available can the plant processes continue.
Outdoors, breezes and the exchange of gasses in the air constantly replace the CO2 that plants consume. This provides enough CO2 for vigorous growth, and outdoor growers rarely think of the gas as a limiting factor, even though growth of some plants, including cannabis, is not maximized in the Earth’s present atmosphere. In fact, the 380 ppm of CO2 found in earth’s atmosphere is on the low end of the continuum of most plants’ ability to use it as fuel for photosynthesis.
Outdoor plants growing in the bright light of summer grow heavier and faster when supplemented with CO2. Raising the level of CO2 up to 0.15% (1500 ppm), or a little more than four times the amount usually found in the atmosphere, increases plant growth rate significantly. Enhancing growth outdoors using increased CO2 is discussed in the supplementation section.