Below 200 PPM of CO2, plants do not have enough CO2 to carry on the photosynthesis process and essentially stop growing.
Because 500 PPM is the average CO2 content, this amount is chosen as our “Ambient CO2” level.
Above 2,000 PPM, CO2 starts to become toxic to plants and above 4,000 PPM it becomes toxic to people.
With the advent of ideal growing conditions provided by high-intensity discharge (H.I.D.) lighting systems, hydroponics, controls such as temp., humidity, etc. and complete, balanced plant nutrients, the limiting factor on plant growth rate, quality, size and time to maturity becomes the amount of carbon dioxide available to the plants.
This is easily fixed with CO2 enrichment.