Photosynthesis is the process which involves a chemical reaction between water and carbon dioxide in the presence of light, to make food (sugars) for plants and as a byproduct releases oxygen in the atmosphere. Carbon dioxide currently comprises .04% (400 ppm) of the atmospheric volume. //

The ambient CO2 (naturally occurring level of CO2) concentration of 400 parts per million can occur in a properly vented greenhouse. However, the concentration is much lower than ambient during the day and much higher at night in sealed greenhouses. The carbon dioxide level is higher at night because of plant respiration and microbial activities. The carbon dioxide level may drop to 150 to 200 parts per million during the day in a sealed greenhouse, because CO2 is utilized by plants for photosynthesis during daytime. Exposure of plants to lower levels of CO2 even for a short period can reduce rate of photosynthesis and plant growth. Generally, doubling ambient CO2 level (i.e. 700 to 800 parts per million) can make a significant and visible difference in plant yield. Plants with a C3 photosynthetic pathway (geranium, petunia, pansy, aster lily and most dicot species) have a 3-carbon compound as the first product in their photosynthetic pathway, thus are called C3 plants and are more responsive to higher CO2 concentration than plants having a C4 pathway (most of the grass species have a 4-carbon compound as the first product in their photosynthetic pathway, thus are called C4 plants). An increase in ambient CO2 to 800-1,000 ppm can increase yield of C3 plants up to 40%-100% percent and C4 plants by 10%-25% while keeping other inputs at an optimum level. Plants show a positive response up to 700 to need of 1,800 parts per million, but higher levels of CO2 may cause plant damage (Figure 1). //

Adding CO2 one to two hours after sunrise and stopping two to three hours before sunset is the ideal duration of supplementation. Plants are photosynthetically active one to two hours after sunrise reaching peak at 2:00 to 3:00 p.m., followed by a decrease in the rate of photosynthesis. However, leafy greens and vegetables in a hydroponic system can be supplemented with CO2 and a grow-lighting system 24 hours a day. Seedlings supplemented with CO2 in flats will be ready to transplant one or two weeks earlier. Supplementing CO2 at an early age reduces the number of days to maturity and plants can be harvested earlier. Young plants are more responsive to supplemental CO2 than more mature plants.