i tried to post a link but it wouldn't work. here's some good info. on co2/plant mechanics.
Carbon Dioxide Enrichment Methods
By Roger H. Thayer, Eco Enterprises
CARBON DIOXIDE (CO2)
Carbon dioxide is an odorless gas and a minor constituent of the air we breathe. It comprises only .03 % (300 parts per million, or PPM) of the atmosphere but is vitally important to all life on this planet!
Plants are made up of about 80-90 % carbon and water with other elements like nitrogen, calcium, magnesium, potassium, phosphorous and trace elements making up only a small percentage. Almost all of the carbon in plants comes from this minor 300 PPM of carbon dioxide in the air.
Plants take in CO through pores, called stomata, in their leaves during daylight hours. They give off oxygen at the same time, the results of a process called photosynthesis. This oxygen that they give off is used by humans and all animal and marine life on this planet. Without it, animal and human life would not be possible.
Oxygen comprises almost 20 % of the earth's atmosphere. Most of it was generated by plant life. The process of photosynthesis combines CO2 and water to form sugars and free oxygen. Simple sugars like C6H12O6 provide plants with energy and are formed into the more complex plant parts such as carbohydrates, amino acids, protein, cellulose, leaves, roots, branches and flowers.
People and animals breathe in oxygen generated by plants and breathe out the CO2 that the plants needóa truly symbiotic relationship. In ancient times, millions of years ago, when there was only plant life on the earth and no animal life, the atmosphere was quite different. There was much more volcanic activity, one of nature's sources of CO2, and the air contained three to four times as much of it than now. Plants thrived. Giant tree ferns reigned supreme and much of our coal, gas and oil deposits were created by them during that long-ago time.
Plants would benefit from more CO2 in the air today, and actually are benefitting as we burn more fuels, one by-product being carbon dioxide. CO2 in the air has increased from 270 PPM to over 300 PPM, more than an 11% increase, in just the last 40 years! This has also worried many scientists because of what is called the greenhouse effect.. The more CO2 there is in the atmosphere, the higher the planet's temperature will go. Too much warming of the planet can melt ice caps, flood coastal cities, spread deserts and famine and drastically change the climate. This effect is somewhat self-regulating however. The oceans absorb a great deal of CO2 giving algae and plankton, 90% of the plant matter on earth, more CO2 to grow on and giving the rest of the plant matter on land more also. This decreases the amount of CO2 in the atmosphere, thereby regulating it. Scientists are just now learning to understand the self-regulating systems that stabilize most factors in our environment.