There is much world-wide controversy regarding the ever increasing level of carbon dioxide (CO2) in the atmosphere that has occurred from 1968 to the present, increasing from 319 ppm in 1968 to the present 399 ppm, primarily due to the burning of fossil fuels (see: CO2Now.org). This increase is thought to be a significant factor contributing to global warming. For some, this increase in CO2 can have a potential positive side effect for it has been demonstrated that plant growth can be enhanced by enriching the atmosphere surrounding the plant with CO2, the degree of plant growth enhancement depending on growing conditions and plant species. Some have suggested that increasing the earth’s surface area with plant cover can be one means to counter the increasing concentration of CO2 in the atmosphere.
Without CO2, there would be no plant life for it is CO2 combined with hydrogen (from water) in the process called “photosynthesis” that a carbohydrate is formed, being the basic building block for all plant life. As with all biological systems, photosynthesis is not particularly simple in terms of how it works as well as the factors that affect its function. But, in simple terms, a molecule of H2O is split and then combined with a molecule of CO2 in the presence of chlorophyll and light to form a carbohydrate as is illustrated in the following chemical equation:
carbon dioxide (6CO2) + water (6H2O)
in the presence of light and chlorophyll yields
carbohydrate (C6H12O6) + oxygen (6O2)
Merriam Webster’s Collegiate Dictionary (10th Edition) defines photosynthesis as “formation of carbohydrates from carbon dioxide and a source of hydrogen (as water) in the chlorophyll-containing tissues of plants exposed to light.”
Photosynthesis primarily takes place within leaf stomata, unique surface leaf structures where the exchange of water and air takes place. There exists what are called “guard cells” surrounding the stoma that control opening and closing. There are both plant physiological factors (nutritional and moisture status) and atmospheric factors (temperature, light intensity, wind) that can affect whether the stoma is either open or closed, and thereby significantly reduce the rate of photosynthesis if closed.
Under optimal conditions, the rate of photosynthesis will increase as the CO2 concentration of the air surrounding the plant increases, although this correlation is not linear, and the level of response decreases with each increment of increasing CO2. Experiments have shown that in many situations, it is the maintenance of a constant level of CO2 in the air surrounding the plant as being equally important as its concentration. Therefore, air movement over plant leaves as well as air movement into and within the plant canopy can significantly affect plant growth and yield. Good examples are the orientation of corn rows so that the predominate directional wind currents will move down between the rows and not be impeded when having to move across the rows, and then the making of provisions for air movement up through a greenhouse tomato plant canopy rather than trying to push or pull air through the canopy. It also should be remembered that the thin layer of air in immediate contact with plant leaves is held in place by surface leaf characteristics as well as the surface tension properties of the leaf itself; therefore that thin air layer is not easily displaced even when there is gentle air movement over the plant leaf surface.
The other significant factor is plant species. When the first product of photosynthesis was determined*, it was found that there exists 2 pathways for carbohydrate formation, one being the formation of a 3-carbon carbohydrate and the other a 4-carbon carbohydrate. From this came the designation of plant species as being either C3 or C4 based on what is the first product of photosynthesis, whether a 3- or 4-carbon carbohydrate.
Is this finding a big deal? Yes indeed. Most plant species are C3, while most grasses, which includes all the major food grain crops, such as corn, wheat, rice, sorghum, etc., are C4 plants. C3 plant species are quite responsive to the concentration of CO2 in air surrounding them, while C4 are less so. C3 species are sensitive to high light intensity, are less drought tolerant and more sensitive to changing growing conditions, both in the rooting medium and surrounding atmosphere as compared to C4 species. So, even with an increasing level of CO2 surrounding the plant, the impact on plant growth may be minimal due to the factors of plant species as well as both environmental and plant physiological conditions.
The beneficial effects of CO2 enrichment in an enclosed structure, such as a greenhouse, have been well established depending on particular growing conditions and with varying results. Simply increasing the CO2 concentration of the air surrounding the plant will not automatically result in a significant increase in plant growth or product yield. Therefore, the grower needs to weigh the potential benefits of CO2 enrichment against costs, and the probable potential for no significant affect as well as possible adverse effects. For example, when the CO2 concentration in the air surrounding the plant is high, there is danger that such high concentrations can result in the closure of stomata. At what CO2 concentration this occurs varies depending on other factors, but experience has shown that stoma closure is more likely to occur when the CO2 air concentration is greater than 800 ppm. As mentioned earlier, it has been shown that the correlation between plant growth rate and CO2 concentration is not linear, with the rate of growth declining with each increasing increment of CO2 concentration (Jones, 2013). Light intensity and duration combined with air temperature and the moisture and nutritional status of the plant are correlated factors that will determine the extent of the “CO2-effect.” Therefore, just increasing the CO2 concentration of the air surrounding the plant does not automatically result in a significant increase in vegetative growth and product yield.
Returning to the global implications of increased atmospheric CO2 concentration on plant growth, it may be worth monitoring as well as determining what impact increased plant growth may have on moderating the increasing concentration of CO2 in the world’s atmosphere. For the C4 grain crops, plant growth increases could have a significant effect on the world supply of these essential grains in relieving the potential for shortages that could lead to famines in areas where population pressure is the greatest. Therefore, the effect of CO2 concentration of the air around a plant can have significant implications, deserving careful monitoring and further study.
*The first product of photosynthesis was discovered by Dr. Melvin Calvin, for which he was awarded the Nobel Prize for Chemistry in 1961.
Jones, Jr., J. Benton. 2013. Instructions for Growing Tomatoes in the Home Garden and Greenhouse. GroSystems, Inc., Anderson, SC (available in soft cover and e-book format for Kindle at Amazon.com)