A number of experiments have found that some plant species that respond positively to elevated CO2 when grown alone experience decreased growth under elevated CO2 when grown in mixed plant communities (Poorter & Navas 2003).
While a great deal of media and public attention has focused on the effects that such higher concentrations of CO2 are likely to have on global climate, rising CO2 concentrations are also likely to have profound direct effects on the growth, physiology, and chemistry of plants, independent of any effects on climate (Ziska 2008). These effects result from the central importance of CO2 to plant metabolism. As photosynthetic organisms, plants take up atmospheric CO2, chemically reducing the carbon. This represents not only an acquisition of stored chemical energy for the plant, but also provides the carbon skeletons for the organic molecules that make up a plants’ structure. Overall, the carbon, hydrogen and oxygen assimilated into organic molecules by photosynthesis make up ~96% of the total dry mass of a typical plant (Marschner 1995). Photosynthesis is therefore at the heart of the nutritional metabolism of plants, and increasing the availability of CO2 for photosynthesis can have profound effects on plant growth and many aspects of plant physiology.
One of the most consistent effects of elevated atmospheric CO2 on plants is an increase in the rate of photosynthetic carbon fixation by leaves.
Carbon dioxide concentrations are also important in regulating the openness of stomata, pores through which plants exchange gasses, with the external environment. Open stomata allow CO2 to diffuse into leaves for photosynthesis, but also provide a pathway for water to diffuse out of leaves. Plants therefore regulate the degree of stomatal opening (related to a measure known as stomatal conductance) as a compromise between the goals of maintaining high rates of photosynthesis and low rates of water loss.
As CO2 concentrations increase, plants can maintain high photosynthetic rates with relatively low stomatal conductance.
will have major implications for plant physiology and growth. Under elevated CO2 most plant species show higher rates of photosynthesis, increased growth, decreased water use and lowered tissue concentrations of nitrogen and protein. Rising CO2 over the next century is likely to affect both agricultural production and food quality.