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Interactive Effects of CO2 and Ozone on C3 and C4 Plants
Volin, J.C., Reich, P.B. and Givnish, T.J.  1998.  Elevated carbon dioxide ameliorates the effects of ozone on photosynthesis and growth: species respond similarly regardless of photosynthetic pathway or plant functional group.  New Phytologist 138: 315-325.

What was done
The authors grew two C4 grasses, two C3 grasses and two C3 trees for approximately three months in controlled environment growth rooms that received either ambient (366 ppm) or ambient plus 300 ppm atmospheric CO2.  The plants of each CO2 treatment were also exposed to either severely reduced (3 ppb) or twice the ambient concentration (95 ppb) of ozone (O3) to determine the interactive effects of elevated CO2 and O3 on plants with different growth forms that utilize different physiological mechanisms for extracting carbon from the air and incorporating it into their tissues.

What was learned
In general, plants grown at ambient CO2 manifested lower growth and photosynthetic rates at elevated O3 than they did at reduced O3 concentrations.  However, the deleterious effects of the high ozone concentration were reduced or eliminated when plants were exposed to elevated levels of CO2.  This amelioration was primarily due to reduced stomatal conductance, which was a consequence of atmospheric CO2 enrichment in these species.

What it means
As the air's CO2 content continues to rise, earth's vegetation should experience less deleterious effects of O3 pollution, regardless of their photosynthetic pathways or growth forms.  Because O3 damage results from the indiscriminate uptake of this pollutant through leaf stomata, it is likely that the damages caused by other aerial pollutants (such as sulfur and nitrogen-based oxides) will decrease as well, as the stomatal conductances of most plants tend to decrease in response to increasing levels of atmospheric CO2.

Reviewed 15 December 1998