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Response of Spring Wheat to Elevated CO2 and Ozone
Cardoso-Vilhena, J., Balaguer, L., Eamus, D., Ollerenshaw, J. and Barnes, J.  2004.  Mechanisms underlying the amelioration of O3-induced damage by elevated atmospheric concentrations of CO2Journal of Experimental Botany 55: 771-781.

What was done
Individual spring wheat (Triticum aestivum L. cv. Hanno) plants were grown in 3-dm3 pots in controlled environment chambers for 77 days at atmospheric CO2 concentrations of either 350 or 700 ppm and at ozone (O3) concentrations of either less than 5 or 75 ppb, while gas exchange measurements of leaves 4 and 7 on the plants' main shoots were made at regular intervals throughout the study, after which the plants were harvested and their total dry weights determined.  In parallel with the gas exchange measurements, Rubisco activity and chlorophyll fluorescence were also assessed throughout the experiment.

What was learned
In air of less than 5 ppb O3, the doubling of the air's CO2 concentration increased total plant dry weight by 66%; while in air of 75 ppb O3, it increased total plant dry weight by 189%.  Over the lifespans of leaves 4 and 7, elevated CO2 also reduced cumulative O3 uptake by 10 and 35%, respectively, due to the decrease it caused in leaf stomatal conductance, while it protected against the decline in apparent quantum yield of CO2 assimilation caused by high O3 in the ambient CO2 treatment.  In addition, elevated CO2 protected against the reduction in the maximum in vivo rate of Rubisco carboxylation induced by high O3 in both leaves 4 and 7.

What it means
The authors say their data "revealed that rising atmospheric CO2 concentrations are likely to afford protection against the adverse effects of O3 on plant growth and photosynthesis, with the effect due, at least in part, to the decline in stomatal conductance triggered by increases in atmospheric CO2."  In addition, they report that their study "suggested that rising atmospheric CO2 concentrations may also enhance the tolerance of leaf tissue to O3-induced oxidative stress," and that "this finding is consistent with reported shifts in the antioxidant status of leaves under the combined influence of elevated CO2+O3 (Rao et al., 1995)."  Last of all, when we consider the bottom line of total plant dry weight production, we once again see that atmospheric CO2 enrichment more than completely ameliorated the deleterious effect of ozone pollution.  See Ozone (Effects on Plants) in our Subject Index for other examples of this phenomenon.

Rao, M.V., Hale, B.A. and Ormrod, D.P.  1995.  Amelioration of ozone-induced oxidative damage in wheat plants grown under high carbon dioxide.  Plant Physiology 109: 421-432.

Reviewed 23 June 2004