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Interactive Effects of Elevated CO2, Ozone, and Leaf Rust Infection on Productivity in Wheat
Tiedemann, A.V. and Firsching, K.H.  2000.  Interactive effects of elevated ozone and carbon dioxide on growth and yield of leaf rust-infected versus non-infected wheat.  Environmental Pollution 108: 357-363.

What was done
Spring wheat (Triticum aestivum) plants were grown from germination to maturity in controlled environmental chambers receiving combinations of ambient (377 ppm) and enriched (612 ppm) atmospheric CO2 concentrations and ambient (20 ppb) and enriched (61 ppb) atmospheric ozone concentrations to determine the interactive effects of these gases on growth and yield in this species.  In addition, half of the plants in each treatment were inoculated with a leaf rust-causing pathogen (fungus) to determine whether or not this biotic stress influences overall plant responses to elevated CO2 and ozone concentrations.

What was learned
In non-inoculated plants, elevated CO2 significantly increased average photosynthetic rates at both ozone concentrations.  In fact, the greatest absolute photosynthetic rates were achieved in the elevated CO2/elevated ozone treatment combination, as were the largest CO2-induced percentage increases in photosynthesis.  This phenomenon invariably impacted total grain yields, which were also greatest in the elevated CO2/elevated ozone treatment combination.  In fact, elevated CO2 increased total grain yield at high ozone concentration by 38%, relative to that observed at ambient CO2 and elevated ozone concentrations.  Moreover, the absolute value of total grain yield in this treatment combination was not significantly different from that produced at ambient ozone concentrations, regardless of atmospheric CO2 concentration.  Thus, the deleterious effects of ozone on photosynthesis and yield were completely ameliorated in non-inoculated plants by atmospheric CO2 enrichment.

In inoculated plants, the percentage leaf area infected by rust was strongly reduced by elevated ozone concentration, while it was largely unaffected by atmospheric CO2 enrichment.  Elevated CO2 increased photosynthetic rates in inoculated plants by 20 and 42% at ambient and elevated ozone concentrations, respectively.  Although inoculated plants displayed less absolute yield than non-inoculated plants, atmospheric CO2 enrichment tended to stimulate yield in infected plants, even increasing it by 57% at high ozone concentrations.  Thus, the beneficial effects of elevated CO2 on photosynthesis and yield in wheat continue to persist under conditions of abiotic ozone and biotic pathogenic stresses.

What it means
As the CO2 content of the air continues to rise, it is likely that wheat plants will exhibit significant increases in photosynthesis and yield even under less than favorable growing conditions characterized by elevated atmospheric ozone concentrations and/or pathogenic infections by rust-causing organisms.  Thus, it is likely that such detrimental environmental and biotic factors will not threaten world grain production in the years to come, as long as the air's CO2 content is allowed to increase unhindered by mankind's misguided legislative attempts to stabilize the atmospheric concentration of this vital life-supporting gas.

Reviewed 15 July 2000