The combustion of fossil fuels is profoundly altering the chemical composition of the atmosphere. Beginning with the Industrial Revolution, the concentration of carbon dioxide in the atmosphere has increased from approximately 280 to 370(micro)l l(sup -1) in 2004, and it is expected to exceed 550(micro)l l(sup -1) by 2050. Tropospheric ozone has risen even more rapidly than CO(sub 2) and average summer concentrations in the Northern Hemisphere are expected to continue to increase by 0.5-2.5% per year over the next 30 years. Although elevated CO(sub 2) stimulates photosynthesis and productivity of terrestrial ecosystems, ozone (O(sub 3)) is deleterious. In addition to directly affecting the physiology and productivity of crops, increased concentrations of tropospheric CO(sub 2) and O(sub 3) are predicted to lower the nutritional quality of leaves, which has the potential to increase herbivory as insects eat more to meet their nutritional demands. We tested the hypothesis that changes in tropospheric chemistry affect the relationship between plants and insect herbivores by changing leaf quality. The susceptibility to herbivory of soybean grown in elevated CO(sub 2) or O(sub 3) was examined using free air gas concentration enrichment (SoyFACE). FACE technology has the advantage that plants are cultivated under realistic field conditions with no unwanted alteration of microclimate or artificial constraints on the insect community.
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