Human activities are causing many simultaneous changes to the chemistry of Earth’s atmosphere. These changes include increasing concentrations of ozone (O₃), nitrogen dioxide (NO₂), and carbon dioxide (CO₂), all of which have the potential to profoundly affect plant performance. In general, elevated CO₂ has been found to increase plant growth in the short term, but the growth response often decreases over time due to some other resource limitation or plant acclimation. In contrast, NO₂ enters plant leaves through the stomata and is converted to nitrate in the apoplast, making it a source of nitrogen for the plant potentially augmenting growth, but most laboratory studies suggest the oxidative damage caused by NO₂ counteracts any growth effect. Finally, O₃ is a strong oxidant and has nearly always been found to be detrimental to plants. We investigated the single and combined effects of elevated O₃ (100 ppb during the daylight hours five days per week), NO₂ (40 ppb), and CO₂ (560 ppm) on Arabidopsis thaliana grown in open-top chambers. We measured growth, phenology, and reproductive output throughout an 80-day growing period. 

Results/Conclusions

The single-gas effects on growth concurred with observations from previous studies. CO₂ increased growth and reproductive output as well as accelerating seed production. NO₂ had no effect on growth or reproductive output and only slightly delayed seed production when soil nitrogen was low. O₃ decreased growth and reproductive output, and delayed seed production. When under fumigation by two gases, NO₂ significantly decreased the effects of CO₂ fumigation, but only when soil nitrate was low. Under fumigation by all three gases, CO₂ fumigation largely eliminated the effects of NO₂ and O₃ fumigation observed in the single-gas fumigations. The most pronounced effect observed in this study was the combined fumigation of NO₂ and O₃. This treatment caused significantly decreased growth compared to O₃ fumigation alone and greatly delayed the production of seeds especially when soil nitrate was high. Given that O₃ and NO₂ tend to rise together in the atmosphere, our findings suggest that the influence of air pollution on plant performance or even the effect of ozone alone cannot be predicted by experiments examining the effect of single-gas fumigation. The combined effects of gases appear to be large and it will only be by quantifying plant performance under simulated future atmospheric conditions that we will be able to rigorously parameterize predictive models.