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 plant growth. 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 and reproductive output throughout an 80-day growing period.  

Results/Conclusions

The effects of the CO₂, NO₂, and O₃ treatments are the result of changes in carboxylation vs oxidative effects.  Elevated CO₂ increased the carboxylation capacity of the plants, enhancing growth and increasing reproductive output.  Elevated O₃ caused oxidative damage to the plants which resulted in reduced biomass and decreased reproductive output.  When these gases were applied in combination, the balance of the increased carboxylation and the increased oxidation resulted in growth and reproductive output that were similar to the control plants.  The responses to elevated NO₂ were less straightforward.  Alone, elevated NO₂ had no effect on A. thaliana growth or reproductive output; however, it eliminated both the negative effects of elevated O₃ and the positive effects of elevated CO₂.  This suggests that NO₂ increases both carboxylation and oxidation, but that balance of the processes depends on the other conditions that the plant is experiencing.  These findings suggest that while future CO_2­ levels may help mediate the effects of rising O₃, the rising O₃ will in turn dampen the effects of elevated CO₂.  The rising concentrations of NO₂ are likely to lessen or eliminate the effects of both CO₂ and O₃.