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.
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 CO2 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.