A major contributor to the global carbon cycle is plant respiration. Elevated atmospheric CO 2 concentrations may either accelerate or decelerate plant respiration for reasons that have been uncertain. We recently established that elevated CO 2 during the daytime decreases plant mitochondrial respiration in the light and protein concentration because CO 2 slows the daytime conversion of nitrate (NO 3-) into protein. This derives in part from the inhibitory effect of CO 2 on photorespiration and the dependence of shoot NO 3- assimilation on photorespiration. Elevated CO 2 also inhibits the translocation of nitrite into the chloroplast, a response that influences shoot NO 3- assimilation during both day and night. Here, we exposed Arabidopsis ( Arabidopsis thaliana) and wheat ( Triticum aestivum) plants to daytime or nighttime elevated CO 2 and supplied them with NO 3- or ammonium as a sole nitrogen (N) source. Six independent measures (plant biomass, shoot NO 3-, shoot organic N, 15N isotope fractionation, 15NO 3- assimilation, and the ratio of shoot CO 2 evolution to O 2 consumption) indicated that elevated CO 2 at night slowed NO 3- assimilation and thus decreased dark respiration in the plants reliant on NO 3-. These results provide a straightforward explanation for the diverse responses of plants to elevated CO 2 at night and suggest that soil N source will have an increasing influence on the capacity of plants to mitigate human greenhouse gas emissions.
