Plant Responses to Rising CO2 Depend on Nitrogen Source and Nighttime CO2 Levels.

A major contributor to the global carbon cycle is plant respiration. Elevated atmospheric CO₂ concentrations may either accelerate or decelerate plant respiration for reasons that have been uncertain. We recently established that elevated CO₂ during the daytime decreases plant respiration in the light and protein concentration because CO₂ slows the daytime conversion of nitrate into protein. This derives from the inhibitory effect of CO₂ on photorespiration and the dependence of shoot nitrate assimilation on photorespiration. Here, we exposed Arabidopsis and wheat plants to daytime or nighttime elevated CO₂ and supplied them with nitrate or ammonium as a sole nitrogen source. Elevated CO₂ at night slowed nitrate assimilation and thus decreased dark respiration in the plants reliant on nitrate. These results provide a straightforward explanation for the diverse responses of plants to elevated CO₂ and suggest that nitrogen source will have an increasing influence on primary productivity during the next few decades.