Elevated atmospheric CO2 concentration favors nitrogen partitioning into roots of tobacco plants under nitrogen deficiency by decreasing nitrogen demand of the shoot

Anthropogenic increase of atmospheric CO2 concentration is likely to affect plant growth in natural and agricultural ecosystems. Since nitrogen (N) is one of the major factors limiting agricultural plant production, we investigated the effect of elevated atmospheric CO2 concentration on N partitioning at the whole-plant level and the cellular level at limited N supply. Tobacco was grown at ambient (400 ppm) and elevated (800 ppm) concentrations of atmospheric CO2 under conditions of defined N supply with the same amount of N supplied to all plants, independent of CO2-induced changes of the actual growth rate. Under conditions of N deficiency, high CO2 concentration promoted root growth whereas shoot growth was only slightly increased which resulted in an increased root/shoot ratio. At low N supply, elevated atmospheric CO2 concentration decreased N concentrations in the shoot tissue, but not in toots. Obviously elevated CO2 supply stimulated N partitioning into roots relative to the shoots, which coincided with relatively stronger root growth. At the cellular level, Rubisco (ribulose-1,5-bisphosphate carboxylase-oxygenase) protein decreased under N deficiency and elevated CO2. This was associated with increasing starch concentrations. while sugar concentrations were not affected. We suggest that root growth under N limitation is restricted by the internal N availability rather than by other factors such as carbon supply. The present results suggest that N partitioning into roots is favored by elevated CO2 supply due to a decreased N demand of the shoot.