Reference
Takatani, N., Ito, T., Kiba, T., Mori, M., Miyamoto, T., Maeda, S.-i. and Omata, T. 2014. Effects of high CO2 on growth and metabolism of Arabidopsis seedlings during growth with a constantly limited supply of nitrogen. Plant & Cell Physiology 55: 281-292.
Background
The authors write that "elevated CO2 has been reported to stimulate plant growth under nitrogen-sufficient conditions," but they say that "the effects of CO2 on growth in a constantly nitrogen-limited state, which is relevant to most natural habitats of plants, remain unclear."
What was done
As a first step to better understanding plant responses to "a high-CO2 and low-nitrogen environment," as Takatani et al. describe it, they established "a high-CO2-induced, constantly nitrogen-limited growth state in A. thaliana," where they maintained seedlings of two strains - a wild type (WT) and a mutant generated by Kiba et al. (2012) - under nitrogen-limited conditions, growing them on a medium that contained nitrate as the sole nitrogen source at atmospheric CO2 concentrations of 280 and 780 ppm.
What was learned
The seven Japanese scientists report that (1) under nitrogen-sufficient conditions, the growth of shoots and roots of both the WT and the mutant increased by approximately two-fold in the elevated CO2 treatment, that (2) growth stimulation of both shoots and roots by elevated CO2 was observed in the WT when nitrate was the sole source of nitrogen, while only root growth was stimulated in the mutant grown with nitrate, and that (3) "in the mutant, elevated CO2 caused well-known symptoms of nitrogen-starved plants, including decreased shoot/root ratio, reduced nitrate content and accumulation of anthocyanin."
What it means
Takatani et al. say their results indicate that elevated CO2 causes certain limitations in seedlings grown with a constantly-limited supply of nitrogen; but they state that the chlorophyll content and root biomass of the plants still "increase to enhance the activities of both photosynthesis and nitrogen uptake, while maintaining normal metabolism."
Reference
Kiba, T., Feria-Bourrellier, A.B., Lafouge, F., Lexhneva, L., Boutet-Mercey, S., Orsel, M., Brehaut, V., Miller, A., Daniel-Vedele, F., Sakakibara, H. and Krapp, A. 2012. The Arabidopsis nitrate transporter NRT2.4 plays a double role in roots and shoots of nitrogen-starved plants. The Pant Cell 24: 245-258.