ELEVATED ATMOSPHERIC CO₂ CONCENTRATION FAVORS NITROGEN PARTITIONING INTO ROOTS OF TOBACCO PLANTS UNDER NITROGEN DEFICIENCY BY DECREASING NITROGEN DEMAND OF THE SHOOT

Abstract

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 roots. 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.

ACKNOWLEDGMENTS

This paper is dedicated to the memory of Prof. Dr. Dres. h.c. Horst Marschner. We thank Dr. Nicolaus von Wirén and Dr. Heidi-Jayne Hawkins for constructive discussion and critical reading of the manuscript. This project was financially supported by the Deutsche Forschungsgemeinschaft.