Journal of Experimental Botany, Vol. 53, No. 379, pp. 2351-2367,
December 1, 2002
© 2002 Oxford University Press
Elevated pCO2 favours nitrate reduction in the roots of wild-type tobacco (Nicotiana tabacum cv. Gat.) and significantly alters N-metabolism in transformants lacking functional nitrate reductase in the roots
Received 18 April 2002; Accepted 18 July 2002
1 Universität Freiburg, Institut für Forstbotanik und Baumphysiologie, Professur für Baumphysiologie, Georges-Köhler-Allee, Geb. 053/054, D-79085 Freiburg, Germany
2 Botanisches Institut der TU Braunschweig, Humboldtstr. 1, D-38106 Braunschweig, Germany
3 Universität Hohenheim, Institut für Pflanzenernährung, Fruwirthstr. 20, D-70593 Stuttgart, Germany
4 Universität Bayreuth, Fachgruppe Geowissenschaften, Abteilung Agrarökologie D-95440 Bayreuth, Germany
5 To whom correspondence should be sent. Fax: +49 761 203 8302. E-mail: here{at}uni-freiburg.de
The impact of elevated pCO2 on N-metabolism of hydroponically grown wild-type and transformed tobacco plants lacking root nitrate reduction was studied in order to elucidate the effects on (i) nitrate uptake, (ii) long-distance transport of N, (iii) nitrate reduction with emphasis on root-NR, and (iv) the allocation of N between the root and shoot. The findings were related to alterations of growth rates. At elevated pCO2 the wild type exhibited higher growth rates, which were accompanied by an increase of NO3–-uptake per plant, due to a higher root:shoot ratio. Furthermore, elevated pCO2 enhanced nitrate reduction in the roots of the wild type, resulting in enhanced xylem-loading of organic N (amino-N) to supply the shoot with sufficient nitrogen, and decreased phloem-transport of organic N in a basipetal direction. Transformed tobacco plants lacking root nitrate reduction were smaller than the wild type and exhibited lower growth rates. Nitrate uptake per plant was decreased in transformed plants as a consequence of an impeded root growth and, thus, a significantly decreased root:shoot ratio. Surprisingly, transformed plants showed an altered allocation of amino-N between the root and the shoot, with an increase of amino-N in the root and a substantial decrease of amino-N in the shoot. In transformed plants, xylem-loading of nitrate was increased and the roots were supplied with organic N via phloem transport. Elevated pCO2 increased shoot-NR, but only slightly affected the growth rates of transformed plants, whereas carbohydrates accumulated at elevated pCO2 as indicated by a significant increase of the C/N ratio in the leaves of transformed plants. Unexpectedly, the C/N balance and the functional equilibrium between root and shoot growth was disturbed dramatically by the loss of nitrate reduction in the root.
Key words: C:N balance, elevated pCO2, functional equilibrium, long-distance transport, N-uptake, nitrate reduction, root:shoot ratio.
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