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ENVIRONMENTAL STRESS AND ADAPTATION

Nitrate Does Not Result in Iron Inactivation in the Apoplast of Sunflower Leaves¹

Institut für Pflanzenernährung (330), Universität Hohenheim, D–70593 Stuttgart, Germany

It has been hypothesized that nitrate (NO₃^–) nutrition might induce iron (Fe) deficiency chlorosis by inactivation of Fe in the leaf apoplast (H.U. Kosegarten, B. Hoffmann, K. Mengel [1999] Plant Physiol 121: 1069–1079). To test this hypothesis, sunflower (Helianthus annuus L. cv Farnkasol) plants were grown in nutrient solutions supplied with various nitrogen (N) forms (NO₃^–, NH₄⁺ and NH₄NO₃), with or without pH control by using pH buffers [2-(N-morpholino)ethanesulfonic acid or 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid]. It was shown that high pH in the nutrient solution restricted uptake and shoot translocation of Fe independently of N form and, therefore, induced Fe deficiency chlorosis at low Fe supply [1 µM ferric ethylenediaminedi(O-hydroxyphenylacetic acid)]. Root NO₃^– supply (up to 40 mM) did not affect the relative distribution of Fe between leaf apoplast and symplast at constant low external pH of the root medium. Although perfusion of high pH-buffered solution (7.0) into the leaf apoplast restricted ⁵⁹Fe uptake rate as compared with low apoplastic solution pH (5.0 and 6.0, respectively), loading of NO₃^– (6 mM) showed no effect on ⁵⁹Fe uptake by the symplast of leaf cells. However, high light intensity strongly increased ⁵⁹Fe uptake, independently of apoplastic pH or of the presence of NO₃^– in the apoplastic solution. Finally, there are no indications in the present study that NO₃^– supply to roots results in the postulated inactivation of Fe in the leaf apoplast. It is concluded that NO₃^– nutrition results in Fe deficiency chlorosis exclusively by inhibited Fe acquisition by roots due to high pH at the root surface.

¹ This work was supported by the Deutsche Forschungsgemein-schaft as part of the special research project "The Apoplast of Higher Plants: Compartment for Storage, Transport, and Reactions."

² Present address: Centre for Multidisciplinary Studies, University of Belgrade, Kneza Viseslava 1, 11000 Belgrade, Yugoslavia.

^* Corresponding author; e-mail roemheld{at}uni-hohenheim.de; fax 49–711–459–3295.

Received November 15, 2002; returned for revision January 20, 2003; accepted March 23, 2003.

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