It has been hypothesized that
nitrate (NO(3)(-)) 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 (NO(3)(-), NH(4)(+) and NH(4)NO(3)), 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 micro M ferric
ethylenediaminedi(O-hydroxyphenylacetic acid)]. Root NO(3)(-) 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 (59)Fe uptake rate as compared with low apoplastic
solution pH (5.0 and 6.0, respectively), loading of NO(3)(-) (6 mM) showed no effect on (59)Fe uptake by the symplast of leaf cells. However, high light intensity strongly increased (59)Fe uptake, independently of apoplastic pH or of the presence of NO(3)(-) in the apoplastic
solution. Finally, there are no indications in the present study that NO(3)(-) supply to roots results in the postulated inactivation of Fe in the leaf apoplast. It is concluded that NO(3)(-) nutrition results in Fe deficiency
chlorosis exclusively by inhibited Fe acquisition by roots due to high pH at the root surface.