Nitrate does not result in iron inactivation in the apoplast of sunflower leaves

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 mum 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 Fe-59 uptake rate as compared with low apoplastic solution pH (5.0 and 6.0, respectively), loading of NO3- (6 mM) showed no effect on Fe-59 uptake by the symplast of leaf cells. However, high light intensity strongly increased Fe-59 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.