It has been hypothesized that nitrate (NO 3 Ϫ ) nutrition might induce iron (Fe) deficiency chlorosis by inactivation of Fe in the leaf apoplast (H.U. Kosegarten, B. Hoffmann, K. Mengel [1999] Ϫ 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 Ϫ (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 Ϫ in the apoplastic solution. Finally, there are no indications in the present study that NO 3 Ϫ supply to roots results in the postulated inactivation of Fe in the leaf apoplast. It is concluded that NO 3 Ϫ nutrition results in Fe deficiency chlorosis exclusively by inhibited Fe acquisition by roots due to high pH at the root surface.When grown on highly calcareous soils, most plant species of the so called Strategy I type (Marschner and Rö mheld, 1994) respond to lack of iron (Fe) by developing characteristic symptoms of Fe deficiency chlorosis, primarily in young leaves. The concentration of Fe expressed on a dry weight leaf basis and the amount of Fe per leaf frequently decreases in chlorotic leaves, although the Fe concentration can sometimes be the same or even higher in chlorotic leaves as compared with green ones, as has been reported in field-grown woody plants ("the chlorosis paradox"; Morales et al., 1998;Rö mheld, 2000;Nikolic and Rö mheld, 2002). Various soil factors (e.g. CO 2 , ethylene, low temperature, high water content, and drought) resulting in severe inhibition of root growth might be responsible for triggering a restriction in leaf expansion growth, which in turn elevates the Fe concentration in these chlorotic leaves as a consequence of the diminished dilution of Fe concentration (Rö mheld, 2000, and refs. therein). This explanation would appear to account for the phenomenon of the "chlorosis paradox" that occurs occasionally under field conditions and is always associated with restricted leaf expansion growth. On the other hand, as has been hypothesized by Mengel and coworkers for many years, the higher Fe concentrations in the leaves of chlorotic plants could be caused by an increase in the pH of leaf apoplast induced by bicarbonate (HCO 3 Ϫ ) or nitrate (NO 3 Ϫ ) or both of these ions in the soil solution of calcareous soils (Mengel, 1994). These authors argue that because a high leaf apoplastic pH depresses the activity of Fe III reductase, less Fe 2ϩ can be transported across the plasma membrane into the leaf symplast, resulting in Fe deficiency chlorosis (Kosegarten et al., 2001). Despite this explanation, however, chlorotic plants with higher concentrations of Fe in younger leaves have never been found so far in nutrient solutions under controlled environmental...