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Abstract
Iron chlorosis of plants grown on calcareous soils is not induced by an absolute Fe deficiency but rather results from a physiological disorder which affects the mobility of Fe in the entire plant. Evidence is provided that this Fe immobility is caused by an alkaline nutrition which means NO‐ 3”; as the major, if not as the sole N source combined with HCO‐ 3. Such alkaline nutritional conditions prevail in calcareous soils. It is supposed that alkaline nutrition results in high pH levels in the leaf apoplast which may bring about a precipitation of Fe. It is also feasible that a high leaf apoplast pH inhibits the plasmalemma located FeIII reductase which is responsible for the Fe transfer across the plasmamembrane. Measurements which decrease the apoplastic pH such as NH4 + nutrition, the application of indole acetic acid or fusicoccin resulted in a regreening of Fe chlorotic leaves.