Summary Young sunflower plants ( Helianthus annuus L. cv. Topflor), grown in iron (Fe)-free nutrient solution, developed Fe deficiency symptoms characteristic for dicotyledonous and non-graminaceous monocotyledons, such as chlorosis, reduced shoot growth, changes in root morphology, increased ferricyanide reductase activity and net proton extrusion. The ferricyanide reductase activity of roots was low in the acidic pH range and increased with increasing pH, reaching maximal activity above pH 7. Five to ten mM Ca 2+ and 0.25 mM NAD(P)H caused a 2- to 3-fold increase in the reductase activity. Plasma membrane (PM) vesicles were purified from the microsomal fraction (MF) of roots of Fe deficient and Fe sufficient sunflower using an aqueous polymer two-phase system. Fe deficiency induced an increase in NAD(P)H-ferricyanide oxidoreductase activity both in the microsomal (2-fold) and plasma membrane (3-fold) fractions. ATPase activity in PM from Fe deficient roots was not significantly changed. These data demonstrate that the linkage between PM ATPase and reductase systems occurs via changes in membrane polarization and acidification rather than via direct coupling. However, the activity of the ATPase under physiological conditions may also be altered due to structural changes in root cells and tissues under Fe deficiency.