In Plantago lanceolata L. the effect of Cu(II) additions to the nutrient solution on root-associated Fe(III) reductase was studied in a factorial design with different Cu(II) and Fe(III) concentrations. Iron starvation resulted in approx. an eightfold increase in root Fe reduction at the level of intact plants and twofold enhancement in the specific activity of both NADH-linked FeEDTA reductase and H+-ATPase in isolated root plasma membrane vesicles. In plants exposed to low (0·3–0·7 μM) Cu and suboptimal Fe levels, reduction activity at the root surface was further increased and associated with more severe interveinal chlorosis than plants grown in Cu-free medium. In Fe-sufficient plants, withholding Cu over a prolonged period slightly enhanced the reduction activity.

Addition of high (5 μM) Cu concentrations to Fe-free medium inhibited the induction of the physiological responses by Fe-deficiency stress. In plants without Fe supply but with adequate Cu supply, short-term application of 5 μM CuSO4 completely inhibited the reduction activity. Neither incubation of the plasma membrane vesicles before measurement nor incubation of intact plants with Cu before isolation caused a significant decrease in reductase activity. The results are interpreted as indicating different mechanisms underlying Cu-induced alterations in iron nutrition.