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.