Experiments with two maize (Zea mays L.) hybrids were conducted to det
ermine (a) if the inhibition of nitrate uptake by aluminium involved a
restriction in the induction (synthesis/assemblage) of nitrate transp
orters, and (b) if the magnitude of the inhibition was affected by the
concurrent presence of ambient ammonium. At pH 4.5. the rate of nitra
te uptake from 240 muM NH4NO3 was maximally inhibited by 100 muM alumi
nium, but there was little measurable effect on the rate of ammonium u
ptake. Presence of ambient aluminium did not eliminate the characteris
tic induction pattern of nitrate uptake upon first exposure of nitroge
n-depleted seedlings to that ion. Removal of ambient aluminium after s
ix hours of induction resulted in recovery within 30 minutes to rates
of nitrate uptake that were similar to those of plants induced in abse
nce of aluminium. Addition of aluminium to plants that had been induce
d in absence of aluminium rapidly restricted the rate of nitrate uptak
e to the level of plants that had been induced in the presence of alum
inium. The data are interpreted as indicating that aluminium inhibited
the activity of nitrate transporters to a greater extent than the ind
uction of those transporters. When aluminium was added at initiation o
f induction, the effect of ambient ammonium on development of the inhi
bition by aluminium differed between the two hybrids. The responses in
dicate a complex interaction between the aluminium and ammonium compon
ents of high acidity soils in their influence on nitrate uptake.