INHIBITION OF NITRATE UPTAKE BY ALUMINUM IN MAIZE

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.