EFFECTS OF ALUMINUM ON NITRATE UPTAKE AND ASSIMILATION

A study was conducted to examine the hypothesis that the effects of ex ternal Al on NO3- uptake and assimilation depend upon the concentratio n of Al present, Young soybean seedlings [Glycine max (L.) Merrill, cv . Essex], growing under moderate acidity stress at pH 4.2, were expose d to a range of {Al3+} in solution for 3 d, and to labelled 99 atom % (NO3-)-N-15 during the final hour of Al exposure, Uptake of (15)NO(3)( -)g(-1) root dry weight was increased by about 28% in the presence of Al at {Al3+} below 10 mmol m(-3), and NO3- uptake was decreased by abo ut 12 % when the {Al3+} increased to 44 mmol m(-3). The stimulation ph ase closely paralleled stimulation of root elongation, At higher {Al3}, the inhibition of root elongation was much more severe than that of NO3- uptake, There was no indication of a separate effect of Al on ro ot (NO3-)-N-15 reduction in situ, as the accumulation of reduced N-15 in the root remained a similar percentage of (NO3-)-N-15 uptake at all {Al3+}, At higher {Al3+}, the atom % N-15 enrichment of the insoluble reduced-N (protein) fraction of root tips increased, This suggests th at the Al inhibition of root elongation did not result from disruption of the N supply to the root apex.