Restricted nitrate influx and reduction in corn seedlings exposed to ammonium

The effect of ambient ammonium (0.5 millimolar ¹⁴NH₄]₂SO₄) added to a nutrient solution containing 1.0 millimolar K¹⁵NO₃, 99 atom per cent ¹⁵N, upon ¹⁵N]nitrate assimilation and utilization of previously accumulated ¹⁴N]nitrate was investigated. Corn seedlings, 5-day-old dark-grown decapitated (experiment I) and 10-day-old light-grown intact (experiment II), which had previously been grown on K¹⁴NO₃ nutrient solution, were used. In both experiments, the presence of ambient ammonium decreased ¹⁵N]nitrate influx (20% after 6 hours) without significantly affecting the efflux of previously accumulated ¹⁴N]nitrate. In experiment I, relative reduction of ¹⁵N]nitrate (reduction as a percentage of influx) was inhibited more than was ¹⁵N]nitrate influx. Nevertheless, in experiment I, where all reduction could be assigned to the root system, the absolute inhibition of reduction during the 12 hours (13 micromoles/root) was less than the absolute inhibition in influx (24 micromoles/root). The data suggest that the influence of ammonium on ¹⁵N]nitrate influx could not be totally accounted for by the decrease in the potential driving force which resulted from restricted reduction; an additional impact on the influx process is indicated. Reduction of ¹⁵N]nitrate in experiment II after 6 hours accounted for 30 and 18% of the tissue excess ¹⁵N in the control and ammonium treatments, respectively. Relative distribution of ¹⁵N between roots and exudate (experiment I), or between roots and shoots (experiment II) was not affected by ammonium. On the other hand, the accumulation of ¹⁵N]nitrate in roots, shoots, and xylem exudate was enhanced by ammonium treatment compared to the control, whereas the accumulation of reduced ¹⁵N was inhibited.