Inhibition of Nitrate Assimilation in Roots in the Presence of Ammonium: The Moderating Influence of Potassium

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Abstract
Experiments were conducted to investigate the effect of concentration of NH4+ in nutrient solution on root assimilation of NO3− and to determine whether the NH4+NO3− interaction was modified in the presence of K+. Dark-grown, detopped corn seedlings (cv. Pioneer 3369A) were exposed for 8 h to 0.15 mM Ca(NO3)2 and varying concentrations of (NH4)2SO4 in the absence or presence of 0.15 mM K2SO4. The accelerated phase of NO3− uptake appeared most sensitive to restriction by additions of 0.15 mM (NH4)2SO4. In the absence of K+, the restriction increased only slightly even when solution (NH4)2SO4, was increased from 0.15 mM to 12.5 mM which was accompanied by an increase of NH4+ in the tissue from about 7.0 to 35 μmol g−1 fr. wt. of root. Increasing concentrations of solution NH4+ progressively inhibited net K+ uptake. At the highest solution NH4+ concentrations, there was an initial net efflux of K+ and no net influx occurred during the treatment period. The severity of the NH4)SO4 restriction of NO3− uptake was moderated considerably in the presence of K+ as long as a net influx of K+ occurred. However, net influx of K+ was not associated with alteration of NH4+ uptake, assimilation, or accumulation in the root tissue. The lack of correlation between the severity of restriction of NO3− uptake and endogenous NHJ suggested the restriction resulted from an effect exerted by exogenous NH4+ which tended to saturate at lower solution NHJ concentrations or by inhibitory factors generated during assimilation of NH4+. Several mechanisms were postulated to account for the moderating influence of K+. In all experiments, root NO3− reduction was restricted by the presence of ambient NH4+. The quantitative decreases in reduction tended to be less than decreases in NO3− uptake and therefore, could result from inhibition solely of uptake with subsequent limitation in availability of substrate for the reduction process, but the possibility of a direct effect on reduction could not be excluded.
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