Source and magnitude of ammonium generation in maize roots

Studies with N-15 indicate that appreciable generation of NH4+ from endogenous sources accompanies the uptake and assimilation of exogenous NH4+ by roots. To identify the source of NH4+ generation, maize (Zea mays L.) seedlings were grown on (NH4+)-N-14 and then exposed for 3 d to highly labeled (NH4+)-N-15. More of the entering (NH4+)-N-15 was incorporated into the protein-N fraction of roots in darkness (approximately 25%) than in the light (approximately 14%). Although the (NH4+)-N-14 content of roots declined rapidly to less than 1 mu mol per plant, efflux of (NH4+)-N-14 continued throughout the 3-d period at an average daily rate of 14 mu mol per plant. As a consequence, cumulative (NH4+)-N-14 efflux during the 3-d period accounted for 25% of the total N-14 initially present in the root. Although soluble organic N-14 in roots declined during the 3-d period, insoluble N-14 remained relatively constant. In shoots both soluble organic N-14 and (NH4+)-N-14 declined, but a comparable increase in insoluble N-14 was noted. Thus, total N-14 in shoots remained constant, reflecting little or no net redistribution of N-14 between shoots and roots. Collectively, these observations reveal that catabolism of soluble organic N, not protein N, is the primary source of endogenous NH4+ generation in maize roots.