Source and Magnitude of Ammonium Generation in Maize Roots

Studies with (15)N indicate that appreciable generation of NH(4)(+) from endogenous sources accompanies the uptake and assimilation of exogenous NH(4)(+) by roots. To identify the source of NH(4)(+) generation, maize (Zea mays L.) seedlings were grown on (14)NH(4)(+) and then exposed for 3 d to highly labeled (15)NH(4)(+). More of the entering (15)NH(4)(+) was incorporated into the protein-N fraction of roots in darkness (approximately 25%) than in the light (approximately 14%). Although the (14)NH(4)(+) content of roots declined rapidly to less than 1 μmol per plant, efflux of (14)NH(4)(+) continued throughout the 3-d period at an average daily rate of 14 μmol per plant. As a consequence, cumulative (14)NH(4)(+) efflux during the 3-d period accounted for 25% of the total (14)N initially present in the root. Although soluble organic (14)N in roots declined during the 3-d period, insoluble (14)N remained relatively constant. In shoots both soluble organic (14)N and (14)NH(4)(+) declined, but a comparable increase in insoluble (14)N was noted. Thus, total (14)N in shoots remained constant, reflecting little or no net redistribution of (14)N between shoots and roots. Collectively, these observations reveal that catabolism of soluble organic N, not protein N, is the primary source of endogenous NH(4)(+) generation in maize roots.