Maize plants (Zea mays L. cv. R201) were grown to 21 d in pH-controlled gravel culture with 2 mM inorganic N supplied as nitrate alone, ammonium alone or 1:1 nitrate + ammonium. At 21 d, the 14N feeding solutions were replaced with 15N solutions, and xylem sap collections were made 4 and 8 h after the commencement of feeding. Leaf and root material was harvested also for in vitro nitrate reductase and glutamine synthetase activity assays.
Xylem sap analyses showed that in nitrate-only fed plants the major supply of nitrogen from root to shoot was in the nitrate form (60%) with 35 % carried as amino compounds. However, 93% of 15N was transported to the shoot as nitrate and only 6% in amino compounds, indicating the more direct routing of newly absorbed nitrogen to the shoot via the former. Leaf NRA was seven-fold greater than that of the root, confirming the shoot as the major site of nitrogen assimilation in plants fed only nitrate. In ammonium-only fed plants, 84% of xylem N was found in organic form (66%16N), the remainder translocating as ammonium, identifying the root as the major site of ammonium N assimilation. In ammonium + nitrate fed plants, 64% of xylem N was present as organic N (55%16N), 34% as nitrate (43 %16N), indicating shared N assimilation between shoot and root, with root assimilation predominating. In plants receiving nitrate, glutamine was the major N compound translocated, in plants receiving only ammonium, asparagine predominated. GS activity was approximately the same in root and shoot and showed no response to N source.
The significance of these results is discussed with respect to the reported increased productivity of maize fed a mixed nitrate-ammonium N source.