EFFECT OF NITROGEN FEEDING SOURCE ON THE SUPPLY OF NITROGEN FROM ROOT TO SHOOT AND THE SITE OF NITROGEN ASSIMILATION IN MAIZE (ZEA MAYS L. CV. R201).

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 N feeding solutions were replaced with N 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 N 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% N), 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% N), 34% as nitrate (43 % N), 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.