DISTRIBUTION OF N-15-LABELED UREA INJECTED INTO FIELD-GROWN CORN PLANTS

Nitrogen (N) assimilate supply to developing corn (Zea mays L.) ears p lays a critical role in grain dry weight accumulation. The use of stem -perfused/ injected N-15 labeled compounds to determine the effects of an artificial N source on the subsequent distribution of injected N a nd grain weight of field-grown corn plants has not been reported previ ously. Our objective was to assess the distribution of N added via an artificial source. Three soil N fertilizer levels (0, 180, and 270 kg N ha(-1)) and three N solutions (distilled water control and N-15 enri ched urea at 15 and 30 mM N) were arranged in a split-plot design. Thr ee N concentrations were injected using a pressurized stem injection t echnique. The injection started fifteen days after silking and continu ed until immediately prior to plant physiological maturity. The averag e uptake volume was 256 mL over the 30-day injection period. The N sup plied via injection represented 1.5 to 3% of the total plant N. Neithe r soil applied N fertilizer nor injected N altered dry matter distribu tion among plant tissues. As the concentration of N in the injected so lutions increased, N concentrations increased in the grain and upper s talks, and % N-15 atom excess in ear+1 leaves and leaves increased. Th e relative degree of N-15 enrichment for each of the tissues measured was injected internode>grain>upper stalks>leaves>lower stalks>cob>husk >ear+1 leaf>ear leaf. This study indicated that the exogenous N suppli ed via stem-injection, was incorporated into all the measured plant pa rts, although not uniformly. The distribution of the injected N-15 was affected both by the proximity of sinks to the point of injection and the strength of the various sinks. The lack of increase in grain N co ncentration as soil N fertility increased, combined with the increase in grain N concentration as the concentration of the injected N soluti ons increased demonstrated that the ability of the plant roots to take up from the soil was more limiting to grain N accumulation than was t he ability of grain to take up N from the rest of plant.