Distribution of 15N-labeled urea injected into field-grown corn plants

Nitrogen (N) assimilate supply to developing corn (Zea mays L.) ears plays a critical role in grain dry weight accumulation. The use of stem-perfused/injected 15N labeled compounds to determine the effects of an artificial N source on the subsequent distribution of injected N and grain weight of field-grown corn plants has not been reported previously. 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 15N enriched urea at 15 and 30 mM N) were arranged in a split-plot design. Three N concentrations were injected using a pressurized stem injection technique. The injection started fifteen days after silking and continued until immediately prior to plant physiological maturity. The average uptake volume was 256 mL over the 30-day injection period. The N supplied via injection represented 1.5 to 3% of the total plant N. Neither soil applied N fertilizer nor injected N altered dry matter distribution among plant tissues. As the concentration of N in the injected solutions increased, N concentrations increased in the grain and upper stalks, and % 15N atom excess in ear+1 leaves and leaves increased. The relative degree of 15N 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 supplied via stem-injection, was incorporated into all the measured plant parts, although not uniformly. The distribution of the injected 15N 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 concentration as soil N fertility increased, combined with the increase in grain N concentration as the concentration of the injected N solutions increased demonstrated that the ability of the plant roots to take up from the soil was more limiting to grain N accumulation than was the ability of grain to take up N from the rest of plant.