LOCALIZATION OF NITRATE ABSORPTION AND TRANSLOCATION WITHIN MORPHOLOGICAL REGIONS OF THE CORN ROOT

The absorption of NO3- was characterized in six regions of a 7-d-old corn root (Zea mays L. cv W64A x W182E) growing in a complete nutrient solution. Based on changing rates of N-15 accumulation during 15-min time courses, translocation of the concurrently absorbed N through each region of the intact root was calculated and distinguished from direct absorption from the medium. Of the N-15 accumulated in the 5-mm root tip after 15 min, less than 15 and 35% had been absorbed directly from the external solution at 0.1 and 10 mM NO3- concentration of the external solution, respectively. The characterization of the apical portion of the primary root as a sink for concurrently absorbed N was con-confirmed in a pulse-chase experiment that showed an 81% increase of N-15 in the 5-mM root tip during a 12-min chase (subsequent to a 6-min labeling period). The lateral roots alone accounted for 60% of root influx and 70% of 15-min whole root N-15 accumulation at either 0.1 or 10 mM. NO3- concentration of the external solution. Because relatively steady rates of N-15 accumulation in the shoot were reached after 6 min, the rapidly exchanging pools in lateral roots must have been involved in supplying N-15 to the shoot. The laterals and the basal primary root also showed large decreases (24 and 17%) in N-15 during the chase experiment, confirming their role in rapid translocation.