Localization of Nitrate Absorption and Translocation within Morphological Regions of the Corn Root

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Abstract
The absorption of NO3− was characterized in six regions of a 7-d-old corn root (Zea mays L. cv W64A × W182E) growing in a complete nutrient solution. Based on changing rates of 15N 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 15N 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 conconfirmed in a pulse-chase experiment that showed an 81% increase of 15N 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 15N accumulation at either 0.1 or 10 mm. NO3− concentration of the external solution. Because relatively steady rates of 15N accumulation in the shoot were reached after 6 min, the rapidly exchanging pools in lateral roots must have been involved in supplying 15N to the shoot. The laterals and the basal primary root also showed large decreases (24 and 17%) in 15N during the chase experiment, confirming their role in rapid translocation.