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Articles

Nitrate Accumulation, Assimilation, and Transport by Decapitated Corn Roots ¹

EFFECTS OF PRIOR NITRATE NUTRITION

Department of Soil Science, North Carolina State University, Raleigh, North Carolina 27650

The effects of accumulated [¹⁴N]nitrate and its utilization in decapitated, 5-day-old dark-grown corn roots on influx, accumulation, xylem deposition, and reduction of concurrently absorbed nitrate during an 18-hour exposure to 0.5 millimolar K¹⁵NO₃ nutrient solution were examined. A 20-hour pretreatment in 15.0 millimolar K¹⁴NO₃ high nitrate (HN) resulted in a 2-fold greater tissue nitrate level than pretreatment in 0.5 millimolar K¹⁴NO₃ low nitrate (LN). Upon transfer to the 0.5 millimolar K¹⁵NO₃ solution, the net nitrate uptake rate in HN roots after 2 hours was 52% of the LN rate, but increased to 93% at the end of the uptake period. Despite an enhanced [¹⁴N]nitrate efflux from HN roots to the uptake solution, the efflux differences between the two pretreatments did not compensate for the decrease in net nitrate uptake. The [¹⁵N]nitrate influx rate was initially restricted by 33% in the HN roots compared to LN roots, but it had decreased to 7% by the end of the 18-hour uptake period. At this time, the total tissue nitrate levels were similar for both pretreatments. The rate of accumulation of [¹⁵N]nitrate in the tissue was relatively constant for both pretreatments, but was 25% less in HN roots. Of the previously accumulated [¹⁴N]nitrate, 52 and 46% remained after 18 hours in the LN and HN roots, respectively. The [¹⁴N]nitrate decline for HN roots was initially more rapid than in the LN roots which was linear over time. Xylem transport and efflux more than accounted for the decline in [¹⁴N]nitrate of LN roots and all but 4% in the HN roots which was attributed to reduction. Compartmentation of the previously accumulated nitrate was evident from the higher atom per cent ¹⁵N of xylem nitrate compared to that of the tissue nitrate of both LN and HN roots. During the first 2 hours, xylem transport of [¹⁴N]nitrate by the HN roots was 49% greater than for LN roots, while [¹⁵N]nitrate transport was 9% less in HN roots compared to LN roots. Even though the reduction of [¹⁵N]nitrate in HN roots was 31% less than LN roots during the first 2 hours, [¹⁵N]nitrate was reduced more rapidly than the previously accumulated [¹⁴N]nitrate. After the first 4 hours, the relative partitioning of absorbed [¹⁵N]nitrate between accumulation, reduction, and translocation was similar regardless of pretreatment.

¹ This work was supported by Grant PCM-7703152 from the National Science Foundation. Paper No. 6496 of the Journal Series of the North Carolina Agricultural Research Service at Raleigh.