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Nitrate Uptake and Assimilation by Wheat Seedlings during Initial Exposure to Nitrate ¹

^a Department of Soil Science, North Carolina State University, Raleigh, North Carolina 27607

Nitrate uptake, reduction, and translocation were examined in intact, 14-day-old, nitrogen-depleted wheat (Triticum vulgare var. Knox) seedlings during a 9-hour exposure to 0.2 mM Ca (NO₃)₂. The nitrate uptake rate was low during the initial 3-hour period, increased during the 3- to 6-hour period, and then declined. By the 3rd hour, 14% of the absorbed nitrate had been reduced, and this increased to 36% by the 9th hour. Shoots accumulated reduced ¹⁵N more rapidly than roots and the ratio of reduced ¹⁵N to ¹⁵N-nitrate was higher in the shoots. A significant proportion of the total reduction occurred in the root system under these experimental conditions. Accumulation of ¹⁵N in ethanol-insoluble forms was evident in both roots and shoots by the 3rd hour and, after 4.5 hours, increased more rapidly in shoots than in roots.

An experiment in which a 3-hour exposure to 0.2 mM Ca (¹⁵NO₃)₂ was followed by a 12-hour exposure to 0.2 mM Ca (¹⁴NO₃)₂ revealed a half-time of depletion of root nitrate of about 2.5 hours. A large proportion of this depletion, however, was due to loss of ¹⁵N-nitrate to the ambient ¹⁴N-nitrate solution. The remaining pool of ¹⁵N-nitrate was only slowly available for reduction. Total ¹⁵N translocation to the shoot was relatively efficient during the first 3 hours after transfer to Ca (¹⁴NO₃)₂ but it essentially ceased after that time in spite of significant pools of ¹⁵N-nitrate and -amino-¹⁵N remaining in the root tissue.

¹ Paper No. 4475 of the Journal Series of the North Carolina Agricultural Experiment Station, Raleigh. These investigations were supported in part by the United States Atomic Energy Commission, Grant AT-(40-1)-2410.

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