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Metabolism and Enzymology

Endogenous NO₃^– in the Root as a Source of Substrate for Reduction in the Light ¹

United States Department of Agriculture, Agricultural Research Service, North Carolina State University, Raleigh, North Carolina 27695, Department of Crop Science, North Carolina State University, Raleigh, North Carolina 27695, Department of Botany, North Carolina State University, Raleigh, North Carolina 27695, Department of Soil Science, North Carolina State University, Raleigh, North Carolina 27695, Department of Agronomy, University of Kentucky, Lexington, Kentucky 40546

An experiment was conducted to investigate the reduction of endogenous NO₃^–, which had been taken up by plants in darkness, during the course of the subsequent light period. Vegetative, nonnodulated soybean plants (Glycine max [L]. Merrill, `Ransom') were exposed to 1.0 millimolar <sup>15</sup>NO<sub>3</sub><sup>–</sup> for 12 hours in darkness and then returned to a solution containing 1.0 millimolar <sup>14</sup>NO<sub>3</sub><sup>–</sup> for the 12 hours `chase' period in the light. Another set of plants was exposed to ¹⁵NO₃^– during the light period to allow a direct comparison of contributions of substrate from the endogenous and exogenous sources. At the end of the ¹⁵NO₃^– exposure in the dark, 70% of the absorbed ¹⁵NO₃^– remained unreduced, and 83% of this unreduced NO₃^– was retained in roots. The pool of endogenous ¹⁵NO₃^– in roots was depleted at a steady rate during the initial 9 hours of light and was utilized almost exclusively in the formation of insoluble reduced-N in leaves. Unlabeled endogenous NO₃^–, which had accumulated in the root prior to the previous dark period, also was depleted in the light. When exogenous ¹⁵NO₃^– was supplied during the light period, the rate of assimilation progressively increased, reflecting an increased rate of uptake and decreased accumulation of NO₃^– in the root tissue. The dark-absorbed endogenous NO₃^– in the root was the primary source of substrate for whole-plant NO₃^– reduction in the first 6 hours of the light period, and exogenous NO₃^– was the primary source of substrate thereafter. It is concluded that retention of NO₃^– in roots in darkness and its release in the following light period is an important whole-plant regulatory mechanism which serves to coordinate delivery of substrate with the maximal potential for NO₃^– assimilation in photosynthetic tissues.

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