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Metabolism and Enzymology

Effects of Altered Carbohydrate Availability on Whole-Plant Assimilation of ¹⁵NO₃^–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 40546-0091

An experiment was conducted to investigate the relative changes in NO₃^– assimilatory processes which occurred in response to decreasing carbohydrate availability. Young tobacco plants (Nicotiana tabacum [L.], cv NC 2326) growing in solution culture were exposed to 1.0 millimolar ¹⁵NO₃^– for 6 hour intervals during a normal 12 hour light period and a subsequent period of darkness lasting 42 hours. Uptake of ¹⁵NO₃^– decreased to 71 to 83% of the uptake rate in the light during the initial 18 hours of darkness; uptake then decreased sharply over the next 12 hours of darkness to 11 to 17% of the light rate, coincident with depletion of tissue carbohydrate reserves and a marked decline in root respiration. Changes also occurred in endogenous ¹⁵NO₃^– assimilation processes, which were distinctly different than those in ¹⁵NO₃^– uptake. During the extended dark period, translocation of absorbed ¹⁵N out of the root to the shoot varied rhythmically. The adjustments were independent of ¹⁵NO₃^– uptake rate and carbohydrate status, but were reciprocally related to rhythmic adjustments in stomatal resistance and, presumably, water movement through the root system. Whole plant reduction of ¹⁵NO₃^– always was limited more than uptake. The assimilation of ¹⁵N into insoluble reduced-N in roots remained a constant proportion of uptake throughout, while assimilation in the shoot declined markedly in the first 18 hours of darkness before stabilizing at a low level. The plants clearly retained a capacity for ¹⁵NO₃^– reduction and synthesis of insoluble reduced-¹⁵N even when ¹⁵NO₃^– uptake was severely restricted and minimal carbohydrate reserves remained in the tissue.

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