Endogenous NO(3) in the Root as a Source of Substrate for Reduction in the Light |
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Authors: | Rufty T W Volk R J Mackown C T |
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Institution: | United States Department of Agriculture, Agricultural Research Service, North Carolina State University, Raleigh, North Carolina 27695. |
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Abstract: | 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 15NO3− for 12 hours in darkness and then returned to a solution containing 1.0 millimolar 14NO3− 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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