Endogenous NO3− in the Root as a Source of Substrate for Reduction in the Light

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 hte 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.