An evaluation of the evidence for, and implications of, cytoplasmic nitrate homeostasis

A review of literature, reporting values of cytoplasmic/cytosolic NO₃^–] in plant cells, identified two major areas of disagreement: (1) disparity in the absolute values within the same system, and (2) constancy versus variability in cytoplasmic/cytosolic NO₃^–] with varying NO₃^–]_o. These differences are related to the techniques used by the different authors. Estimates of cytoplasmic NO₃^–] by compartmental analysis and by cell fractionation were consistently higher than the estimates by NO₃^–selective microelectrodes and by techniques based upon in vivo and in vitro nitrate reductase activity (NRA). A model recognizing more than one cytoplasmic ionic pool would satisfactorily reconcile the differences in both aspects, i.e. absolute values and constancy. Compartmental analysis and cell fractionation techniques may measure the amount of NO₃^– in the cytoplasm as a whole (including organelles); by contrast, NO₃^– selective microelectrodes and NRA estimate only the cytosolic NO₃^– and, hence, may result in lower estimates. Thus, variable organellar pool(s) may maintain a constant cytosolic pool as estimated by microelectrodes. However, certain observations remain at odds with the notion of a constant cytosolic NO₃^–].