Homeostasis of ammonium and nitrate in plants

Nitrogen (N) is a major plant nutrient, and its supply is very often limiting growth.The mainforms of inorganic N in soil supplying plants are ammonium and nitrate ions. Although thesoil availability of N can vary greatly, the cytoplasmic nutrient ion activities in a typical plantcell are maintained at set points that are independent of changes in supply. By contrast, thestorage of N as protein and vacuolar nitrate depends on the external supply. Measurements ofcellular homeostasis of ammoniumand nitrate are limited bymethodology.The upper limits forcytoplasmic set points are likely to depend on toxicity, and for ammonium this is well knownbut less clear for nitrate. An intracellular set point for N must be maintained by membranetransport systems and assimilation processes. CropNuse efficiency has uptake and assimilationcomponents, and understanding homeostasis is fundamentally important for improving thisimportant trait.