Homeostasis of ammonium and nitrate in plants

Nitrogen (N) is a major plant nutrient, and its supply is very often limiting growth. The main forms of inorganic N in soil supplying plants are ammonium and nitrate ions. Although the soil availability of N can vary greatly, the cytoplasmic nutrient ion activities in a typical plant cell are maintained at set points that are independent of changes in supply. By contrast, the storage of N as protein and vacuolar nitrate depends on the external supply. Measurements of cellular homeostasis of ammonium and nitrate are limited by methodology. The upper limits for cytoplasmic set points are likely to depend on toxicity, and for ammonium this is well known but less clear for nitrate. An intracellular set point for N must be maintained by membrane transport systems and assimilation processes. Crop N use efficiency has uptake and assimilation components, and understanding homeostasis is fundamentally important for improving this important trait.