Regulation of nitrogen uptake on the whole plant level

The largest part of nitrogen requirements of crops is mostly covered by nitrate. The uptake of this ion is thermodynamically uphill and thus dependent on metabolism. This article considers regulation of N uptake in higher plants putting emphasis on NO3- and the whole plant level. In field conditions the transport rate depends on the concentration at the root surface in Michaelis-Menten-Kinetics. Maximum net influx of NO3- (I-max) was often reported at concentrations of 100 mu M NO3- and even lower. concentration at root surface has to be in the order of There are indications that for unrestricted growth the NO3- magnitude allowing I-max if plants are not able to compensate for lower NO3- concentrations by increasing root surface per unit of shoot. I-max is not a constant but depends for a given variety on N status of plants, the availability of NO3- and plant age. The decrease of I-max with increasing plant age is closely related to relative growth rate as long as the relationship between N demand and new growth is linear and the root:shoot ratio keeps constant. It seems that I-max is a meaningful physiological characteristic of NO3- uptake reflecting absolute N demand. There is evidence that shoot demand is linked to NO3- uptake of the root through an amino acid transport pool cycling in the plant via phloem and xylem. The N demand of a crop depends on increase of dry mass and might not be linear if the ''critical level'' of nitrogen in plant dry matter changes during crop development or if retranslocation of nitrogen from older leaves to meristematic tissue occurs. Radiation and temperature drive plant growth and thus N demand of crops. These relationships can be described by mathematical models.