Regulation of nitrogen uptake on the whole plant level

Abstract

The largest part of nitrogen requirements of crops is mostly covered by nitrate. The uptake of this ion is thermody-namically uphill and thus dependent on metabolism. This article considers regulation of N uptake in higher plants putting emphasis on NO;-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 NO;-(lmax) was often reported at concentrations of 100 pM NO;-and even lower. There are indications that for unrestricted growth the NO;-concentration at root surface has to be in the order of magnitude allowing Imax if plants are not able to compensate for lower NO;-concentrations by increasing root surface per unit of shoot. Imax is not a constant but depends for a given variety on N status of plants, the availability of NO;-and plant age. The decrease of Imax 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 rootshoot ratio keeps constant. It seems that Imax is a meaningful physiological characteristic of NO;-uptake reflecting absolute N demand. There is evidence that shoot demand is linked to NO;-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.