Nitrogen dynamics in plant growth systems.

The predominant nitrogen source for the plants in closed environmental systems is the mineral nitrogen (i.e., nitrate and/or ammonium) in the nutrient medium. The following focuses on the processes through which plants obtain nitrate and ammonium from the rhizosphere and on the influences that each form has upon plant performance. Most plant species can sustain full growth at nitrate or ammonium concentrations that are over two orders of magnitude lower than those provided in most plant growth systems. Under the high concentrations (mM) normally used, root nitrogen absorption is downregulated: a) both the affinity and capacity of the transport systems for ammonium or nitrate are diminished, b) efflux of either ion becomes a significant percentage of influx, and c) root growth is inhibited. High concentrations also promote accumulation of ammonium or borate in plant tissues to potentially deleterious levels and foster microbial outbreaks. Several lines of evidence argue that roots in natural soils are normally exposed to lower concentrations (micromoles) of nitrate or ammonium: models of root nutrient absorption indicate that roots deplete rhizosphere nitrate and ammonium to such levels; the high-affinity transport systems for nitrate and ammonium have optimal control in this range; and root growth and development is maximized under such conditions. The high-affinity transport systems are distinct for nitrate and ammonium. In general, the affinity of the nitrate system for nitrate is less than the ammonium system for ammonium. Nitrate absorption is induced by the presence of ammonium or nitrate. Roots most rapidly absorb nitrate in the zone where root hairs emerge and ammonium in the zone of division near the apex. Nitrate absorption tends to alkalinize the rhizosphere, whereas ammonium absorption acidifies the rhizosphere. The energy requirements for absorption and assimilation of nitrate are several fold higher than those of ammonium. Root growth and elongation are more extensive when ammonium is provided as the sole nitrogen source, perhaps as a consequence of the lower energy requirements or the increased rhizosphere acidity.