Mechanisms and Regulation of Ammonium Uptake in Higher Plants

Abstract

In soils, ammonium (NH₄ ⁺) mainly results from the mineralisation of organic matter and represents besides nitrate (NO₃ ⁻) the quantitatively most important source of nitrogen (N) for plant nutrition. In well-aerated agricultural soils, however, average annual NH₄ ⁺ concentrations are often 10 to 1000 times lower than those of NO₃ ⁻, rarely exceeding 50.µM (Marschner 1995). Despite these low concentrations in soils, NH₄ ⁺ uptake by plant roots can proceed at very high rates, due to the presence of transport systems in the root plasma membrane with a particularly high substrate affinity. Indeed, NH₄ ⁺ uptake is of major importance for N nutrition under numerous circumstances. On the one hand, NH₄ ⁺ nutrition plays an essential role in waterlogged and acid soils, or in cold climates where nitrification is inhibited (Marschner 1995). On the other hand, under mixed N nutrition (NO₃ ⁻ plus NH₄ ⁺), NH₄ ⁺ is often the preferential form of N taken up by the plant (Sasakawa and Yamamoto 1978; Gojon et al. 1986; Glass and Siddiqi 1995; Gazzarrini et al. 1999). NH₄ ⁺ is also probably the main form of N exported from symbiotic N₂-fixing microorganisms to their host plants, thereby making a major contribution to N nutrition in several plant families (Udvardi and Day 1997). In root nodules, NH₄ ⁺ is transported across the symbiosome membrane which segregates the bacteroids from the plant cytosol (see Chap. 3). In this case, NH₄ ⁺ concentrations in the plant cytosol can be about 50-fold lower than in the bacteroids (Streeter 1989), requiring low-affinity and high-capacity transport systems on the plant side, to ensure an efficient import of microbially fixed N (Tyerman et al. 1995).