Nitrogen assimilation by legumes - processes and ecological limitations

Legumes may feed on three different sources of nitrogen: nitrate, ammonium, and, due to symbiotic N₂ fixation, atmospheric dinitrogen. In all three cases ammonium is finally assimilated by the glutamine synthetase (GS) / glutamate synthase (GOGAT) system. NH ₄ ⁺ produced by nitrogenase in symbiosomes of legume nodules is released into the host cell cytosol where it is incorporated into amino acids and amides. The release of NH ₄ ⁺ into the cytosol appears to occur purely by diffusion. Therefore, the activity of the GS / GOGAT enzymes is decicive to avoid product inhibition of nitrogenase by NH ₄ ⁺ . No information is available on the mechanism of xylem loading with amides or ureides, a process that may play a key role in avoiding accumulation of amino acids in infected nodule cells. The same applies to phloem unloading of sucrose. Both transport processes, however, may determine the efficiency of N₂ fixation by legumes.There is no convincing evidence that N₂ fixation by legumes is generally limited by energy supply to nodules. On the other hand, N₂ fixation is often restricted by environmental constraints. Environmental stresses may limit N₂ fixation of legumes at four different levels:Rhizobium (Bradyrhizobium) multiplication in soil, rhizobial infection of roots, nodulation, and N₂ fixation. There is increasing evidence that, sufficient infection by effective rhizobial strains provided, N demand of the host plant determines the potential of N₂ fixation. Various environmental stresses and supply of mineral N reduce nodulation and nitrogenase activity without affecting total N concentration of the plant tissue. Stress-induced reduction of plant growth, however, results in an accumulation of free amino acids, amides, or ureides in shoots, roots, and nodules which may be responsible for the regulation of nodulation and nitrogenase activity via a feedback system. This implies that enhancement of N₂ fixation by legumes can be realized in two different ways: either by improvement of stress resistance and dry matter accumulation or by uncoupling of the feedback control.