Nitrogen use efficiency. 3. Nitrogen fixation: genes and costs

The nitrogen use efficiencies (NUE) of N-2 fixation, primary NH (+)(4) assimilation and NO (-)(3) assimilation are compared. The photon and water costs of the various biochemical and transport processes involved in plant growth, N-assimilation, pH regulation and osmolarity generation, per unit N assimilated are respectively likely to be around 5 and 7% greater for N-2 fixation than for a combination of NH (+)(4) and root and shoot NO (-)(3) assimilation as occurs with most crops. Studies on plant and rhizobial genes involved in nodulation and N-2 fixation may lead to more rapid nodulation, production of more stress-tolerant N-2 fixing systems and transfer of the hydrogenase system to rhizobium/legume symbioses which currently do not have this ability. The activity of an uptake hydrogenase is predicted to decrease the photon cost of diazotrophic plant growth by about 1%.