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

The nitrogen use efficiencies (NUE) of N2 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 N2 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 N2 fixation may lead to more rapid nodulation, production of more stress‐tolerant N2 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%.