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

Abstract

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%.

Photosynthetic Energy Conversion Efficiency: Setting a Baseline for Gauging Future Improvements in Important Food and Biofuel Crops

“…One study reports a 5% greater photon energy requirement for nitrogen fixation compared with the combined cost of NH 4 + and NO 3 2 assimilation that occurs in most nonlegume crops (Andrews et al, 2009). In terms of carbon usage, the proportion of assimilated carbon that is diverted to the nodules for nitrogen fixation is reported as 7% to 19% (Gordon et al, 1987;Vessey et al, 1988;Hansen et al, 1992Hansen et al, , 1993Fujikake et al, 2003;Ito et al, 2006).…”

Improving Nitrogen Use Efficiency in Crops for Sustainable Agriculture

“…Thus, NUE and energy input are seen as important indicators for the environmental impact of the production of conventional crops but also of energy crops, since they have a large capacity to produce biomass with the minimal amount of N fertilizer [29]. Comparatively, the net energy cost of N 2 fixation in leguminous species is lower than that necessary for an equivalent production of synthetic N fertilizers [30,31]. Therefore, it will be advantageous to the farmer to include more legumes both in crop rotations and in cover crops, whether the main cultivated crop is grown for grain or biomass.…”

“…There are accurate estimations of annual inputs of symbiotically fixed N by legume crops. However, the amount of N fixed by other agricultural production systems involving non-symbiotic N 2 fixing associations, such as rice, sugar cane and cereals is much more difficult to estimate (see [30,33,34] for reviews).…”

AFestschriftin honour of Professor John A. Raven

“…One of us was fortunate enough to have had John as a lecturer at undergraduate level, as a Ph.D. supervisor, and then to have worked with him over the following 25 years building on some of his earlier work (e.g. Andrews et al 1984Andrews et al , 1999Andrews et al , 2005Andrews et al , 2009aRaven et al 2004Raven et al , 2005a. We highlight two areas of John's current research -firstly, 'Interactions among resources in the growth of phytoplankton', and secondly, 'The potential for photosynthesis on other planets'.…”

“…He concluded, for example, that photon and water savings can be made with nitrate assimilation in comparison with nitrogen fixation but this is dependent on site of nitrate assimilation (root or shoot) and mechanism of pH regulation. Subsequent work has altered some of the stoichiometries used by Raven (1985) but recent revisions of the calculations indicate that they are an underestimate by only a few percent at most, and that the order of costs is unaltered as a function of pathway or site of nitrogen assimilation or the mechanism of pH regulation (Andrews et al , 2005(Andrews et al , 2009a A further area is that of how the earliest embryophytic plants functioned in their early-and mid-Palaeozoic environment (Raven 1977(Raven , 1993(Raven , 1995(Raven , 2002Raven and Edwards 2001). Here, John has considered the evolution of important extant higher plant features such as roots, long-distance transport processes, intercellular gas spaces, cuticle and stomata.…”