Project: Biochemical and physiological mechanisms of legume nitrogen fixation under higher atmospheric CO2 concentrations


Principal investigators:

Associated with:

  • The University of Melbourne
  • Agriculture Victoria Research
  • Grain Research and Development Corporation

Project Summary:

Elevated CO2 concentrations (e[CO2]) stimulate photosynthesis and greater carbon (C) supply to all C3 species. The result is vigorous plant growth which requires additional nitrogen (N) in proportion to extra C fixed at e[CO2]. Long term exposure of C3 plants under e[CO2] can cause N limitation. Legumes could overcome N limitation by their ability to fix additional N from the atmosphere. N fixation is regulated by the exchange of C and N compounds between bacteroids harboured in root nodules and host plants. Higher nodule biomass and greater supply of C to nodules under e[CO2] can increase N2 fixation.

Among the relatively large numbers of Free Air Carbon-dioxide Enrichment (FACE) studies analysing e[CO2] effect on growth and yield, only few studies focus on legume N2 fixation. Availability of carbohydrates and redistribution of N compounds to other plant tissues are the key factors modulating N2 fixation of legumes and these relations are likely to change under e[CO2]. Concentrations and types of C- and N- metabolites involved provide important clues about these changes at the biochemical/physiological levels.

Metabolism of legume nodules is highly sensitive to environmental stresses such as drought and heat. These stress factors decrease leaf photosynthesis and C supply to nodules.  Synthesis and transport of N metabolites through the xylem is also inhibited by stresses. Among several metabolites, arginine, glutamine and sucrose can accumulate in nodules and cause an “N feedback mechanism” to inhibit N2 fixation. There is currently no information available about how these metabolites concentrations change in response to stress under elevated CO2 and especially in a FACE facility. In addition to N2 fixation itself, the supply and translocation of N compounds to the maturing grain is another point of interest, because it is crucial in maintaining grain protein quality and quantity. Stress factors and e[CO2] alone and their interactions are likely to change which N pools are available and to what extent for grain filling.

The aims of this PhD project are to investigate the effect of e[CO2] on i)  the C and N assimilation, fixation and remobilization to grain in dryland legumes ii) how C and N metabolite profiles in nodules change under drought and how this is linked to N2 fixation of legumes and iii) how the change of  C and N metabolites during grain filling under e[CO2] as affected by heat stress. This project will expand our knowledge to underpin potential adaptations to increase N2 fixation and conserve grain protein quality in response to the stress factors (drought and heat) projected to accompany the rise in atmospheric CO2.