School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemBiochemical and physiological mechanisms of legume nitrogen fixation under higher atmospheric CO2 concentrations( 2019)Atmospheric CO2 concentration ([CO2]) is expected to rise from a current level of ~400 to 550 µmol mol-1 by 2050. It is well established that elevated [CO2] enhances plant growth and yield. However, the stimulation of plant growth at elevated [CO2] requires additional nitrogen (N) and prolonged exposure to elevated [CO2] potentially risks N limitation. Legumes can overcome such limitations by fixing aerial N. Previous studies under Free Air CO2 Enrichment (FACE) have shown that elevated [CO2] can stimulate N2 fixation, but it is unknown to what extent this applies to dryland Mediterranean environments or what impact environmental interactions have. Legumes grown in dryland environments frequently experience terminal drought accompanied by high temperature during reproductive phases. It has been suggested that elevated [CO2] delays the effect of drought by conserving soil water, maintaining N2 fixation mechanisms for longer under drought. This thesis addresses this gap by investigating the growth and N economy of three legumes (lentil, field pea and faba bean) in a FACE facility in a semi-arid environment where seasonal and experimentally controlled drought was imposed. 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 N concentration. This thesis investigated N2 fixation, remobilization and grain quality of dryland legumes under predicted future e[CO2] atmosphere conditions, including interactions with drought, heat waves, and genotypes. Free Air CO2 Enrichment technology was used to simulate future growing conditions in the field with target [CO2] as expected by the middle of this century. Elevated [CO2] stimulated N2 fixation through increased nodule number, nodule biomass, and nodule activity to a greater extent under unstressed conditions. Soil water savings under elevated [CO2] were only temporary, so that drought reduced nodule activity due to lower C/sucrose supply and therefore decreased N2 fixation. Consequently, elevated [CO2] was found to stimulate N2 fixation of all three species of legumes, but this effect was smaller under drought or heat stress. The decrease of N2 fixation under drought caused depletion of grain N concentration under elevated [CO2]. In contrast, when soil water was sufficient, N2 fixation continued throughout the grain filling period, and grain N concentration was maintained under elevated [CO2]. Traits that allow N2 fixation for longer throughout the growing season, e. g. by exploiting potential water savings mechanisms under elevated [CO2], may confer benefits under future climatic conditions. Findings of this study are now available to underpin new strategies for improvement of the N2 fixation potential of legumes as atmospheric [CO2] continues to increase in the future.