The aim of this thesis was to explore the physiological basis for differential yield characteristics of a barley QTL mapping population.  In 2002, 1.1 million tonnes of inorganic N fertiliser was applied in the UK, most of it to cereal crops.  Economic considerations aside, reduction of fertiliser applications, whilst maintaining yield, is a desirable goal, as a nitrogen, especially as nitrate, can be leached from the soil, causing environmental damage. 5.6 million tonnes of barley was grown in the UK during 2002.  Barley requires less fertiliser-N than wheat, much of which is recovered in biomass at harvest; however significant quantities of N are lost from the system. Initial pot-based trails of the QTL mapping population parental lines suggested phenotypic plasticity in their response to N application.  This was followed up by a further three experimental studies investigating the role of the chloroplast enzyme, Rubisco, in N storage, and its importance in the plants N economy.  Rubisco comprises approximately 25-30% of leaf N, or 15 - 20% of total plant N.  Rubisco is also important in the plants C metabolism, catalysing the Calvin cycle reaction between ribulose-1,5-bisphosphate and carbon dioxide, and represents the sole plant of inorganic carbon entry into biological systems. Field and laboratory-based trials were conducted to explore the hypothesis that Rubisco protein turnover is a mechanism of plant N loss, and that the regulation of turnover would have an impact on plant yield and grain properties.  Results suggest that barley genotypes exhibit a range of Rubisco turnover rates, and this affects not only the efficiency with which N is stored in the plant, but also the photosynthetic capacity of the plants, which correlates, ultimately, with yield.