Improving nitrogen use efficiency (NUE) in cereals is a key target for sustainable intensification of agriculture. This is particularly the case under conditions with rising atmospheric carbon dioxide (CO2) levels, which may have negative consequences for grain protein concentration. Cytosolic glutamine synthetase (GS1) plays an important role in N remobilization and is determinant for grain yield, grain protein concentration and NUE. The PhD study investigated the interactions between N supply and atmospheric CO2 levels with respect to yield structure and NUE in barley (Hordeum vulgare L.) overexpressing the GS1 isogene HvGS1-1 under the control of a constitutive 35S promoter (transgenic approach) or the native HvGS1-1 gene with its own promoter (cisgenic approach). Overexpression of GS1-1 by either the transgenic or cisgenic approach resulted in higher grain yield and NUE. The extra capacity for N assimilation obtained by GS1 overexpression also prevented the decline in grain protein levels under CO2 enrichment.

A second part of the PhD study focused on grain quality of cereals under elevated CO2 conditions, as altered amino acid composition and lowered mineral elements concentration are commonly observed under CO2 enrichment. One of the major grain storage protein fractions in barley is C-hordein, which contains the lowest proportion of sulphur-rich amino acids and is poorest in binding of zinc. The work demonstrated that barley lines in which the C-hordein content was reduced by RNA-interference or antisense technology not only showed altered amino acid composition, but also 50% increased zinc concentration in the core endosperm.

Overall, the PhD study provides new information on the potential to improve NUE by manipulation of one of the bottlenecks in N assimilation. In addition, the work shows that the zinc concentration in the core endosperm of barley grain can be improved by manipulation of the hordein fraction of storage proteins, particularly under elevated atmospheric CO2.