The effect of nitrogen nutrition on the lysine content and protein composition of barley seeds

Increases in the total grain nitrogen of barley (cv. Julia and Bomi), resulting from increased N fertilisation, are accompanied by decreases in the relative lysine content of the grain proteins. Extraction of the grain proteins shows that the high‐N grain have increased relative and total amounts of the lysine‐poor storage protein (hordein) fraction. There are only small increases in the total amounts of the other more lysine‐rich protein fractions (salt‐soluble proteins and glutelin+residue proteins). There is also little or no effect on the amino acid compositions of the fractions. Consequently, it is concluded that the decreased relative lysine content of the whole grain is due to the increased synthesis of hordein. Further evidence for this hypothesis comes from similar studies with the high‐lysine mutant, Risø 1508, in which the high‐lysine character results from a decrease in the relative and total amount of hordein. The hordein fraction does not increase disproportionately with increasing grain N, nor does the relative lysine content of the grain decrease. There is no effect of grain N on the polypeptide compositions of the salt‐soluble proteins of either line. However, in Bomi and Julia increased grain N does result in an increase in the relative amount of sulphur‐poor ‘C’ hordein polypeptides in the hordein fraction. It is speculated that this results from limitation of S relative to N.