The effects of sulphur starvation on the amino acid and protein compositions of barley grain

Sulphur starvation resulted in decreases in the total sulphur content, and in the proportions of the sulphur-containing amino acids, cysteine and methionine, in the grain of pot-grown plants of the barley cultivars Sundance and Athos. The S-deficient grain contained a decreased proportion of storage protein (hordein) and increased non-protein nitrogen (NPN). Comparison of the amino acid compositions of the whole grain, and of the individual protein fractions, indicated that aspartate or asparagine was a major component of the increased NPN. Electrophoretic analyses showed that the hordein fraction was depleted in S-rich ‘B’ and ‘D’ hordein polypeptides, which was consistent with its low content of sulphur-containing amino acids. The salt-soluble protein fraction also contained low levels of cysteine and methionine, and SDS-PAGE showed that some bands, notably those of low molecular weight, were either greatly reduced in amount or were absent. It is suggested that these are non-essential components, possibly storage proteins. In contrast there was little effect on the amino acid or polypeptide compositions of the glutelin fraction.