The amino acid composition of wheat grain as a function of nitrogen content

Abstract

Variation in the amino acid compositions of 30 samples of wheat (Triticum aestivum) grains from 12 different varieties was determined. Total nitrogen contents ranged from 1·4 to 3·3 % of grain dry matter. Amino acid contents were determined with maximum accuracy by employing three different times of hydrolysis in 6 m HCl for unoxidised samples so as to account for losses due either to degradation or incomplete release of amino acids. Accurate quantification of cyst(e)ine and methionine was achieved by performic acid oxidation prior to acid hydrolysis, and tryptophan was quantified after basic hydrolysis. Amide nitrogen content was determined following a fifth dilute acid hydrolysis. On a dry matter basis, amino acid content increased linearly as a function of nitrogen content with correlation coefficients often higher than 0·98. For a given amino acid, the same linear relationship was obeyed for the samples of a given genotype resulting from different environmental conditions as well as for samples from different genotypes. This enables prediction of the amino acid composition of any wheat from its nitrogen content. On a protein basis, amino acids varied as hyperbolic functions of nitrogen content. They increased for (glutamine+glutamic acid) proline and phenylalanine, they were roughly constant for leucine, isoleucine, serine, tyrosine, methionine and histidine and they decreased for the other amino acids. The degree of amidation (i.e. molar ratio of (Gln+Asn)/(Glx+Asx)), nitrogen to protein conversion factor, kA, and non-protein nitrogen to total nitrogen ratio (NPN/N) also varied as hyperbolic functions of nitrogen content. The degree of amidation and kA increased so slightly that they appeared to be practically constant (0·79 and 5·62, respectively) whereas NPN/N decreased from 61 × 10−3 to 33 × 10−3 within the range investigated.