The Amino Acid Composition of Whole Sorghum Grain in Relation to Its Nitrogen Content

Cereal Chem. 65(4):271-277 Sorghum grains (12 samples from seven different lines or hybrids) with phenylalanine, and glutamine plus glutamic acid, remained constant for total nitrogen, N, content ranging from 1.5 to 3 g/ 100 g grain dry matter serine, tyrosine, tryptophan, and asparagine plus aspartic acid, and were accurately analyzed for their amino acid composition from six decreased for the other amino acids. The nonprotein-to-total nitrogen ratio different hydrolysates per sample. Amino acid levels in grain increased as remained practically constant and close to 5%, and the nitrogen-to-protein linear functions of N, with correlation coefficients close to one for most conversion factor (kA) was close to 5.81 within the N range investigated. amino acids regardless of sorghum genotype or phenotype. As a result, the The results also showed that the composition of storage proteins amino acid composition of any grain sample of normal sorghum can be accumulated in sorghum grain remained constant, with the rate of predicted from its N. Amino acids in grain protein changed as hyperbolic deposition of kafirins roughly 1.5 times that of glutelins. functions of N, which increased for alanine, leucine, isoleucine, Since 1970, sorghum has represented only about 4% of total world cereal grain production. However it ranks fifth behind wheat, corn, rice, and barley because it is widely used as food in many countries of Africa and Asia, and as major feed grain in United States, where it ranks third behind corn and wheat, reaching 8% of cereal grain production (FAO 1986). Furthermore, the discovery of the high-lysine mutant gene by Singh and Axtell (1973) has stimulated much interest in improvement of protein quality and quantity. As in cereal grains, the total nitrogen N content (on a dry basis) of sorghum ranges from about 1 to at least 3% (Singh and Axtell 1973). Several studies have investigated amino acid compositions of normal sorghum genotypes (Ajakaiye 1984, Axtell et al 1975, Bressani and Rios 1962, Deosthale et al 1970, Deyoe and Shellenberger 1965, Haikerwal and Mathieson 1971, Hoseneyetal 1974, Jambunathan and Mertz 1973) and of high-lysine mutants (Guiragossian et al 1978, Hassen et al 1986, Paulis and Wall 1979). The influence of location or of fertilizers has been studied by Ajakaiye (1984), Deyoe and Shellenberger (1965), Eppendorfer et al (1985), and Waggle et al (1967). The amino acid composition of one or more sorghum samples has been determined for other purposes by Busson et al (1966), Chibber et al (1978), FAO (1970), Jones and Beckwith (1970), Pedersen and Eggum (1983), Pion (1971), Skoch et al (1970), Waggle et al (1966), and Wu and Wall (1980). Several of these publications have been reviewed by Chung and Pomeranz (1985), Hoseney et al (1981), and Wall and Paulis (1978). A careful analysis of this literature shows that the influence of total sorghum grain N on amino acid composition still calls for clarification. Many authors agree that, for a given genotype, amino acid composition can change as a function of N, but few (Singh and Axtell 1973, for instance) take this into account in comparing different genotypes. Relationships between amino acids and N were first examined by Waggle and Deyoe (1966), who showed that amino acid level in sorghum grain is linearly correlated with N. This means that amino acids in protein change according to quadratic relationships as a function of N. However, Eppendorfer et al (1985) concluded that it is not linearly correlated with N. The influences of genotype, culture conditions, and environment remain uncertain. However, linear relations between N and the levels of each amino acid in seeds, with correlation coefficients often higher than 0.99, have been shown for normal genotypes and phenotypes of wheat (Moss6 et al 1985), rye, corn (Baudet et al 'I.N.R.A. Laboratoire d'Etude des Proteines. Departement de Physiologie et Biochimie vWgetales, 78026 Versailles, France. ©1988 American Association of Cereal Chemists, Inc. 1987, 1986b), and legume seeds such as broad bean (Baudet and Mosse 1980), pea (Huet et al 1987), and lupin (Mosse et al 1987). These studies show that the changes of some amino acids in protein can be considerable. For example, proteins of corn grain with N= 1% are 50% richer in glycine and tryptophan and 60% richer in lysine than proteins of corn with N = 3% (Baudet et al 1986b). When Nis doubled in pea seed from 3 to 6%, the histidine level in seed proteins remains constant, arginine increases by 53%, and tyrosine and cystine decrease by 20 and 33%, respectively (Huet et al 1987). A similar study was thus undertaken with sorghum, using grain samples covering a wide N range and employing amino acid analysis conditions of highest possible accuracy. MATERIALS AND METHODS Sorghum Samples The 12 grain samples analyzed corresponded to seven Sorghum bicolor (L.) Moench genotypes from field-grown sorghums. One cultivar (Monitor) was grown under different conditions in six locations. The other samples were inbred lines chosen for their wide N distribution. Grain sampling (from 1 kg), milling, and meal subsampling for analysis were performed as described previously