Protein quality and amino acid‐protein relationships of maize, sorghum and rice grain as influenced by nitrogen, phosphorus, potassium and soil moisture stress

Abstract

AbstractIn pot and field experiments cereals were grown with greatly differing rates of N, P and K applications. For maize and sorghum, soil moisture levels were also varied. N applications, P‐ and K‐deficiency, and moisture stress generally increased the total N content of grain, from 1.06 to 2.68, 1.01 to 2.42 and 0.81 to 2.33% (as % of DM) for maize, sorghum and rice, respectively. P, K and moisture stress affected the amino acid composition only indirectly through their effects on N concentration. In all three cereals increasing N concentrations were associated with decreases in crude protein of lysine, methionine, cystine, threonine, tryptophan and, generally, with increases of isoleucine, leucine, phenylalanine and glutamate. Lysine as a percentage of dry matter increased up to the highest N concentration in all three cereals. Linear regressions with significant correlation coefficients were found between concentrations of most (g 16 g−1 N) or all (mg 100 g−1 DM) amino acids and N content of grain. Based on all experimental data equations for lysine were: g 16 g−1 N=4.44–0.89 × %N; 4.23–0.86 × % N; 4.39–0.42 × % N for maize, sorghum and rice, respectively. Correction of P‐ and K‐deficiency decreased N content of grain and resulted in higher lysine content and better nutritional value of grain protein, whereas correction of N‐deficiency had the opposite effect. The amino acid composition of rice was much more balanced than that of maize and sorghum with a leucine/isoleucine ratio of 2 compared to that of 3 in maize and sorghum. In rat feeding experiments true digestibility of maize and sorghum protein was high (about 95%) but increased only slightly with increasing N in grain, whereas the biological value decreased considerably, from 63 to 55 and from 70 to 58, respectively. The chemical score underestimated the nutritional value of both cereal proteins.