Tryptophan and methionine levels in quality protein maize breeding germplasm

Because maize (Lea mays L.) is often used either as food for humans or as feed for monogastric animals, essential amino acid levels are important. Maize kernels containing the opaque-2 (o2) mutation have improved amino acid balance and poor agronomic qualities including opaque kernels that are soft and susceptible to mechanical and biological damage. Quality Protein Maize (QPM) developed through plant breeding has improved amino acid balance conferred by the opaque-2 (o2) mutation, but lacks the agronomic deficiencies normally associated with this mutation. To characterize the amino acid balance in QPM breeding germplasm, we determined the levels of nutritionally limiting amino acids tryptophan and methionine. Tryptophan levels were negatively correlated with endosperm translucence, a measure of kernel hardness suggesting the process of selection for hard-kernels reduces tryptophan levels. On average, germplasm containing the o2/ o2 mutation had lower methionine levels than 02/ 02 germ plasm regardless of kernel hardness, suggesting methionine levels could be reduced by the o2/ o2 mutation. A series of inbred lines was test-crossed to the o2/o2 soft endosperm inbred line Tx804. The predictive value of the characteristics of the inbred line for the characteristics of the hybrids was examined. The amino acid levels of the inbred lines were significantly correlated with those of the hybrids, although the predictive value was low (1{2 = 0.13 and 0.27 for methionine and t1yptophan, respectively). The reduction in tryptophan during conversion to the hard-kernel phenotype and the reduction in methionine in o2 germplasm both reduce the nutritional value of QPM. It may be possible to correct these deficiencies by breeding and selection for levels of tryptophan and methionine