Combining Ability of Quality Protein Maize (Zea mays L.) Inbred Lines for Tryptophan and Quality Traits Across Diverse Soil Nitrogen Environments

Maize (Zea mays L.) is the most important cereal crop on which many small-scale farmers depend as a sole source of protein. However, it contains lower amounts of the essential amino acids lysine and tryptophan compared to other crops. Quality Protein Maize (QPM), developed through conventional plant breeding methods, has higher levels of lysine and tryptophan than non-QPM maize varieties. This study was designed to assess the combining ability of QPM inbred lines for tryptophan content, endosperm modification score, protein, and protein quality index under low and optimum soil nitrogen environments. We crossed eleven QPM inbred lines using a complete diallel mating scheme to generate 110 F1 hybrids. Including five maize hybrid checks, we evaluated the 110 F1 hybrids and 11 parental lines under both low and optimum soil nitrogen environments in an 18 × 7 alpha-lattice design with two replications. We estimated tryptophan content, kernel modification scores, total protein contents and protein quality index in the F2 grain of F1 hybrids using Near-Infrared Reflectance Spectroscopy (NIRS 6500). The results of the study indicate that the mean squares for general and specific combining ability were highly significant (p ≤ 0.01) for all measured quality traits under both low and optimum soil nitrogen environments. The GCA sum of squares was greater than the SCA sum of squares for the endosperm modification score under low, optimum, and across soil nitrogen environments. However, the SCA sum of squares was higher than the GCA sum of squares for the protein quality index under low, optimum and across soil nitrogen environments. The contributions of GCA, SCA, and reciprocal effects were significant for all measured traits under both environments, indicating that the inheritance of the quality traits is controlled by both additive and non-additive gene action.