GENETIC ANALYSIS OF QUALITY PROTEIN MAIZE (Zea mays L.) INBRED LINES UNDER LOW AND OPTIMUM SOIL NITROGEN ENVIRONMENTS IN ETHIOPIA
Maize is the most important cereal crop on which many people in developing countries depend on as a sole source of protein. However, normal maize is deficient in two essential amino acids, lysine and tryptophan, which necessitated the development of Quality Protein Maize (QPM) developed. QPM plays an important role in addressing malnutrition in countries where maize is consumed as a major staple. In addition to malnutrition in sub-Saharan Africa, low soil nitrogen (low N) is the most limiting factor in maize production and productivity. Therefore, to identify QPM genotypes that are tolerant to low N, this study was conducted to specifically to (i) determine genetic diversity among QPM and non-QPM maize inbreds under low and optimum N environments, (ii) determine combining ability and heterosis of QPM inbreds for grain yield and other agronomic traits under low and optimum N environments, (iii) identify single cross QPM hybrids with high grain yield performance and yield stability under low and optimum N environments, and (iv) estimate combining ability of QPM inbreds for tryptophan and kernel modification score under low and optimum N environments. To determine genetic diversity among QPM and non-QPM maize inbreds under low and optimum N environments, one hundred and sixth eight QPM and non-QPM with five QPM inbred line checks were evaluated using an augmented experimental design under optimum N soil environment (100 kg N ha-1) for one year in 2018 cropping season and for two years under low N soil environment (30 kg N ha-1) in 2017 and 2018 cropping seasons at Haramaya University research site (Raare). The result indicated significant differences among inbred lines, under both optimum and low N environments for all characters except anthesis-silking interval. Under optimum soil N environment, inbred lines BQL36, VL06373, TL145744, BQL95 and BQL56 were identified as high yielding inbreds
while inbred lines VL06373, BNL79, BQL8, BNL86 and TL147070 identified as high yielding inbreds under low N environment. Principal component analysis (PCA) computed for 168 QPM and non-QPM maize inbreds extracted the first five PCA, which accounted for about 74.32% under optimum N environments and 72.45% under low N environment of the cumulative variations existing among the inbred lines. Cluster analysis grouped inbreds into three cluster groups under optimum N environments and four cluster groups under low N environments. To determine combining ability and heterosis of QPM inbreds for grain yield and other morphological traits under low and optimum N environments, 11 QPM inbreds were crossed in a complete diallel mating design (method-I of Griffing's) in 2017 cropping season to generate 121 genotypes. The one hundred and twenty one genotypes with five checks were evaluated using alpha-lattice design under both low and optimum N environments at three locations in 2018 cropping season. Significant differences were observed among genotypes for major characters under both low and optimum N environments. Under combined low and optimum N environments, significant mean squares of GCA, SCA and reciprocal effects were found for yield and other traits. Under low and optimum N environments, non-additive gene actions were more important than additive gene action for grain yield, number of ears per plant, plant and ear aspects, plant and ear height, ear length and diameter. Under low N environments, more contributions of reciprocal effects than GCA effects were observed for number of ears per plant, plant aspect, ear length and diameter. Inbred lines were classified into three heterotic groups under low, optimum and across all N environments. To identify single cross QPM hybrids with high grain yield performance and yield stability under low and optimum N environments, the grain yield performance of the top 25 and five hybrid checks were subjected to AMMI and GGE biplot model. Significant effects of genotype, environment, and genotype x environment interaction (GEI) were found for grain yield. AMMI stability value identified TL155976 x TL156583, TL156583 x TL155932, and TL156579 x TL156583 as stable genotypes, and TL156583 x VL05128, TL156583 x TL147078, and TL156579 x TL155976 identified as unstable. GGE-biplot identified genotype TL156583 x VL05128 won at E6 (Dire-Dawa optimum N) and genotype L156583 x TL156612 won at E3 (Fedis low N) and E4 (Fedis optimum N) whereas genotype TL156612 x TL148288 won at E2 (Raare optimum N) and E5 (Dire-Dawa low N). Genotype TL156579 x TL155976 won at E1 (Raare low N) and E5. To determine the effects of soil nitrogen on combining ability of tryptophan, endosperm modification score, protein and protein quality index (QI), 121 genotypes with five checks were sib-mated to generate F2 grains under low and optimum N environments at Raare in 2018 cropping season. Then, 100 grains of each F2 generation was used for endosperm modification scores and tryptophan and protein concentration analysis in maize kernel endosperm. The result of the study indicated that genetic variabilities were found among genotypes due to genotypes and GEI under both low and optimum N environments. The contribution of GCA, SCA and reciprocal effects were important for all measured traits under both environments indicating quality traits inheritance were controlled by both additive and non-additive gene action. Under both low and optimum N environments, the importance of reciprocal effects was also identified for endosperm modification score, tryptophan, protein, and quality index.