DEPARTMENT OF AGRONOMY
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Item GENETIC VARIABILITY FOR STEM BORER RESISTANCE IN TWO ADAPTED EARLY-MATURING MAIZE (Zea mays L.) POPULATIONS(2013-03) OLOYEDE-KAMIYO, Q. O.Stem borers are among the major biotic stresses limiting the grain yield of maize, an important cereal crop in Africa. Breeding for multiple resistance to maize stem borers has been reported to be a promising method of control. Understanding the genetic variability of crop populations will help in improving them for pest resistance. Thus, genetic variability for dual resistance to two stem borer species were determined in a white (DMR ESR-W) and a yellow (DMR ESR-Y) maize populations to provide information required for improving their levels of resistance to the borers. Two experiments were conducted. In Experiment 1, a set of 100 S1 plants of DMR ESR-W were selected to produce 250 full-sib and half-sib progenies using the North Carolina Design II (NCD II) mating scheme. The progenies with six checks were evaluated under artificial infestation with two borer species (Sesamia calamistis and (Eldana saccharina) at Ibadan, and non-infested conditions at Ibadan and Ikenne in 2008 and 2009 using Randomized Incomplete Block Design with two replications. Experiment 2 was with DMR ESR-Y and the same methodology was used. In both experiments, days to 50% anthesis and silking, plant and ear height, plant and ear aspect, ear length and grain yield were measured. Resistance was measured according to levels of leaf feeding damage, dead heart, stalk breakage, cob damage and stem tunneling. Analyses of variance for NCD II were conducted to estimate genetic variances and Narrow-Sense Heritability (NSH). Correlation coefficients were determined and partitioned into direct and indirect effects. Predicted responses to selection were estimated to measure expected genetic gains. Correlated response was used to determine traits that could hasten selection progress. Tests of significance were conducted at p < 0.05. Infestation significantly reduced plant height (6.0 -11.1%), ear length (20.9 - 25.6%) and grain yield (23.9 - 30.4%) in both maize populations. Additive variance was significant for grain yield and stalk breakage in DMR ESR-W, and for stalk breakage, cob damage and stem tunneling in DMR ESR-Y. Narrow-sense heritability was low to moderate, but low for damage parameters except stalk breakage (40.6%) in DMR ESR-W, and cob damage (40.1%) in DMR ESR-Y. Negative correlations exist between grain yield and increasing levels of stem borer damage, with genotypic correlation between grain yield and stem tunneling being the highest (-0.52*) in DMR ESR-Y. Stem tunneling and cob damage had high positive direct effects on grain yield reduction. Estimated genetic gain per generation was 4.0 - 6.1% (= 210kg/ha) for grain yield under infestation in DMR ESR-W, but low for damage parameters in both maize populations. Direct selection for individual trait gave better response than indirect selection through other traits. The significant additive variances and moderate heritability estimates obtained for stalk breakage and cob damage indicate that the traits are heritable, therefore, improvement of the maize populations for stem borer resistance using these traits is feasible. Direct selection for grain yield is recommended in improving the maize populations. Stem tunneling and cob damage by the borers are major causes of grain yield reduction.Item Nitrogen-Use Efficiency and Gene Expression Profiling of Tropical Maize Hybrids Selected for Contrasting Responses to Nitrogen Fertilizer(2012) Abe, A.Low soil Nitrogen (N) and sub-optimal N fertilizer application result in low Grain Yield (GY) in maize. Genotypes with improved N-Use Efficiency (NUE) are beneficial to low-input agriculture. To facilitate the genetic improvement of tropical maize for NUE, information is required on the relative importance of N-Uptake Efficiency (NUpE) and N-Utilization Efficiency (NUtE), both components of NUE, the relationships among the gene networks in the developing cob tissue and agronomic traits under limited N. This study was undertaken to evaluate genetic variation for NUE and investigate gene responses in the developing maize cob under sub-optimal N fertilizer application. Fourteen tropical maize hybrids differing in GY under low-N conditions were grown at no-N (0 kg N/ha), low-N (30 kg N/ha), and high-N (90 kg N/ha) in a randomized complete block design with four replications from 2006 to 2008. Data collected on GY and its components, NUE, NUpE, NUtE, and N-related traits were analysed using ANOVA at p=0.05. Path analysis was carried out to capture the intricate relationships among traits related to GY and NUE. Microarray technique using Maize Oligonucleotide array slides containing ~57000 probes were used to identify differentially expressed genes in developing cob tissues harvested from three N-use efficient hybrids compared to an N-use inefficient reference hybrid. Differentially expressed genes were validated by quantitative Real Time Polymerase Chain Reaction (qRT-PCR). Genotype and Genotype × Nitrogen interaction were significant for GY and NUE-related traits. Mean GY across years was reduced by 76.5% at no-N and 35.4% at low-N. It varied from 2.0 to 3.2 Mg/ha at low-N and 3.3 to 4.4 Mg/ha at high-N. Number of kernels (KN) was the GY component most severely reduced under nitrogen stress. Nitrogen use efficiency, NUtE and NUpE increased by 61.4, 42.1 and 21.0% respectively from high-N to low-N. Grain yield was positively and significantly correlated with NUE, NUtE and NUpE at both low-N and high-N. Stover nitrogen content at silking and NUE had strong positive direct effects on GY under low-N and high-N. Both NUtE and NUpE had significant positive direct effects on NUE. The relative importance of NUpE and NUtE varied depending on genotype and environment. Four hybrids (4001/4008, KU1409/9613, KU1409/4008 and 4008/1808) produced similar above average GY at low-N and high-N but differed in their NUpE and NUtE. Gene expression profiling of developing cob tissues revealed that at low-N, 163 of the probes on the array showed differential expression across the test hybrids. Under low-N, the probe MZ00019244 corresponding to the L-asparaginase gene showed the strongest relative mRNA expression in the highest yielding hybrid (KU1409/9613). L-asparaginase mRNA expression level was positively and significantly correlated with GY, KN and NUtE. Genetic variation for nitrogen use efficiency and its components was present among the tropical hybrids. L-asparaginase mRNA expression would be a valuable tool for selecting maize genotypes with high nitrogen utilization efficiency and superior grain yield under low soil nitrogen