Abstract:
Bread wheat is one of the most important cereal crop produced in almost all agro-ecologies of Ethiopia. Assessment of genetic variability in crop species is one of the major activities of plant breeding which helps to design breeding methods of genotypes for further evaluation to meet the diversified goals.Therefore the field experiment was conducted to estimategenetic variability, association of traits, determining their direct and indirect effecton grain yield and to identify principal drought indices. The experiment was laid out in 8x8 simple lattice design under both optimum and stress conditions at middle Awash during 2019/20 post-rainy season. Analysis of variance revealed highly significant (p≤0.01) variation for studied traits under both condition except for chlorophyll content and canopy temperature under optimum condition.The variation observed for grain yield varied from 2.30-6.0t ha-1 and 1.01-4.36 t ha-1 under optimum and stress condition respectively. Genotypic and phenotypic coefficient of variation ranged from 3.88(days to maturity) to 20.84% (grain yield) and 4.76(days to maturity)to 24.73% (fertile tiller plant-1 )under optimum condition respectively. Under stress condition GCV and PCV ranged from 2.91(canopy temperature) to 18.41% (fertile tiller plant-1 ) and 3.32(canopy temperature) to 22.27% (fertile tiller plant-1 )respectively.Broad sense heritability and genetic advance as percentage of mean ranged from 43(spike length) to 89% (biomass yield) and 6.51(days to maturity) to 40.33% (grain yield) under optimum condition respectively, whereas under stress condition it ranged from 23.40(harvest index) to 86.1% (days to heading) and 4.51(canopy temperature)to 33.34%(biomass yield) respectively. High heritability coupled with high genetic advance as percentage of mean wasrecorded for biomass yield and grain yield under optimum condition, whereas for biomass yield and fertile tiller plant-1 under stress condition.Under both conditions grain yield had positive and highly significant correlation with biomass yield, harvest index, fertile tiller plant -1 and spike length at both genotypic and phenotypic level.Under stress condition, grain yield had negative and highly significantly correlation with days to maturity and canopy temperature.Geometric mean productivity (GMP), mean productivity (MP), stress tolerance index (STI) and yield index (YI) showed positive and highly significant correlation with grain yield under both conditions indicating suitability of these indices for selecting drought tolerant genotypes.The highestpositive direct effect was exerted by biomass yield and harvest index on grain yield at both genotypic and phenotypic level under both conditions.Cluster analysis based on Unweighted Pair Group Method with Arithmetic Means (UPGMA) estimated six clusters of 64 bread wheat genotypes under both conditions.Under optimum condition cluster I, II,III,IV, V andVI accounted 31.25%, 9.37%, 37.5%, 1.56%, 10.99% and 9.37% of the tested genotypes in that order.Under stress condition, cluster I, II,III,IV, V and VI accounted 3.12%, 43.75%, 42.19%, 6.25%, 1.56% and 3.12%respectively.Under optimum condition the maximum inter cluster distance was observed between cluster I and IV, II and IV and IV and V, while under stress condition cluster I and V, IV and V and V and VI had highest inter cluster distance to be used as source for hybridization program. Principal components analysis revealed five principal components with Eigenvalues greater than unity accounted 74% and 69% of total variations under optimum and stress conditions respectively.According to mean performance of genotypes G3, G24, G26, G24 and G45 had yield advantage over checks under both conditions. Generally, the variation observed among the tested genotypes confirmed that possibility of improving bread wheat genotypes through selection and hybridization to improve the yield for the study area.