Analyses of drought tolerance in durum wheat (Triticum turgidum var. durum) genotypes

English: In order to identify yield components associated with drought tolerance in durum wheat and evaluate the performace of Ethiopian durum wheat genotypes, 26 durum wheat genotypes, from different agro-ecologies of Ethiopia were evaluated under simulated moisture stress conditions. Stress caused dramatic reductions in grain yield and harvest index. Yield was significantly correlated primarily with number of kernels per spike and 100 kernel weight. Further decomposition of simple correlation coefficients into direct and indirect effects showed that number of kernels per spike and 100 kernel weight had the largest direct effects on grain yield, under both stressed and non-stressed conditions' The expression of drought tolerance in the F1generation obtained from all possible combinations among genotypes differing in their responses to moisture deficit stress was studied. Analysis of variance revealed significantly high variability among genotypes for yield, yield components and drought tolerance measurements due to the effects of treatments. Drought tolerance was expressed in the crosses involving tolerant parents. The diallel analysis showed that mean squares for both GCA and SCA were significantly high at both moisture regimes. GCA:SCA ratios indicated predominance of additive gene action for all characteristics positively correlated with grain yield under stress or negatively correlated with drought susceptibility index. Differences in water use and transpiration efficiencies and interrelationships among water use and transpiration efficiencies and associated traits were investigated among durum wheat genotypes with differing responses to moisture stress. Significantly high genotypic variability in the amount of cumulative water used before (ETba) and after (ETpa) anthesis was observed. Susceptible genotypes used higher amounts of water before anthesis and lower amounts after anthesis. In contrast, tolerant genotypes used a higher proportion of water during the post-anthesis period. Significantly high variability among the genotypes was observed for various measures of water use and transpiration efficiencies, total dry matter and harvest index. Ranking of cultivars for water use efficiency based on grain yield (WUEG) and transpiration efficiency based on grain yield, was consistent with ranking of cultivars for drought susceptibility indices. Drought susceptibility index was significantly but negatively correlated with harvest index, WUEG and grain yield. However, it was positively and significantly correlated with the ETba:ETpa ratio. A high positive correlation of WUEG with harvest index and grain yield with harvest index was found. Differences in flag leaf water potential were significant only for genotypes under stress treatments. Water potential declined with age under both treatment conditions, however, the fall was faster in stress sensitive types than in tolerant types in more advanced growth stages. Inheritance of water use and transpiration efficiencies was studied in a hybrid population obtained from six parents, selected for their different responses to moisture stress. GCA and SCA effects were significant at both the moisture levels. The interactions of GCA and SCA with moisture levels were also highly significant. Analysis of the genetic components of variation demonstrated that WU E and T were under the control of additive and dominance type of genes. Narrow sense heritability estimates for water use and transpiration efficiencies based on grain yield (WUEG) were higher at the moisture stress level. Measures of water use and transpiration efficiencies showed significantly high and positive genotypic and phenotypic correlations among them as well as with grain yield and harvest index. The impact of the effect of moisture stress on growth and phenological development was examined among durum wheat genotypes differing in their tolerance to moisture stress. Drought stress was found to delay major growth stages and shorten the grain filling period. Drought tolerant genotypes had fast early growth, whereas susceptible ones had slow RGR initially. Variation in RGR was associated with NAR and LAR. Differences in hybrid performance were due to significant GCA and SCA effects. Interactions of GCA and SCA with moisture level were also highly significant. Moderate to high levels of broad sense heritability estimates were found for most of the traits. Significantly high genetic and phenotypic correlations between NAR and RGR, and LAR and RGR were found. The genetic and phenotypic correlations of grain yield with total dry matter, harvest index, RGR and LAR were significant. The effect of moisture stress on the content of water soluble carbohydrate (sucrose and D-glucose) was investigated in durum wheat (Triticum turgidum L. var. durum) above ground organs to assess and characterise the responses of genotypes with differing responses to drought stress. In all organs examined, drought tolerant genotypes accumulated more glucose and sucrose, particularly, at an early age. Stress caused an overall increase in the level of these carbohydrates, but the levels were highest in the stems compared to other organs. Level of drought (drought susceptibility index, 'S') was strongly related with the level of carbohydrates in the various plant organs Genetic differences among genotypes with differing responses to drought stress and their progenies obtained from all possible cross combinations were assessed using AFLP markers. Moderate to high level polymorph isms were obtained with the primer combinations evaluated. Euclidean genetic distance estimates showed that there was considerable genetic diversity among the materials studied. The correlation coefficient for pair wise genetic distance estimates based on AFLP data and yield, yield components, and morphophysiological traits was significant and positive. Cluster analysis showed that grouping of genotypes on the basis of AFLP data and agro-morphological data fairly agreed. A number of AFLP fragments were significantly correlated with the various traits evaluated under stress conditions. AFLP fragments were observed to affect a number of traits that were strongly correlated with drought susceptibility index.