Studies on genotypic variability and inheritance of waterlogging tolerance in wheat

English: Analysis of genetic relationships in crop species can provide a relative measure of genetic diversity, an index of parental selection and structure for stratified sampling of populations. Seed storage proteins of 38 Ethiopian-grown and four advanced CIMMYT lines were fractionated by one-step sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) to assess the composition of the three major endosperm proteins, determine the level of genetic diversity and search for the number of clusters among the genotypes of hexaploid wheat which is believed to be a relatively recent introduction to the Ethiopian highlands. The results indicated that there was a wide range of allelic variation in the composition of gliadins, low molecular weight (LMWGS) and high molecular weight (HMW-GS) glutenin subunits among the different genotypes studied. A total of 82 polymorphic bands, i.e., 32 gliadins, 35 LMW-GS and 15 HMW-GS bands, were detected among the 42 genotypes. The mean protein-based genetic distance estimate was 0.609 unit with values ranging from 0.376 to 0.744 units. Over 80% of pairwise comparisons had genetic distance values between 0.440 and 0.700 units. Cluster analysis also resulted in five genetically distinct groups of genotypes. Soil waterlogging is a serious environmental stress affecting wheat production in the high rainfall or irrigated areas with heavy clay Vertisols. Sixteen bread wheat genotypes were evaluated for their tolerance to prolonged transient and continuous waterlogging treatments. The results indicated that increased severity of soil waterlogging stress significantly reduced grain yields and growth of wheat; continuous waterlogging, in particular, resulted in greater damage to the plants in terms of most of the characteristics studied. The results also demonstrated that there were marked genotypic differences among the wheat genotypes studied for waterlogging tolerance. Ducula xl, PRL/Sara, HAR 604 and Vee/Myna were relatively tolerant whereas Et-13 and K 6290-Bulk were most sensitive to waterlogging stress. Under continuous waterlogging, grain yield was correlated positively with grains per spike and spikelet, kernel mass, biomass yield, number of green leaves on the main stem, the stress tolerance index, and negatively with percentage leaf chlorosis, area under chlorosis progress curve and the stress tolerance index. Percentage leaf chlorosis, biomass yield, number of grains per spike and spikelet, and kernel mass accounted for over 88% of the total variation in the grain yield under continuous waterlogging. Heritability values of these characteristics and indices were also fairly large, indicating a promising gain from selection based on these characteristics. Waterlogging of soils restricts crop performance by altering the soil mineral nutrient. availability and uptake by roots. Two experiments were conducted to determine the effects of soil waterlogging on nutrient availability, and on the concentration and uptake of nutrients by wheat genotypes that differ in tolerance, and to assess the response of waterlogged wheat seedlings to foliar applications of selected nutrients. Root zone oxygenation was significantly depressed by the waterlogging treatments as indicated by significantly reduced soil redox potentials. All the IN Nlia-acetate (pH 7.0) extractable mineral nutrient concentrations in the soil increased as waterlogging severity increased. A significant differential response of wheat genotypes to the waterlogging treatments was detected for several mineral nutrient concentration and uptake parameters. Compared to the tolerant genotypes, the sensitive genotypes appeared to accumulate more Fe, Mn, and Na although the concentrations were far below the level of toxicity reported for these nutrients, and less Cu, Zn, K, Pand N with concentrations lower than the "critical" values previously reported for wheat. The results from the foliar application study indicated that waterlogging tolerance of wheat could be improved with the foliar application of Zn, Cu and P. A study was undertaken to assess the combining ability effects, variance components, heterotic responses, heritability and correlations of waterlogging tolerance in a diallel cross involving five diverse bread wheat genotypes with contrasting tolerance response to waterlogging stress. The results showed that genotypic variability accounted for most of the variation among genotypes studied. Highly significant general combining ability (GCA) and specific combining ability (SCA) were observed for most of the characteristics studied, indicating that both additive and dominance gene effects were important in the inheritance of the waterlogging tolerance. GCA effects, however, were relatively more prevalent than SCA effects on most of the characteristics studied. The tolerant genotypes PRL/Sara and Ducula were the best combiners for waterlogging stress tolerance and for most other agronomic characteristics studied. Relatively high estimates of heritability and predictability ratios were also observed for most characteristics relevant to waterlogging tolerance, confirming the importance of both additive and non-additive gene effects in controlling the waterlogging tolerance in wheat genotypes studied. The degree of dominance for waterlegging tolerance characteristics was estimated to be in the partial and complete dominance range. Under waterlogging conditions, grain yield correlated with genotypically and phenotypically positively with all its components, biomass yields, plant height, days to heading, number of green leaves on the main stem and stress tolerance index, and negatively with percentage leaf chlorosis, area under leaf chlorosis progress curve and stress susceptibility index. Substantial mid- and high- parent heterotic responses were observed for most characteristics in some crosses.