Breeding bread wheat (Triticum aestivum L.) for drought- tolerance in Ethiopia.
Belete, Yared Semahegn.
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Wheat (Triticum aestivum L.; 2n=6x=42; AABBDD) is one of the major staple cereal crops in the world. The demand for wheat has rapidly increased in sub-Saharan Africa including Ethiopia because of the growing population, changing food preferences and socio-economic change associated with urbanization. However, production and productivity of wheat in the region is affected by various biotic, abiotic and socio-economic factors. Drought is one of the major abiotic constraints limiting wheat productivity in Ethiopia. Adoption of new improved varieties by farmers is relatively low in marginal, drought-prone areas of the country because of a lack of improved varieties with desirable agronomic and drought-adaptive traits. Developing wheat cultivars, which are drought-tolerant, high and stable yielding with farmers’ preferred traits, is a sustainable strategy to ensure food security and to improve livelihoods of farmers in marginal areas. Therefore, the objectives of this study were: i) to assess farmers’ production practices, perceived production constraints and preferred traits of bread wheat varieties as a guide to variety development and deployment in drought-prone areas of Ethiopia; ii) to screen bread wheat genotypes for drought-tolerance using phenotypic analysis to select promising lines for use in breeding for drought-tolerance; iii) to estimate the genetic parameters and association of yield and yield components and thus determine the selection criteria to increase genetic gains under drought stress conditions; iv) to assess the genetic diversity and relationships among the selected wheat genotypes using simple sequence repeat (SSR) markers in order to complement the phenotypic data in identifying complementary parents for further breeding for drought-tolerance; and v) to determine combining ability effects of the selected wheat genotypes, thereby deducing gene action controlling traits of interest and identifying promising families for drought-stress conditions, and to advance these families through the single seed descent selection method. Separate but complementary research activities were conducted to attain the aforementioned objectives. A participatory rural appraisal (PRA) study was conducted involving 170 randomly selected wheat producing farmers in selected districts of Arsi zone in the Oromia Regional State of Ethiopia during 2018. Moisture-stress, disease (wheat rusts) and the high cost of fertilizers were the first, second and third ranked production constraints in the study areas. Varietal attributes such as early maturity (p<0.01) and tall plant height (p<0.05) had positive and significant effects on the adoption of new, improved varieties, while poor adaptation and poor baking quality had negative and significant (p<0.05) influences. High grain yield was the most preferred trait as perceived by the farmers in the study areas, followed by stress adaptation (drought and heat stresses-tolerance), disease-resistance and early maturity. In the second part of the study, 120 genotypes were evaluated at five test sites in the 2018/19 cropping season using a 10 x 12 alpha lattice design with two replicates. The level of drought-stress was imposed using different sowing dates (early planting resulted in non-stressed plants, while late planting created drought-stressed conditions) following the onset of the main seasonal rain at each site. Grain yield and yield components were recorded, and drought indices were calculated for each genotype. Genotypes such as ‘YS-39’, ‘YS-119’ and ‘YS-109’ were the earliest to mature and can be used in droughttolerance breeding. Among the drought-tolerance indices, Geometric Mean Productivity (GMP), Mean Productivity (MP), Harmonic Mean (HM), Stress Tolerance Index (STI) and Yield Index (YI) were found to be the most suitable for predicting drought-tolerance because they had significant and positive correlations with yield under drought-stressed and non-stressed conditions. Genotypes ‘YS-41’, ‘YS- 92’, ‘YS-115’, ‘YS-34’ and ‘YS-93’ were found to be drought-tolerant, and exhibited dynamic stability, with relatively high yield under both drought-stressed and non-stressed conditions. ‘YS-90’, ‘YS-106’, ‘YS-96’, ‘YS-102’ and ‘YS-101’ were susceptible to drought-stress, while ‘YS-32’, ‘YS-29’, ‘YS-14’, ‘YS-53’ and ‘YS-11’ were relatively drought-tolerant, but exhibited static stability under non-stressed conditions. In the third part of the study, the extent of the genetic parameters and associations of yield and yield components were determined among the aforementioned 120 genotypes in order to design appropriate breeding strategies for yield improvement in wheat. The highest estimates for genetic variance were obtained for days to heading (DH; 54.0%), followed by Spike length (SL; 38.3%). The high heritability estimated for DH (94.4%), SL (90.2%) and spikeletes per spike (SS; 85.2%), coupled with a high rate of genetic advance, suggest that direct selection for these traits would be effective. Grain yield (GY) exhibited low genetic advancement (9%) and heritability (41.5%) estimates, which were concomitant with its polygenic and complex inheritance pattern. Correlation and path analyses revealed that plant height (PH) and 1000-kernel weight (TKW) were the most important contributing traits for improving grain yield under drought-stress conditions. In the fourth part of the study, 52 selected bread wheat genotypes were assessed using 20 SSR markers. SSR analysis identified a total of 181 alleles, with a mean of 10.1 alleles per locus. Population structure analysis grouped the test genotypes into three main populations. Analysis of molecular variance revealed that 85% of the variance emanated from intra-population differences. Cluster analysis also grouped the test genotypes into three major groups. In the fifth part of the study, eight parental lines and 28 crosses obtained from a half-diallel mating design were evaluated at two sites representing drought-stressed and non-stressed conditions. The genotypic effects were significant for all traits studied except grain-filling period (GFP) across the test environments. The parental line ‘YS-32’ was the best general combiner for DH, days to maturity (DM), GFP and TKW, enabling direct selection for improved grain yield under drought-stress conditions. Parent ‘YS-85’ can also be used for improving grain yield under drought-stress conditions due to its positive and significant GCA effect on GY. The highest specific combining ability (SCA) effects under drought-stressed for improving GY were obtained in families ‘YS-32’ x ‘YS-85’, ‘YS-102’ x ‘YS-82’ and ‘YS-102’ x ‘YS-92’. Overall, the present study revealed drought-stress was the major bread wheat production constraint in drought-prone agro-ecologies of Ethiopia. And, farmers had varying varietal preferences for adopting newly improved varieties. The tested genotypes proved to be valuable genetic resources to enhancing drought-tolerance and improving farmers’ preferred traits. In future, these genetic resources will be used either for developing mapping populations for quantitative trait loci (QTL) analysis underlying traits of interest under drought-stress conditions to serve as long-term breeding materials or release directly as cultivars incorporating farmers’ preferred traits.