Pre-breeding of tef [Eragrostic tef (Zucc.) trotter] for tolerance to aluminium toxicity.
Tef [Eragrostis tef (Zucc.) Trotter] is the most widely grown cereal crop in Ethiopia. Its grain is used for human consumption and the straw is an important and highly valued livestock feed. Soil acidity and Al toxicity are among the major production constraints affecting tef in Ethiopia. Utilization of lime and other non-genetic acid soil management options is constrained by various socio-economic factors. Development of cultivars with tolerance to Al-toxicity is a complementary approach to liming in the production of globally important crops such as wheat, rice, maize, barley, sorghum and rye. However, no breeding for tolerance to Al toxicity in tef had been undertaken previously. Hence, this research project was initiated in order to address the following objectives: 1. To assess the perceptions, challenges and coping mechanisms of farmers dealing with soil acidity and Al-toxicity in problem areas of north western Ethiopia; 2. To characterize the reactions of released tef varieties to soil acidity and the associated Al-toxicity; 3. To determine the extent of genetic diversity among tef germplasm collected from areas of Ethiopia with acid soil; 4. To isolate and characterize EMS-induced mutants of tef for tolerance to Al-toxicity and other important agronomic traits; 5. To evaluate the use of hydroponics system as a phenotyping platform to screen for Al-tolerance in tef, using root measurement and haematoxylin assay methods. There is no information on breeding for Al-tolerance in tef. Therefore, relational background literature was collated on other cereals on their mechanisms of Al-toxicity, tolerance mechanisms, genetic control, screening methods and marker assisted breeding. The information obtained from such sources was used to develop and undertake the subsequent breeding activities on tef. In order to meet the set objectives, several laboratory, greenhouse, and field experiments were conducted at the Amhara Regional Agricultural Research Institute (ARARI), Ethiopia, from December 2012 to June 2015. A Participatory Rural Appraisal (PRA) study was conducted in three Districts of north western Ethiopia that are affected by acid soils, in order to assess the state of soil acidity, and to determine its perceived causes and indicators, and to document the coping strategies of the farmers. Semi-structured interviews, group discussions and soil analyses were the main techniques used to generate data in this background study. Farmers’ perceived the causes of soil acidity to include: soil erosion; poor nutrient recycling; the abandoning of traditional fertility management practices; the unbalanced and/or minimal use of external inputs; and the exclusive use of acid-forming, inorganic fertilizers. Soil erosion, soil acidity, the high cost of mineral fertilizers and lime, cash shortages, and a lack of acid tolerant crop varieties were ranked as the top constraints. Species tolerance to soil acidity was found to be one of the major factors that influenced crop choice by farmers. A decline in genetic diversity and the rapid expansions of newly introduced, acid tolerant crops such as oat and triticale were noticed. The pH (H2O) of most of the soils in the study sites was in a strongly acidic range (4.6–5.5). Gashena Akayita of Banja District was the most acidic of all and had high levels of exchangeable Al. The limitations of the current coping strategies suggested the need to introduce compatible technologies that would ensure the sustainable management of the soils in the region, by the small-scale farmers there. Thirty three Released Varieties and selected accessions of tef were evaluated for their tolerance to soil acidity in pot trials. Twenty eight of these were then evaluated under field conditions. The results revealed the presence of significant genetic variability within the test genotypes. Nearly all the test genotypes were highly sensitive to soil acidity and Al-toxicity. However, a local landrace that is widely grown in Banja, a District severely affect by soil acidity, consistently outperformed the other genotypes both under pot and field conditions. There were changes in the ranking of the tef genotypes tested under pot and field conditions, which suggested the need to consider other edaphic and climatic factors when breeding for Al-tolerance. Overall, the grain yield of the test genotypes and the tolerant local landrace were less than the national mean yield of tef, identifying the need to develop varieties with better tolerance of acid soils and the associated Al-toxicity, aiming for superior agronomic performances in acid soils. Twenty-seven tef accessions collected from three regions of Ethiopia that are affected by acid soils were evaluated, together with released breeders’ varieties, and selected breeding materials for genetic diversity, using 16 selected and highly polymorphic SSR markers. Analysis of molecular variance (AMOVA) showed highly significant differences (P<0.001) among and within populations. Despite the wide geographical separation of the collection sites, 88.5% of the accessions from acid soils were grouped into two clusters (Clusters II and III) while 90% of the breeding materials and the Released Varieties were grouped into Cluster I. A significant degree of genetic differentiation was observed among the populations. Accessions from the north western Ethiopia exhibited a significant level of variation for most of the genetic diversity parameters. The number of private alleles was significantly higher for tef plants from acid soils than the Released Varieties and the breeding materials the Pair-wise estimates of genetic identity and gene flow showed higher values existed between the Released Varieties and breeding materials. About 15,000 M2 seeds were screened under acid soil conditions along with the M0 mutagenized seeds of the parent variety Tsedey and an Al-tolerant local landrace, Dabo banja. Twenty one M2 plants with root lengths of greater than the mean plus standard deviation of the tolerant check were selected and their M3 progenies were characterized for Al-tolerance and morpho-agronomic traits under greenhouse and field conditions, respectively. There were highly significant differences for Al-tolerance between the M3 mutant lines and the parent (P<0.001); and between the M3 mutant lines and the sensitive check (P<0.001). However, there was no significant difference between the M3 mutant lines and the tolerant check. The result of the morpho-agronomic characterization revealed the presence of significant differences between the M3 mutants for 16 of the 20 quantitative traits measured. Five levels of AlK(SO4)2.12H2O were evaluated (0, 150, 250,350, 450, 550 μM) in order to select the optimal concentration of Al that can most efficiently discriminate between sensitive and tolerant tef genotypes, using a hydroponic growing facility and measuring root lengths. The haematoxylin staining method was also assessed as a tool for the visual evaluation of tef varieties for Al-tolerance using selected test genotypes. There were highly significant differences (P<0.001) between the treatments, both for dose of Al and for genotype sensitivity to Al. The maximum differences in relative root length (RRL) (%) and root length (RL) (mm) between the sensitive and the tolerant genotypes were observed at the Al level of 150 μM Al. This concentration efficiently discriminated between 28 test genotypes with different levels of sensitivity to Al-toxicity. A visual assessment of the reactions of two sensitive and two tolerant genotypes to haematoxylin staining using 0, 150 and 250 μM of AlK(SO4)2.12H2O showed differential staining reactions in their roots that were consistent with their prior root growth measurements.
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