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One hundred and eighty-nine (189) wheat genotypes were evaluated in multi-environments (Tel-hadya (Syria), Dongola (Sudan) and Wadmedani (Sudan)) for heat tolerance from 2011 to 2012. Genomic mapping of the quantitative trait loci underlying heat tolerance in the crop was also performed. The field experiment was laid out in an alpha lattice design. The data obtained were subjected to restricted maximum likelihood (REML) for generation of best linear unbiased estimates (BLUEs). The heat tolerance study in the two seasons (early and late) in Tel-hadya, Syria showed that days to heading, days to maturity and grain filling duration, plant height and grain yield were significantly (p <0.05) reduced in late season when compared with their performance in the early season. The grain loss due to heat stress in the late season compared to the early season was in the range of 58 to 88%.The effect of heat stress on days to heading, days to maturity and grain filling duration, plant height and grain yield of the crop varied across the sites (Late season in Tel-hadya, Dongola and Wadmedani). The decreased effect of heat stress was most pronounced in late season in Tel-hadya than in the two sites in Sudan. The Additive Main effects and Multiplicative Interaction (AMMI) estimates showed 20 top yielding genotypes in all the sites with a grain yield range of 2.593 t/ha in Gen135 to 2.893t/ha in Gen117. In terms of their AMMI stability values ranking, Gen 60, Gen 68, Gen101, Gen155 and Gen118 were ranked first, second, third, fourth and fifth, respectively. The broad sense heritability estimates for grain yield ranged from 0.297 (Dongola) to 0.449 (Late season in Tel-hadya). Other traits like grain filling duration, plant height and one thousand kernel weight showed moderate to low broad sense heritability across the sites. The path coefficient analyses across the three sites showed that days to heading, canopy temperature and grain filling duration had reduced direct effect on the grain yield, while biomass and harvest index showed positive direct effect on the grain yield. Days to maturity showed negative direct influence on the grain yield in the late season in Tel-hadya, but positive direct effect was observed in the two environments in Sudan. The broad sense heritability of days to heading ranged from 0.804 (Wadmedani) to 0.908 (Late season in Tel-hadya), while days to maturity ranged from 0.68 (Dongola) to 0.793 (Late season in Tel-hadya). The structure analysis revealed that the wheat germplasm studied had seven sub-populations. The linkage disequilibrium (LD) analysis obtained showed that the LD decay was approximately at 25cM. The genome wide association mapping of the quantitative trait loci associated with heat tolerance showed that few markers were consistently detected in the three environments, while many were environment- specific.
Bread wheat (Triticum aestivum L.) is unarguably one of the world’s most important and widely consumed cereal crop (Asif et al., 2005; Bushuk, 1998). The flour is used for making bread, biscuits, confectionary products, noodles, wheat gluten among others. The world population is expected to reach about 9 billion by the end of the 21st century, and it has been predicted that the demand for cereals, especially wheat, will increase by approximately 50% by 2030 (Borlaug and Dowswell, 2003).Wheat production attracts increasing attention globally owing to its importance as a staple food crop, such that the availability of wheat and wheat products are seen as a food security issue in many countries. This has led to growing of wheat in many parts of the world even where it was not formerly grown. Paliwal et al. (2012) indicated that wheat is one of the most broadly adapted cereals. Although wheat is a thermo sensitive long day crop that requires relatively low temperature for its optimal yield, it is being grown in the tropics and subtropics despite the relatively high temperature that is associated with the areas (Rehman et al., 2009). In spite of the growing attention on the crop globally, Ali (2011) reported that its production in many regions of the world is below average because of adverse environmental conditions. High temperature which imposes heat stress on wheat is a major limitation to its productivity in arid, semi- arid, tropical and subtropical regions of the world (Ashraf and Harris, 2005). It affects the different growing stages of the crop especially during anthesis and grain filling (Rehman et al., 2009) leading to poor grain yield and quality. This is exacerbated by the increasing temperature associated with global warming, thus breeding for high temperature tolerance in wheat is a major challenge globally.
A detailed understanding of the genetics and morpho-physiology of heat tolerance and use of effective breeding strategies to address the situation would be ideal. Sikder and Paul (2010) reported that identification of wheat varieties suitable for heat stressed condition would be an important step toward achieving high yield potentials in wheat. Major gains have been achieved in the improvement of economic traits of wheat through conventional breeding, and more recently; through marker assisted selection (MAS) that has transformed plant breeding. Advances in molecular technologies have resulted in the mapping and identification of quantitative trait loci (QTLs) controlling traits of
importance in wheat, thereby permitting improvement beyond the upper limit of conventional breeding approaches. The two most commonly used approaches in mapping and identification of QTLs are bi-parental and association mapping (AM). Association mapping, which is more recent, has been utilized in overcoming some limitations associated with bi-parental mapping approach in exhaustive genomic dissection of putative QTLs of interest in plants. These limitations that are associated with bi-parental mapping approach are time consuming in generation of mapping population from a cross between two parents, low recombination events in the mapping population which leads to poor mapping resolution, and detection of only few QTL, among others. AM has the potential to identify a single polymorphism within a gene that is responsible for phenotypic differences (Braulio et al., 2012).
Although significant variation for heat tolerance exists among wheat germplasm (Reynolds et al., 1994; Joshi et al., 2007a, b), no direct selection criteria for heat tolerance are available (Paliwal et al., 2012). This is probably because of lack of detailed understanding of the morphological, physiological and genetic bases for heat tolerance in wheat. Phenotypic selection for heat tolerance has been performed using grain filling duration (Yang et al., 2002); one thousand grain weight, canopy temperature depression (Reynolds et al., 1994a; Ayeneh et al., 2002) and grain yield. Despite these attempts, Ortiz et al. (2008) and Ashraf (2010) reported that breeding for heat tolerance using trait- based selection is still in its infancy stage and warrants more attention.
Wheat developmental phases such as ear emergence, anthesis and maturity are controlled by three groups of vernalization (Vrn), photoperiod (Ppd), and the earliness per se genes (Kosner and Pankova, 1998) and their expression plays a significant role in wheat adaptation to different locations (Gororo et al., 2001). These three sets of genes together influence flowering time, and the suitability of genotypes for flowering under particular environmental conditions (Snape et al., 2001; Dubcovsky et al., 2006). Differences in flowering time could be of vital physiological implication in heat tolerance in wheat crop. The paucity of knowledge of the underlying physiological basis of heat tolerance as well as the genomic regions associated with heat tolerance in wheat prompted this research. This study was carried out to characterize the physiological bases of heat tolerance and identify QTLs/genomic regions underlying these traits in a
collection of 189 elite wheat germplasm in diverse multi-environmental regions. The information gathered from the characterization would enable wheat breeders to effectively combine, accumulate or pyramid several desirable traits into locally adapted germplasm leading to the development of wheat varieties with high yield, stability and enhanced heat tolerance. This research was initiated to achieve the following objectives:
§ to evaluate the grain yield performance and stability of 189 elite ICARDA wheat genotypes in multi-heat stressed environments.
§ to determine the morphological and physiological bases for heat tolerance in wheat.
§ to map the putative quantitative trait loci (QTLs) underlying heat tolerance in wheat.
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