ABSTRACT
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.
TABLE OF CONTENTS
TABLE OF CONTENTS
LIST OF TABLES
LIST OF FIGURES
APPENDICES
ABSTRACT
INTRODUCTION
LITERATURE REVIEW
Botany and Adaptation of Wheat
Genome of Wheat
Importance of Wheat
Effects of Heat stress on Wheat Crop
Bases for Screening Wheat for Heat Tolerance
Molecular Markers
Genetic Mapping Approaches
MATERIALS AND METHODS
RESULTS
DISCUSSION
CONCLUSION
REFERENCES
APPENDICES
INTRODUCTION
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
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....
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