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
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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 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.....
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