Introduction affairs of the United Nations, 20151. Therefore,

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Introduction affairs of the United Nations, 20151. Therefore,

Introduction                                                                                                                               
It is expected that world population will continue to
grow and exceed nine billion by 2050 (Department of Economic and Social affairs
of the United Nations, 20151. Therefore, the global food production must increase
substantially to ensure food security for the growing population. However, food
production is seriously threatened by various environmental factors and soil
salinity is one of the major stresses adversely affecting plant growth and crop
productivity, especially in arid and semi-arid regions. Worryingly, these
regions continue to expand and they represent today 40% of the world’s land
surface where two billion people are living, mostly in developing countries (UNEP, 1992; Flowers and Yeo,
1995). As a
result, more irrigation with brackish water is unavoidable and salinization
becomes a serious agricultural concern worldwide. Therefore, engineering crops
with enhanced salt stress tolerance traits is one of the most important
challenges for modern agriculture.

      Salt
tolerance refers to the ability of plants to prevent reduce or overcome the
injurious effect of soluble salts present in their root zone. The salt
tolerance capacity differs from species to species. For example, crops like
barley, cotton, sugarcane, oilseeds and grasses are known to be more salt
tolerant than other crops. The salt tolerance capacity also differs within a
crop based on ploidy level. For instance, in wheat, hexaploid bread wheat is
more tolerant to salt than durum (tetraploid) and einkorn (diploid) wheat. In
Brassica, tetraploid species are more tolerant than diploids. In rice, late
maturing, tall and coarse grain varieties exhibit maximum tolerance to salinity
conditions.The problem of salinity is of global significance because saline and
alkaline soils are found in almost all countries of the world.

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The location of Pakistan is in arid and
semi-arid climatic zones. Generally high evapo-transpiration in semi-arid and
arid zones is the basic cause for salt accumulation on the soil surface. The
average summer temperature is about 40°C and the minimum winter temperature
remains between 2°C to 5°C. The annual rainfall varies between 100 mm to 700
mm throughout the country. The evaporation rate is generally very high and
exceeds that of precipitation. Thus, the insufficient rainfall followed by
high evaporative demand and shallow ground water depth, enhances the movement
of salts towards soil surface.Salinity is an important problem affecting
irrigated agriculture of Pakistan. Improper irrigation practices and lack of
drainage have generally led to accumulation of salts in the soil in
concentrations, which are harmful to the crops. There is a major imbalance in
the amount of salt entering and leaving the soil in Pakistan. Each year about
120 million tonnes of salts are added to the land in canal water and brackish
underground water. Only about 1/5th of this salt finds its ways to the sea.
The remainder accumulates in the soil it continues to decrease the growth and
survival of crops.

There are two ways to overcome the problem
of salinity.

Ø  One is reclaim the salt affected soils

Ø  The other is to develop salt tolerant
genotypes/ cultivars

The first method is very costly, time
consuming and short method, on the other hand, is long lasting, more effective
and less costly. Genetic differences exist among cultivars for their salt
tolerance capacity moreover plants adapt themselves to adverse environments for
their survival. High salt tolerant plants in crop are found in the salt
affected areas.

Breeding Approach;                                                  

There are two approaches by using these
approaches we overcome the effect of salinity. These approaches include

·        
Improving
yield level of salt tolerant cultivars

·        
Transfer
of salt tolerant genes to high yielding cultivars.

In the first traditional cultivars of salt
affected areas are improved for their productivity without affecting their salt
tolerance ability. In the second approach, salt tolerances genes from locally
adapted (Salt tolerant) cultivars are transferred to high yielding cultivars
through hybridization and selections. Breeders require close cooperation of
geneticist, physiologist, biochemist, and soil scientist in developing salinity
resistant varieties. The new approaches such as tissue culture and genetic
engineering may be rewarding in developing more salt tolerant genotypes. The
main limitations in the breeding for salt tolerance are:

                   1) Lack of efficient
selection criteria

                   2) Lack of inter disciplinary
cooperation                                                                                                          

                   3) Paucity of funds, etc.

Salinity tolerance in Wheat:   
                               

Wheat (Triticum aestivum L.) is one of most
important crop plants worldwide with annual production of about 736 million
metric tons (FAO 2015), but suffers signi?cant
grain yield losses due to soil salinity. Although, there are several strategies
to increase wheat production in the salt-affected areas (such as leaching and
drainage), the cultivation of tolerant genotypes is recognized as the most
effective way to overcome this limitation. The prerequisite is the identi?cation of wheat genotypes
with proven wide adaptation under saline conditions. The cultivar, Kharchia 65,
is one of the very few reputed donors of salt tolerance (ST) in wheat and has
been extensively used in breeding for ST cultivars globally (Chatrath et al.
2007). Thus, there is an urgent need to identify new sources of ST to broaden
the gene base and to provide donor parents in locally adapted genetic
backgrounds.

Wheat (Triticum aestivum) is a
moderately salt-tolerant crop (Maas and Hoffman, 1977). In the field, where the
salinity rises to 100 mM NaCl (about 10 dS m?1), rice (Oryza
sativa) will die before maturity, while wheat will produce a reduced yield .Durum
wheat (Triticum turgidum ssp. durum) is less salt tolerant than
bread wheat, as are maize (Zea mays) and sorghum (Sorghum bicolor)
(Maas and Hoffman, 1977; Salt Tolerance Database reproduced on USDA-ARS, 2005).

Genetic
improvement in salt tolerance of durum wheat using the trait for sodium
exclusion

Cultivated
durum (pasta) wheat (Triticum turgidum ssp. durum) is more salt
sensitive than bread wheat (Triticum aestivum), a feature that restricts
the production of durum wheat on farms with sodic or saline soils. To increase
the salt tolerance of durum wheat, attempts have been made to improve its
sodium exclusion, building on the earlier work of John Gorham and Jan Dvorak (Gorham et al., 1990; Dvorak et al., 1994). In the bread and
durum wheat, salt tolerance is associated with low rates of transport of Na+
to shoots with high selectivity for K+ over Na+; there is
little genotypic variation in rates of Cl? transport (Gorham et al., 1990;
Husain et al., 2004). To introduce the trait of Na+ exclusion
into current durum wheat varieties, genetic variation in salt tolerance was
investigated across a wide range of ancient durum-related accessions and landraces
representing five Triticum turgidum subspecies. Selections were screened
non-destructively for low Na+ concentration in leaves, and the
associated enhanced K+/Na+ discrimination (Munns et al.,
2000b). Wide genetic variation in Na+ accumulation (and K+/Na+
discrimination) was found, and a landrace named Line 149 was selected for
breeding.

Proof of
the concept that Line 149 would provide a source of salt tolerance for modern
durum wheat was obtained by comparing it with another durum landrace, Line 141 with
a high rate of sodium transport to leaves, to assess the effects of the sodium
exclusion trait on preventing leaf injury and death in saline soil (Husain et
al., 2003). Leaves of Line 149 lived longer than leaves of Line 141, the
start of leaf senescence being delayed by 1 week or more. High Na+
lines lost chlorophyll more rapidly and died earlier than the low Na+
lines. Other leaves showed comparable results (Husain et al., 2003), so
by the time the grain was developing, all the leaves of Line 141 were dead but
some were still alive in Line 149.

Improvement
of Salt tolerance in Durum wheat by Ascorbic Acid Application

Two
weeks old seedling, grown in plastic pots of 1kg, were subjected to salt stress
by adding 25ml of NaCl (150mm), and treated or not with the addition of
ascorbic acid (0.7 mM). Two weeks after salt stress, plants were harvested and
the various measures were recorded. The effects of salt stress, in the presence
and absence of vitamin C, on the leaf growth, leaf area (LA) and some physiological
and biochemical changes were investigated. It was established that the
application of vitamin C mitigate to variable extent the adverse effect of salt
stress on plant growth, may be due, in part, to increased leaf area, improved
chlorophyll and carotenoid contents, enhanced proline accumulation and
decreased H2O2 content. In conclusion, we can say that treatment with ascorbic
acid improve salt tolerance in durum wheat through the enhancement of multiple
processes.

Conclusion:                                                                                                                                Salt tolerance is a very complex trait, both
at the physiological and at the genetic levels, and is also very influenced by
other environmental factors acting on the plant at the same time. Over the last
two decades, research made on different crops shed light on several aspects of
the molecular mechanisms controlling the salt stress tolerance. However, many
challenges still lie ahead before successfully improving crop yield under
saline conditions. Hopefully, available tools including molecular breeding and
advanced biotechnology methods combined to the exploitation of the potential of
soil microorganisms can speed up the release of salt-tolerant crop varieties. A
combination of approaches will accelerate the identification and
characterization of specific loci involved in tolerance to salinity that can be
introgressed into elite sensitive varieties through molecular marker-assisted
breeding. To achieve this goal, we need to have diversity in germplasm
resources, high-throughput phenotyping platforms, genome sequencing of crops
and their relatives. Furthermore, molecular genetic resources including
mutation detection, gene discovery and expression profile, genome wide
association studies, and powerful omics databases are also needed. 

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