**2. Global wheat production**

Wheat is one of the most important cereal and staple food crop around the world. It ranks first due to its area and production and contributes more calories to the world's human diet than any other crop. On the other hand, wheat also maintains its first rank among major cereals due to its higher protein and gluten contents [8–10]. In 1986–1987, the wheat production across the world, which was 521 million metric tons, was increased to approximately 572 million metric tons in 2005–2006 from an area of 220 million hectares [11] and 694 million metric tons in 2011–2012. In 2011, the European Union (137 million tons) was top ranking in wheat production countries followed by China (118 million tons) and the United States of America (54 million tons). Further, Canada, Australia, India, Pakistan and Argentina contribute about 79% of the total wheat production. The world trade market was very feasible for wheat in 2011, and 129 million tons of wheat was traded in the world market [12].

### **3. Drought stress**

Drought is defined as water deficiency in the root zone of crops that result to decrease in yield during the plant life cycle [13]. The capability of a plant to grow and reproduce in waterlimited area is referred to as drought tolerance. Drought stress is changeable in its intensity, length and effectiveness, and crop plants are required not only be able to survive, but their ability to produce a harvestable yield under drought stress is of practical importance [14]. Drought tolerance is a quantitative trait, influenced by complex phenotype and genetic interactions. Understanding the genetic basis of drought tolerance in crop plants is a prerequisite for developing superior genotypes. High temperatures, radiation, water and nutrient deficiencies are commonly encountered under normal growing conditions also pose somewhat similar challenges. Further, certain soil properties such as composition and structure can also affect the balance of these different stresses; see, *for example*, [15, 16].

Drought is the main environmental problem that causes high negative effect on cereal crops particularly wheat. During drought conditions, plants show a wide range of behaviors varying from great sensitivity to high tolerance [17]. Seasonal cyclic drought has great involvement in reduction of wheat, barley and other cereal yields [18]. Drought stress greatly affects plant growth, gene expression, distribution, yield and quality of crop in arid and semiarid areas around the world [19]. About 60% of crop production around the world is from arid and semiarid regions. The rate of rainfall critically fluctuates in these areas. In developing countries 37% of wheat is commonly grown in drought susceptible areas [20]. The major constraint to wheat production around the world is inadequate supply of water. Within the United States of America alone, about 67% of crop losses over the last 50 years have been due to drought. The 2012 drought in the United States of America was the worst in the last 60 years, and more frequent occurrences of water shortages are expected due to climate projections and increasing competition for water among urban, industrial and agricultural demand.

The plants' reaction to drought stress depends on plant growth (development), stress period and plant genetics [21, 22]. Drought can also influence morphophysiological features of plant such as growth, anatomy, morphology, physiology (stomatal closure, low photosynthesis, transpiration rate), biochemistry and ultimately productivity [23, 24]. Yield is the basic criteria for cultivation of crop varieties under drought conditions. Therefore, it is a great challenge for crop breeders to produce cultivars having good potential of survival in stressed (drought, salinity, cool) environment [14, 15, 25]. Breeding for drought tolerance is further complicated by the fact that several types of abiotic stress can challenge crop plants simultaneously. Further, given the complexity of drought tolerance, marker-assisted selection has not contributed significantly to cultivar improvement, and breeding for dry environments has relied on direct phenotypic selection. However, recent technological advances and the great potential in wheat to ensure sustainable food production have driven research programmes to improve this crop genetically despite the size and complexity of the genome. Nonetheless, drought tolerance breeding may be effective if the markerassisted selection-based molecular linkage maps for crop species are available [15, 26].

#### **3.1. Drought stress in Pakistan**

cereals and modified to different climatic conditions [3, 4]. Bread wheat (2n = 6x = 42) and durum wheat (4x = 28) are the two common cultivated species around the world. Bread wheat supplies about 95% wheat globally, while durum and other wheats (emmer (4x = 28), einkorn (2x = 14) and spelt (6x = 42)) provide only 5% of the world wheat [2, 5]. Human population is increasing rapidly and is estimated to reach 9.4 billion by 2050. Therefore, food production will require a greater yield from the present cropland without horizontal expansion [6]. Population growth, environmental pollution and utilization of croplands for other purposes may reduce the croplands by 10–20% [7]. To meet the growing demand of global food shortage of 2050, total food production must increase by 50% at least to meet out demands of 2050. Among the crop plants, wheat is an economic and rich source of energy and proteins and supplies one fifth of all human calories for the world population [8]. Plant breeders are always trying to find wheat germplasm having desirable traits such as tolerance to diseases and other abiotic stresses [7, 9]. There is no doubt that cereals such as wheat, rice and maize are the world's leading food crops for all humans and are the principal resources that have led to the

Wheat is one of the most important cereal and staple food crop around the world. It ranks first due to its area and production and contributes more calories to the world's human diet than any other crop. On the other hand, wheat also maintains its first rank among major cereals due to its higher protein and gluten contents [8–10]. In 1986–1987, the wheat production across the world, which was 521 million metric tons, was increased to approximately 572 million metric tons in 2005–2006 from an area of 220 million hectares [11] and 694 million metric tons in 2011–2012. In 2011, the European Union (137 million tons) was top ranking in wheat production countries followed by China (118 million tons) and the United States of America (54 million tons). Further, Canada, Australia, India, Pakistan and Argentina contribute about 79% of the total wheat production. The world trade market was very feasible for wheat in

Drought is defined as water deficiency in the root zone of crops that result to decrease in yield during the plant life cycle [13]. The capability of a plant to grow and reproduce in waterlimited area is referred to as drought tolerance. Drought stress is changeable in its intensity, length and effectiveness, and crop plants are required not only be able to survive, but their ability to produce a harvestable yield under drought stress is of practical importance [14]. Drought tolerance is a quantitative trait, influenced by complex phenotype and genetic interactions. Understanding the genetic basis of drought tolerance in crop plants is a prerequisite for developing superior genotypes. High temperatures, radiation, water and nutrient deficiencies are commonly encountered under normal growing conditions also pose somewhat similar challenges. Further, certain soil properties such as composition and structure can also

2011, and 129 million tons of wheat was traded in the world market [12].

affect the balance of these different stresses; see, *for example*, [15, 16].

emergence of human civilization.

40 Wheat Improvement, Management and Utilization

**2. Global wheat production**

**3. Drought stress**

Diverse climatic and soil conditions are available for wheat growing in Pakistan. About one third of the total land area comes under rain-fed regions where rainfall is unusual [27]. Drought and salinity are very common around the world and are among the most serious problems to the agriculture in Pakistan [28]. Arid and semiarid regions of the world are badly affected by water stress, and as result crop production is reduced. Irrigated areas sometimes face drought conditions due to inadequate supply of water and canal closures [23, 29]. Drought-tolerant varieties are those, where grain yield is least affected by drought stress, or drought-tolerant crops are those that take up maximum amount of water and lose minimum of water during dry conditions [1–5, 30]. To ensure high crop production in rain-fed areas, different aspects of agriculture like holding precipitation, reducing evapotranspiration and sowing of drought tolerant varieties are important. Wheat varieties cultivated in rain-fed areas of Pakistan are usually low yielding as well as pests and diseases that are susceptible but are well adapted and flourish in dry conditions. Still, the need to increase yield to meet the demands of growing population to ensure food security requires well-integrated efforts. Although global water scarcity may be an abstract concept to many and a reality for others. But with no confusion, it is the result of myriad environmental, political, economic and social issues. The current global climatic conditions are to hit Pakistan, and therefore, the search for diverse and drought-tolerant sources of crop plants is of paramount significance to feed its ever-growing population. Marker-assisted selection is a cry of the day for yield improvement in drought stress areas of Pakistan. Thus, the use of molecular markers for tagging of drought resistance genes is needed [14, 15, 31].
