Hindawi Publishing Corporation The Scientific World Journal Volume 2013, Article ID 610721, 12 pages http://dx.doi.org/10.1155/2013/610721 Review Article Drought Tolerance in Wheat Arash Nezhadahmadi, Zakaria Hossain Prodhan, and Golam Faruq Institute Biological Sciences, Faculty Science, Universiti Malaya, 50603 Kuala Lumpur, Malaysia Correspondence should be addressed to Golam Faruq; faruq@um.edu.my Received 29 July 2013; Accepted 6 September 2013 Academic Editors: R. S. Boyd and N. Rajakaruna Copyright © 2013 Arash Nezhadahmadi et al. This is an open access article distributed under Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided original work is properly cited. Drought is one most important phenomena which limit crops' production and yield. Crops demonstrate various morphological, physiological, biochemical, and molecular responses to tackle drought stress. Plants' vegetative and reproductive stages are intensively influenced by drought stress. Drought tolerance is complicated trait which is controlled by polygenes and their expressions are influenced by various environmental elements. This means that breeding for this trait is so difficult and new molecular methods such as molecular markers, quantitative trait loci (QTL) mapping strategies, and expression patterns genes should be applied to produce drought tolerant genotypes. In wheat, there are several genes which are responsible for drought stress tolerance and produce different types enzymes and proteins for instance, late embryogenesis abundant (lea), responsive to abscisic acid (Rab), rubisco, helicase, proline, glutathione-S-transferase (GST), and carbohydrates during drought stress. This review paper has concentrated on study water limitation and its effects on morphological, physiological, biochemical, and molecular responses wheat with possible losses caused by drought stress. 1. Introduction Drought is one most common environmental stresses that affect growth and development plants. Drought continues to be an important challenge to agricultural researchers and plant breeders. It is assumed that by year 2025, around 1.8 billion people will face absolute water shortage and 65% world's population will live under water-stressed environments. Tolerance to water stress is complicated parameter in which crops' performance can be influenced by several characteristics [1]. Tolerance can be divided into two parts including drought avoidance and dehydration tolerance [2]. Drought avoidance includes root depth, reasonable use available water by plants, and changes in plants' lifestyle to use rainfall. Dehydration tolerance consists plants' capability to partially dehydrate and grow again when rainfall continues [3]. Adaption plants to drought stress is vital issue to develop new improve methods for increasing stress tolerant plants [4]. Many factors can affect plants' responses to drought stress such as plant genotype, growth stage, severity and duration stress, physiological process growth [5], different patterns genes expression [6], different patterns activity respiration [7], activity photosynthesis machinery [8], and environmental factors [4, 9]. Drought stress can have effects on genes expression, and detection genes during water stress is crucial to observe their responses. In this regard, various drought responsive genes were distinguished [1]. Ouvrard et al. [10] believed that role genes can be distinguished by expression gene to high resistance levels among varieties. Drought stress can also influence plants in terms protein changes, antioxidant production, osmotic adjustment, hormone composition, root depth and extension, opening and closing stomata, cuticle thickness, inhibition photosynthesis, decrease in chlorophyll content, reduction in transpiration, and growth inhibition [11–14] to stand with some osmotic changes in their organs. Drought can also cause pollen sterility, grain loss, accumulation abscisic acid in spikes drought-susceptible wheat genotypes, and abscisic acid synthesis genes in anthers [15]. In many biochemical studies, role reactive oxygen species (ROS) has been identified. Dat et al. [16] claimed that increase in ROS can be caused by drought stress in which oxidative balance cell is changed. rise in generation ROS prompts to generation ABA (abscisic acid) which is general signal under drought [17–20] and can consequently 2 regulate antioxidant genes expressions by producing superoxide dismutase (SOD) and catalase (CAT) [21]. Several physiological studies have been completed on impact drought stress on wheat. Rosenberg et al. [22] observed that transpiration decreased significantly under drought stress; then heat can slowly be lost from leaves and leaf temperature can be increased. As result, CO2 concentrations and photosynthesis are increased which affect plant's growth and finally, water use efficiency can be improved. The same studies demonstrated that plants' development can be promoted more with CO2 [23–26]. respiratory terminal oxidase, alternative oxidase (AOX), plays important roles in optimizing photosynthesis and protecting chloroplast under drought stress [27]. Ribas-Carbo et al. [7] suggested that increase in AOX pathway under water stress could be prompted by inhibition cytochrome pathway. In this review paper, an attempt is made to explore different research information on wheat drought tolerance in various aspects, namely, morphological, physiological, biochemical, and molecular responses. 2. Physiological Derivations Drought Tolerance in Wheat Physiological responses include closure stomata, decrease in activity photosynthesis, development oxidative stress, alteration in integrity cell wall, production metabolites which are toxic and cause plants' death [28], signal recognition roots, turgor loss and adjustment osmosis, reduction in water potential leaf, decrease in stomata conductance to CO2 , reduction internal CO2 concentration, and reduction growth rates. According to researchers, there is relationship between different physiological responses crops and their resistance functions under drought such as high amount relative water and potential water [29, 30] and integrity membrane [31, 32]. For measuring drought tolerance, various scientists considered maintenance membrane integrity and its role under water stress [33, 34]. Sink strength can be reduced in drought stress during early grain filling which results in reducing endosperm cell number and metabolic activity [35]. Grudkowska and Zagda´ ska [36] indicated that cysteine n proteinase plays an imperative function in plant signalling pathways, growth and development, and in response to various kinds stress. Cysteine is expressed in wheat leaf organs and its contribution in proteolysis activity rises under drought [37]. Wi´niewski and Zagda´ ska [38] also s n observed that role cysteine was improved, but its role was negatively related to degree drought tolerance ten lines spring wheat. Transpiration efficiency (TE) is indispensable phenomenon in plants. Various researchers proposed that TE can be influenced by cultivar and drought [39, 40]. So, selection high TE crops is most important action to produce drought tolerant plants. Growth is one physiological processes which is sensitive to drought and can be affected by reduction in turgor pressure. Because low turgor pressure, water stress quenches cell expansion and growth. However, when turgor pressure is The Scientific World Journal Table 1: Yield losses at vegetative growth stages under drought in wheat. Vegetative stage Early season stress Midseason stress Booting stage Tillering stage 1000-grain weight (vegetative stage) Earlier stages Spike length (vegetative stage) Number spikelets per spike (vegetative stage) Grains number (vegetative stage) Grain yield (vegetative stage) Yield loss (%) Reference 22 58 20.74 46.85 38.67 79.7 16.90 [48] [48] [49] [49] [50] [51, 52] [50] 28.63 [50] 72.51 61.38 [50] [50] Table 2: Yield losses at reproductive growth stages under drought in wheat. Reproductive stage Higher grain protein content, fewer days to physiological maturity, smaller kernel weight and diameter, less grain yield Less grain yield (drought-tolerant variety) Less grain yield (drought-sensitive variety) 1000-grain weight 1000-grain weight (anthesis stage) Biological yield Maximum grain yield Decreased seed number Grain formation stage Grain formation stage Number spikes Number spikes (anthesis stage) Spike length (anthesis stage) Number spikelets per spike (anthesis stage) Grains number (anthesis stage) Grain yield (anthesis stage) Yield loss (%) Reference Not applicable [53] 43 [54] 26 [54] 18.29 5 38.67 10 22 64 101.23 65.5 19.85 15.79 16.90 [1] [3] [50] [1] [1] [3] [49] [51, 52] [50] [50] [50] 26.20 [50] 72.51 64.46 [50] [50] bigger than cell wall yield, cell expansion can occur [41, 42]. Osmotic adjustment is remarkable part plants' physiology by which they respond to water deficits [5, 43– 47]. Yield losses at vegetative growth and reproductive stages under drought in wheat are provided in Tables 1 and 2. 3. Biochemical Derivations Drought Tolerance in Wheat reduction in efficiency photochemical, reduced Rubisco efficiency, gathering stress metabolites (glutathione, The Scientific World Journal MDHA, glybet, and polyamines), antioxidative enzymes (superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), glutathione-S-transferase (GST), glutathione peroxidase (GP)), monodehydroascorbate reductase (MDHAR), and reduced ROS accumulation are biochemical responses plants to water stress. Tolerance to drought correlates with positive response plants' antioxidant system. According to study Li and Staden [55], in drought condition, some reactive oxygen species (ROS) such as hydroxyls (OH), superoxide (O2 − ), peroxide hydrogen (H2 O2 ), and oxygen which is singlet (1 O2 ) are created. These ingredients may initiate disturbing lipid peroxidation, chlorophyll, protein oxidation, and nucleic acids [56]. Changes in activity these enzymes are crucial for resistance various plants to drought stress [57]. Evidences suggest that drought causes oxidation damage from increased production ROS with deficit defense system antioxidant in plants [54, 58–61]. Osmotic regulators include small molecules (Pro), ions (K+), and soluble sugar, which help crops to absorb water in drought environments. In wheat, various studies exhibited that wheat genotypes with higher osmotic regulators and lower malondialdehyde (MDA) content have better tolerance to drought [5, 43, 47, 60, 62–66]. Polyamines (PAs) have role in completeness membranes and nucleic acid under water stress environments [11]. Malabika and Wu [67] mentioned that higher levels polyamines can make crops have higher growth under water stress conditions [18, 68, 69]. CAT is one most rapidly reversible proteins in leaf cells especially in stress conditions and its activity is reduced in drought condition [70]. 4. Morphological Derivations Drought Tolerance in Wheat According to study Deˇ ci´ et al. [71], wheat is n c paid special attention due to its morphological traits during drought stress including leaf (shape, expansion, area, size, senescence, pubescence, waxiness, and cuticle tolerance) and root (dry weight, density, and length). Shi et al. [72] expressed that drought can affect vegetative and reproductive stages. Therefore, understanding plants' responses to drought at every life stage is crucial to progress in genetic engineering and breeding. Rizza et al. [50] observed that early maturity, small plant size, and reduced leaf area can be related to drought tolerance. Lonbani and Arzani [73] claimed that length and area flag leaf in wheat increased while width flag leaf did not significantly change under drought stress. Leaf extension can also be limited under water stress in order to get balance between water absorbed by roots and water status plant tissues [74]. According to study Rucker et al. [75], drought can reduce leaf area which can consequently lessen photosynthesis. Moreover, number leaves per plant, leaf size, and leaf longevity can be shrunk by water stress [76]. Singh et al. [53] observed that leaf development was more susceptible to water stress in wheat. Root is an important organ as it has capability to move in order to find water [77]. It is first organ to be 3 induced by drought stress [78]. In drought stress condition, roots continue to grow to find water, but airy organs are limited to develop. This different growth response shoots and roots to drought is an adaptation to arid conditions [79, 80]. To facilitate water absorption, root-to-shoot ratio rises under drought conditions [81, 82] which are linked to ABA content roots and shoots [83]. The growth rate wheat roots was diminished under moderate and high drought conditions [84]. In wheat, root growth was not markedly decreased under drought [85]. Plant biomass is crucial parameter which was decreased under drought stress in spring wheat [86]. The same outcomes were observed in previous studies in wheat and other crops [86–88]. In winter wheat, yield was decreased or changed under drought and, in contrast, water use efficiency was boosted [89, 90]. 5. Molecular Responses Drought Tolerance in Wheat Some genes are known to be drought influenced and produced different types drought stress related proteins and enzymes including dehydrins [91], vacuolar acid invertase [92], glutathione S-transferase (GST) [93], and late embryo abundant (LEA) protein [94]; expression ABA genes and production proteins like RAB, rubisco, helicase, proline, and carbohydrates are molecular basis drought tolerance. Plants respond to stress environments with altering their gene expressions and protein productions. In contrast, available information on drought-responsive genes is still limited as their roles have not been thoroughly determined [28]. In wheat seedling stage, lot studies are done in gene expression, but it is junction stage that is susceptible to drought [72]. This is because junction phase is linkage point in vegetative and flowering growth stages and it is important for development and reproduction [72]. Sivamani et al. [52] indicated that HVA1 gene assists to increase wheat growth under drought stress. HVA1 gene produce kind protein which is in group 3 LEA and has 11 amino acid motifs in nine repeats. Proline is crucial protein that has vital function in water stress tolerance. It can be created from pyrroline-5-carboxylate synthetase or P5CR, and responsible gene for this enzyme has been distinguished in some crops, namely, petunia, soybean, and tobacco [95–97]. Hong-Bo et al. [98] investigated role proline as wheat antidrought defence protein under drought. In photosystem II (PS II) reaction center, psbr has an indispensable task in oxidation water [99], and in Calvin cycle, rubisco is key enzyme under drought stress [100]. Some plant proteins can be over-expressed including late embryogenesis abundant (LEA) that are saved in vegetative tissues during desiccation seeds under drought stress. LEA proteins are influenced by drought stress and their size in wheat reaches 200 kDa (Wcs200) [101]. These proteins have been detected through their sequence amino acid [102] and they help other proteins retrieve after denaturation during water stress [103]. There have been lot works during last two decades to engineer LEA producing genes for promoting crop water stress resistance. For instance, 4 wheat LEA genes, PMA1959 (encoding group one LEA protein) and PMA80 (encoding LEA protein's second group) improved water deficit resistance in rice [104]. In wheat, protein contents groups one, two, and three LEA have been detected. The Em gene wheat which encodes LEA protein first group has been vastly researched [105–107]. Group three LEA protein has also been distinguished in seedlings wheat [108, 109]. In durum wheat, protein groups two (dehydrins) and four LEA proteins were studied by Ali-Benali et al. [110]. Td27e, Td29b, and Td16 gene transcripts were saved late in embryogenesis and throughout seed development. Transcripts Td11 gene were presented whereas no transcripts Td25a gene were detected in seeds [110]. Vacuolar H+ -translocating pyrophosphatase (V-PPase) is an important enzyme linked to plant development as well as resistance to abiotic stress. Wheat V-PPase genes, TaVP3, TaVP2, and TaVP1 were investigated by Wang et al. [111]. Kam et al. [112] also detected responsible genes in wheat for water stress. They observed that TaRZF70 as RING-H2 zinc finger gene presented various responses to drought stress which was upregulated in leaf and downregulated in root [113]. TaRZF38 and TaRZF70 were expressed in wheat root while TaRZF74 and TaRZF59 were expressed in embryo and endosperm at highest level. TACCGACAT, 9-bp consensus sequence, was first distinguished in promoter Arabidopsis rd29A/lti78 and presented to be vital for drought induction in abscisic acid absence [114]. Then, this element could be bent by family transcription elements and therefore named DRE-binding (DREB) proteins [115]. Lucas et al. [116] used sequence putative DREB labelled DREB3A from wheat (TaDREB3A, Gen bank ID: AY781349) to seclude DREB from wild wheat (T. turgidum ssp. dicoccoides) and to detect its function in higher drought resistance. They also concluded that DREB proteins are numerous and vastly upregulated in reaction to drought in root tissue rather than leaf [116]. Drought stress influences RD gene (responsive to desiccation) [117, 118]. This gene has been divided into two major parts. The first group includes expression regulatory gene and signal direction during crops' reaction to stress, and second group involves proteins which directly protect cells from stresses [119]. In wheat, among 265 genes detected at junction phase and 146 genes distinguished at seedling stage in response to drought stress, more than half them were thought to be involved in abiotic or biotic stress responses [72]. 6. Breeding for Drought Tolerance through Conventional and Biotechnological Breeding Methods Conventional breeding needs detection genetic variability under drought between plant genotypes, or between sexually compatible cultivars, and introduction tolerance line with proper agronomic traits. Although conventional breeding for water stress resistance has had some prosperity, it is slow process which is limited by availability proper genes for breeding. In traditional breeding, crosses are The Scientific World Journal partially uncontrolled and breeders select parents to cross, but at genetic approach, outcomes are unpredictable [120]. Conventional breeding strategies are labour-intensive which requires great efforts to separate undesirable traits from desirable traits, and this is not economically suitable. For instance, crops must be back-crossed again over lots growing seasons to breed undesirable traits generated by random mixing genomes [120]. On other hand, improvement resistant plants through genetic engineering needs detection important genetic dominants to respond as stress resistance crops by transferring novel genes into plants. Drought affects activity vast number genes, and gene expression experiments have detected various genes that are induced and repressed under drought stress [121]. The nature drought tolerance makes management difficult in traditional breeding techniques. Novel biotechnological strategies have increased information on crop responses to drought at whole crop and molecular levels [122]. lot drought stress-induced genes were detected and cloned. Crop genetic engineering and molecular-marker methods make improvement drought-resistant germplasm possible [122]. Transgenic crops are also being improved to manage water stress. Structural and regulatory genes including dehydration-responsive, element-binding (DREB) factors, zinc finger proteins, and NAC transcription factor genes are already being applied [122]. Agrobacterium and particle gun techniques for transgenes related to drought resistance were applied in different crops such as rice, wheat, maize, sugarcane, tobacco, Arabidopsis, groundnut, tomato, and potato. Drought-tolerant genetically modified (GM) plants are being produced and molecular markers are used to detect drought-related quantitative trait loci (QTL) which were successfully transferred into rice, wheat, maize, pearl millet, and barley [122]. 7. Breeding for Drought Tolerance through Molecular Markers in Wheat Nowadays, molecular markers are widely used to detect location drought-induced genes. Different molecular marker are currently available for genome mapping and tagging different traits which is useful for Marker-assisted breeding (MAB) technique in wheat in stress conditions [123]. It is intensively used to create stress-tolerant lines in different crops. Marker-assisted selection (MAS) refers to selection by DNA markers linked to QTLs that are very powerful. Thus, DNA markers can track presence QTLs for drought tolerance [124, 125]. For development drought tolerance in plants through molecular linkage maps, marker-assisted selection (MAS) is best procedure. In winter wheat, with use amplified fragment length polymorphism (AFLP) and simple sequence repeat (SSR) markers, QTL mappings for senescence flag leaf (FLS) in normal and water-stressed environments have been studied. The responsible gene for this characteristic is revealed and QTL is also detected on chromosome 2D associated with better performance under drought [126]. In another study by Quarrie et al. [127], DNA markers like restriction fragment The Scientific World Journal length polymorphism (RFLP), AFLP, and SSR have been used to tag QTLs for drought stress in wheat. During last few decades, molecular markers such as SDS-protein, isozymes, and DNA sequences have assisted to select quantitative traits especially drought tolerance. These molecular markers are used in wheat to evaluate diversity genes and identify genotype and genetic mapping [128–130]. Some markers in durum wheat are linked to grain yield and morphophysiological characteristics for drought tolerance [131]. Leaf water potential, canopy temperature, chlorophyll inhibition, and proline content showed strong relationships with molecular markers [131]. Ashraf et al. [132] prepared various DNA markers to estimate inheritance stress tolerance such as PCR indels, RAPDs, RFLPs, CAPS, AFLPs, microsatellites (SSRs), SNPs and sequences DNA. In cereals, RAPDs with use DNA primer were vastly used [133, 134]. ISSRs were used in mapping genome in wheat and other crops [135, 136]. Milad et al. [48] identified RAPD and ISSR markers related to flag leaf senescence gene in wheat under drought stress. RAPDs were found to be helpful in hexaploid wheat as genetic markers [134, 137]. When correlation between molecular marker and trait is greater than heritability trait, marker assisted selection may be advantageous. These results suggest usefulness molecular markers to enhance drought tolerance in durum wheat in drought condition [131]. 8. Mapping QTL for Drought Tolerance in Wheat Quantitative trait loci (QTL) is location from where some genes influence phenotype quantitatively inherited trait. Genetic variations crop can be explored through QTL mapping (polygenes) [132]. Mapping QTL allows estimation places, quantity, size effects for phenotype, and gene activity pattern [138]. In 2005, first activity was conducted for cloning QTL [139] to know and operate characteristics which are responsible for drought resistance [51, 140, 141]. QTL mapping for water stress resistance traits has been done in wheat and other crops [142–147]. In wheat, due to drought stress, place genes which had influence on ABA concentration was detected [142]. It is detected that 5A chromosome transports gene(s) for ABA concentration. Quarrie et al. [127] conducted mapping QTLs for drought resistance in hexaploid wheat placed on chromosomes 1A, 1B, 2A, 2B, 2D, 3D, 5A, 5B, 7A, and 7B. Double haploid populations serve as permanent source QTL mappings. Recombinant inbred lines from crossing drought-resistant and drought-susceptible cultivars were used to create mapping populations for QTL analysis regulating yield under drought [148]. QTL analysis is so important to target genes and for doing this some steps are required. Firstly, phenotypic evaluation relatively large population for markers which are polymorphic is needed. Secondly, genotyping population is important. Thirdly, there is need for statistical analysis to detect loci that are influencing target trait. On other hand, QTL for drought tolerance has some drawbacks like genetic and 5 environmental interactions, numerous numbers genes, and using mapping populations which are wrong. These have limited plans for mapping QTL for high yield under drought condition [149]. 9. Drought Management 9.1. Drought-Tolerant Varieties. In past decade, there have been several efforts to generate drought-tolerant wheat through breeding methods. Cross-breeding among wild wheat species at International Centre for Agricultural Research in Dry Areas (ICARDA) created germplasm that creates higher yields under drought. In wheat breeding programs, seeking for increased yield has been priority to improve drought tolerance plants. However, before successful genetic manipulation can be made, it is important to characterize physiological parameters droughttolerant or -sensitive cultivars [150]. Analysing physiological determinants for yield which responds to water stress may also be helpful in breeding for higher yields and stability genotypes under drought conditions. Traits to select either for stress escape, avoidance or tolerance, and framework where breeding for drought stress is addressed will depend on level and timing stress in targeted areas. However, selecting for yield itself under stress-alleviated conditions appears to produce superior cultivars, not only for optimum environments, but also for those characterized by frequent mild and moderate stress conditions [150]. This implies that broad avoidance/tolerance to mild/moderate stresses is given by constitutive traits also expressed under stress-free conditions [151]. Keeping in view importance identifying water-stress tolerant wheat genotypes, water stress conditions can be imposed to wheat at various stages crop growth and development. The stresses can be given at tillering, booting, and grain forming stages. Root system size (RSS) wheat can be selection target for drought tolerance. During dry periods, crops expand their roots to deeper soil regions and they are able to alter their morphology. For instance, airy organ mass is decreased but mass roots is increased. Wheat genotypes with good water management are able to bear high yields in drought conditions [152]. Genotypes with proper water management could be used to create new breeding lines and cultivars with developed drought resistance. 9.2. Agronomic Practices. Drought stress includes different agronomic, soil, and climatic factors which vary in time occurrence, duration, and intensity. It has effect on yield and can also diminish benefits crop handling performances including management fertilizer or pest and disease [49]. Drought management strategies are very important and have to concentrate on extraction available soil moisture, crop establishment, growth, biomass, and grain yield. There are many agronomical ways to manage drought stress such as control field irrigation methods (surface or furrow, sprinkled, and drip) and identification drought resistance sources through developing screening methods under environmental conditions. So, for drought screening, 6 The Scientific World Journal Table 3: Research scenario physiological traits under drought stress in wheat. Table 5: Research scenario morphological traits under drought stress in wheat. Traits Physiological Stomata closure Cell wall integrity Synthesis metabolites Oxidative stress Photosynthesis Turgor pressure CO2 concentration Growth rate Osmotic adjustment Stomata conductance Relative water content Membrane integrity Transpiration Water use efficiency Transpiration efficiency Total biomass Alternative oxidase (AOX) Traits Morphological Reference [16] [16] [16] [16] [16, 45, 62, 115] [64, 71] [30, 46, 73, 84] [92] [21, 42, 62, 81, 88, 89, 97] [62] [26, 110] [36, 72, 102, 119] [115] [13, 69, 153] [121, 124] [13, 154, 155] [108, 156] Reference Small plant size Leaf area [112] [112, 116] Root extension Roots dry weight, density, and length Early maturity [75, 135, 143, 160, 161] [35] [112] Yield Leaf extension Leaf size [69, 90, 128, 153] [95] [133] Leaf number Leaf longevity [133] [133] Root-to-shoot ratio [88, 91] Table 6: Research scenario biochemical traits under drought stress in wheat. Traits Reference Biochemical Table 4: Research scenario molecular traits under drought stress in wheat. Traits Reference Molecular CAT gene expression SOD gene expression Proline Dehydrins Vacuolar acid invertase Glutathione S-transferase (GST) Late embryo abundant (LEA) DRE-binding proteins Rd29A/Lti78 Psbr Rubisco QTL mapping Molecular markers [66] [66] [57, 114, 157] [2, 27] [150] [5] [17, 40, 98] [78] [158] [142] [43] [8, 9, 12, 19, 44, 103, 122, 123, 125, 126, 145, 146, 149, 151, 152, 159] [9, 44, 146] not only analysing sources replications, variation among plots, and repeated experiments are needed, but also sprinkler irrigation, rainout shelters, and evaluation drought susceptibility index (DSI) are important [49]. In drought management strategies, increasing biomass and seed yield, crop establishment, and maximum crop growth have to be considered. For example, to improve yield in drought-prone area, these steps are essential: frequency drought stress occurrence in target environment, matching phenology crop (sowing, growth period, flowering, and seed filling) with period soil moisture and climatic regimes, developing way for better use irrigation, and increasing soil water to crop through agronomic management practices. Chlorophyll content Superoxide Dismutase (SOD) Catalase (CAT) Polyamines (PAs) Reactive oxygen species (ROS) Abscisic acid (ABA) [75, 100, 127, 143, 160, 161] [66] [59, 66] [4, 14, 82, 136, 143] [22, 24, 31, 32, 56, 76, 96, 107, 127, 132, 136, 139, 162] [32, 56, 96, 136] Furthermore, good knowledge what type stress is more frequent in target environment is essential in drought breeding. Yield stability under water shortage condition and crop water productivity should be goal. In drought stress condition, aim is to preserve source water. These sources include snow, rain, and irrigation water. Water conservation can be achieved by surface residue during growing season. Todd et al. [163] claimed that wheat residue diminished evaporation rate during season. Residue also slows movement water and allows much time for water to penetrate into soil. Rotation crop can preserve total water needs by irrigation. In winter wheat, it can be decline requirements for irrigation. Schneekloth et al. [159] claimed that with irrigation for 6 inches, corn following wheat produced 8 percent more than corn following corn. Rotation crops also makes irrigation season to have much time frame in comparison with single crop. In breeding for drought resistance, productions biomass and water use efficiency (WUE) are imperative elements agronomy [155]. There is risen interest in improving WUE plant genotypes so that plants can develop and bear better under drought condition [154, 164]. Figure 1 shows effects drought stress on different wheat traits. Detailed information on physiological, molecular, biochemical, and morphological traits under drought stress in wheat is demonstrated in Tables 3, 4, 5, and 6. The Scientific World Journal 7 Water stress Morphological changes - Small plant size - Early maturity - Reduced leaf area - Reduced yield - Limited leaf extension - Diminished leaf size - Decreased number leaves - Reduced leaf longevity - Increased root-to-shoot ratio - Reduced total shoot length - Decreased plant height Physiological changes - Stomata closure - Diminish in photosynthesis - Increase in oxidative stress - Cell wall integrity changes - Leaf water potential reduction - Decrease in stomata conductance - Internal CO2 concentration reduction - Diminished growth rates - Decline in transpiration - Developed water use efficiency - Enhance AOX pathway - Reduced relative water content Biochemical changes - Reduction in rubisco efficiency - Declined photochemical efficiency - Produced reactive oxygen species (ROS) - Oxidation damage - Antioxidant defense - ABA generation - Diminished Chlorophyll content - Proline production - Polyamines generation - Increase in antioxidative enzymes - Carbohydrates production - ABA accumulation Overcome - Classical genetic methods - Responsible genes detection and transgenic plants production which lead to overexpression compatible solutes in transgenic plants to improve stress tolerance - Manipulation genes - Improved plant breeding techniques (water extraction efficiency, water use efficiency, hydraulic conductance, osmotic and elastic adjustments, and modulation leaf area) - Appropriate agronomical practices Figure 1: The effect drought stress on wheat. The information are provided from observations Powel et al. [128], Lawlor and Cornic [13], Shiran and Wan [15], Karthikeyan et al. [41], and Russell et al. [129]. 10. Conclusion Detection genomic responses plants to water stress is so important. Firstly, it prepares intensive information about transcriptional reactions plants to drought stress. Secondly, it makes possible to know functions genes in stress environments. Thirdly, it assists to distinguish promoters which react to stress and related cis-elements, which are both crucial for primitive studies and crop engineering [165]. Rapid improvements can be performed in drought resistance by manipulating genes which are responsible for plant growth regulators, antioxidants, proteins, and transcriptional factors [149]. QTL analysis and molecular mapping are also proper methods which have been done for qualitative and quantitative characteristics including resistance for stress. But, there are some limitations in this issue. For example, there is challenge for QTL detection, for instance, interaction between genotype and environment, inconsistent repeatability, numerous genes that regulate yield, and use wrong populations for mapping. Furthermore, other elements also limit efficiency QTL for genetic development parameter because improper interaction epistasis, it is difficult to carry influences an allele to extract substance [156, 166]. Moreover, in several circumstances, QTL does not present marked impacts and stop thoroughly in various groundwork, even in similar growth conditions [153, 156]. This high variability in nature water stress and inadequate information about its complicatedness have caused it to be hard to identify specific physiological traits needed for improved crop performance. Acknowledgment The authors wish to express their gratitude to University Malaya, Kuala Lumpur, Malaysia, for providing IPPP Grant no. PV0138-2012A for this paper. References [1] J. Ingram and D. 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