**2. Genetic diversity of water stress tolerance in soybean**

#### **2.1. Different response of soybean to water stress**

#### *2.1.1. Morphological performance*

Drought induces morphological changes in plants, enabling them to sense and rapidly adapt to the stress. Root-related traits are crucial in maintaining crop yield in soybean [45]. Drought alters the root system architecture (root depth, root angle and root branching density) [27]. For instance, root architecture was characterized in field under normal and water deficit conditions using three soybean cultivars (Jackson, Prima 2000 and A5409RG). As a result, Prima 2000 (drought-tolerant cultivar) has an intermediate root phenotype with a root angle of 40–60°, while a shallow root phenotype along with root angle of <40° has been observed in drought-sensitive cultivar A5409RG [27].

species, especially in soybean, secondary aerenchyma having a spongy parenchyma cell layer develops through cell division of phellogen [44, 62]. Secondary aerenchyma is morphologically and anatomically different from cortical aerenchyma (lysigenous and schizogenous aerenchyma) [33]. Waterlogging stimulated the formation of aerenchyma and adventitious roots in soybean plants facilitating transport of oxygen from shoot to root [62–64]. Under waterlogging condition, adventitious roots are formed in several flooded plants including soybean [61, 62, 65]. However, adventitious roots are absent in soybean seedlings under complete submergence [66]. Under flooding conditions, secondary aerenchyma consisting of white and spongy tissues develops within a few weeks in stems, roots and root nodules of soybean [33]. Aerenchyma formations initiated by ethylene, Ca2+, and ROS signalling through a pro-

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37

Rapid shoot elongation is another escape mechanism for adaptation in waterlogging stress [68]. It has been reported that lower stem of soybean having hypertrophic lenticels helps oxygen entry into the aerenchyma [64]. Flooding also causes a significant reduction in leaf number, leaf

Stress-responsive mechanisms have been studied at the physiological and biochemical level in soybean under drought and flooding stress. To optimize the use of water under water deficit conditions, stomatal control is considered as major physiological indicator. For instance, in soybean, stomatal conductance decreased by 42% in drought-stressed leaves rather than normal leaves [69]. Owing to dehydration, MG/BR46 (drought tolerant soybean variety) showed faster decline in stomatal conductance as compared to BR16 (drought-susceptible variety) (65 versus 50% reduction) [55]. In same study, prolonged drought stress (45 days) exhibited no profound impact on stomatal conductance of BR16 while it had reached 79% in the MG/BR46. Several studies have provided strong evidence that drought-tolerant soybean genotypes (C12 and W05) exhibited a higher reduction in stomatal conductance rather than the susceptible one (C08) [25]. In soybean, ABA is involved in the reduction of stomatal conductance and photosynthesis. For instance, after imposition of exogenous application of ABA under soil drying, leaf stomatal con-

ductance of soybean tolerant genotype C12 declined than the susceptible one (C08).

Maintenance of cell turgidity and water-use efficiency are important indicators to cope with drought stress [26]. Soybean introduction line PI 416937 is an excellent example of drought tolerance by limiting transpiration rate and maintaining a lower osmotic potential. An increase in WUE was observed in drought-tolerant genotype (C12) by regulating stomatal closure during the entire period of water deficiency [25, 52].The maintenance of cell turgidity under waterlimited conditions may be achieved by adjusting the osmotic potential in response to the accumulation of proline, sucrose, soluble carbohydrates, glycine betaine and other solutes [70]. The accumulation of solutes under water deficit condition is known as osmotic adjustment. Some authors have reported higher proline content in drought-tolerant crop species such as bean [71]. In soybean, water stress exhibited significant increase in proline contents in drought tolerance as well as susceptible genotype, but tolerant genotypes recovered to pre-stress levels

grammed cell death process are involved in aerenchyma development [60, 67].

area, canopy height and dry weight at maturity in soybean crops.

*2.1.2. Physiological and biochemical response under drought stress*

more quickly after rehydration [25].

Depth of rooting system influenced by the elongation of taproot also plays an important role for plant survival under water deficit [27, 46]. An increase in number of root tips, root length, root surface area and root volume was observed under water limited conditions. Several studies have proposed that roots having large xylem number, diameters, lateral root systems with more root hairs are indicators of drought tolerance [31, 47, 48]. Jackson is considered as drought escaping cultivar with long and deep roots into the soil permitting better water uptake compared with drought-sensitive cultivars [27, 49]. Under water-limited conditions, Plant Introduction (PI) 578477A and 088444 exhibited higher yield due to higher lateral root number in clay soil [50]. It was reported that deeper region of soil has high root density under seasonal drought as compared to dry surface of soil [51]. In addition, total root length/ plant weight, dry root weight/plant weight and root volume/plant weight were positively correlated with drought tolerance [52]. Therefore, studying the relationship between root traits and drought is helpful to develop drought-resistant cultivar.

Root-to-shoot ratio is also a good indicator to allocate the resources between different plant components. The water-limited environment increases the root-to-shoot ratio. For example, in soybean, root-to-shoot ratio increased by 13% indicates the flow of biomass towards roots [53]. The drought-tolerant soybean genotype (C12) showed a higher root-to-shoot ratio than the susceptible genotype (C08) under restricted soil water with application of exogenous ABA. To cope with drought stress, leaf morphology also plays an important role. Under water-limited conditions, plants reduce their leaf area by closing stomata. Due to water scarcity, reduction in soybean plant leaf area has been reported [54]. In contrast, drought-tolerant soybean cultivar exhibited a greater leaf area rather than less-tolerant cultivar under hydric stress condition [55].

Aerenchyma formation is a major indicator that facilitates gas exchange between aerial and submerged plant parts (shoots and/or roots) to avoid flooding stress [56, 57]. Flooding stress induces two kinds of aerenchyma i.e. primary (cortical) [58] and secondary (white and spongy tissues) [33]. A number of aquatic plants develop cortical aerenchymatous tissue by cell disintegration (lysigenous aerenchyma) and cell separation (schizogenous aerenchyma) [59]. In rice, barley, maize and wheat, lysigenous aerenchyma is induced by flooding [60, 61]. In some species, especially in soybean, secondary aerenchyma having a spongy parenchyma cell layer develops through cell division of phellogen [44, 62]. Secondary aerenchyma is morphologically and anatomically different from cortical aerenchyma (lysigenous and schizogenous aerenchyma) [33]. Waterlogging stimulated the formation of aerenchyma and adventitious roots in soybean plants facilitating transport of oxygen from shoot to root [62–64]. Under waterlogging condition, adventitious roots are formed in several flooded plants including soybean [61, 62, 65]. However, adventitious roots are absent in soybean seedlings under complete submergence [66]. Under flooding conditions, secondary aerenchyma consisting of white and spongy tissues develops within a few weeks in stems, roots and root nodules of soybean [33]. Aerenchyma formations initiated by ethylene, Ca2+, and ROS signalling through a programmed cell death process are involved in aerenchyma development [60, 67].

Rapid shoot elongation is another escape mechanism for adaptation in waterlogging stress [68]. It has been reported that lower stem of soybean having hypertrophic lenticels helps oxygen entry into the aerenchyma [64]. Flooding also causes a significant reduction in leaf number, leaf area, canopy height and dry weight at maturity in soybean crops.

#### *2.1.2. Physiological and biochemical response under drought stress*

**2. Genetic diversity of water stress tolerance in soybean**

Drought induces morphological changes in plants, enabling them to sense and rapidly adapt to the stress. Root-related traits are crucial in maintaining crop yield in soybean [45]. Drought alters the root system architecture (root depth, root angle and root branching density) [27]. For instance, root architecture was characterized in field under normal and water deficit conditions using three soybean cultivars (Jackson, Prima 2000 and A5409RG). As a result, Prima 2000 (drought-tolerant cultivar) has an intermediate root phenotype with a root angle of 40–60°, while a shallow root phenotype along with root angle of <40° has been observed in

Depth of rooting system influenced by the elongation of taproot also plays an important role for plant survival under water deficit [27, 46]. An increase in number of root tips, root length, root surface area and root volume was observed under water limited conditions. Several studies have proposed that roots having large xylem number, diameters, lateral root systems with more root hairs are indicators of drought tolerance [31, 47, 48]. Jackson is considered as drought escaping cultivar with long and deep roots into the soil permitting better water uptake compared with drought-sensitive cultivars [27, 49]. Under water-limited conditions, Plant Introduction (PI) 578477A and 088444 exhibited higher yield due to higher lateral root number in clay soil [50]. It was reported that deeper region of soil has high root density under seasonal drought as compared to dry surface of soil [51]. In addition, total root length/ plant weight, dry root weight/plant weight and root volume/plant weight were positively correlated with drought tolerance [52]. Therefore, studying the relationship between root traits and

Root-to-shoot ratio is also a good indicator to allocate the resources between different plant components. The water-limited environment increases the root-to-shoot ratio. For example, in soybean, root-to-shoot ratio increased by 13% indicates the flow of biomass towards roots [53]. The drought-tolerant soybean genotype (C12) showed a higher root-to-shoot ratio than the susceptible genotype (C08) under restricted soil water with application of exogenous ABA. To cope with drought stress, leaf morphology also plays an important role. Under water-limited conditions, plants reduce their leaf area by closing stomata. Due to water scarcity, reduction in soybean plant leaf area has been reported [54]. In contrast, drought-tolerant soybean cultivar exhibited a greater leaf area rather than less-tolerant cultivar under hydric stress condition [55]. Aerenchyma formation is a major indicator that facilitates gas exchange between aerial and submerged plant parts (shoots and/or roots) to avoid flooding stress [56, 57]. Flooding stress induces two kinds of aerenchyma i.e. primary (cortical) [58] and secondary (white and spongy tissues) [33]. A number of aquatic plants develop cortical aerenchymatous tissue by cell disintegration (lysigenous aerenchyma) and cell separation (schizogenous aerenchyma) [59]. In rice, barley, maize and wheat, lysigenous aerenchyma is induced by flooding [60, 61]. In some

**2.1. Different response of soybean to water stress**

*2.1.1. Morphological performance*

36 Plant, Abiotic Stress and Responses to Climate Change

drought-sensitive cultivar A5409RG [27].

drought is helpful to develop drought-resistant cultivar.

Stress-responsive mechanisms have been studied at the physiological and biochemical level in soybean under drought and flooding stress. To optimize the use of water under water deficit conditions, stomatal control is considered as major physiological indicator. For instance, in soybean, stomatal conductance decreased by 42% in drought-stressed leaves rather than normal leaves [69]. Owing to dehydration, MG/BR46 (drought tolerant soybean variety) showed faster decline in stomatal conductance as compared to BR16 (drought-susceptible variety) (65 versus 50% reduction) [55]. In same study, prolonged drought stress (45 days) exhibited no profound impact on stomatal conductance of BR16 while it had reached 79% in the MG/BR46. Several studies have provided strong evidence that drought-tolerant soybean genotypes (C12 and W05) exhibited a higher reduction in stomatal conductance rather than the susceptible one (C08) [25]. In soybean, ABA is involved in the reduction of stomatal conductance and photosynthesis. For instance, after imposition of exogenous application of ABA under soil drying, leaf stomatal conductance of soybean tolerant genotype C12 declined than the susceptible one (C08).

Maintenance of cell turgidity and water-use efficiency are important indicators to cope with drought stress [26]. Soybean introduction line PI 416937 is an excellent example of drought tolerance by limiting transpiration rate and maintaining a lower osmotic potential. An increase in WUE was observed in drought-tolerant genotype (C12) by regulating stomatal closure during the entire period of water deficiency [25, 52].The maintenance of cell turgidity under waterlimited conditions may be achieved by adjusting the osmotic potential in response to the accumulation of proline, sucrose, soluble carbohydrates, glycine betaine and other solutes [70]. The accumulation of solutes under water deficit condition is known as osmotic adjustment. Some authors have reported higher proline content in drought-tolerant crop species such as bean [71]. In soybean, water stress exhibited significant increase in proline contents in drought tolerance as well as susceptible genotype, but tolerant genotypes recovered to pre-stress levels more quickly after rehydration [25].

The production of ROS, such as superoxide radical (O<sup>2</sup> − ), hydroxyl radical (OH˙) and hydrogen peroxide (H<sup>2</sup> O2 ), is one of the biochemical responses causing damage to DNA, proteins and lipids [72] under drought stress. The toxicity of ROS may be limited by antioxidant enzymatic (superoxide dismutase, catalase, and glutathione peroxidase) and non-enzymatic scavengers [73, 74]. For instance, drought stress increased activities of some antioxidant enzymes (catalase, glutathione reductase and superoxide dismutase) in soybean varieties which were positively correlated to seed yield [75].

shrunken and wrinkled, and hard seeds were produced in soybean [84, 85, 87]. Same study pointed out 30–40% reduction in proportion of seed having diameter > 4.8 mm. In contrast, the

Adaptation to Water Stress in Soybean: Morphology to Genetics

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Germination is a complex process that consists of several metabolic events. Numerous studies reported that negative correlation exists between germination percentage and flooding stress [88, 89]. Seeds are usually germinated under optimum conditions within 1 or 2 days. But, seed germination is delayed due to the quick absorption of water, collapse of seed structure, and outflow of internal seed contents under flooding stress. When seeds were flooded for 3 days after imbibition, germination percentage was drastically dropped out and seed injury was observed [90]. Flooding causes mechanical damage on the soybean seeds and prohibits germination. Seed coat and seed weight are fundamental factors to evaluate a positive effect on seed flooding tolerance. For example, germination rate (GR) and normal seedling rate (NS) was higher in pigmented varieties as compared to yellow varieties of soybean (**Table 1**). These parameters (GR and NS) were negatively correlated with seed weight (SW) in the combined population [91]. Therefore, pigmented seed coat and small seed weight could be key param-

Root length, shoot length and leaf area are considered as major determinants to evaluate drought response during vegetative stage. A positive relationship exists between root traits and resistance to drought [52, 92]. At seedling stage, drought stress affects leaf expansion rate, leaf water potential, relative water content of leaves (%RWC) and relative growth. The degradation of chlorophyll contents of soybean leaves was correlated with the different levels of drought stress [75]. Water deficit stress also decreased the number of nodes and intermodal length while the reduction in inter-nodal length depends upon the duration of drought stress. For example, drought stress showed no profound impact on number of internodes in drought tolerant soybean cultivar (C12), whereas drought-susceptible cultivar (C08) showed higher number of internode [93].

Essential traits, root length and shoot length are also important indicators in response to flooding stress. The insufficient allocation of water, minerals, nutrients, and hormones led to root and shoot damage [94]. The first symptom usually appears in soybean is wilting of leaves in response to flooding. Soybean shoot growth under flooded conditions is significantly decreased due to inability of the root system regarding water transport, hormones, nutrients and assimilates [95, 96]. Flooding tolerance in soybean is strongly correlated to root surface area, root length and dry weight [97]. It has been reported that root tips are extremely sensitive to flooding in soybean and pea seedlings [98–100]. Under complete submergence,

In soybean, pod number per plant, seed number per pod and 100-seed weight are major determinants of yield under water stress [101], and these yield components are the important sink for assimilates at reproductive stages [102]. Drought stress especially during flowering (R1)

soybean root growth is absolutely repressed due to the death of root tips.

ratio of seeds of diameter < 3.2 mm was increased by 3–15% [85].

eters in response to seed-flooding tolerance.

*2.2.2. Parameters related to vegetative tissues*

*2.2.3. Parameters related to adult plants*

Under flooding stress, plant undergoes different physiological and biochemical adaptations. For instance, in soybean, a significant reduction in photosynthetic activity and stomatal conductance was observed in Essex and Forrest within 48 h of flooding at vegetative and reproductive growth stages. Waterlogging also decreases biological nitrogen fixation, as nodules need adequate oxygen to maintain nitrogenase activity for aerobic respiration and contributing adenosine triphosphate [41]. As a consequence of flooding stress, a reduction in root hydraulic conductivity has also been reported [76]. Several studies have provided the correlation between stomatal conductance and carbon fixation. In flooded plants, photosynthetic activities were reduced by restricting CO<sup>2</sup> due to stomatal closure [77, 78]. Furthermore, due to the higher concentration of CO<sup>2</sup> assimilation in flooded soil, biomass and soybean root elongation eventually repressed [79].

Tamang et al. [66] reported that submergence stimulates starch degradation, soluble carbohydrates and ATP in cotyledons and hypocotyls of soybean seedlings. Extensive submergence degrades the chlorophyll contents in aerial parts of several terrestrial plants [80, 81]. However, under submergence, abundance of chlorophyll *a* and *b* remained nearly constant in soybean [66]. The decrease in photosynthetic activity with long-term flooding may be triggered by the reduction in chlorophyll, transpiration and ribulose-1,5-biphosphate (RuBP) carboxylase activity. These combined effects against flooding declined the crop growth, net assimilation and leaf expansion of plants. Blocking of hypertrophic lenticels at the base of stem restricted O<sup>2</sup> transport into the roots resulting in reduction of plant growth under hypoxic conditions [82]. Flooding stress causes higher production of ROS resulting in oxidative damage to proteins related to photosynthetic apparatus [83]. As a result, the scavenging activity is overpassed under flooding stress.

#### **2.2. Parameters for measuring the tolerance degree of water stress**

#### *2.2.1. Parameters related to seed tolerance*

Seeds need a suitable condition to have a good germination. The germination rate and percentage of different cultivars were affected by levels of drought stress. In soybean, drought stress simulated by polyethylene glycol PEG-6000 significantly reduced seed germination percentage (**Table 1**). An increase in the PEG concentration reduced root growth by two to three times for different genotypes. Seed weight and seed size, and seed weight distribution are key indicators to evaluate the genotypic response to drought stress [84, 85]. A positive correlation between 100-seed weight per plant and seed yield were reported in soybean under water limited conditions. For instance, Habit (soybean drought-tolerant cultivar) exhibited higher 100-seed weight and seed yield under drought stress [29, 86].Water deficit conditions lead to a significant reduction in seed weight and seed size. It also had little effect on seed shape as shrunken and wrinkled, and hard seeds were produced in soybean [84, 85, 87]. Same study pointed out 30–40% reduction in proportion of seed having diameter > 4.8 mm. In contrast, the ratio of seeds of diameter < 3.2 mm was increased by 3–15% [85].

Germination is a complex process that consists of several metabolic events. Numerous studies reported that negative correlation exists between germination percentage and flooding stress [88, 89]. Seeds are usually germinated under optimum conditions within 1 or 2 days. But, seed germination is delayed due to the quick absorption of water, collapse of seed structure, and outflow of internal seed contents under flooding stress. When seeds were flooded for 3 days after imbibition, germination percentage was drastically dropped out and seed injury was observed [90]. Flooding causes mechanical damage on the soybean seeds and prohibits germination. Seed coat and seed weight are fundamental factors to evaluate a positive effect on seed flooding tolerance. For example, germination rate (GR) and normal seedling rate (NS) was higher in pigmented varieties as compared to yellow varieties of soybean (**Table 1**). These parameters (GR and NS) were negatively correlated with seed weight (SW) in the combined population [91]. Therefore, pigmented seed coat and small seed weight could be key parameters in response to seed-flooding tolerance.

#### *2.2.2. Parameters related to vegetative tissues*

The production of ROS, such as superoxide radical (O<sup>2</sup>

which were positively correlated to seed yield [75].

O2

38 Plant, Abiotic Stress and Responses to Climate Change

gen peroxide (H<sup>2</sup>

ing CO<sup>2</sup>

−

and lipids [72] under drought stress. The toxicity of ROS may be limited by antioxidant enzymatic (superoxide dismutase, catalase, and glutathione peroxidase) and non-enzymatic scavengers [73, 74]. For instance, drought stress increased activities of some antioxidant enzymes (catalase, glutathione reductase and superoxide dismutase) in soybean varieties

Under flooding stress, plant undergoes different physiological and biochemical adaptations. For instance, in soybean, a significant reduction in photosynthetic activity and stomatal conductance was observed in Essex and Forrest within 48 h of flooding at vegetative and reproductive growth stages. Waterlogging also decreases biological nitrogen fixation, as nodules need adequate oxygen to maintain nitrogenase activity for aerobic respiration and contributing adenosine triphosphate [41]. As a consequence of flooding stress, a reduction in root hydraulic conductivity has also been reported [76]. Several studies have provided the correlation between stomatal conductance and carbon fixation. In flooded plants, photosynthetic activities were reduced by restrict-

due to stomatal closure [77, 78]. Furthermore, due to the higher concentration of CO<sup>2</sup>

assimilation in flooded soil, biomass and soybean root elongation eventually repressed [79].

apparatus [83]. As a result, the scavenging activity is overpassed under flooding stress.

Seeds need a suitable condition to have a good germination. The germination rate and percentage of different cultivars were affected by levels of drought stress. In soybean, drought stress simulated by polyethylene glycol PEG-6000 significantly reduced seed germination percentage (**Table 1**). An increase in the PEG concentration reduced root growth by two to three times for different genotypes. Seed weight and seed size, and seed weight distribution are key indicators to evaluate the genotypic response to drought stress [84, 85]. A positive correlation between 100-seed weight per plant and seed yield were reported in soybean under water limited conditions. For instance, Habit (soybean drought-tolerant cultivar) exhibited higher 100-seed weight and seed yield under drought stress [29, 86].Water deficit conditions lead to a significant reduction in seed weight and seed size. It also had little effect on seed shape as

**2.2. Parameters for measuring the tolerance degree of water stress**

*2.2.1. Parameters related to seed tolerance*

Tamang et al. [66] reported that submergence stimulates starch degradation, soluble carbohydrates and ATP in cotyledons and hypocotyls of soybean seedlings. Extensive submergence degrades the chlorophyll contents in aerial parts of several terrestrial plants [80, 81]. However, under submergence, abundance of chlorophyll *a* and *b* remained nearly constant in soybean [66]. The decrease in photosynthetic activity with long-term flooding may be triggered by the reduction in chlorophyll, transpiration and ribulose-1,5-biphosphate (RuBP) carboxylase activity. These combined effects against flooding declined the crop growth, net assimilation and leaf expansion of plants. Blocking of hypertrophic lenticels at the base of stem restricted O<sup>2</sup> transport into the roots resulting in reduction of plant growth under hypoxic conditions [82]. Flooding stress causes higher production of ROS resulting in oxidative damage to proteins related to photosynthetic

), is one of the biochemical responses causing damage to DNA, proteins

), hydroxyl radical (OH˙) and hydro-

Root length, shoot length and leaf area are considered as major determinants to evaluate drought response during vegetative stage. A positive relationship exists between root traits and resistance to drought [52, 92]. At seedling stage, drought stress affects leaf expansion rate, leaf water potential, relative water content of leaves (%RWC) and relative growth. The degradation of chlorophyll contents of soybean leaves was correlated with the different levels of drought stress [75]. Water deficit stress also decreased the number of nodes and intermodal length while the reduction in inter-nodal length depends upon the duration of drought stress. For example, drought stress showed no profound impact on number of internodes in drought tolerant soybean cultivar (C12), whereas drought-susceptible cultivar (C08) showed higher number of internode [93].

Essential traits, root length and shoot length are also important indicators in response to flooding stress. The insufficient allocation of water, minerals, nutrients, and hormones led to root and shoot damage [94]. The first symptom usually appears in soybean is wilting of leaves in response to flooding. Soybean shoot growth under flooded conditions is significantly decreased due to inability of the root system regarding water transport, hormones, nutrients and assimilates [95, 96]. Flooding tolerance in soybean is strongly correlated to root surface area, root length and dry weight [97]. It has been reported that root tips are extremely sensitive to flooding in soybean and pea seedlings [98–100]. Under complete submergence, soybean root growth is absolutely repressed due to the death of root tips.

#### *2.2.3. Parameters related to adult plants*

In soybean, pod number per plant, seed number per pod and 100-seed weight are major determinants of yield under water stress [101], and these yield components are the important sink for assimilates at reproductive stages [102]. Drought stress especially during flowering (R1) and pod-filling stages reduces soybean yield [30] (**Table 1**). Under water deficit conditions, an increase in rate of abortion has been reported during early pod-filling stage in soybean [54, 103]. Soybean yield is also affected by the occurrence of drought stress during seed filling (R6) period [93]. Water stress at flowering stage decreased the pod number and seed number resulting in yield loss [104]. Kobraei et al. [29] conducted experiment on eight soybean cultivars to asses yield under normal and drought conditions. This study pointed out that drought reduced the yield components resulting in yield loss. In addition, more yield loss was observed during R1 stage as compared to R6 stage [104].

using 21 soybean varieties for flooding tolerance in both screen-house and field tests. Three soybean germplasm, Nam Vang from Cambodia, VND2 from China and ATF15-1 from Australia were identified as most flood-tolerant varieties which survived better, grew taller, produced more pods/plants and heavier seed weight as compared to sensitive varieties [114]. A total of 192 soybean germplasm lines were screened for flooding tolerance at seedling stage. Among them, Jangbaegkong, Danbaegkong, Sowonkongkong, Socheong2 and Suwon269 were identified as donor line for flooding tolerance, whereas Shillog, T201, T181, NTS1116 and HP-963 exposed flooding sensitivity [115]. Several cultivated germplasm lines (*Glycine max*) including PI 408105A, PI 561271, PI 567343, PI 407184, PI603910C, PI 567394B, PI 567651, Archer and Misuzudaiz have been identified as a source of potential source for

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Wild soybean (*Glycine soja*), is a valuable genetic resource for the tolerance to water stress by reintroducing alleles. Wild soybean PI 483463 (*G. soja*) had favourable donor alleles for root angle, while PI 468917 predicted to contribute to slow wilting. Hence, it can be used for development of drought-resistant soybean cultivars [112, 116]. In another study, the wild parent, PI 407162 had favourable alleles for fibrous roots, thus enhancing the soybean ability to survive under drought stress. These studies suggested that it is possible to enhance genetic variation in cultivated soybean by introducing alleles from wild soybeans [117]. For flooding, different wild soybean accessions, PI 467162, PI 479751, PI 407229, PI 597459C, PI 424082, PI 378699A, PI 424107A, PI 366124, PI 378699A were identified, which showed tremendous waterlogging tolerance than *G. max* [112]. Therefore, wild populations can offer useful in breeding program for improving

**3. Genetic regulation mechanisms for tolerance to water stress**

photosynthesis [136, 137], and nitrogen fixation [138–141] have been reported.

The application of QTL helps in identification of chromosomal regions, detecting phenotypic variation associated with drought-resistance traits and to determine the desirable alleles at these QLs for marker-assisted breeding. Progress towards the identification of droughtrelated QTLs is needed [118], only a few QTLs have been reported for drought (**Table 2**). Du et al. [128] identified 19 QTLs associated with seed yield under normal and water-limited conditions and 10 QTLs associated with drought susceptibility index (DSI) in soybean. To develop drought-tolerant varieties, the role of secondary traits associated with yield stability has been accelerated. In crops, under water deficit condition, several secondary traits i.e. early seedling vigor [129], canopy wilting [119, 130], root system architecture (RSA) [117, 131, 132], canopy temperature depression [133], carbon isotope discrimination [134, 135], alterations in

*3.1.1. Genetic and QTL structure of morpho-physiological performance*

flooding tolerance [112].

**3.1. Drought tolerance**

drought and flooding resistance of soybean.

*2.3.2. Wild soybean*

One of the major traits conferring tolerance to waterlogging is yield and production of good quality seeds [105]. A significant decline in pod number, pods per node, branch number, and seed size was observed following 7 days of flooding at different vegetative and regenerative development stages [106]. Sullivan et al. [107] confirmed reduction in pod number and plant height at early vegetative growth stages. Soybean crops flooded with excessive water at early flowering stage showed severe chlorosis and stunting growth [108]. Schöffel et al. [109] showed a decreased number of pods per plant at the reproductive stage (R4) in pot trails. A field experiment was conducted in flooded soil and obtained yield reduction from 20–39% in the different soybean cultivars when subjected during the R5 stage. During flooding, a significant reduction in soybean yield was observed at R5 stage as compared to the R2 stage [110].

#### **2.3. Genetic variation of tolerance to water stress**

#### *2.3.1. Cultivated soybean*

Considerable genetic variation in seed yield was observed in soybean genotypes under drought stress. A total of 50 soybean genotypes were screened under rain-fed condition in Bangladesh. Among them, genotypes BARI Soybean 5, BARI Soybean 6, Shohag and BD2331 were identified as drought-tolerant genotypes [32]. In another study, response of eight cultivars of soybean (Clark, Hobbit, Pershing, Williams, Hood, DPX, M7 and M9) was investigated in Iran. Williams cultivar was predicted as drought-tolerant, having highest number of nodes and pods/plant in normal and water deficit conditions [29]. Genetically and geographically, diverse soybean germplasm lines i.e. from Korea (PI085355, PI339984, PI407778A, PI407973A, PI423841, PI424460, PI424608A, PI603170, PI458020), China (PI088444, PI567398, PI567561, PI594410, PI578477A), Japan (PI243548, PI417092, PI507066) were screened to examine root response under water deficit condition in clay and sandy soil. Plant Introduction PI578477A, PI088444 (high lateral root number in clay soil) and PI458020 (thick lateral roots in sandy soil) were found to have higher yield under water-limited conditions [50]. Brazilian cultivars BR-4 and Ocepar 4 were considered as drought-tolerant [111]. Several cultivated germplasm lines (*Glycine max*) including Williams, Jackson, Prima 2000, Jindou 21(C12), PI416937, PI 427136, PI 408105A, PI 471938, PI 424088, PI 081041, N04-9646, DT51 and R02-1325 have promising performance under water deficit conditions and can be used in breeding program [25, 27, 112, 113].

Genetic variation in soybean germplasm was observed in response to flooding tolerance to overcome yield loss. Elite lines conserve genomic regions that can inhibit extensive yield losses during flooding stress. An experiment was conducted to determine genetic variations using 21 soybean varieties for flooding tolerance in both screen-house and field tests. Three soybean germplasm, Nam Vang from Cambodia, VND2 from China and ATF15-1 from Australia were identified as most flood-tolerant varieties which survived better, grew taller, produced more pods/plants and heavier seed weight as compared to sensitive varieties [114]. A total of 192 soybean germplasm lines were screened for flooding tolerance at seedling stage. Among them, Jangbaegkong, Danbaegkong, Sowonkongkong, Socheong2 and Suwon269 were identified as donor line for flooding tolerance, whereas Shillog, T201, T181, NTS1116 and HP-963 exposed flooding sensitivity [115]. Several cultivated germplasm lines (*Glycine max*) including PI 408105A, PI 561271, PI 567343, PI 407184, PI603910C, PI 567394B, PI 567651, Archer and Misuzudaiz have been identified as a source of potential source for flooding tolerance [112].

#### *2.3.2. Wild soybean*

and pod-filling stages reduces soybean yield [30] (**Table 1**). Under water deficit conditions, an increase in rate of abortion has been reported during early pod-filling stage in soybean [54, 103]. Soybean yield is also affected by the occurrence of drought stress during seed filling (R6) period [93]. Water stress at flowering stage decreased the pod number and seed number resulting in yield loss [104]. Kobraei et al. [29] conducted experiment on eight soybean cultivars to asses yield under normal and drought conditions. This study pointed out that drought reduced the yield components resulting in yield loss. In addition, more yield loss was

One of the major traits conferring tolerance to waterlogging is yield and production of good quality seeds [105]. A significant decline in pod number, pods per node, branch number, and seed size was observed following 7 days of flooding at different vegetative and regenerative development stages [106]. Sullivan et al. [107] confirmed reduction in pod number and plant height at early vegetative growth stages. Soybean crops flooded with excessive water at early flowering stage showed severe chlorosis and stunting growth [108]. Schöffel et al. [109] showed a decreased number of pods per plant at the reproductive stage (R4) in pot trails. A field experiment was conducted in flooded soil and obtained yield reduction from 20–39% in the different soybean cultivars when subjected during the R5 stage. During flooding, a significant reduction

Considerable genetic variation in seed yield was observed in soybean genotypes under drought stress. A total of 50 soybean genotypes were screened under rain-fed condition in Bangladesh. Among them, genotypes BARI Soybean 5, BARI Soybean 6, Shohag and BD2331 were identified as drought-tolerant genotypes [32]. In another study, response of eight cultivars of soybean (Clark, Hobbit, Pershing, Williams, Hood, DPX, M7 and M9) was investigated in Iran. Williams cultivar was predicted as drought-tolerant, having highest number of nodes and pods/plant in normal and water deficit conditions [29]. Genetically and geographically, diverse soybean germplasm lines i.e. from Korea (PI085355, PI339984, PI407778A, PI407973A, PI423841, PI424460, PI424608A, PI603170, PI458020), China (PI088444, PI567398, PI567561, PI594410, PI578477A), Japan (PI243548, PI417092, PI507066) were screened to examine root response under water deficit condition in clay and sandy soil. Plant Introduction PI578477A, PI088444 (high lateral root number in clay soil) and PI458020 (thick lateral roots in sandy soil) were found to have higher yield under water-limited conditions [50]. Brazilian cultivars BR-4 and Ocepar 4 were considered as drought-tolerant [111]. Several cultivated germplasm lines (*Glycine max*) including Williams, Jackson, Prima 2000, Jindou 21(C12), PI416937, PI 427136, PI 408105A, PI 471938, PI 424088, PI 081041, N04-9646, DT51 and R02-1325 have promising performance under

in soybean yield was observed at R5 stage as compared to the R2 stage [110].

water deficit conditions and can be used in breeding program [25, 27, 112, 113].

Genetic variation in soybean germplasm was observed in response to flooding tolerance to overcome yield loss. Elite lines conserve genomic regions that can inhibit extensive yield losses during flooding stress. An experiment was conducted to determine genetic variations

observed during R1 stage as compared to R6 stage [104].

40 Plant, Abiotic Stress and Responses to Climate Change

**2.3. Genetic variation of tolerance to water stress**

*2.3.1. Cultivated soybean*

Wild soybean (*Glycine soja*), is a valuable genetic resource for the tolerance to water stress by reintroducing alleles. Wild soybean PI 483463 (*G. soja*) had favourable donor alleles for root angle, while PI 468917 predicted to contribute to slow wilting. Hence, it can be used for development of drought-resistant soybean cultivars [112, 116]. In another study, the wild parent, PI 407162 had favourable alleles for fibrous roots, thus enhancing the soybean ability to survive under drought stress. These studies suggested that it is possible to enhance genetic variation in cultivated soybean by introducing alleles from wild soybeans [117]. For flooding, different wild soybean accessions, PI 467162, PI 479751, PI 407229, PI 597459C, PI 424082, PI 378699A, PI 424107A, PI 366124, PI 378699A were identified, which showed tremendous waterlogging tolerance than *G. max* [112]. Therefore, wild populations can offer useful in breeding program for improving drought and flooding resistance of soybean.
