**1. Introduction**

Soybean is an important leguminous crop in the world, providing an essential source of protein to human diet, feed for live-stock and as bio-diesel for industry [1, 2]. Soybean seeds consist of 40% protein, 20% oil, 35% carbohydrate and ~5% ash [3]. As compared to other oilseed crops, soybean collectively occupies around 6% of the world's land under cultivation [4].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Due to the rapid rise in the commercial value of soybean in an international market, the total area under soybean cultivation has been increasing from last three decades. Soybean is an important cash crop with a total production of over 313.05 million metric tons in 2015–2016 (USDA data). During this year, the USA has been the world's leading producer of soybean representing 35% of the world production, followed by Brazil with 31%, Argentina with 17%, China with 4%, India with 3%, Paraguay with 3% and Canada with 2% (USDA data).

Water stress including drought and flooding is considered as a major threat, limiting growth and yield of plants [5, 6]. Drought is caused by insufficient rainfall or irrigation which results in soil drying, whereas, in flooding, water exists in soil solution causing water logging and submergence. In response to drought and flooding stress, 40–60% yield losses have been reported in soybean [7, 8]. High temperature, low humidity in atmosphere and water deficiency are the main causes of drought [9, 10]. Drought stress affects germination rate and early seedling growth of the plant [11, 12]. Under water deficit conditions, a significant reduction in germination, hypocotyl length, root and shoot fresh and dry weight were observed whereas the root length is increased [13]. It also affects the carbon assimilation and phenology of the plant [10]. Prolonged drought stress at different growth stages has profound effect on soybean growth and yield [14].

To counteract the adverse effects of drought, the soybean plant adopts three mechanisms i.e. escape, tolerance, and avoidance [15]. In the escape mechanism, the plant completes its life cycle before the onset of drought. Normally, the plants complete their life cycle very quickly and produce few seeds. For instance, early planting of soybean helps to avoid drought, and is largely practiced in the USA—planting in March to April affords escape from water stress [16, 17]. Drought avoidance is performed by maintaining high water potential, grow deeper in soil, stomatal control of transpiration rate, and by reduction of water loss from tissues. The tolerance mechanism includes low tissue water potential, maintenance of turgor through osmotic adjustments [18, 19].

Flooding ranks second after drought, causing yield reduction in soybean [20, 21]. Flooding stress can be categorized as waterlogging or submergence. In waterlogging stress, root goes under water while shoots remains above ground, whereas, during submergence, plant is completely immersed in water saturated soil. As plants are aerobic, hypoxia (insufficient oxygen) or anoxia (complete absence of oxygen) causes losses in crop production. Soybean is more sensitive to flooding stress resulting in yield decline by reducing photosynthesis nitrogen fixation and biomass accumulation. Flooding stress can happen during any growing stage, especially in the seed germination and vegetative stages leads to substantial decrease of soybean grain yield [22] (**Table 1**). In addition, flooding stress hampers yield production during vegetative (17–43%) and reproductive stage (50–56%) [41].

For mitigating the negative impact of flooding stress, plants use a number of strategies for their survival, mainly escape and quiescence strategies [42, 43]. In escape strategy, morphological (aerenchyma development, shoot elongation and adventitious root formation) and anatomical alterations allow the plant to exchange gas between cells and atmosphere. The Quiescence strategy suppresses morphological changes to save energy and resources and retard plant growth. This strategy depends on anaerobic energy production [42, 44].

Understanding the genetic base for water stress tolerance in diverse soybean is a fundamental issue that contributes for the genetic improvement. This chapter will present the research progress about the situation of soybean tolerance to water stress at germination, seedling and adult plant stages. It also includes the current knowledge about QTL mapping, gene discovery and 'omic' technologies relevant to drought and flooding tolerance that will be helpful to understand

Seedling stage 162 soybean accessions Root development [40]

**Growth stage Experimental material Indicator Ref.**

weight

Germination, shoot and root length, fresh and dry

Adaptation to Water Stress in Soybean: Morphology to Genetics

http://dx.doi.org/10.5772/intechopen.72229

Root and shoot length, germination rate and

Gas exchange, water relation parameters, total chlorophyll, proline contents of leaves, root xylem

Root architecture, shoot parameters [26]

relative performance (RP), tolerance (TOL), drought

Fermentative metabolism and carbohydrate contents

content, carbon exchange rate, dry matter accumulation and nitrogen content

percentage of germination

Habit, L17 and M17 Leaf relative water content, chemical osmolytes and chlorophyll content

Eight soybean cultivars Highest number of node/plant, number of pod/main

Adult PI578477A,PI088444,PI458020 Yield, root architecture [31]

Seedlings stage Soybean Secondary aerenchyma formation [33]

Seedling stage 11 soybean genotypes Primary/adventitious roots and root nodules, stem

Flowering stage Five soybean cultivars Nodule number, nodule dry weight, chlorophyll

**Table 1.** A list of drought- and flooding-related parameters at different growth stages of soybean.

41 soybean accessions increases in metaxylem number [29,

susceptibility index (DSI)

and leaf biomass

Taekwang and Asoaogari Root morphological traits, adventitious roots and Photosynthesis

92 Soybean Lines Root architecture [37]

in roots and nodules

pH, plant growth and root traits

stem, pod/sub stem and pod/plant

[23]

35

[24]

[25]

[27]

[28]

30]

[32]

[34] [35]

[36]

[38]

[39]

Under drought stress

Second trifoliate

Third trifoliate leaf (V3)

Flowering and pod-filling stage

V4, R1 and R3 growth stages.

Reproductive stage (R6–R7)

Under flooding stress

Vegetative and flowering stage

Cotyledon-stage seedlings

leaves

Germination 4 Bulgarian lines & one USA variety

Germination L17, M9, Clark, M7, Hobbit and Williams

2000

Adult BARI Soybean 5, BARI Soybean 6, Shohag and BD2331

Flowering stage Cultivars Fundacep 53 RR and BRS Macota

Jindou 21 (C12), Mengjin 1 (W05) and Union (C08)

A5409RG, Jackson and Prima

drought and flooding-tolerance mechanisms in soybean.


Due to the rapid rise in the commercial value of soybean in an international market, the total area under soybean cultivation has been increasing from last three decades. Soybean is an important cash crop with a total production of over 313.05 million metric tons in 2015–2016 (USDA data). During this year, the USA has been the world's leading producer of soybean representing 35% of the world production, followed by Brazil with 31%, Argentina with 17%,

Water stress including drought and flooding is considered as a major threat, limiting growth and yield of plants [5, 6]. Drought is caused by insufficient rainfall or irrigation which results in soil drying, whereas, in flooding, water exists in soil solution causing water logging and submergence. In response to drought and flooding stress, 40–60% yield losses have been reported in soybean [7, 8]. High temperature, low humidity in atmosphere and water deficiency are the main causes of drought [9, 10]. Drought stress affects germination rate and early seedling growth of the plant [11, 12]. Under water deficit conditions, a significant reduction in germination, hypocotyl length, root and shoot fresh and dry weight were observed whereas the root length is increased [13]. It also affects the carbon assimilation and phenology of the plant [10]. Prolonged drought stress at different growth stages has profound effect on

To counteract the adverse effects of drought, the soybean plant adopts three mechanisms i.e. escape, tolerance, and avoidance [15]. In the escape mechanism, the plant completes its life cycle before the onset of drought. Normally, the plants complete their life cycle very quickly and produce few seeds. For instance, early planting of soybean helps to avoid drought, and is largely practiced in the USA—planting in March to April affords escape from water stress [16, 17]. Drought avoidance is performed by maintaining high water potential, grow deeper in soil, stomatal control of transpiration rate, and by reduction of water loss from tissues. The tolerance mechanism includes low tissue water potential, maintenance of turgor through

Flooding ranks second after drought, causing yield reduction in soybean [20, 21]. Flooding stress can be categorized as waterlogging or submergence. In waterlogging stress, root goes under water while shoots remains above ground, whereas, during submergence, plant is completely immersed in water saturated soil. As plants are aerobic, hypoxia (insufficient oxygen) or anoxia (complete absence of oxygen) causes losses in crop production. Soybean is more sensitive to flooding stress resulting in yield decline by reducing photosynthesis nitrogen fixation and biomass accumulation. Flooding stress can happen during any growing stage, especially in the seed germination and vegetative stages leads to substantial decrease of soybean grain yield [22] (**Table 1**). In addition, flooding stress hampers yield production during vegetative

For mitigating the negative impact of flooding stress, plants use a number of strategies for their survival, mainly escape and quiescence strategies [42, 43]. In escape strategy, morphological (aerenchyma development, shoot elongation and adventitious root formation) and anatomical alterations allow the plant to exchange gas between cells and atmosphere. The Quiescence strategy suppresses morphological changes to save energy and resources and retard plant

growth. This strategy depends on anaerobic energy production [42, 44].

China with 4%, India with 3%, Paraguay with 3% and Canada with 2% (USDA data).

soybean growth and yield [14].

34 Plant, Abiotic Stress and Responses to Climate Change

osmotic adjustments [18, 19].

(17–43%) and reproductive stage (50–56%) [41].

**Table 1.** A list of drought- and flooding-related parameters at different growth stages of soybean.

Understanding the genetic base for water stress tolerance in diverse soybean is a fundamental issue that contributes for the genetic improvement. This chapter will present the research progress about the situation of soybean tolerance to water stress at germination, seedling and adult plant stages. It also includes the current knowledge about QTL mapping, gene discovery and 'omic' technologies relevant to drought and flooding tolerance that will be helpful to understand drought and flooding-tolerance mechanisms in soybean.
