2. Salinity stress and its causes

The ecological anxieties (biotic and abiotic stresses) have turned into essential threats to plant growth, development, and survival. Among these ecological anxieties, abiotic stresses, for example, drought, chilling or high temperature, and salinity inactively influencing the growth, biomass generation, and yield of many field crops. These threats are ending up more deteriorated by regular or humanmade activities, which result in the excessive soluble salts accumulation in the underground water and soil. As concern salinity stress, about 20% of the world's land, and about 33% of the world's irrigated zone is under the impact of salinity [9]. Besides, salinity influenced areas are expanding at a rate of 10% yearly. The expanding of salinity issues are because of low precipitation, high surface evaporation, weathering of native rocks, irrigation with saline water, and poor agronomic practices. Salt influenced soils have various sorts that negative effect on agricultural production, for instance, irrigation-induced salinity and 'transient' dry-land salinity have been arranged in detail with different perspectives considered by [10], and illuminate that salinity in the soil is one of the vast abiotic stress that hamper the agricultural production in the world. The estimation has been done that >50% of the agricultural land would be affected by agricultural till 2050 [11].

Salinity is the issue of almost all the continents and under a wide range of climates. However, the salinity issue is more in arid and semi-arid climate contrasted with the humid climate where yearly precipitation is not as much as evapotranspiration in the world. It is need of great importance to comprehend the mode and sources of salinity with classification, and its role in the plant life cycle. The characteristic critical source of salinity is the primary minerals in exposed layers of the earth crust by weathering process with the assistance of atmospheric

also need to produce more food up-to 70% till 2050 to feed the increasing mouths of

maize (Zea mays L.), sorghum (Sorghum bicolor (L.) Moench), cotton (Gossypium hirsutum), and sugarcane (Saccharum officinarum), etc. show negative response towards salinity. However, plant performance and grain yield may not decrease until a 'threshold'salinity level is reached. Threshold levels of salinity are generally defined as the maximum amount of salt that a plant can tolerate in its root zone without impacting growth (Table 2). Plant physiology is very susceptible to high salinity in its rhizosphere and affects germination rate, growth stages, and ultimately plant yield [1]. Similarly, many other growth hampering effects on plants due salinity are low net CO2 assimilation to plant tissues, leaf area, leaf cell enlargement, dry matter production, and relative growth, poor development of spikelets (rice and wheat), boll (cotton), etc. [4, 5]. There are many reasons for hampering of plant production under salinity. Generally, salinity affects plant growth in three ways, such as osmotic stress, ionic stress or ion imbalance, and oxidative stress [6]. Osmotic stress disturbs the salt water balance, which results in a high concentration of salts and loses of water in plant

Soil types ECe (dS/m) ESP SAR pHs Normal soil <4 <15 <15 4.5–7.5 Saline soil >4 <15 <15 <8.5 Sodic soil <4 >15 >15 >8.5 Saline-sodic soil >4 >15 >15 >8.5 Whereas, ECe = electrical conductivity, ESP = exchangeable sodium percentage, SAR = sodium adsorption ratio, and

Country Salt-affected area of irrigated in the world

China 6.7 15 India 7.0 17 Soviet Union 3.7 18 United States 4.2 23 Pakistan 4.2 26 Iran 1.7 30 Thailand 0.4 10 Egypt 0.9 33 Australia 0.2 9 Argentina 0.6 34 South Africa 0.1 9 Subtotal 29.6 20 World 45.4 20

where, mha = million hectare, % = percentage area. Source: Ghassemi et al. [3].

Global estimate of secondary salinity in irrigated lands of the world.

Mha %

Many major field crops such as wheat (Triticum aestivum L.), rice (Oryza sative L.),

the world [2].

Table 2.

198

pHs = negative log of H+ ion [12].

Climate Change and Agriculture

The USDA classification system of salt affected soils.

Table 1.

CO2. The weathering of these primary mineral rocks in the earth crust is the primary source of all the dissolvable salts present in the soils and ocean. However, there are several other anthropogenic sources of salinity in the soil or water. Under arid to and semi-arid climates, the products from the weathering procedure of mineral and rocks accumulate in the soil and result in the advancement of saltinfluenced soils (saline or sodic soil). Though, under a humid atmosphere, salt could not collect in-situ and filter down through the soil and transport to the close-by streams and waterways and caused the salinity in the water bodies [12]. The US Salinity Lab staffs (1954) group the salt-influenced soils (Table 2). These saltaffected soils types have unique nature of soluble salts. For instance, saline soil has Cland SO4 <sup>2</sup> and CO3 <sup>2</sup> present and sodic or alkali soil has HCO3 of Na<sup>+</sup> , and in exceptional cases with high CO3 <sup>2</sup> concentration with the capacity of alkaline hydrolysis. So also, saline sodic soil has predominant soluble salts of Na<sup>+</sup> with Cl and SO4 <sup>2</sup> with an average intensity of NaHCO3 and Na2CO3 in a trace concentration. An ordinary soil has maximum nutrients for development and improvement of the plant. On the inverse, Salinity is one of the significant environmental element influencing plant growth and production. As indicated by FAO report, a saline soil is characterized as having a high concentration of soluble salts for the most of Sodium (Na<sup>+</sup> ), Calcium (Ca2+), magnesium (Mg2+) chloride (Cl) and sulfate (SO4 <sup>2</sup>). Magnesium sulphate (MgSO4) and sodium chloride (NaCl, table salt), are among the most well-known soluble salts which are sufficiently high to influence plant growth and development.

### 2.1 Salinity effects on plant growth

Salinity influence crops in these ways: osmotic effect, specific ion effect, ion imbalance, and oxidative stress [6]. Salinity decline water uptake limit of plant, and causes a decrease in plant development. It might be explicit salt effects. If a high concentration of salt enters the plant, this high concentration of salt will increase at last ascent to a toxic level in older leaves causing early senescence and diminished the photosynthetic leaf area of a plant to a dimension that cannot support plant development [14]. Salinity seems to influence plant growth mechanism in two different ways, water relations, and ionic relations. Firstly, plants face water stress, which in cause decline leaf expansion. Secondly, long-term salt stress in soil and plant, plants involvement (Na<sup>+</sup> and Cl) ionic stress, which can prompt early senescence of older leaves [15] (Figure 1).

outside the root surface, causing changes in osmotic impacts. In the wake of taking some days, weeks or even months the other slower impact (explicit salt impact), bringing about the aggregation of salt in leaves, basically in older leaves and salt toxicity in the plant. This salt toxicity in the plant can cause the death of leaves and decrease the total photosynthetic leaf area. Thus, there is a decrease in the availability of photosynthate to the plant and influence the overall carbon (CO2) balance essential

Salinity response adaptations in plant. Extracted from Kumar et al. [17] and Hussain et al. [18].

Salinity Stress in Arid and Semi-Arid Climates: Effects and Management in Field Crops

DOI: http://dx.doi.org/10.5772/intechopen.87982

The important harmful effect of salinity is the sodium and chloride ions accumulation in plant tissues and soil [19]. The higher concentration of soluble salts in the soil profile may cause physiological drought to plant, that is, reduction in uptake of water due to salt accumulation in the root zone [20]. The entrance of sodium and chloride ions into the plant cell from soil causes ion imbalance in plant and soil and excessive uptake of these ions by plant causing many problems related to the physiology of plant's tissues such as root, leaf, grain, fruit, or fiber [21]. Similarly, the reduction of plant osmotic potential, excessive uptake of Na+ and Cl in the cell, and disruption of cell metabolic functions is due to ion toxicity [21]. Excessive sodium ion in plant tissues harms the cell membrane and plant organelles, and as a result, cell death of plant [22]. These physiological changes in the plant include the membranes disruption, reactive oxygen species (ROS) production, reduction of photosynthesis rate (Pn), and scavenging of antioxidants [21]. Consequently, the accumulation of soluble

salts in the rhizosphere is one of the main reasons for low crop productivity.

for sustainable plant growth and development [16] (Figure 1).

2.1.1 Salinity and ion toxicity in plant

Figure 1.

201

Plants experience the ill effects of the presentation of salinity until maturity [16]. Generally, the markers of salinity impacts in plants are impeded growth and small plants with fewer and smaller leaves. Munns [16] depicted salinity consequences for various plant development stages under a different period of the plant growth mechanism and development. After a couple of minute's introduction of salinity stress, dehydration and shrinkage of the cell begin, and following a couple of hours after the fact recovers their original volume. Regardless of this recovering of the original volume, cell elongation and cell division are diminished, prompting slower rates of root and leaf development. On the following days, a diminishing in cell division and lengthening change into slower leaf inception and size. Plants that are harshly salt influenced regularly build up obvious salt damage. As exposure of salinity extends to half a month, secondary shoot growth is influenced, and following a couple of months, clear changes observed in development and injury between salt-stressed plants and control. To comprehend these time-sensitive changes in light of salinity in plant development stages, the 'two-phase growth response to salinity idea created by [16]. The first phase of growth decline occurs within minutes after exposure to salinity. The decline of growth is because of the osmosis stress, osmotic changes

Salinity Stress in Arid and Semi-Arid Climates: Effects and Management in Field Crops DOI: http://dx.doi.org/10.5772/intechopen.87982

Figure 1. Salinity response adaptations in plant. Extracted from Kumar et al. [17] and Hussain et al. [18].

outside the root surface, causing changes in osmotic impacts. In the wake of taking some days, weeks or even months the other slower impact (explicit salt impact), bringing about the aggregation of salt in leaves, basically in older leaves and salt toxicity in the plant. This salt toxicity in the plant can cause the death of leaves and decrease the total photosynthetic leaf area. Thus, there is a decrease in the availability of photosynthate to the plant and influence the overall carbon (CO2) balance essential for sustainable plant growth and development [16] (Figure 1).

#### 2.1.1 Salinity and ion toxicity in plant

The important harmful effect of salinity is the sodium and chloride ions accumulation in plant tissues and soil [19]. The higher concentration of soluble salts in the soil profile may cause physiological drought to plant, that is, reduction in uptake of water due to salt accumulation in the root zone [20]. The entrance of sodium and chloride ions into the plant cell from soil causes ion imbalance in plant and soil and excessive uptake of these ions by plant causing many problems related to the physiology of plant's tissues such as root, leaf, grain, fruit, or fiber [21]. Similarly, the reduction of plant osmotic potential, excessive uptake of Na+ and Cl in the cell, and disruption of cell metabolic functions is due to ion toxicity [21]. Excessive sodium ion in plant tissues harms the cell membrane and plant organelles, and as a result, cell death of plant [22]. These physiological changes in the plant include the membranes disruption, reactive oxygen species (ROS) production, reduction of photosynthesis rate (Pn), and scavenging of antioxidants [21]. Consequently, the accumulation of soluble salts in the rhizosphere is one of the main reasons for low crop productivity.

CO2. The weathering of these primary mineral rocks in the earth crust is the primary source of all the dissolvable salts present in the soils and ocean. However, there are several other anthropogenic sources of salinity in the soil or water. Under arid to and semi-arid climates, the products from the weathering procedure of mineral and rocks accumulate in the soil and result in the advancement of saltinfluenced soils (saline or sodic soil). Though, under a humid atmosphere, salt could not collect in-situ and filter down through the soil and transport to the close-by streams and waterways and caused the salinity in the water bodies [12]. The US Salinity Lab staffs (1954) group the salt-influenced soils (Table 2). These saltaffected soils types have unique nature of soluble salts. For instance, saline soil has

<sup>2</sup> present and sodic or alkali soil has HCO3

<sup>2</sup> with an average intensity of NaHCO3 and Na2CO3 in a trace concentration. An ordinary soil has maximum nutrients for development and improvement of the plant. On the inverse, Salinity is one of the significant environmental element influencing plant growth and production. As indicated by FAO report, a saline soil is characterized as having a high concentration of soluble salts for the most of

), Calcium (Ca2+), magnesium (Mg2+) chloride (Cl) and sulfate

<sup>2</sup>). Magnesium sulphate (MgSO4) and sodium chloride (NaCl, table salt), are among the most well-known soluble salts which are sufficiently high to influence

Salinity influence crops in these ways: osmotic effect, specific ion effect, ion imbalance, and oxidative stress [6]. Salinity decline water uptake limit of plant, and causes a decrease in plant development. It might be explicit salt effects. If a high concentration of salt enters the plant, this high concentration of salt will increase at last ascent to a toxic level in older leaves causing early senescence and diminished the photosynthetic leaf area of a plant to a dimension that cannot support plant development [14]. Salinity seems to influence plant growth mechanism in two different ways, water relations, and ionic relations. Firstly, plants face water stress, which in cause decline leaf expansion. Secondly, long-term salt stress in soil and plant, plants involvement (Na<sup>+</sup> and Cl) ionic stress, which can prompt early

Plants experience the ill effects of the presentation of salinity until maturity [16]. Generally, the markers of salinity impacts in plants are impeded growth and small plants with fewer and smaller leaves. Munns [16] depicted salinity consequences for various plant development stages under a different period of the plant growth mechanism and development. After a couple of minute's introduction of salinity stress, dehydration and shrinkage of the cell begin, and following a couple of hours after the fact recovers their original volume. Regardless of this recovering of the original volume, cell elongation and cell division are diminished, prompting slower rates of root and leaf development. On the following days, a diminishing in cell division and lengthening change into slower leaf inception and size. Plants that are harshly salt influenced regularly build up obvious salt damage. As exposure of salinity extends to half a month, secondary shoot growth is influenced, and following a couple of months, clear changes observed in development and injury between salt-stressed plants and control. To comprehend these time-sensitive changes in light of salinity in plant development stages, the 'two-phase growth response to salinity idea created by [16]. The first phase of growth decline occurs within minutes after exposure to salinity. The decline of growth is because of the osmosis stress, osmotic changes

hydrolysis. So also, saline sodic soil has predominant soluble salts of Na<sup>+</sup> with Cl

<sup>2</sup> concentration with the capacity of alkaline

of Na<sup>+</sup>

, and in

Cland SO4

Sodium (Na<sup>+</sup>

(SO4

200

and SO4

<sup>2</sup> and CO3

exceptional cases with high CO3

Climate Change and Agriculture

plant growth and development.

2.1 Salinity effects on plant growth

senescence of older leaves [15] (Figure 1).

#### 2.1.2 Salinity and nutrient imbalance in plant

Salinity has direct effects on nutrients imbalance between soil and plant. The most important harmful effect of salinity is the sodium and chloride ions accumulation in plant tissues and soil [19]. High sodium ion (Na<sup>+</sup> ) concentration has an antagonistic effect on potassium (K<sup>+</sup> ) ions [23]. Moreover, N uptake reduction by the plant has also been observed under high salt conditions [24]. Similarly, salinity has an antagonistic effect on P, K<sup>+</sup> , Zn, Fe, Ca2+, and Mn while it has a synergistic effect on N and Mg in field crops such as rice [23, 25].

#### 2.1.3 Salinity and oxidative stress in plant

The production of reactive oxygen species (ROS), like oxygen radical (O2), superoxide (OH), and H2O2 under salinity is high [30]. These oxidative species can interrupt the routine functions of various cellular plant modules. For example, DNA, proteins, and lipids, are interfering metabolism of the plant [26].

> (21%), and Pakistan (18%) respectively, while remaining 30% is contribution belongs to Japan, Thailand, Indonesia, and Burma [39]. Rice is a high yielding crop. However, the current average yield is 8–10 t/ha for indica rice, 10–15% yield is lower than its potential [40]. This rice production gap is due to many reasons, such as environmental stresses (biotic or abiotic), management strategies, and nutrients

25% yield loss (dS m<sup>1</sup> )

Salinity Stress in Arid and Semi-Arid Climates: Effects and Management in Field Crops

Wheat Grain yield 6–8 6.3 10 16–24 MT [31] Rice Grain yield 3 3.2 3.5–4 8–16 S [32] Maize Ear FW 1.8 2.5–6.8 8.6 15.3 MS [33] Sorghum Grain yield 6.8 7 10 30 MT [34] Cotton Seed cotton 7.7 8.3 7 17.0 16–24 T [35] Sugarcane Shoot DW 1.7 3.9 13.3 16–24 MS [36] Where EC = electrical conductivity, FW = fresh weight, DW = dry weight, S = sensitive, MS = moderately sensitive,

50% yield loss (dS m<sup>1</sup> )

Zero yield (dS m<sup>1</sup> ) Ranking References

Among the abiotic stresses, especially salinity is among the essential causes of this low yield. The morphological characteristics of rice are severely affected by salinity [41]. Rice plant responds differently against salinity compared to other field crops. The intent of salinity in rice plant life cycle varies from growth stages, and cultivar to cultivar, that is, the early seedling growth stage is more sensitive than the tillering stage in rice plant [14]. The threshold level of salt stress for rice is 3 dS m<sup>1</sup> [42]. However, a significant reduction in seedling growth and fresh weight were observed with increased salt stress from 1.9 to 6.1 dSm<sup>1</sup> and 5 to 7.5 dSm1, respectively [43]. Many studies also exposed that salinity stress decrease rice stand density and production of seedling biomass, which shows the high sensitivity

The first organ of the rice plant that keeps in contact with soluble salt is a root [45]. The root is responsible for the entrance of hydrogen peroxide (H2O2) and solutes by through different pathways such as symplastic, apoplastic, and

transcellular, respectively. So, transport of water and solutes through the apoplastic pathway is vital in rice [46]. Mostly Na<sup>+</sup> transport in rice shoots via the apoplastic passage where Na+ transports by apoplast through Casparian tubes [47]. As a result of this Na<sup>+</sup> accumulation, a significant reduction in numbers of root per plant, root length, and shoot length occurred under increased salinity [48]. Based on these proofs, the reduced root and shoot lengths are considered two indicators of rice

Moreover, cell division and cell elongation in rice plant are severely affected by salinity, which results in a reduction of the root, leaf growth, and yield [16]. Rice plant shows response very soon after the exposure of salinity stress and affects plant growth. For example, rice leaf mortality boosted with increased salinity in almost all rice cultivars at early seedling stage [14]. Some rice cultivars showed leaf mortality up to 0–300% after 1 week of salinity exposure [16]. Salinity effect cause panicle sterility and poor development of inferior and superior spikelets, which result in the reduction of rice grain yield [4]. Many rice cultivars showed panicle sterility at pollination and fertilization stages due to some genetic mechanisms and nutrient

deficiencies.

Table 3.

Crop type

Tolerance based on

DOI: http://dx.doi.org/10.5772/intechopen.87982

MT = moderately tolerant, and T = tolerant.

Salt tolerance classification of major field crop.

Threshold EC levels (dS m<sup>1</sup> )

against salinity [4, 44].

plant response to salinity.

203

#### 2.1.4 Salinity and hormonal response in the plant

The phytohormones are naturally produced in a chemical form called plant growth regulators. The phytohormones are active signal compounds which show response against salinity stress and reduce the plant growth [27]. Under salinity stress, the ethylene, cytokinin, and gibberellic acid concentration decreased, and abscisic acid contents increased. This alteration of hormones effects plant growth, such as germination, tiller formation, and reproductive growth. For example, poor development of rice and wheat spikelets, boll of cotton, etc.
