**7. Salinity stress**

ability to retain water in the vacuole and in the apoplast against the low water potential in the apoplast by modifying aquaporin density and activity [101]. In this sense, this characteristic of turgor retention is complex from the genomic, biochemical, metabolic, and structural perspective; is different between ecotypes or varieties of the same species, obtaining the differences through natural selection or genetic selection. Therefore its manipulation corresponds to techniques of plant genetics and transgenic crops [12]. During the induction of water stress, this process of turgor loss and loss of photosynthetic tissues is the last to occur, since it is preceded by the responses associated with the decrease in productive metabolism

One way to mitigate the adverse effect of a high VPD in a field is to increase the concentration

capacity, including partial closure of the stomata which decreases the water vapor loss of the mesophyll [68]. It is known that a greater amount of OM in the soil and the planting of high

buffer effect on the loss of moisture in the canopy by wind and convective processes caused

In practical terms, the edaphic water storage is the one within reach for manipulation and control in agricultural production systems. The water absorption and retention capacity of the

Ψ = Ψ<sup>g</sup> + Ψ<sup>p</sup> + Ψ<sup>o</sup> + Ψ<sup>m</sup> (1)

balance with the components that allow the conservation of the water, which are the matric

actions between water and the structural components (such as soil pores), physicochemical (inorganic colloids) and biological (organic colloids) of the soil, as well as the ions dissolved in the water of the soil pores. These interactions occur at different scales, from nanometric to

The practical way to maintain or increase ability of the soil to absorb and conserve water in the edaphic profile accessible to crop plants (0.1–1.5 m) has been described with different techniques of rainwater harvesting [102] and soil conservation, among which we can mention low tillage to conserve soil pore structure and the use of cover crops [103], the promotion of beneficial microorganisms in the soil [70] and the use of hydrophilic polymers. A relatively simple way to increase the soil ability to provide water to crops at medium and long term is to increase the amount of organic matter, which increases the soil matric potential. Different reports indicate the direct relationship between a higher concentration of organic matter, higher water retention capacity, and plant response in the form of less impact on growth when irrigation water or atmospheric precipitation is reduced [104]. In that sense, any strategy aimed at raising crop tolerance to water deficits under the current climate change scenario must take into account the increase in soil organic matter as well as the counterpart

soil depend on the set of forces between the components of the water potential:

] results in an increase in photosynthetic

), which refer to the molecular inter-

] and decrease of evaporation in the soil, in addition to the

) and evapotranspiration of the soil and plants are in dynamic

in the canopy of plants, as more elevated [CO2

or capillary potential (Ψm) and the osmotic potential (Ψ<sup>o</sup>

and growth.

density crops allow higher [CO2

162 Plant, Abiotic Stress and Responses to Climate Change

The losses due to leaching (Ψ<sup>g</sup>

of its biological activity [70].

micrometric.

by the proximity of other plants [30, 92].

of CO2

Until a few decades ago the cultivation in saline soils was not considered as an alternative for food, fiber, or biomass production. However, the stress induced by the presence of large amounts of salts in soils and water has taken on current importance due to the progressive salinization of agricultural soils, resulting in the extraction of water from the subsoil, the higher evapotranspiration resulting from the increase in temperatures and by rainfall regimes, which occur more erratically. Along with the gradual loss of organic matter from soils, salinization is considered an increasingly common symptom of soil degradation [111, 112].

The first step to cultivate in saline or salinization soil due to the use of irrigation water in combination with high evapotranspiration is to determine whether the crop will be destined to produce food, fiber, or biomass. The point is important because it is more feasible to find a species with some tolerance to salinity to produce fiber or biomass (which could later be transformed into biofuel) than one for the production of food such as corn, wheat, or tomato. Part of the strategy to achieve greater agricultural production in saline or salinized soils is perhaps to correctly select the plant species to be cultivated in such a way that the natural abilities of the different plant species are part of the solution to the growing problem of salinity in agricultural systems. As has been said for other stresses, there is an intrinsic incompatibility between high productivity and high yields and stress tolerance, since both processes depend on the same budget of photosynthates for successful development. The case of salinity is complex regarding its management and the obtaining of improved varieties since it involves two stresses in one: osmotic and ionic, affecting many different aspects of the growth, development, physiology, and biochemistry of plants (**Figure 2**).

A relevant part of the solution is to give greater attention to halophytic species, both as a source of genetic resources to improve glycophytic crops, and for direct use in the rehabilitation processes of salinized soils or for cultivation in saline soils seeking to produce fodder, pigments, or biomass for industrial processing, seems to be of increasing importance [113].

#### **7.1. Responses at the molecular level**

Salinity-induced stress occurs in two phases: the first occurs very rapidly (the plant's perception takes place in in seconds and the signaling and response in minutes or hours) and is a result of the decrease in the water potential of soil pore water or nutrient solution by the high concentration of dissolved ions present. The low water potential makes it difficult for water to be absorbed by the roots and a water deficit is induced, with signaling and responses as described in the previous Section of Water Stress, including decreased growth, ABA synthesis, and stomatal closure [114]. The second phase occurs at a later time to the first and is a result of the gradual intoxication of the cells mainly by Na<sup>+</sup> and Cl<sup>−</sup> , which interfere with the ionic balance in the cytoplasm that depends mostly on K+ , Mg2+ and NO<sup>3</sup> − [115].

At the molecular level, there are three defense mechanisms of the plant against the high concentration of salts in the soil solution or nutrient solution: (i) osmotic tolerance and root exclusion, (ii) osmotic tolerance and foliar exclusion, and (iii) Na<sup>+</sup> tolerance. The first mechanism depends on the interruption of the production of radical hairs and new leaves, stomatal closure, and increased root growth toward new soil volumes. Root exclusion depends on the control of Na<sup>+</sup> and Cl<sup>−</sup> flux in the endodermis (which depends significantly on the availability of silicon). However, as Na<sup>+</sup> and Cl<sup>−</sup> enter the cytoplasm via nonselective channels and transporters, it is inevitable that they will reach the xylem and thence to the rest of the plant [111, 115, 116]. An important part of the osmotic response in the root is based on the increase in aquaporin density in the cells of the epidermis; it is believed that this response improves the water status of the root [116].

When the first root exclusion mechanism fails, Na<sup>+</sup> and Cl<sup>−</sup> accumulate in stem and leaf apoplast, activating a foliar osmotic tolerance response that depends on the synthesis of enzymatic and non-enzymatic antioxidants, to counteract the most of ROS production, and synthesis of osmolytes such as proline, polyols, and glycine betaine to preserve the functionality of membranes, proteins, and other biomolecules. On the other hand, Na<sup>+</sup> and Cl<sup>−</sup> of the apoplast are transported to the cytoplasm of the cells by means of nonspecific channels and the Na<sup>+</sup> /Ca2+, Na<sup>+</sup> /K+ , Ca2+/Mg2+, and Cl<sup>−</sup> /NO<sup>3</sup> − ionic balance breaks, which interferes with

**Figure 2.** Environmental factors and agronomic management to be used for agricultural production in soils or substrates

Tolerance-Induction Techniques and Agronomical Practices to Mitigate Stress in Extensive Crops…

http://dx.doi.org/10.5772/intechopen.71771

165

with high amounts of salts.

agricultural systems. As has been said for other stresses, there is an intrinsic incompatibility between high productivity and high yields and stress tolerance, since both processes depend on the same budget of photosynthates for successful development. The case of salinity is complex regarding its management and the obtaining of improved varieties since it involves two stresses in one: osmotic and ionic, affecting many different aspects of the growth, devel-

A relevant part of the solution is to give greater attention to halophytic species, both as a source of genetic resources to improve glycophytic crops, and for direct use in the rehabilitation processes of salinized soils or for cultivation in saline soils seeking to produce fodder, pigments, or biomass for industrial processing, seems to be of increasing impor-

Salinity-induced stress occurs in two phases: the first occurs very rapidly (the plant's perception takes place in in seconds and the signaling and response in minutes or hours) and is a result of the decrease in the water potential of soil pore water or nutrient solution by the high concentration of dissolved ions present. The low water potential makes it difficult for water to be absorbed by the roots and a water deficit is induced, with signaling and responses as described in the previous Section of Water Stress, including decreased growth, ABA synthesis, and stomatal closure [114]. The second phase occurs at a later time

and Cl<sup>−</sup>

tolerance. The first mechanism

accumulate in stem and leaf

ionic balance breaks, which interferes with

and Cl<sup>−</sup>

of

, Mg2+

,

to the first and is a result of the gradual intoxication of the cells mainly by Na<sup>+</sup>

which interfere with the ionic balance in the cytoplasm that depends mostly on K+

At the molecular level, there are three defense mechanisms of the plant against the high concentration of salts in the soil solution or nutrient solution: (i) osmotic tolerance and root exclu-

depends on the interruption of the production of radical hairs and new leaves, stomatal closure, and increased root growth toward new soil volumes. Root exclusion depends on the

porters, it is inevitable that they will reach the xylem and thence to the rest of the plant [111, 115, 116]. An important part of the osmotic response in the root is based on the increase in aquaporin density in the cells of the epidermis; it is believed that this response improves the

apoplast, activating a foliar osmotic tolerance response that depends on the synthesis of enzymatic and non-enzymatic antioxidants, to counteract the most of ROS production, and synthesis of osmolytes such as proline, polyols, and glycine betaine to preserve the func-

the apoplast are transported to the cytoplasm of the cells by means of nonspecific channels

/NO<sup>3</sup> −

tionality of membranes, proteins, and other biomolecules. On the other hand, Na<sup>+</sup>

, Ca2+/Mg2+, and Cl<sup>−</sup>

flux in the endodermis (which depends significantly on the availability

and Cl<sup>−</sup>

enter the cytoplasm via nonselective channels and trans-

opment, physiology, and biochemistry of plants (**Figure 2**).

sion, (ii) osmotic tolerance and foliar exclusion, and (iii) Na<sup>+</sup>

and Cl<sup>−</sup>

When the first root exclusion mechanism fails, Na<sup>+</sup>

tance [113].

and NO<sup>3</sup>

− [115].

control of Na<sup>+</sup>

and the Na<sup>+</sup>

and Cl<sup>−</sup>

of silicon). However, as Na<sup>+</sup>

water status of the root [116].

/Ca2+, Na<sup>+</sup>

/K+

**7.1. Responses at the molecular level**

164 Plant, Abiotic Stress and Responses to Climate Change

**Figure 2.** Environmental factors and agronomic management to be used for agricultural production in soils or substrates with high amounts of salts.

the membrane's charge balance on which the activity of the integral proteins depends, and also competitively inhibits a large number of enzymes which are activated by K+ , Mg2+ and Ca2+. Plant cells turns on an exclusion process dependent on the pumping of toxic ions to mitigate the accumulation of Na<sup>+</sup> . An example of such mechanisms is the SOS proteins that are responsible for transporting the Na<sup>+</sup> from the cytoplasm to the vacuole or the apoplast and is even associated with long-distance transport of Na<sup>+</sup> [112]. These toxic ion-pumping systems, however, have a very high-energy expenditure. As salinity significantly interferes with metabolic processes, the energy budget is getting lower, and over time the cells ability to keep toxic ions out of the cytoplasm volume is exceeded. Then the point where the concentration of Na<sup>+</sup> and Cl<sup>−</sup> in the cytoplasm grows in such a way that it causes the death of the cells.

It is usually determined on a saturated past extract soil:water (ECe), in soil taken from the root region of the plant and averaged over depth and time. The ECe of a saturation extract for a heavy soil or medium texture multiplied by 2 marks the approximate EC for soil solution at field capacity. In contrast, for sandy soils, ECe is multiplied by 3. For direct determination in field or greenhouse, filtered extracts of soil:water in relation 1:1, 1:2 or 1:5 volume/volume are used. EC and osmotic potential are linearly related (1 mS cm−1 = −0.036 Mpa). The productivity of salinity-sensitive plants decreases if soil EC exceeds 4 mS cm−1 (4000 μS cm−1) therefore it is

Tolerance-Induction Techniques and Agronomical Practices to Mitigate Stress in Extensive Crops…

There are several physical-chemical techniques for the management of saline soil conditions that favor crop yields. When soil salinity is not intrinsic but results from the application of fertilizer or irrigation water with high EC, soil management techniques can be used such as drainage improvement and leaching practices, with or without gypsum applications, limestone, and sulfuric acid, as well as deep tillage, subsoiling, and inversion of the soil profile. Several factors, such as the availability of water, the quality of the water, the access to the machinery and the necessary economic resources must be considered before applying the methods above mentioned. The application of localized organic matter (used on rows or seed beds) or throughout the complete soil profile is also useful as a technique to dilute the concentration of salts in the soil explored by the root. This latter technique is also useful when soils

Other methods that mitigate salinity are the incorporation of crop residues, as well as crop rotation and the application of biosolids and biochar to dilute ion concentration in soil and to promote plant microbiome, the latter has been shown to have a positive effect on plants subjected to high salt concentration either by the production of growth hormones, osmolytes and other stress relieving compounds, or because the microorganisms themselves capture part of the salts present and sequester them in their biomolecules over a period. The greater

of partial closure of the stomata, allows the maintenance of photosynthesis, which in turn is associated with greater availability of energy and biomolecules such as antioxidants and

On the other hand, an adequate regulation of the nutrients in plants can improve the acclimatization to the saline environment. Application to the soil of silicon fertilizers has shown to be an effective technique to improve tolerance to salinity in plants. The contribution of other mineral nutrients such as K and Ca combined with compost or other sources of organic mat-

plant growth and yield. The use of nanofertilizers, which has been shown to be more efficient to feed plants compared to traditional fertilizers [46], could be another alternative to reduce the supply of salts to agricultural soils, thus decreasing the process of salinization that every day increases the surface of degraded soils. Another suitable alternative to mitigate this type of stress is the application of zeolite and humic substances that capture the salts in the soil, reducing the EC of the soil solution, increasing the growth of the roots as well as the uptake

and improves the K+

:Na<sup>+</sup>

, which, even under conditions

http://dx.doi.org/10.5772/intechopen.71771

167

balance, resulting in higher

recommended that irrigation water does not exceed 2 mS cm−1 [13].

amount of organic matter increases the availability of CO<sup>2</sup>

absorption, increases K+

of other mineral elements [119, 120].

are inherently saline [117].

osmolytes [117, 118].

ter reduces Na<sup>+</sup>

Crops are not very tolerant to Na<sup>+</sup> and Cl<sup>−</sup> , but this character is variable from one species to another. Tolerance capacity can be increased if plants have enough Ca2+, K+ , Mg2+, and NO<sup>3</sup> − to mitigate the imbalances caused by Na<sup>+</sup> and Cl<sup>−</sup> . Also, the use of antioxidants and osmolytes such as proline and glycine betaine applied exogenously, or the use of enhanced or genetically modified varieties may be useful during the osmotic phase of salinity-induced stress [111, 114]. However, obtaining crops with high productivity and halophytic character is a challenge, since the exclusion, compartmentalization, and extrusion of Na<sup>+</sup> and other ions that reach toxic concentrations requires a high-energy expenditure and therefore a high percentage of the photosynthates produced.

#### **7.2. Responses at the cellular and physiological-morphological level**

The osmotic and toxicity effects on the molecular scale are transferred to the upper levels, causing a rapid stomatal closure dependent on ABA, decreased photosynthesis and interruption of growth in young leaves, which is the first symptom observed in the plant. In case the adaptive response is successful, the growth can be restarted at a later time, once the adjustments in the cellular development programs that allow the osmotic tolerance and the Na<sup>+</sup> tolerance occur. If the mechanisms mentioned above of osmotic balance and exclusion are not sufficient, the plant will initiate a gradual process of intoxication characterized by the senescence of mature leaves, a result of the accumulation of Na<sup>+</sup> and Cl<sup>−</sup> [111].

#### **7.3. Management of salinity-induced stress**

As the primary factor that induces stress is the high concentration of salts in the soil or substrate, stress mitigation is mainly directed to the application of soil management techniques. However, the use of tolerance-inducing compounds such as salicylic acid, antioxidants, osmolytes and growth regulators applied by foliar spraying or in seedlings or seeds, are useful for improving plant response. Also, the use of genetically improved plants is an alternative that can be combined with soil management to obtain better results [112].

From an agronomic perspective, salinity is expressed in terms of electrical conductivity (EC) in units of dS m−1 (1 dS m−1 = 1000 μS cm−1) or mmhos cm−1 (equivalent to 1 dS m−1 or 1 mS cm−1) It is usually determined on a saturated past extract soil:water (ECe), in soil taken from the root region of the plant and averaged over depth and time. The ECe of a saturation extract for a heavy soil or medium texture multiplied by 2 marks the approximate EC for soil solution at field capacity. In contrast, for sandy soils, ECe is multiplied by 3. For direct determination in field or greenhouse, filtered extracts of soil:water in relation 1:1, 1:2 or 1:5 volume/volume are used. EC and osmotic potential are linearly related (1 mS cm−1 = −0.036 Mpa). The productivity of salinity-sensitive plants decreases if soil EC exceeds 4 mS cm−1 (4000 μS cm−1) therefore it is recommended that irrigation water does not exceed 2 mS cm−1 [13].

the membrane's charge balance on which the activity of the integral proteins depends, and

Ca2+. Plant cells turns on an exclusion process dependent on the pumping of toxic ions to

systems, however, have a very high-energy expenditure. As salinity significantly interferes with metabolic processes, the energy budget is getting lower, and over time the cells ability to keep toxic ions out of the cytoplasm volume is exceeded. Then the point where the

. An example of such mechanisms is the SOS proteins that

in the cytoplasm grows in such a way that it causes the death

and Cl<sup>−</sup>

[111].

from the cytoplasm to the vacuole or the apoplast

, but this character is variable from one species to

. Also, the use of antioxidants and osmolytes

[112]. These toxic ion-pumping

, Mg2+, and NO<sup>3</sup>

and other ions that

− to

, Mg2+ and

also competitively inhibits a large number of enzymes which are activated by K+

and Cl<sup>−</sup>

and Cl<sup>−</sup>

such as proline and glycine betaine applied exogenously, or the use of enhanced or genetically modified varieties may be useful during the osmotic phase of salinity-induced stress [111, 114]. However, obtaining crops with high productivity and halophytic character is a

reach toxic concentrations requires a high-energy expenditure and therefore a high percent-

The osmotic and toxicity effects on the molecular scale are transferred to the upper levels, causing a rapid stomatal closure dependent on ABA, decreased photosynthesis and interruption of growth in young leaves, which is the first symptom observed in the plant. In case the adaptive response is successful, the growth can be restarted at a later time, once the adjustments in the cellular development programs that allow the osmotic tolerance and the Na<sup>+</sup> tolerance occur. If the mechanisms mentioned above of osmotic balance and exclusion are not sufficient, the plant will initiate a gradual process of intoxication characterized by the senes-

As the primary factor that induces stress is the high concentration of salts in the soil or substrate, stress mitigation is mainly directed to the application of soil management techniques. However, the use of tolerance-inducing compounds such as salicylic acid, antioxidants, osmolytes and growth regulators applied by foliar spraying or in seedlings or seeds, are useful for improving plant response. Also, the use of genetically improved plants is an alternative that

From an agronomic perspective, salinity is expressed in terms of electrical conductivity (EC) in units of dS m−1 (1 dS m−1 = 1000 μS cm−1) or mmhos cm−1 (equivalent to 1 dS m−1 or 1 mS cm−1)

another. Tolerance capacity can be increased if plants have enough Ca2+, K+

challenge, since the exclusion, compartmentalization, and extrusion of Na<sup>+</sup>

**7.2. Responses at the cellular and physiological-morphological level**

cence of mature leaves, a result of the accumulation of Na<sup>+</sup>

can be combined with soil management to obtain better results [112].

**7.3. Management of salinity-induced stress**

mitigate the accumulation of Na<sup>+</sup>

166 Plant, Abiotic Stress and Responses to Climate Change

Crops are not very tolerant to Na<sup>+</sup>

mitigate the imbalances caused by Na<sup>+</sup>

age of the photosynthates produced.

concentration of Na<sup>+</sup>

of the cells.

are responsible for transporting the Na<sup>+</sup>

and is even associated with long-distance transport of Na<sup>+</sup>

and Cl<sup>−</sup>

There are several physical-chemical techniques for the management of saline soil conditions that favor crop yields. When soil salinity is not intrinsic but results from the application of fertilizer or irrigation water with high EC, soil management techniques can be used such as drainage improvement and leaching practices, with or without gypsum applications, limestone, and sulfuric acid, as well as deep tillage, subsoiling, and inversion of the soil profile. Several factors, such as the availability of water, the quality of the water, the access to the machinery and the necessary economic resources must be considered before applying the methods above mentioned. The application of localized organic matter (used on rows or seed beds) or throughout the complete soil profile is also useful as a technique to dilute the concentration of salts in the soil explored by the root. This latter technique is also useful when soils are inherently saline [117].

Other methods that mitigate salinity are the incorporation of crop residues, as well as crop rotation and the application of biosolids and biochar to dilute ion concentration in soil and to promote plant microbiome, the latter has been shown to have a positive effect on plants subjected to high salt concentration either by the production of growth hormones, osmolytes and other stress relieving compounds, or because the microorganisms themselves capture part of the salts present and sequester them in their biomolecules over a period. The greater amount of organic matter increases the availability of CO<sup>2</sup> , which, even under conditions of partial closure of the stomata, allows the maintenance of photosynthesis, which in turn is associated with greater availability of energy and biomolecules such as antioxidants and osmolytes [117, 118].

On the other hand, an adequate regulation of the nutrients in plants can improve the acclimatization to the saline environment. Application to the soil of silicon fertilizers has shown to be an effective technique to improve tolerance to salinity in plants. The contribution of other mineral nutrients such as K and Ca combined with compost or other sources of organic matter reduces Na<sup>+</sup> absorption, increases K+ and improves the K+ :Na<sup>+</sup> balance, resulting in higher plant growth and yield. The use of nanofertilizers, which has been shown to be more efficient to feed plants compared to traditional fertilizers [46], could be another alternative to reduce the supply of salts to agricultural soils, thus decreasing the process of salinization that every day increases the surface of degraded soils. Another suitable alternative to mitigate this type of stress is the application of zeolite and humic substances that capture the salts in the soil, reducing the EC of the soil solution, increasing the growth of the roots as well as the uptake of other mineral elements [119, 120].
