**5. Low temperature stress**

Another alternative for handling high-temperature stress in field crops is the use of chemi-

demonstrated effectiveness in inducing protection against stress caused by high temperature. Given the complexity of managing this stress and the projection that it will be increasingly

Metabolic processes such as photosynthesis, respiration, transpiration, absorption, transport and assimilation of nutrients, among others, also have temperature spectra for their proper functioning. These responses depend in part on the phenomena described for the molecular domain, but also on the interactions between the biomolecules and their complexes that form the different metabolic pathways, as well as on the interactions between the different metabolic pathways that produce precursor compounds or which are source and information for

In the scale of the interaction of the plant structures with the canopy atmosphere, the high temperature causes a very high vapor pressure deficit, which results in high foliar transpiration that competes with the flow of water to other organs such as flowers and fruits, especially when the high temperature also occurs at night time. When transpiration (cooling) capacity is exceeded by the absorption of solar radiation (heating), a condition that occurs most quickly with high irradiance and air temperature >35°C, burns occur on leaves, stems, and fruits, senescence and foliar abscission, inhibition of growth and root damage affecting nutrient uptake, resulting in low yield and poor quality [64]. On the other hand, high temperatures decrease the viability of pollen [65] and shorten the period in which stigmas in flowers are

As mentioned, photosynthesis is more affected in C3 plants than in C4 plants due to the

to reduce photorespiration and increase the photosynthetic rate, decrease transpiration and increase the production of antioxidants in the leaves [67]. The higher concentration of CO2

the canopy of the plants, associated with the maintenance or increase in soil organic matter, appears again as a multifunctional tool for the management of stress [68]. For the mitigation of damages by high temperature, it is also useful to apply hydrophilic polymers, incorporating these materials into the soil or substrate increases the water retention and storage capacity, decreasing the rate of evaporation loss that accompanies the high temperature. As a

On the other hand, the presence of microorganisms associated with soil organic matter has also been found as a factor promoting tolerance to high-temperature stress in plants [70]. Different fungi that form arbuscular mycorrhiza have been shown to be useful in the mitigation of damages induced by high temperature, both by increasing the production of antioxidant metabolites and the activity of antioxidant enzymes, as well as allowing a better response

mitigate the damage caused by high temperatures. The higher concentration of CO2

[61], CeO2

in the canopy is an important factor to

allows

in

[62], and nSe [63],

cal elements in the nanometric form. The application of nTiO2

156 Plant, Abiotic Stress and Responses to Climate Change

common, the use of nanomaterials deserves further exploration.

other metabolic pathways.

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

receptive to pollen, reducing the chances of successful fertilization [66].

consequence, more water is available to be absorbed by plants [69].

in photosynthesis and water use efficiency in crops [71, 72].

high temperature, and again the concentration of CO2

Low-temperature stress is an environmental factor that greatly affects the growth, development, and productivity of plants. This type of environmental stress includes non-freezing temperatures (0°C < T < 10°C) as well as freezing temperatures (T < 0°C). Crop plants originating in the tropics or subtropics die or are severely damaged when exposed to low freezing temperatures, even for short periods (24–48 hours), developing symptoms such as chlorosis, necrosis or stunting. In contrast, species originated from temperate and subarctic zones through an adaptive process that develops during the fall can tolerate freezing temperatures [74]. However, although different species of plants may be more or less resistant to low temperatures depending on the adaptations of each species and their origin or geographical location, they all show a reduction in growth against unexpected events of low temperature as the unexpected nature of the phenomenon does not allow the natural adaptive process to begin [15].

The stress caused by low temperature can be described at the molecular level or in the cellular or physiological-morphological ambits. The primary inducing factor is the impact of low temperature on the reduction of the speed of vibration and translation of the molecules (in the presence of low temperatures) and the total deficit or absence of water when it becomes ice when there are freezing temperatures. Such changes are transferred to other levels of description such as physiological-morphological interactions between biomolecules that give rise to different metabolic pathways and cellular activities such as energy metabolism and transport of ions and metabolites. This fact results in that the process of hardening or cross-resistance (with the exception for the damages caused by the oxidative stress) is difficult to achieve for the stress induced by the low temperatures, especially when they cause freezing [15].

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

From a physical point of view the low non-freezing temperature has an impact contrary to that described for the high temperature because it decreases the speed of vibration of the molecules; in membranes reduces the average distance between molecules and decreases fluidity. This change in fluidity considerably modifies the behavior of integral proteins, many of which are associated with energy metabolism. The result is the production of ROS and consequent oxidative stress [15]. It has been hypothesized that the decrease in membrane fluidity is the primary site of low-temperature stress perception [75] and it has been found that one of the first adaptive responses of cells increases the lipid unsaturation of the membranes, which increases their fluidity [76]. After the initial perception of changes in membrane fluidity, Ca2+ fuses from the apoplast and vacuole stores are triggered into the cytoplasm [77]. Calcium fluxes activate MAPK cascades that result in changes in the activity of transcription factors that initiate an extensive network of transcriptional, posttranscriptional, and posttranslational responses involving more than 2000 genes associated with low-temperature responses [78]. Therefore, maintaining the proper nutritional status of plants, especially concerning calcium concentration in different organs, is critical to ensure an adequate response to changes in temperature.

plant and therefore are factors that facilitate freezing. Once the water from the apoplast begins to freeze at some point, the process spreads rapidly to the rest of the plant. The disappearance of water caused by the formation of ice causes a severe water deficit that causes a rapid denaturation of cellular components and cell death [84]. The damage caused by freezing is very different from that induced by the low temperature and makes it extremely complicated

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The metabolic processes dependent on biochemical reactions are affected by the low temperature more rapidly than the photochemical processes. The presence of PAR aggravates the induced damages by low temperature, and the plants are quickly photo-inhibited, the reason why some of the measures described to mitigate the damage by high irradiance are applicable for low temperature. It has been found that the photo-inhibition process is also present when low temperatures occur at night, this adjustment is thought to be part of the adaptive response to low temperature [85]. Photosynthesis is affected to a large extent, the cessation of growth reduces the capacity of energy utilization, with the consequent production of ROS

Many antioxidant enzymes are involved in low-temperature response machinery. In addition to those associated with the metabolism of osmolytes, detoxification cascades and photosynthesis, the metabolism of lignin (caffeic acid 3-O-methyltransferase), secondary metabolism, remodeling of cell wall polysaccharides, metabolism of starch, sterol biosynthesis and the oligosaccharide of the raffinose family (myoinositol-phosphate synthase and galactinol syn-

Carbohydrates, mainly sucrose, function as osmolytes and antioxidants to protect cells and

is associated with higher amounts of carbohydrates in different plant structures [89]. Soil

magnitude and diversity of the plant microbiome, may be a factor to mitigate damage against cold stress [90, 91]. An additional advantage of organic matter in soil is to increase the water storage capacity, as the thermal capability of the water is much greater than that of the air so a soil with a substantial volume of water will be able to store heat that will radiate to the plants

The available water for crop plants is located in two storages: the edaphic and the atmospheric. The atmospheric storage includes water that precipitates as rain, dew, mist, or snow, in addition to the water contained in the air in the form of water vapor and, together with temperature, determines the vapor pressure deficit (VPD). VPD is strongly associated with stomatal responses and therefore has an impact on photosynthesis and productivity [68].

in the canopy of plants

for plants, and as a factor to increase soil fertility and

regarding its control or genetic improvement of crops.

and oxidative stress [86].

organic matter, as a source of CO<sup>2</sup>

**6. Water stress**

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

thase) are all participants in the overall response to cold stress [87, 88].

their components against oxidative damage. A high value of CO<sup>2</sup>

during the night or a low-temperature period or frost event.

In mitochondria, the low temperature causes a slower rate of consumption of the reducing potential, which results in the production of ROS and the activation of a specialized enzyme called mitochondrial alternative oxidase (chloroplasts also have an alternative oxidase). The mitochondrial alternative oxidase is an indicator of the plant response to low temperature, allowing the dissipation of reducing potential to transform it into heat and reduce the formation of ROS. The mitochondrial alternative oxidase can be activated through exogenous applications of salicylic acid and was demonstrated to work by mitigating oxidative stress in mitochondria against other stresses [79].

The low temperature also causes the elevation of the activation energy of the biochemical reactions, modifying the interactions between the multitude of enzymes and proteins associated with cellular energy processes. Each metabolic pathway is affected differently by low temperature, but the result is an imbalance in the generation and use of energy, which causes oxidative stress and less energy availability for cell growth and maintenance. Among the measures used to mitigate the damage of stress by low temperatures is the exogenous application of tolerance inducers such as salicylic acid, beneficial elements such as silicon and various nanomaterials [79–81], application of osmolytes such as glycine betaine and proline [82], or the use of genetically modified crops with a higher synthesis capacity of these compounds [12].

It has also been determined that phytohormones play a major role in the induction of tolerance at low temperatures. Hormones create a complex network of interactions that are used to integrate external information into endogenous development programs and activate the stress response pathways that lead to resistance. The knowledge of hormone regulatory activities against low temperature is limited, although it is known that they are involved in signaling cascades of other types of biotic and abiotic stress [83].

The stress induced by freezing is different from that caused by low temperature. When freezing of plant tissues occurs, this begins in the apoplast, which is the volume of water that is in contact with the external surfaces of leaves, stems, flowers, and fruits. Typically the freezing process takes place outside the plant toward the interior of the plant and is accelerated by the presence of dust and microorganisms that function as seeds for the formation of the first ice crystals. The presence of mechanical damage (hail, wind) or biotic (pests or pathogens) increases the possibility of contact between the water from the exterior and the interior of the plant and therefore are factors that facilitate freezing. Once the water from the apoplast begins to freeze at some point, the process spreads rapidly to the rest of the plant. The disappearance of water caused by the formation of ice causes a severe water deficit that causes a rapid denaturation of cellular components and cell death [84]. The damage caused by freezing is very different from that induced by the low temperature and makes it extremely complicated regarding its control or genetic improvement of crops.

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

The metabolic processes dependent on biochemical reactions are affected by the low temperature more rapidly than the photochemical processes. The presence of PAR aggravates the induced damages by low temperature, and the plants are quickly photo-inhibited, the reason why some of the measures described to mitigate the damage by high irradiance are applicable for low temperature. It has been found that the photo-inhibition process is also present when low temperatures occur at night, this adjustment is thought to be part of the adaptive response to low temperature [85]. Photosynthesis is affected to a large extent, the cessation of growth reduces the capacity of energy utilization, with the consequent production of ROS and oxidative stress [86].

Many antioxidant enzymes are involved in low-temperature response machinery. In addition to those associated with the metabolism of osmolytes, detoxification cascades and photosynthesis, the metabolism of lignin (caffeic acid 3-O-methyltransferase), secondary metabolism, remodeling of cell wall polysaccharides, metabolism of starch, sterol biosynthesis and the oligosaccharide of the raffinose family (myoinositol-phosphate synthase and galactinol synthase) are all participants in the overall response to cold stress [87, 88].

Carbohydrates, mainly sucrose, function as osmolytes and antioxidants to protect cells and their components against oxidative damage. A high value of CO<sup>2</sup> in the canopy of plants is associated with higher amounts of carbohydrates in different plant structures [89]. Soil organic matter, as a source of CO<sup>2</sup> for plants, and as a factor to increase soil fertility and magnitude and diversity of the plant microbiome, may be a factor to mitigate damage against cold stress [90, 91]. An additional advantage of organic matter in soil is to increase the water storage capacity, as the thermal capability of the water is much greater than that of the air so a soil with a substantial volume of water will be able to store heat that will radiate to the plants during the night or a low-temperature period or frost event.

### **6. Water stress**

is the primary site of low-temperature stress perception [75] and it has been found that one of the first adaptive responses of cells increases the lipid unsaturation of the membranes, which increases their fluidity [76]. After the initial perception of changes in membrane fluidity, Ca2+ fuses from the apoplast and vacuole stores are triggered into the cytoplasm [77]. Calcium fluxes activate MAPK cascades that result in changes in the activity of transcription factors that initiate an extensive network of transcriptional, posttranscriptional, and posttranslational responses involving more than 2000 genes associated with low-temperature responses [78]. Therefore, maintaining the proper nutritional status of plants, especially concerning calcium concentration in different organs, is critical to ensure an adequate response to changes in

In mitochondria, the low temperature causes a slower rate of consumption of the reducing potential, which results in the production of ROS and the activation of a specialized enzyme called mitochondrial alternative oxidase (chloroplasts also have an alternative oxidase). The mitochondrial alternative oxidase is an indicator of the plant response to low temperature, allowing the dissipation of reducing potential to transform it into heat and reduce the formation of ROS. The mitochondrial alternative oxidase can be activated through exogenous applications of salicylic acid and was demonstrated to work by mitigating oxidative stress in

The low temperature also causes the elevation of the activation energy of the biochemical reactions, modifying the interactions between the multitude of enzymes and proteins associated with cellular energy processes. Each metabolic pathway is affected differently by low temperature, but the result is an imbalance in the generation and use of energy, which causes oxidative stress and less energy availability for cell growth and maintenance. Among the measures used to mitigate the damage of stress by low temperatures is the exogenous application of tolerance inducers such as salicylic acid, beneficial elements such as silicon and various nanomaterials [79–81], application of osmolytes such as glycine betaine and proline [82], or the use of genetically modified crops with a higher synthesis capacity of these com-

It has also been determined that phytohormones play a major role in the induction of tolerance at low temperatures. Hormones create a complex network of interactions that are used to integrate external information into endogenous development programs and activate the stress response pathways that lead to resistance. The knowledge of hormone regulatory activities against low temperature is limited, although it is known that they are involved in signal-

The stress induced by freezing is different from that caused by low temperature. When freezing of plant tissues occurs, this begins in the apoplast, which is the volume of water that is in contact with the external surfaces of leaves, stems, flowers, and fruits. Typically the freezing process takes place outside the plant toward the interior of the plant and is accelerated by the presence of dust and microorganisms that function as seeds for the formation of the first ice crystals. The presence of mechanical damage (hail, wind) or biotic (pests or pathogens) increases the possibility of contact between the water from the exterior and the interior of the

temperature.

pounds [12].

mitochondria against other stresses [79].

158 Plant, Abiotic Stress and Responses to Climate Change

ing cascades of other types of biotic and abiotic stress [83].

The available water for crop plants is located in two storages: the edaphic and the atmospheric. The atmospheric storage includes water that precipitates as rain, dew, mist, or snow, in addition to the water contained in the air in the form of water vapor and, together with temperature, determines the vapor pressure deficit (VPD). VPD is strongly associated with stomatal responses and therefore has an impact on photosynthesis and productivity [68]. VPD and water in the atmosphere are difficult to control in open field since they depend on the weather stations, prevailing winds, topography, surrounding vegetation, the presence of nearby bodies of water, etc. On the other hand, according to the models of climate change, the forecast of the availability of atmospheric water will be more and more complicated, and it is expected that the crops in the open field are exposed with increasing frequency and intensity to periods of shortage of atmospheric water [92], this projection is, however, subject to discussion because of the opposite effect that could exert the increase in atmospheric CO<sup>2</sup> on carbon transpiration and metabolism in forest species [93].

direct impact on energy metabolism because it depends on many integral membrane enzymes that are particularly susceptible to loss of functionality due to lack of water or changes in the temperature. Therefore, many of the studies on the induction of tolerance to water deficit refer to the energy metabolism, in particular to the oxidative stress resulting from the imbalances between the supply of reducing potential and ATP and its use in the processes that function

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161

If it is sought to reduce the damage that occurs in plants against the water deficit, it can be achieved by the exogenous application antioxidant compounds, osmolytes, growth regulators such as ABA, IAA, and GA [97, 98], tolerance inducers such as salicylates and other organic acids or amino acids [79]. Their use in specific situations will depend on the application opportunity, the cost and the application capacities in the particular crop in question. On the other hand, for compounds that individually exert a positive effect by increasing tolerance to stress, it is also feasible to produce transgenic crops with advantages over their wild

With this information, it can be concluded that, at the biochemical and metabolic level, the opportunities to mitigate the damage to plants against the water deficit are broad, but again they should be framed in a comprehensive effort that considers soil, irrigation management,

Transpiration is the most important component of water use by plants regarding volume. A typical wheat or corn crop requires 453 and 423 mm per season in the absence of water stress. Of this amount of water, 70% corresponds to transpiration [99]. Unless the temperature or the irradiance is reduced, the light interception is reduced, or the leaf albedo is increased, it is challenging to decrease the transpiration rate since the heat dissipation obtained through the transpiration avoids damages by high temperature in foliar metabolic components, especially those involved in photosynthesis, while on the other hand the decrease in stomatal conductance required to reduce transpiration would lead to a lower photosynthetic rate [68]. It was already mentioned in the subchapter dedicated to high irradiance stress the use of tech-

ing water use efficiency, use of kaolinite as an anti-transpirant and reflector for increase leaf albedo, genetic selection of glossy varieties or to obtain plants with higher density of trichomes and a consequent greater leaf albedo. These same techniques are used to mitigate the

On the other hand water acts as a medium that provides mechanical support to herbaceous plants and photosynthetic and reproductive organs of shrub and tree plants, this is achieved by transporting water to the cells and apoplast to maintain cellular turgor. The water deficit causes loss of turgor that is perceived through mechanoreceptors that trigger part of the stress signaling pathways and ultimately cause the loss of green tissues [100]. The higher tolerance to turgidity loss is associated with changes in the composition and structure of the polymers of the cell wall, or due to the particular composition of the cell walls, as well as the

in the canopy, thus increas-

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

niques such as increasing soil organic matter to release more CO<sup>2</sup>

as energy sinks.

counterparts [12].

water deficit in crops.

and planting systems, among others.

The structure and functionality of biomolecules, membranes, and cytoskeleton, the availability of electrons and protons during photosynthesis, the solubility of gases such as CO2 and mineral ions depend on water [94]. The plants perceive the water deficit through stimuli related to the different functions the water carries out. These include: (a) stabilization of the functional form of proteins, nucleic acids, lipids of membranes, and in general of the different biomolecules and ions with which metabolism occurs; (b) water is a biochemical and electrochemical source for organisms, contributing H+ and e<sup>−</sup> which are used in energy metabolism, as well as in antioxidant metabolism related to productivity, adaptation to the environment and in the development and differentiation; (c) the provision of mechanical support for stems, leaves, flowers, and fruits; (d) the transpiration process for the maintenance of temperature during the absorption of electromagnetic radiation [14].

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

As with high-temperature stress, the water deficit causes the loss of structure and functionality of the biomolecules, creating a general imbalance in the energy metabolism that results in the formation of ROS and oxidative damage to the cellular structures. These oxidative damages are increased in the presence of high irradiance and high temperature, a combination of stresses that is expected to become increasingly common [10].

At the molecular level the response of the plant to this type of stress comes in three forms: the first is to induce the synthesis of antioxidants and chaperone proteins to eliminate ROS and preserve the structure of other biomolecules; the second is the synthesis of osmolytes that function as antioxidants as well as differential exclusion agents that stabilize membranes, proteins, and nucleic acids under water deficit conditions. The same osmolytes serve as a source of N and C to recover cell growth when the stress condition decreases; the third is to increase the rate of degradation of proteins that have undergone oxidative damage or that have aberrant folds, the latter is a response that eliminates non-functional biomolecules and in addition, allows to recover amino acids that can be used for synthesis of other proteins or as a source of C y N in other metabolic pathways [95, 96].

The set of cellular metabolic processes are related to each other through energy signals that constitute redox balance, as well as exchanges of molecules that are products of a particular metabolic pathway, and in others function as regulators or effectors. For this reason, all metabolic processes are sensitive to water deficit, although the level of sensitivity is variable among them [95]. As explained in the Introduction, many of the responses to stress have a direct impact on energy metabolism because it depends on many integral membrane enzymes that are particularly susceptible to loss of functionality due to lack of water or changes in the temperature. Therefore, many of the studies on the induction of tolerance to water deficit refer to the energy metabolism, in particular to the oxidative stress resulting from the imbalances between the supply of reducing potential and ATP and its use in the processes that function as energy sinks.

If it is sought to reduce the damage that occurs in plants against the water deficit, it can be achieved by the exogenous application antioxidant compounds, osmolytes, growth regulators such as ABA, IAA, and GA [97, 98], tolerance inducers such as salicylates and other organic acids or amino acids [79]. Their use in specific situations will depend on the application opportunity, the cost and the application capacities in the particular crop in question. On the other hand, for compounds that individually exert a positive effect by increasing tolerance to stress, it is also feasible to produce transgenic crops with advantages over their wild counterparts [12].

With this information, it can be concluded that, at the biochemical and metabolic level, the opportunities to mitigate the damage to plants against the water deficit are broad, but again they should be framed in a comprehensive effort that considers soil, irrigation management, and planting systems, among others.

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

VPD and water in the atmosphere are difficult to control in open field since they depend on the weather stations, prevailing winds, topography, surrounding vegetation, the presence of nearby bodies of water, etc. On the other hand, according to the models of climate change, the forecast of the availability of atmospheric water will be more and more complicated, and it is expected that the crops in the open field are exposed with increasing frequency and intensity to periods of shortage of atmospheric water [92], this projection is, however, subject to discus-

The structure and functionality of biomolecules, membranes, and cytoskeleton, the availability of electrons and protons during photosynthesis, the solubility of gases such as CO2 and mineral ions depend on water [94]. The plants perceive the water deficit through stimuli related to the different functions the water carries out. These include: (a) stabilization of the functional form of proteins, nucleic acids, lipids of membranes, and in general of the different biomolecules and ions with which metabolism occurs; (b) water is a biochemical and electro-

and e<sup>−</sup>

as well as in antioxidant metabolism related to productivity, adaptation to the environment and in the development and differentiation; (c) the provision of mechanical support for stems, leaves, flowers, and fruits; (d) the transpiration process for the maintenance of temperature

As with high-temperature stress, the water deficit causes the loss of structure and functionality of the biomolecules, creating a general imbalance in the energy metabolism that results in the formation of ROS and oxidative damage to the cellular structures. These oxidative damages are increased in the presence of high irradiance and high temperature, a combination of

At the molecular level the response of the plant to this type of stress comes in three forms: the first is to induce the synthesis of antioxidants and chaperone proteins to eliminate ROS and preserve the structure of other biomolecules; the second is the synthesis of osmolytes that function as antioxidants as well as differential exclusion agents that stabilize membranes, proteins, and nucleic acids under water deficit conditions. The same osmolytes serve as a source of N and C to recover cell growth when the stress condition decreases; the third is to increase the rate of degradation of proteins that have undergone oxidative damage or that have aberrant folds, the latter is a response that eliminates non-functional biomolecules and in addition, allows to recover amino acids that can be used for synthesis of other proteins or as a source of

The set of cellular metabolic processes are related to each other through energy signals that constitute redox balance, as well as exchanges of molecules that are products of a particular metabolic pathway, and in others function as regulators or effectors. For this reason, all metabolic processes are sensitive to water deficit, although the level of sensitivity is variable among them [95]. As explained in the Introduction, many of the responses to stress have a

on carbon

which are used in energy metabolism,

sion because of the opposite effect that could exert the increase in atmospheric CO<sup>2</sup>

transpiration and metabolism in forest species [93].

160 Plant, Abiotic Stress and Responses to Climate Change

chemical source for organisms, contributing H+

**6.1. Responses at the molecular level**

C y N in other metabolic pathways [95, 96].

during the absorption of electromagnetic radiation [14].

stresses that is expected to become increasingly common [10].

Transpiration is the most important component of water use by plants regarding volume. A typical wheat or corn crop requires 453 and 423 mm per season in the absence of water stress. Of this amount of water, 70% corresponds to transpiration [99]. Unless the temperature or the irradiance is reduced, the light interception is reduced, or the leaf albedo is increased, it is challenging to decrease the transpiration rate since the heat dissipation obtained through the transpiration avoids damages by high temperature in foliar metabolic components, especially those involved in photosynthesis, while on the other hand the decrease in stomatal conductance required to reduce transpiration would lead to a lower photosynthetic rate [68]. It was already mentioned in the subchapter dedicated to high irradiance stress the use of techniques such as increasing soil organic matter to release more CO<sup>2</sup> in the canopy, thus increasing water use efficiency, use of kaolinite as an anti-transpirant and reflector for increase leaf albedo, genetic selection of glossy varieties or to obtain plants with higher density of trichomes and a consequent greater leaf albedo. These same techniques are used to mitigate the water deficit in crops.

On the other hand water acts as a medium that provides mechanical support to herbaceous plants and photosynthetic and reproductive organs of shrub and tree plants, this is achieved by transporting water to the cells and apoplast to maintain cellular turgor. The water deficit causes loss of turgor that is perceived through mechanoreceptors that trigger part of the stress signaling pathways and ultimately cause the loss of green tissues [100]. The higher tolerance to turgidity loss is associated with changes in the composition and structure of the polymers of the cell wall, or due to the particular composition of the cell walls, as well as the 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 and growth.

One way to mitigate the adverse effect of a high VPD in a field is to increase the concentration of CO2 in the canopy of plants, as more elevated [CO2 ] results in an increase in photosynthetic 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 density crops allow higher [CO2 ] and decrease of evaporation in the soil, in addition to the buffer effect on the loss of moisture in the canopy by wind and convective processes caused by the proximity of other plants [30, 92].

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

$$
\Psi = \Psi\_{\rm g} + \Psi\_{\rm p} + \Psi\_{\rm o} + \Psi\_{\rm m} \tag{1}
$$

The use of grafts, although applied almost exclusively to horticultural species, offers good results mitigating damage by different stresses, mainly high temperature, salinity, water deficit, and root pathogens [105]. On the other hand, the use of fertilizers with silicon (Na<sup>2</sup>

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

200–800 kg ha−1 to the soil or 123 mg L−1 in the nutrient solution), selenium (10 g ha−1 to the soil or 0.5–3.0 mg L−1 by foliar spraying), or selenium and sulfur (as elemental sulfur S0, applying 20–80 kg ha−1 to the soil) decreases the negative responses of the plant to the water deficit. Although the mechanisms that explain the benefits of these elements in plants are still not well understood, their use has repeatedly been reported obtaining satisfactory results [106–108]. Both silicon and selenium, and sulfur can induce hardening in plants again stress

An effective alternative, although rarely used in field crops because of its high cost, is the application in furrows or seed beds, of biodegradable hydrophilic polymers such as single polyacrylamide (25–100 kg ha−1) or in combination with biochar [69]. Polyacrylamide has a shelf life of 3 years once it is applied to the soil and can absorb 100 or more times its weight in water, conserving the water in its molecular structure against leaching and evaporation processes. The effectiveness of hydrophilic polymers depends on the salinity of the water, being ineffective with electrical conductivities higher than 4000 μS cm−1 or with calcium-rich water [109]. Other techniques used in the field such as the use of natural or plastic mulching and the application of water using drip irrigation systems are also potentially useful as a means of increasing the efficiency of water use [110]. The use of the different techniques mentioned, using an integrated approach to improve the absorption and conservation of soil water, is the

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 deg-

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

caused by water deficit.

best recommendation.

**7. Salinity stress**

radation [111, 112].

SiO<sup>3</sup>

163

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The losses due to leaching (Ψ<sup>g</sup> ) and evapotranspiration of the soil and plants are in dynamic balance with the components that allow the conservation of the water, which are the matric or capillary potential (Ψm) and the osmotic potential (Ψ<sup>o</sup> ), which refer to the molecular interactions 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 micrometric.

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 of its biological activity [70].

The use of grafts, although applied almost exclusively to horticultural species, offers good results mitigating damage by different stresses, mainly high temperature, salinity, water deficit, and root pathogens [105]. On the other hand, the use of fertilizers with silicon (Na<sup>2</sup> SiO<sup>3</sup> 200–800 kg ha−1 to the soil or 123 mg L−1 in the nutrient solution), selenium (10 g ha−1 to the soil or 0.5–3.0 mg L−1 by foliar spraying), or selenium and sulfur (as elemental sulfur S0, applying 20–80 kg ha−1 to the soil) decreases the negative responses of the plant to the water deficit. Although the mechanisms that explain the benefits of these elements in plants are still not well understood, their use has repeatedly been reported obtaining satisfactory results [106–108]. Both silicon and selenium, and sulfur can induce hardening in plants again stress caused by water deficit.

An effective alternative, although rarely used in field crops because of its high cost, is the application in furrows or seed beds, of biodegradable hydrophilic polymers such as single polyacrylamide (25–100 kg ha−1) or in combination with biochar [69]. Polyacrylamide has a shelf life of 3 years once it is applied to the soil and can absorb 100 or more times its weight in water, conserving the water in its molecular structure against leaching and evaporation processes. The effectiveness of hydrophilic polymers depends on the salinity of the water, being ineffective with electrical conductivities higher than 4000 μS cm−1 or with calcium-rich water [109]. Other techniques used in the field such as the use of natural or plastic mulching and the application of water using drip irrigation systems are also potentially useful as a means of increasing the efficiency of water use [110]. The use of the different techniques mentioned, using an integrated approach to improve the absorption and conservation of soil water, is the best recommendation.
