**3. High irradiance stress**

the structure of the membranes, their integral proteins and their interaction with the cytoskeleton [15]. In other organizational scopes, such as ecosystems, similar phenomena have been described where disruption in some system components (by example, a decline in bio-

**Table 1.** Use of management strategies in different stages of cultivation for abiotic stress mitigation in plants.

The proper use of energy by a system is probably the primary process affected during a stressful situation. It is desirable and possible to moderate the damage caused by energy imbalance, not only at the molecular level but also in the description scope of cells, organisms, and ecosystems. At each level, appropriate measures would be applied, depending on the properties that can be manipulated in each scope. Each of the actions in the different fields would synergistically contribute to the mitigation of crop stress. These multiple approaches, which should ideally be comprehensive, contemplate different levels of description and response of the productive system and are expected to improve the ability to adapt and produce food

diversity) has a negative impact on energy efficiency [16, 17].

under the climate change scenarios [18].

**Pre-sowing Sowing or** 

148 Plant, Abiotic Stress and Responses to Climate Change

*Soil management:* • Organic and mineral amendments • Cover crops • Low tillage • Crop rotation

• Zeolites, nanofertilizers

• Transgenic crops

• Organic compounds • Beneficial elements

**transplanting**

*Use of soil microorganisms:*

*Use of genetically improved plants:*

*Tolerance inductors and elicitors*:

• Arbuscular mycorrhizal fungi and rhizobacteria

• Hybrid seeds (traditional breeding)

• Genetic modification (non-transgenic)

• Nanocompounds and nanofertilizers

*Soil management:* • Organic and mineral amendments • Low tillage • Nanofertilizers

**Crop growth Post-harvest Stress factor in which** 

**tolerance is induced**

High irradiance Water stress High temperature

Salinity

Water stress High temperature

Mineral deficiency

Mineral deficiency

Mineral deficiency

High irradiance Water stress High and low temperature Salinity

High irradiance Water stress High and low temperature Salinity

Salinity

The management of the stress condition due to high irradiance, which is very common in C3 crops, depends on two main factors: the capacity of the edaphic system to contribute CO<sup>2</sup> and water in the time of maximum irradiance, as well as the efficiency of the photochemical and biochemical dissipation processes that produce thermal energy and ROS, in addition to the plant ability to reduce the impact of the products of dissipative processes on the biochemical and physiological processes that determine growth and reproduction. For the first factor, the key to management is the soil condition, especially the content of organic matter and the promotion of the microbiome of plants. Also, other measures can be applied such as the reduction of tillage, the use of high-efficiency irrigation systems and the use of hydrophilic polymers. For the second factor, the ability (intrinsic, improved, or genetically modified) to tolerate the stress of each species or variety is considered, as well as the use of various substances or mineral elements that function as tolerance inducers (such as Si, Se, and various nanomaterials of Fe, Zn, etc.), antioxidants, and substances or materials that modify foliar reflectance or the use of radiation (**Figure 1**).

#### **3.1. Irradiance and CO2 availability**

Solar radiation is the primary source of energy for the photosynthetic process. With the current condition of atmospheric CO2 concentration (400 μL L−1), a significant part of the CO<sup>2</sup> used

controlled, cause a decrease in CO2

reduced or if the concentration of CO2

microbiome is a major source of CO<sup>2</sup>

canopy of the plants [30]. However, the CO2

the concentration of CO2

tion of the soil.

of CO2

as K, S, and P, causing nutritional imbalances in the plant [28].

of non-photochemical quenching, is to increase the availability of CO2

in the canopy [14].

by-product. As a consequence, the concentration of CO<sup>2</sup>

assimilation capacity, a response associated with the

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

151

in the canopy is increased and an adequate supply of

in the mesophyll of the

in the soil pores is very high

flow rate depends on the organic matter con-

as a

increase in the stomatal resistance derived from the high transpiration rate, the decline in chlorophyll concentration, the reduction in RUBISCO activity and in the quantum yield of photosystems [23]. In conjunction with increased metabolic expenditure and higher respiratory rate, the result is less availability of photosynthates for growth and defense, which in turn decreases the ability to tolerate other abiotic or biotic stresses [26]. In turn, the drop in the number of photosynthates has an adverse impact on the capacity of the plant to assimilate N, since under sufficient conditions much of the N absorbed is used to be incorporated into amino acids and proteins, where RUBISCO one of the most abundant [27]. This adverse effect on N assimilation subsequently decreases the uptake and assimilation of other nutrients such

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

As mentioned above, it is possible to alleviate stress induced by high PPDF if the irradiance is

water is ensured during the hours of greatest demand. In the photosynthetic process the CO2 is used as the sink of the reducing potential produced by the photochemical reactions, so the way to channel more energy toward the photochemical reactions, decreasing the counterpart

leaf. Under conditions of high irradiance, the only way to achieve this result is by increasing

The reduction of PPDF is possible in some crops using neutral or colored shade cloths that decrease PPFD in different percentages [29], usually 15, 20, or 30%. However, the large-scale use of shade cloths to reduce photosynthetic irradiance in species cultivated in extensive surfaces such as cereals seems unlikely, so a more feasible measure is the management of the soil carbon pool contained in organic matter, which through its transformation by edaphic

for plants [30].

The soil organic matter results from the transformation of the organic remains of living beings, being an important part the root remains and its exudates. A part of this organic matter is available to be metabolized by soil microorganisms, which produce CO<sup>2</sup>

(1500–6500 μL L−1) and moves through diffusion from the soil to the atmosphere and the

centration of the soil, which should be managed in the crop fields to values around 5% or more. The use of cover crops and the use of composites, biosolids or biochar incorporated into the soil are ways to increase soil organic matter [31]. Additionally, more organic matter in soil dilute the salts applied with the water and the fertilizers, mitigating the saliniza-

In soils with organic matter at low levels, the process of reaching the adequate concentration of this component of the soil can take years. However, it is possible to achieve the contribution

 through the soil with the use of humic substances added to the soil. These complex organic compounds provide other advantages such as increasing the availability of minerals to plants, promoting the microbiome of crops and reducing the susceptibility of plants to

**Figure 1.** Oxidative components of high irradiance damage (left), and the factors for crop mitigation.

as a carbon source during photosynthesis comes from soil respiration, and in many cases, CO2 deficiency is found in the canopy of plants during the hours when values of photosynthetic irradiance (PPFD) from 1800 to 2000 μmol m−2 s−1 are reached [22]. In this regard [23], mention that a PPFD of 600–800 μmol m−2 s−1 allow the adequate photosynthetic activity.

As PPFD values increase beyond 800 μmol m−2 s−1, in combination with a low CO2 content in the canopy, an increasing share of the energy captured by the leaves is not used for the photochemical reactions that produce ATP and NADPH<sup>2</sup> in the photosystems, but the excess energy is drifted toward the activation of O2 by the triplet chlorophyll of the antennas or reaction centers to produce singlet oxygen (1 O2 ), or is dissipated in processes of non-photochemical quenching that produce thermal radiation, fluorescence, or formation of free radicals. Singlet oxygen and other radicals such as superoxide (O2 − ) produced by photochemical systems can interact with membranes, proteins, and other biomolecules causing oxidative damage, which requires a high metabolic expenditure in the form of antioxidants for their control [23, 24]. On the other hand, the production of thermal energy increases the leaf temperature which promotes the loss of water by transpiration, increases the respiratory rate and decreases the volume of stored carbon available for growth [25]. This decline in leaf carbohydrates can have a profound impact on the nocturnal growth of the plant and the export of photosynthates to grains, fruits, and tubers.

The combination of high irradiance and low CO2 concentration in the canopy results in the induction of photo-oxidative damage and higher foliar temperature, which, if not adequately controlled, cause a decrease in CO2 assimilation capacity, a response associated with the increase in the stomatal resistance derived from the high transpiration rate, the decline in chlorophyll concentration, the reduction in RUBISCO activity and in the quantum yield of photosystems [23]. In conjunction with increased metabolic expenditure and higher respiratory rate, the result is less availability of photosynthates for growth and defense, which in turn decreases the ability to tolerate other abiotic or biotic stresses [26]. In turn, the drop in the number of photosynthates has an adverse impact on the capacity of the plant to assimilate N, since under sufficient conditions much of the N absorbed is used to be incorporated into amino acids and proteins, where RUBISCO one of the most abundant [27]. This adverse effect on N assimilation subsequently decreases the uptake and assimilation of other nutrients such as K, S, and P, causing nutritional imbalances in the plant [28].

As mentioned above, it is possible to alleviate stress induced by high PPDF if the irradiance is reduced or if the concentration of CO2 in the canopy is increased and an adequate supply of water is ensured during the hours of greatest demand. In the photosynthetic process the CO2 is used as the sink of the reducing potential produced by the photochemical reactions, so the way to channel more energy toward the photochemical reactions, decreasing the counterpart of non-photochemical quenching, is to increase the availability of CO2 in the mesophyll of the leaf. Under conditions of high irradiance, the only way to achieve this result is by increasing the concentration of CO2 in the canopy [14].

The reduction of PPDF is possible in some crops using neutral or colored shade cloths that decrease PPFD in different percentages [29], usually 15, 20, or 30%. However, the large-scale use of shade cloths to reduce photosynthetic irradiance in species cultivated in extensive surfaces such as cereals seems unlikely, so a more feasible measure is the management of the soil carbon pool contained in organic matter, which through its transformation by edaphic microbiome is a major source of CO<sup>2</sup> for plants [30].

as a carbon source during photosynthesis comes from soil respiration, and in many cases, CO2 deficiency is found in the canopy of plants during the hours when values of photosynthetic irradiance (PPFD) from 1800 to 2000 μmol m−2 s−1 are reached [22]. In this regard [23], mention

the canopy, an increasing share of the energy captured by the leaves is not used for the photo-

quenching that produce thermal radiation, fluorescence, or formation of free radicals. Singlet

interact with membranes, proteins, and other biomolecules causing oxidative damage, which requires a high metabolic expenditure in the form of antioxidants for their control [23, 24]. On the other hand, the production of thermal energy increases the leaf temperature which promotes the loss of water by transpiration, increases the respiratory rate and decreases the volume of stored carbon available for growth [25]. This decline in leaf carbohydrates can have a profound impact on the nocturnal growth of the plant and the export of photosynthates to

induction of photo-oxidative damage and higher foliar temperature, which, if not adequately

−

content in

in the photosystems, but the excess energy

) produced by photochemical systems can

concentration in the canopy results in the

by the triplet chlorophyll of the antennas or reaction

), or is dissipated in processes of non-photochemical

that a PPFD of 600–800 μmol m−2 s−1 allow the adequate photosynthetic activity.

**Figure 1.** Oxidative components of high irradiance damage (left), and the factors for crop mitigation.

O2

chemical reactions that produce ATP and NADPH<sup>2</sup>

oxygen and other radicals such as superoxide (O2

The combination of high irradiance and low CO2

is drifted toward the activation of O2

150 Plant, Abiotic Stress and Responses to Climate Change

centers to produce singlet oxygen (1

grains, fruits, and tubers.

As PPFD values increase beyond 800 μmol m−2 s−1, in combination with a low CO2

The soil organic matter results from the transformation of the organic remains of living beings, being an important part the root remains and its exudates. A part of this organic matter is available to be metabolized by soil microorganisms, which produce CO<sup>2</sup> as a by-product. As a consequence, the concentration of CO<sup>2</sup> in the soil pores is very high (1500–6500 μL L−1) and moves through diffusion from the soil to the atmosphere and the canopy of the plants [30]. However, the CO2 flow rate depends on the organic matter concentration of the soil, which should be managed in the crop fields to values around 5% or more. The use of cover crops and the use of composites, biosolids or biochar incorporated into the soil are ways to increase soil organic matter [31]. Additionally, more organic matter in soil dilute the salts applied with the water and the fertilizers, mitigating the salinization of the soil.

In soils with organic matter at low levels, the process of reaching the adequate concentration of this component of the soil can take years. However, it is possible to achieve the contribution of CO2 through the soil with the use of humic substances added to the soil. These complex organic compounds provide other advantages such as increasing the availability of minerals to plants, promoting the microbiome of crops and reducing the susceptibility of plants to certain pathogens [31]. The amounts of humic acids used are 5–15 kg ha−1, up to 50–300 kg ha−1 [32]; the application can be done using the irrigation system or mixed with fertilizers applied to the soil. In the case of soils with high amounts of calcium carbonate and high pH, the use of strong acids such as H2 SO<sup>4</sup> and HNO<sup>3</sup> (15 L ha−1 week−1) produces good results, since decreases soil pH and the reaction of acids with carbonates produces CO2 . In addition to strong acids, citric acid (10−4 M in nutrient solution or 30–90 kg ha−1) has also been used as well as humic substances [33].

The excess of PAR causes excessive production of reducing potential that is signaled by the

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

perception of these compounds or the byproducts of the interaction of the ROS with membranes and biomolecules, causes modifications in the programs of development directed to the defense against stress causing the synthesis of compounds and antioxidant proteins such as ascorbate, superoxide dismutase, and ascorbate peroxidase, osmolytes, chaperone proteins and in general chemical compounds that signal and combat the resulting oxidative damages such as salicylic acid, ABA, and glutathione [39]. An accumulation of anthocyanins and flavonoids is also present in the vacuole, which can absorb and dissipate excess PAR [40]. The activation of these responses to stress decreases the photosynthetic rate since it reduces the

Different techniques have been proposed for the management of damage caused by high irradiance, among which are the use of genetically modified plants [12], the application of antioxidant compounds, tolerance inducers such as proline and salicylic acid or its derivatives [42], the application of silicon or selenium to the soil or by foliar spraying [43, 44], the maintenance of adequate levels of foliar calcium [45], the use of diverse nanomaterials that besides serving as a nutrient for the plant induce stress tolerance [46], the use of beneficial microorganisms [36], the application of particle-films such as kaolinite which increase foliar reflectance [47, 48] or plant varieties with epidermis with high reflectance such as *glossy* sorghum [49]. As far as it is known, there are no

Regarding the need to have a greater volume of water in the soil, in order to counteract the higher rate of leaf transpiration resulting from the increase in leaf temperature, it was already mentioned that organic matter in the soil increases its storage and water retention capacity, but hydrophilic synthetic materials such as polyacrylamide (20–50 kg ha−1) or polyacrylamide combined with biochar are also available which significantly increase water storage capacity

Proper plant nutrition is a key factor for plants to have the resources for signaling and defense against stress induced by high irradiance. Adequate nutrition considers, on the one hand, the necessary amounts of nutrients in such a way as to cause excesses or deficiencies and, on the other hand, adequate nutrient balances in such a way that no induced deficiencies are generated. Taking into account that most of the indispensable mineral elements are directly involved in photosynthesis (Mn, Ca, Cl, Fe, Mg) and biochemical reactions (C, N, S, P, K, Ca, Zn, Mg) a key factor in stress management by high irradiance is based on adequate crop nutrition. When high-efficiency irrigation systems are available in soil-less systems, nutrient management can be carried out very precisely, but when the plants are grown in soil, soil characteristics, especially the amount of organic matter and the associated microbiome, regulate the availability of the mineral elements. In the latter case, management of plant nutrition through the management of soil organic matter would promote a resilient edaphic system

All of the aforementioned techniques have proven useful in mitigating damage caused by high irradiance in plants, but it should not be overlooked that the use of them is directed at the

techniques available to induce hardening in plants against high irradiance stress [10].

that allows the mineral elements necessary for plants to be available.

in soil or substrates [50].

O2 and 1 O2 . The 153

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

redox state of plastoquinone, thioredoxin, and by the generation of ROS, H<sup>2</sup>

synthesis of proteins related to ATP synthesis and the PSII complex [41].

Efficient use of the constant flow of CO<sup>2</sup> from the soil by plants is achieved by using highdensity plantings or, in the case of lower density planting, the use of dry straw coverings or other plant debris and the use of plastic mulching that concentrate the flow of the soil CO<sup>2</sup> in the canopy. Maintaining an adequate concentration of CO2 in the vegetable canopy improves stress tolerance caused by high irradiance [34].

Among the additional advantages of having an adequate amount of organic matter in soil are that it increases the water storage capacity of the soil, increases the availability of mineral elements such as P, S, and Fe, enhances the connectivity between different volumes of the soil which improves the mobility of dissolved minerals [35], a more abundant and more biodiverse microbiome is developed that, among other effects, decreases the susceptibility and the opportunity of invasion by phytopathogenic microorganisms, increases the availability of mineral nutrients of the plants through the association with symbionts and improves plant tolerance to stress through the production of growth regulators, and soluble or volatile metabolites [36].

Soils that have a suitable store of organic matter have the characteristic of behaving like a carbon sink, that is, part of their microbiome can induce the fixation of CO<sup>2</sup> using different metabolic processes. This process seems to establish a balance with the constant loss of CO2 by microbial respiration [37]. The stability of the soil organic matter store appears to be a characteristic of the ecosystem rather than a chemical characteristic of soil carbon compounds [31], which also points to the importance of maintaining the abundance and biodiversity of the edaphic microbiome, as well as to promote the rotation of crops and the biological diversity of vegetal components in the agricultural ecosystems. Among the processes that have a negative impact on the soil carbon store are photo-oxidation and oxidation induced by excessive tillage, which should be avoided as much as possible. Another factor that negatively affects soil organic matter is a high microbial respiratory rate caused by high diurnal and nocturnal temperatures or by the excessive application of fertilizers with N [38].

#### **3.2. Efficiency of the energy dissipation and redox balance processes**

It was explained earlier that when a condition of high irradiance and low availability of CO2 in the canopy prevails, a considerable part of the solar energy absorbed by the leaf is dissipated by photochemical systems in the form of thermal radiation and free radicals. Energy dissipation products disrupt cellular processes, causing the already mentioned adverse effects such as the decrease in the photosynthetic rate and the oxidative damage of cellular components.

The excess of PAR causes excessive production of reducing potential that is signaled by the redox state of plastoquinone, thioredoxin, and by the generation of ROS, H<sup>2</sup> O2 and 1 O2 . The perception of these compounds or the byproducts of the interaction of the ROS with membranes and biomolecules, causes modifications in the programs of development directed to the defense against stress causing the synthesis of compounds and antioxidant proteins such as ascorbate, superoxide dismutase, and ascorbate peroxidase, osmolytes, chaperone proteins and in general chemical compounds that signal and combat the resulting oxidative damages such as salicylic acid, ABA, and glutathione [39]. An accumulation of anthocyanins and flavonoids is also present in the vacuole, which can absorb and dissipate excess PAR [40]. The activation of these responses to stress decreases the photosynthetic rate since it reduces the synthesis of proteins related to ATP synthesis and the PSII complex [41].

certain pathogens [31]. The amounts of humic acids used are 5–15 kg ha−1, up to 50–300 kg ha−1 [32]; the application can be done using the irrigation system or mixed with fertilizers applied to the soil. In the case of soils with high amounts of calcium carbonate and high pH, the use of

citric acid (10−4 M in nutrient solution or 30–90 kg ha−1) has also been used as well as humic

density plantings or, in the case of lower density planting, the use of dry straw coverings or other plant debris and the use of plastic mulching that concentrate the flow of the soil CO<sup>2</sup>

Among the additional advantages of having an adequate amount of organic matter in soil are that it increases the water storage capacity of the soil, increases the availability of mineral elements such as P, S, and Fe, enhances the connectivity between different volumes of the soil which improves the mobility of dissolved minerals [35], a more abundant and more biodiverse microbiome is developed that, among other effects, decreases the susceptibility and the opportunity of invasion by phytopathogenic microorganisms, increases the availability of mineral nutrients of the plants through the association with symbionts and improves plant tolerance to stress through the production of growth regulators, and soluble or volatile

Soils that have a suitable store of organic matter have the characteristic of behaving like a

metabolic processes. This process seems to establish a balance with the constant loss of CO2

microbial respiration [37]. The stability of the soil organic matter store appears to be a characteristic of the ecosystem rather than a chemical characteristic of soil carbon compounds [31], which also points to the importance of maintaining the abundance and biodiversity of the edaphic microbiome, as well as to promote the rotation of crops and the biological diversity of vegetal components in the agricultural ecosystems. Among the processes that have a negative impact on the soil carbon store are photo-oxidation and oxidation induced by excessive tillage, which should be avoided as much as possible. Another factor that negatively affects soil organic matter is a high microbial respiratory rate caused by high diurnal and nocturnal

It was explained earlier that when a condition of high irradiance and low availability of CO2 in the canopy prevails, a considerable part of the solar energy absorbed by the leaf is dissipated by photochemical systems in the form of thermal radiation and free radicals. Energy dissipation products disrupt cellular processes, causing the already mentioned adverse effects such as the decrease in the photosynthetic rate and the oxidative damage of cellular

carbon sink, that is, part of their microbiome can induce the fixation of CO<sup>2</sup>

temperatures or by the excessive application of fertilizers with N [38].

**3.2. Efficiency of the energy dissipation and redox balance processes**

(15 L ha−1 week−1) produces good results, since decreases

from the soil by plants is achieved by using high-

. In addition to strong acids,

in the vegetable canopy improves

in

using different

by

strong acids such as H2

substances [33].

metabolites [36].

components.

SO<sup>4</sup>

Efficient use of the constant flow of CO<sup>2</sup>

152 Plant, Abiotic Stress and Responses to Climate Change

stress tolerance caused by high irradiance [34].

and HNO<sup>3</sup>

soil pH and the reaction of acids with carbonates produces CO2

the canopy. Maintaining an adequate concentration of CO2

Different techniques have been proposed for the management of damage caused by high irradiance, among which are the use of genetically modified plants [12], the application of antioxidant compounds, tolerance inducers such as proline and salicylic acid or its derivatives [42], the application of silicon or selenium to the soil or by foliar spraying [43, 44], the maintenance of adequate levels of foliar calcium [45], the use of diverse nanomaterials that besides serving as a nutrient for the plant induce stress tolerance [46], the use of beneficial microorganisms [36], the application of particle-films such as kaolinite which increase foliar reflectance [47, 48] or plant varieties with epidermis with high reflectance such as *glossy* sorghum [49]. As far as it is known, there are no techniques available to induce hardening in plants against high irradiance stress [10].

Regarding the need to have a greater volume of water in the soil, in order to counteract the higher rate of leaf transpiration resulting from the increase in leaf temperature, it was already mentioned that organic matter in the soil increases its storage and water retention capacity, but hydrophilic synthetic materials such as polyacrylamide (20–50 kg ha−1) or polyacrylamide combined with biochar are also available which significantly increase water storage capacity in soil or substrates [50].

Proper plant nutrition is a key factor for plants to have the resources for signaling and defense against stress induced by high irradiance. Adequate nutrition considers, on the one hand, the necessary amounts of nutrients in such a way as to cause excesses or deficiencies and, on the other hand, adequate nutrient balances in such a way that no induced deficiencies are generated. Taking into account that most of the indispensable mineral elements are directly involved in photosynthesis (Mn, Ca, Cl, Fe, Mg) and biochemical reactions (C, N, S, P, K, Ca, Zn, Mg) a key factor in stress management by high irradiance is based on adequate crop nutrition. When high-efficiency irrigation systems are available in soil-less systems, nutrient management can be carried out very precisely, but when the plants are grown in soil, soil characteristics, especially the amount of organic matter and the associated microbiome, regulate the availability of the mineral elements. In the latter case, management of plant nutrition through the management of soil organic matter would promote a resilient edaphic system that allows the mineral elements necessary for plants to be available.

All of the aforementioned techniques have proven useful in mitigating damage caused by high irradiance in plants, but it should not be overlooked that the use of them is directed at the treatment of symptoms, while the primary reason (the combination of high irradiance with little CO<sup>2</sup> in the canopy) is the one that should receive more attention.

On the other hand, at the molecular level, all the membranes, biomolecules and water associated with these structures have a particular temperature spectrum where the tertiary and quaternary structure of proteins, nucleic acids, and membranes is maintained optimally, as well as the cohesion between biomolecules in multiprotein complexes and protein-nucleic acid complexes. Particularly for the membranes temperature determines an important characteristic known as fluidity, on which the interaction of the integral proteins depends, many of them are sensors or participants in the energy metabolism. However, when it occurs that the thermal vibration exceeds the forces of interaction and cohesion, the result is that the functionality of the biomolecules or their complexes is compromised, initially diminishing their catalytic or functional capacity until they reach inactivation or denaturation when it is exceeded a certain threshold. Similarly, the proteins that are synthesized during the hightemperature period can suffer from misfolding, which makes them non-functional [56]. For terrestrial plants, the threshold where the temperature begins to negatively affect the biomolecules and the processes of interaction between them, is between 35 and 45°C, depending on the specific metabolic activity and the adaptation of each species to a particular environment. For most C3 plants the threshold is at 35°C, whereas C4 species have a response threshold around 40°C. Much of this difference between the two physiological groups is that the solubil-

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

When the temperature exceeds the threshold mentioned above, changes in membrane fluidity, or the lower effectiveness or inactivation of biomolecules associated with electron transport in energy and biosynthetic metabolism, causes the production of ROS in large quantities, leading to oxidative stress. Along with changes in membrane fluidity, ROS production is one of the factors perceived at the molecular level that triggers defense responses against high temperature. It has been observed that acclimatization in plants is possible by exposing them to 5–8°C above the optimal temperature for their growth and development, generating changes in the gene expression associated with the modification of the composition of the membranes, to the production of enzymatic and non-enzymatic antioxidants and osmolytes. For this reason, the exogenous application of some osmolytes and growth regulators such as proline, glycine betaine, salicylic acid, jasmonic acid, IAA, GA, and ABA or the use of sodium

Another response at the molecular level triggered by high temperature is the induction of the synthesis of proteins called heat shock (HSPs). These constitute a family of low molecular weight proteins of 15–30 kDa. HSPs accumulate as granular structures in the cytoplasm protecting the mechanisms of protein synthesis [58]. HSPs work by allowing the appropriate post-transcriptional folding of the new proteins or by maintaining the existing proteins in a functional state [56]. Plant cells respond rapidly to high-temperature stress by accumulating HSPs which in turn trigger increased expression of additional genes related to stress mitigation, whose products can act as chaperonins to stabilize proteins by protecting them from denaturation [59]. This fact indicates that it is possible to obtain thermotolerance by stimulating the accumulation of HSPs, either with the use of genetically modified crops or with the

O2

and salicylic acid [60].

as the temperature rises, and that RUBISCO shows

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155

ity of CO2

higher affinity for O<sup>2</sup>

declines faster than that of O2

at high temperatures [23].

selenite is useful in the mitigation of high-temperature stress [57].

application of tolerance inducers such as H2

On the other hand, it is important to consider that stress due to high irradiance is a characteristic intrinsic to current atmospheric conditions. When the first terrestrial plants made their appearance during the Ordovician, the sun brightness was lower than in the present time, and the CO2 concentration in the air was 14–22 times greater than the current one [51]. That means that existing plants, with virtually the same photosynthetic systems that originated in bacteria billions of years ago and reconfigured to the present form hundreds of millions of years ago, face a condition where it is practically impossible to avoid saturation by light. However, creating plants that have a better adaptation and response to environmental stressors is not an impossibility [52], but will require a great investment of time, human and material resources, with the support of data science, systems biology, synthetic biology, ecology and soil science, among others.
