**4. High temperature stress**

Currently temperate and subtropical agricultural areas can withstand substantial losses in crop yield due to extreme temperature events [53]. Due to global climate change, a high temperature is projected to be a very relevant abiotic stress factor since it adversely affects plant growth and hence crop yields [54]. Although plants may be more or less resistant to high temperatures depending on the adaptations of each species and geographic location, almost all show a reduction in their growth as a result of unexpected extreme temperature fluctuations [55].

As with high irradiance, stress caused by high temperature can be described at the molecular level or in the cellular or physiological-morphological ambits. In this case, the primary inducing factor is the impact of temperature at the molecular level, which is transferred to other scopes of description such as physiological-morphological interactions between the biomolecules that give rise to different metabolic pathways and activities of cellular metabolism such as energy metabolism and the transport of ions and metabolites. This fact results in the hardening or cross-resistance process being difficult to achieve for stress induced by high temperatures [10].

#### **4.1. Responses at molecular level**

From a physical point of view, the temperature is an indicator of the average speed (kinetic energy) of translation or vibration of the molecules that make up the matter. To operate the biochemical reactions biomolecules require a certain temperature to ensure contact with the substrates or receptors with which they interact. This reaction capacity occurs at a very low rate when the temperature falls below a certain threshold (usually 10°C) or occurs at high or excessive rates with temperatures >35°C.

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 solubility of CO2 declines faster than that of O2 as the temperature rises, and that RUBISCO shows higher affinity for O<sup>2</sup> at high temperatures [23].

treatment of symptoms, while the primary reason (the combination of high irradiance with

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

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,

Currently temperate and subtropical agricultural areas can withstand substantial losses in crop yield due to extreme temperature events [53]. Due to global climate change, a high temperature is projected to be a very relevant abiotic stress factor since it adversely affects plant growth and hence crop yields [54]. Although plants may be more or less resistant to high temperatures depending on the adaptations of each species and geographic location, almost all show a reduction in their growth as a result of unexpected extreme temperature fluctua-

As with high irradiance, stress caused by high temperature can be described at the molecular level or in the cellular or physiological-morphological ambits. In this case, the primary inducing factor is the impact of temperature at the molecular level, which is transferred to other scopes of description such as physiological-morphological interactions between the biomolecules that give rise to different metabolic pathways and activities of cellular metabolism such as energy metabolism and the transport of ions and metabolites. This fact results in the hardening or cross-resistance process being difficult to achieve for stress induced by high

From a physical point of view, the temperature is an indicator of the average speed (kinetic energy) of translation or vibration of the molecules that make up the matter. To operate the biochemical reactions biomolecules require a certain temperature to ensure contact with the substrates or receptors with which they interact. This reaction capacity occurs at a very low rate when the temperature falls below a certain threshold (usually 10°C) or occurs at high or

concentration in the air was 14–22 times greater than the current one [51]. That means

in the canopy) is the one that should receive more attention.

little CO<sup>2</sup>

the CO2

among others.

tions [55].

temperatures [10].

**4.1. Responses at molecular level**

excessive rates with temperatures >35°C.

**4. High temperature stress**

154 Plant, Abiotic Stress and Responses to Climate Change

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

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 application of tolerance inducers such as H2 O2 and salicylic acid [60].

Another alternative for handling high-temperature stress in field crops is the use of chemical elements in the nanometric form. The application of nTiO2 [61], CeO2 [62], and nSe [63], 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 common, the use of nanomaterials deserves further exploration.

As we have seen, there is a high amount of work that describes the adjustments made by plants against heat, both at the molecular level with studies in the transcriptome and proteome as well as in the metabolic and physiological field with biochemical studies of specific metabolic pathways, physiological studies of photosynthesis, respiration and growth [56, 57]. However, an important aspect to which less attention is given is that in other scopes of description, for example in ecosystems the high temperatures also impose modifications in the interactions of its components, causing changes in the structure and dynamics, it is still

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

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

157

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

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].

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

poorly understood and difficult to predict [73].

**5. Low temperature stress**

**5.1. Responses at the molecular level**

begin [15].

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

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 other metabolic pathways.

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

As mentioned, photosynthesis is more affected in C3 plants than in C4 plants due to the high temperature, and again the concentration of CO2 in the canopy is an important factor to mitigate the damage caused by high temperatures. The higher concentration of CO2 allows 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 in 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 consequence, more water is available to be absorbed by plants [69].

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

As we have seen, there is a high amount of work that describes the adjustments made by plants against heat, both at the molecular level with studies in the transcriptome and proteome as well as in the metabolic and physiological field with biochemical studies of specific metabolic pathways, physiological studies of photosynthesis, respiration and growth [56, 57]. However, an important aspect to which less attention is given is that in other scopes of description, for example in ecosystems the high temperatures also impose modifications in the interactions of its components, causing changes in the structure and dynamics, it is still poorly understood and difficult to predict [73].
