**1. Introduction**

Global climate change means that extreme environmental conditions are now being experienced more frequently. The Intergovernmental Panel on Climate Change [1] has suggested that global warming increases the incidence of various natural disasters, such as extreme temperatures, flood, and drought; agriculture is particularly susceptible to the influence of such events, because plants are organisms that show great sensitivity to changes in their environment.

As sessile organisms, plants are constantly exposed to widely varying and unfavorable environmental conditions, such as drought and extreme temperatures, which are major limiting factors in crop production [2]. Plants therefore have an inherently complicated response

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

mechanism against environmental stresses, including developmental, physiological, and biochemical changes that are regulated by abiotic-related gene expression. In this response process, environmental physical stimuli are perceived and transduced to biochemical processes, resulting in the induction of a series of abiotic stress-related gene expressions. The involvement of chemicals such as phytohormones, abscisic acid (ABA), salicylic acid (SA), jasmonic acid (JA), and ethylene has been shown to be important in the stress signaling process [3]. In addition, recent research suggests that a central role in the various causes of environmental stress is played by oxidized chemicals, which are produced in response to oxidative stress, an unavoidable stress for plants.

by physicochemical reactions. On the other hand, the Calvin-Benson cycle is a complex process comprising various enzymes; its rate is therefore restricted by enzymatic properties, such as the maximal velocity (Vmax) value of each enzyme and the rate-limiting steps, and eventually reaches a plateau. Under balanced conditions such as low and moderate light intensity conditions, the quantity of NADPH and ATP supplied via the photosystem is almost equal to the NADP<sup>+</sup>

Integrated Chemical Control of Abiotic Stress Tolerance Using Biostimulants

http://dx.doi.org/10.5772/intechopen.74214

ADP returned from the Calvin-Benson cycle; this is not, however, the case under conditions of high light intensity, such as sunny weather. High light levels enhance photochemical reac-

under drought stress conditions, and its enzyme activity reduces under heat or cold conditions. When facing these stresses, the NADPH and ATP supplied by photosystem and their consumption in the Calvin-Benson cycle become imbalanced. This "energy gap," that is, the difference between energy supply and consumption, is usually eliminated by thermal energy dissipation.

as well as enzyme inactivation during heat or cold stress, causes the rate of the Calvin-Benson cycle to lower, thereby increasing the energy gap. When this gap exceeds the capacity required for thermal energy to dissipate, the excess energy causes the production of ROS, potentially

Consequently, disturbance of the photochemical reaction leads to the production of ROS, an

heat and cold stress, and a combination of these conditions with high light intensity [4, 5]. ROS are primarily toxic compounds that damage cellular components because of their high reactivity, resulting in a decrease in plant production. Under oxidative stress conditions, ROS attack polyunsaturated fatty acids (PUFAs) in the thylakoid membrane; PUFAs are easily oxidized by ROS, releasing various degraded products. Malondialdehyde, which is representative of these degraded products and is easily produced by the oxidation of PUFAs [6], chemically modifies proteins, especially in conditions of high light intensity and heat stress [7]. Decreases in photosynthetic activity are partly due to the modification of malondialdehyde by reaction center proteins in photosystem II [8]. On the other hand, ROS [9], ROS-related chemicals such as carotenoid oxidation products [10], and lipophilic reactive electrophilic species [11] are recognized as important signal chemicals involved in the responses to environmental stress.

**2. Potential chemicals involved in abiotic stress responses and their** 

As described above, chloroplasts are the organelles that are most susceptible to damage under conditions of oxidative stress. Therefore, chloroplasts are also potential sensors of environmental stress, assimilating environmental changes, and transmitting information about the changes to other organelles using infochemicals. Recently, we have found evidence to support the premise that chloroplasts produce signal chemicals that induce gene expression and

**2.1. Reactive short-chain leaf volatiles as potential signaling chemicals**

deficiency due to stomatal closure under conditions of drought or high salinity,

tions in the photosystem; in contrast, the Calvin-Benson cycle is inhibited by CO<sup>2</sup>

However, CO<sup>2</sup>

damaging many bioprocesses.

**use as biostimulants**

enhance stress tolerance.

effect that is further enhanced by conditions limiting CO<sup>2</sup>

and

135

deficiency

fixation, such as drought, salinity,

Stress-related disturbance of the metabolic balance in oxidative organelles often results in the enhanced production of reactive oxygen species (ROS) [4]. The sensitivity of plants to environmental stress partly arises because the cause of the damage derived from almost all abiotic stressors is related to photosynthesis. In terms of plant energy metabolism, photosynthesis is the process that is most sensitive in the presence of abiotic stress, because any imbalance between energy production in photochemical reactions and energy consumption in the Calvin-Benson cycle is often a result. As shown in **Figure 1**, the rate of photochemical reactions is almost dependent on a linear function with light intensity, because the photochemical reactions are mirrored

**Figure 1.** Chloroplasts comprise the most sensitive site in plants in responding to various environmental stresses. (A) Photosynthesis comprises two distinct processes: the photochemical reaction mediated by the photosystem, and CO<sup>2</sup> assimilation mediated by the Calvin-Benson cycle. Under environmental stresses such as high light levels, drought and temperature stresses, the NADPH and ATP supplied by photosystem and their consumption in the Calvin-Benson cycle become imbalanced [4]. (B) The energy gap between the photosystem and Calvin-Benson cycle is normally eliminated by thermal dissipation, but the energy imbalance occurred under environmental stresses enlarges the energy gap, often exceeding the dissipation capacity.

by physicochemical reactions. On the other hand, the Calvin-Benson cycle is a complex process comprising various enzymes; its rate is therefore restricted by enzymatic properties, such as the maximal velocity (Vmax) value of each enzyme and the rate-limiting steps, and eventually reaches a plateau. Under balanced conditions such as low and moderate light intensity conditions, the quantity of NADPH and ATP supplied via the photosystem is almost equal to the NADP<sup>+</sup> and ADP returned from the Calvin-Benson cycle; this is not, however, the case under conditions of high light intensity, such as sunny weather. High light levels enhance photochemical reactions in the photosystem; in contrast, the Calvin-Benson cycle is inhibited by CO<sup>2</sup> deficiency under drought stress conditions, and its enzyme activity reduces under heat or cold conditions. When facing these stresses, the NADPH and ATP supplied by photosystem and their consumption in the Calvin-Benson cycle become imbalanced. This "energy gap," that is, the difference between energy supply and consumption, is usually eliminated by thermal energy dissipation. However, CO<sup>2</sup> deficiency due to stomatal closure under conditions of drought or high salinity, as well as enzyme inactivation during heat or cold stress, causes the rate of the Calvin-Benson cycle to lower, thereby increasing the energy gap. When this gap exceeds the capacity required for thermal energy to dissipate, the excess energy causes the production of ROS, potentially damaging many bioprocesses.

mechanism against environmental stresses, including developmental, physiological, and biochemical changes that are regulated by abiotic-related gene expression. In this response process, environmental physical stimuli are perceived and transduced to biochemical processes, resulting in the induction of a series of abiotic stress-related gene expressions. The involvement of chemicals such as phytohormones, abscisic acid (ABA), salicylic acid (SA), jasmonic acid (JA), and ethylene has been shown to be important in the stress signaling process [3]. In addition, recent research suggests that a central role in the various causes of environmental stress is played by oxidized chemicals, which are produced in response to oxidative stress, an

Stress-related disturbance of the metabolic balance in oxidative organelles often results in the enhanced production of reactive oxygen species (ROS) [4]. The sensitivity of plants to environmental stress partly arises because the cause of the damage derived from almost all abiotic stressors is related to photosynthesis. In terms of plant energy metabolism, photosynthesis is the process that is most sensitive in the presence of abiotic stress, because any imbalance between energy production in photochemical reactions and energy consumption in the Calvin-Benson cycle is often a result. As shown in **Figure 1**, the rate of photochemical reactions is almost dependent on a linear function with light intensity, because the photochemical reactions are mirrored

**Figure 1.** Chloroplasts comprise the most sensitive site in plants in responding to various environmental stresses. (A) Photosynthesis comprises two distinct processes: the photochemical reaction mediated by the photosystem, and CO<sup>2</sup> assimilation mediated by the Calvin-Benson cycle. Under environmental stresses such as high light levels, drought and temperature stresses, the NADPH and ATP supplied by photosystem and their consumption in the Calvin-Benson cycle become imbalanced [4]. (B) The energy gap between the photosystem and Calvin-Benson cycle is normally eliminated by thermal dissipation, but the energy imbalance occurred under environmental stresses enlarges the energy gap, often

unavoidable stress for plants.

134 Plant, Abiotic Stress and Responses to Climate Change

exceeding the dissipation capacity.

Consequently, disturbance of the photochemical reaction leads to the production of ROS, an effect that is further enhanced by conditions limiting CO<sup>2</sup> fixation, such as drought, salinity, heat and cold stress, and a combination of these conditions with high light intensity [4, 5]. ROS are primarily toxic compounds that damage cellular components because of their high reactivity, resulting in a decrease in plant production. Under oxidative stress conditions, ROS attack polyunsaturated fatty acids (PUFAs) in the thylakoid membrane; PUFAs are easily oxidized by ROS, releasing various degraded products. Malondialdehyde, which is representative of these degraded products and is easily produced by the oxidation of PUFAs [6], chemically modifies proteins, especially in conditions of high light intensity and heat stress [7]. Decreases in photosynthetic activity are partly due to the modification of malondialdehyde by reaction center proteins in photosystem II [8]. On the other hand, ROS [9], ROS-related chemicals such as carotenoid oxidation products [10], and lipophilic reactive electrophilic species [11] are recognized as important signal chemicals involved in the responses to environmental stress.
