**6. Drought stress**

Drought is a common occurrence for many areas that recurs without noticeable regularity. Drought is one of the abiotic factors that have the greatest impact on agricultural production. It occurs when soil moisture decreases to an amount that negatively affects the yield and profitability of agricultural production [57]. Drought as an abiotic stress can directly affect agricultural production and even in some extreme situations lead to the complete destruction of yields. It is very important to distinguish the meaning of drought in agronomy and the definitions of drought in meteorology, hydrology, and the socio-economic concept of drought. The amount of physiologically active water in the soil, which is the only available water for plants, and the ratio of capillary and noncapillary pores in the soil are also important. The water content in the soil also depends on the texture of a particular soil type (i.e., on the water balance in the soil; fine sand has a lower possibility of water retention than clay loam). The ratio of humus in the soil is significant, because it has a great ability to absorb water (it acts like a sponge). The term drought should not be confused with the term "aridity," which refers to the permanent property of a naturally dry (waterless, arid) climate. A dry or dry area ("aryland") is an environment that was constantly, seasonally, or occasionally exposed to a significant lack of moisture. It was estimated that about 36% of the planet's continental surface, which is approximately 45 million km2 , can be classified as dry area. It is estimated that between 15 and 21% of the earth's population live in this area [4].

Regarding the extreme climatic conditions and rising temperatures, the need for irrigation of agricultural land is increasing; according to some statistic report, as much as 69% of drinking water is used in agriculture (irrigation of plants, watering livestock, etc.). FAO estimates that water consumption for plantation irrigation will increase by 5.5% from 2008 to 2050. These data show that the world's demand for water is increasing, and the increase in agricultural production results in higher consumption of drinking water. In order to reduce the need for irrigation, scientists studied the drought resistance of plants and sought to investigate the associated adaptation mechanisms. Geneticists are already creating varieties that contain drought-resistant genes; however, drought tolerance is only possible up to a certain percentage of moisture reduction. For example, chickpea (*Cicero arietinum* L.) is extremely drought resistant, its root is spindle-shaped and branched, and it reaches a depth of 1 m where there is a higher amount of moisture in the soil, and also many Mediterranean plant species are resistant to both high temperatures and lack of moisture in the soil.

Different time of drought periods also cause different changes in plants. Some plants increases synthesis of secondary metabolite as a response to drought occurrence resulting in leaf or fruit abscission and leading a plant cell, tissue, or organs to death (**Figure 8**). Transient wilting occurs during the hottest part of the day, when there is increased transpiration in plants. Permanent wilting occurs due to the low water content in the soil, root hairs die off, which blocks the plant's connection with nutrients from the soil (lost water can no longer be compensated). It is necessary to distinguish: desiccation (delay of drying ability to maintain hydrated tissue), tolerance to drying (retention of functions during drying), and avoidance of drought completion of the plant life cycle before drought occurs [13]. The point of wilting is the phase in which the plant begins to die, resulting in permanent death, and the plant when it reaches this stage (critical lack of moisture depends on the plant

**97**

**Figure 8.**

*Water Plant and Soil Relation under Stress Situations DOI: http://dx.doi.org/10.5772/intechopen.93528*

*avoid excess transpiration rate under stress condition (right).*

species and type of cultivar) will not recover after irrigation. When a critical water deficit occurs, necrotic changes first appear on the plants, and finally, the lethal phase occurs. Plants have the greatest need for water during the growth phase, and during fruit formation, when plants are most sensitive to drought. According to the requirements of plants against water, they are divided into three main groups: hygrophytes (wetland plants), mesophytes (plants of temperate areas, which include most agricultural plants), and xerophytes (plants of arid areas).

*Citrus fruit abscission under the drought occurrence in critical phase (left) and leaf scrolling in tomato plant to* 

One of the first defense mechanisms of the plant on drought is the reduction of the leaf surface, because with the reduction of water in the soil, the turgor pressure in the cells decreases, which ultimately results in a decrease in the concentration of cell content and cell volume. Turgor affects cell growth, and by reducing it, cell growth also decreases affecting the growth of the leaf surface and thus transpiration. Plants can reduce the surface area of transpiration by leaf scrolling and abscission (leaf rejection) or increased secretion of ethylene, which affects cell death. One of the most effective adaptations to drought is the closure of the stomata, to prevent further dehydration, this mechanism occurs when the plants have already fully developed their leaf surface. However, many plants lose water through the stomata because they remain open, and much of the water is lost through the

Although it can be found in almost all parts of the world, the characteristics of drought vary from region to region. Defining drought is therefore difficult and depends on regional differences and needs, but also on the perspective from which this phenomenon is viewed. Regardless of the needs for which drought is defined, it is necessary that this definition includes the deviation of the current relationship between precipitation and evapo-transpiration in an area from the normal value of this relationship determined for a multi-year data set. It is also important to take into account the time distribution (precipitation regime, delay of the beginning of the rainy season, the relationship between precipitation, and phenological phases of the most important field crops in the observed area), as well as precipitation efficiency (precipitation intensity and number of rain episodes). Other climatic factors, such as high temperatures, high wind speeds and strengths, and low relative humidity, are often associated with drought in many parts of the world and can significantly worsen its consequences (**Figure 9**). Drought is an insidious natural disaster that, unlike other natural disasters, occurs slowly, lasts a long time, and affects large areas. It can be considered from four aspects (meteorological, hydro-

epidermis by the cuticle, especially if the cuticle is thin.

logical, agricultural, and socio-economic).

#### **Figure 8.**

*Soil Moisture Importance*

**6. Drought stress**

is approximately 45 million km2

between 15 and 21% of the earth's population live in this area [4].

Regarding the extreme climatic conditions and rising temperatures, the need for irrigation of agricultural land is increasing; according to some statistic report, as much as 69% of drinking water is used in agriculture (irrigation of plants, watering livestock, etc.). FAO estimates that water consumption for plantation irrigation will increase by 5.5% from 2008 to 2050. These data show that the world's demand for water is increasing, and the increase in agricultural production results in higher consumption of drinking water. In order to reduce the need for irrigation, scientists studied the drought resistance of plants and sought to investigate the associated adaptation mechanisms. Geneticists are already creating varieties that contain drought-resistant genes; however, drought tolerance is only possible up to a certain percentage of moisture reduction. For example, chickpea (*Cicero arietinum* L.) is extremely drought resistant, its root is spindle-shaped and branched, and it reaches a depth of 1 m where there is a higher amount of moisture in the soil, and also many Mediterranean plant species are resistant to both high temperatures and lack of moisture in the soil.

Different time of drought periods also cause different changes in plants. Some plants increases synthesis of secondary metabolite as a response to drought occurrence resulting in leaf or fruit abscission and leading a plant cell, tissue, or organs to death (**Figure 8**). Transient wilting occurs during the hottest part of the day, when there is increased transpiration in plants. Permanent wilting occurs due to the low water content in the soil, root hairs die off, which blocks the plant's connection with nutrients from the soil (lost water can no longer be compensated). It is necessary to distinguish: desiccation (delay of drying ability to maintain hydrated tissue), tolerance to drying (retention of functions during drying), and avoidance of drought completion of the plant life cycle before drought occurs [13]. The point of wilting is the phase in which the plant begins to die, resulting in permanent death, and the plant when it reaches this stage (critical lack of moisture depends on the plant

and called physiologically active water. Within this interval, not all water is equally accessible, so the soil should maintain a moisture state between the water capacity and the lentocapillary point, which corresponds to the optimal moisture interval.

Drought is a common occurrence for many areas that recurs without noticeable regularity. Drought is one of the abiotic factors that have the greatest impact on agricultural production. It occurs when soil moisture decreases to an amount that negatively affects the yield and profitability of agricultural production [57]. Drought as an abiotic stress can directly affect agricultural production and even in some extreme situations lead to the complete destruction of yields. It is very important to distinguish the meaning of drought in agronomy and the definitions of drought in meteorology, hydrology, and the socio-economic concept of drought. The amount of physiologically active water in the soil, which is the only available water for plants, and the ratio of capillary and noncapillary pores in the soil are also important. The water content in the soil also depends on the texture of a particular soil type (i.e., on the water balance in the soil; fine sand has a lower possibility of water retention than clay loam). The ratio of humus in the soil is significant, because it has a great ability to absorb water (it acts like a sponge). The term drought should not be confused with the term "aridity," which refers to the permanent property of a naturally dry (waterless, arid) climate. A dry or dry area ("aryland") is an environment that was constantly, seasonally, or occasionally exposed to a significant lack of moisture. It was estimated that about 36% of the planet's continental surface, which

, can be classified as dry area. It is estimated that

**96**

*Citrus fruit abscission under the drought occurrence in critical phase (left) and leaf scrolling in tomato plant to avoid excess transpiration rate under stress condition (right).*

species and type of cultivar) will not recover after irrigation. When a critical water deficit occurs, necrotic changes first appear on the plants, and finally, the lethal phase occurs. Plants have the greatest need for water during the growth phase, and during fruit formation, when plants are most sensitive to drought. According to the requirements of plants against water, they are divided into three main groups: hygrophytes (wetland plants), mesophytes (plants of temperate areas, which include most agricultural plants), and xerophytes (plants of arid areas).

One of the first defense mechanisms of the plant on drought is the reduction of the leaf surface, because with the reduction of water in the soil, the turgor pressure in the cells decreases, which ultimately results in a decrease in the concentration of cell content and cell volume. Turgor affects cell growth, and by reducing it, cell growth also decreases affecting the growth of the leaf surface and thus transpiration. Plants can reduce the surface area of transpiration by leaf scrolling and abscission (leaf rejection) or increased secretion of ethylene, which affects cell death. One of the most effective adaptations to drought is the closure of the stomata, to prevent further dehydration, this mechanism occurs when the plants have already fully developed their leaf surface. However, many plants lose water through the stomata because they remain open, and much of the water is lost through the epidermis by the cuticle, especially if the cuticle is thin.

Although it can be found in almost all parts of the world, the characteristics of drought vary from region to region. Defining drought is therefore difficult and depends on regional differences and needs, but also on the perspective from which this phenomenon is viewed. Regardless of the needs for which drought is defined, it is necessary that this definition includes the deviation of the current relationship between precipitation and evapo-transpiration in an area from the normal value of this relationship determined for a multi-year data set. It is also important to take into account the time distribution (precipitation regime, delay of the beginning of the rainy season, the relationship between precipitation, and phenological phases of the most important field crops in the observed area), as well as precipitation efficiency (precipitation intensity and number of rain episodes). Other climatic factors, such as high temperatures, high wind speeds and strengths, and low relative humidity, are often associated with drought in many parts of the world and can significantly worsen its consequences (**Figure 9**). Drought is an insidious natural disaster that, unlike other natural disasters, occurs slowly, lasts a long time, and affects large areas. It can be considered from four aspects (meteorological, hydrological, agricultural, and socio-economic).

**Figure 9.**

*Current trends of most abundant abiotic stress occurrence in agriculture sector estimated for around 50% of world's arable land.*

Short-term water shortage over a period of several weeks in the surface layer of the soil, which occurs at a critical time for plant development, can cause agronomic drought. The agronomic droughts may lag behind meteorological droughts, depending on the condition of the surface layer soil. High temperatures, low relative humidity, and wind amplify the negative consequences agronomic droughts. The agronomic droughts, precipitation deficits are taken into account along with the physical and biological aspects of plants, interactions within the soil-plantatmosphere system and the balance between plants' water needs and available water reserves, which may result in declining yields. Beside agronomical drought, we can also distinct the meteorological and hydrological drought.

Meteorological drought occurs as a consequence of lack or complete absence of precipitation over a long period of time in a certain area. This deficiency is defined as the deviation of precipitation from normal, that is, from the multi-year average. Meteorological drought can develop abruptly and stop abruptly.

Deficit of precipitation over a long period of time affects surface and groundwater supplies: to the flow of water in rivers and streams, level of water in lakes, and level of groundwater. When flows and levels decrease, we talk about hydrological drought. The onset of hydrological drought may lag a few months behind the beginning of the meteorological drought, but also continue after the end of the meteorological droughts. Finally, socio-economic drought could be defined as an event when the need for water is greater than the possibility to provide it with agrotechnical measures. The mentioned concept of drought reflects a strong connection between drought and human activities. The droughts in recent years have had a significant impact on the economies and environment of agricultural production, increasing the vulnerability of society as well as a whole ecosystem.

#### **6.1 Mechanisms of plant adaptation to drought**

The lack of water especially in agricultural sector presents important limitation of world food production. The strategies for promoting the mechanisms of plant tolerance to drought:


**99**

*Water Plant and Soil Relation under Stress Situations DOI: http://dx.doi.org/10.5772/intechopen.93528*

water scarcity are:

• reduction of leaf area,

• leaf rejection (abscissa),

• increased root growth,

• retaining the stomata,

• osmotic adjustment,

• thickening of the cuticle.

It is already clear from this division that the mechanisms and strategies of drought resistance may differ. Water deficiency could be explained as amount of water in a cell tissue that is lacking until optimal hydration. When water deficit gradually occurs, the impact of water scarcity on plant growth and development comes to the fore. The basic acclimatization strategies that occur in conditions of

Decrement of water plant status affects the turgor pressure, which also decreases. As turgor falls, cell volume decreases, cell contents become more concentrated, and cell membrane becomes less tense and thicker. Cell growth if highly influenced by turgor and consequently declining of turgor will restrict cell growth. In addition to reducing turgor, the lack of water also reduces the elasticity of cell walls, which also affects cell growth. Decreased cell growth results in smaller leaves, that is, reduced leaf area. Reduced leaf area helps conserve water because smaller leaves breathe less (lose water more slowly). Therefore, a plant usually starts to decrease leaves surface and afterwards its number as a result on water stress conditions. The leaves age and fall off faster (there is an increased synthesis of ethylene, which encourages leafs fall). Beside leaves, plant root is also susceptible to the lack of water. The balance between the uptake of water through the root and the photosynthetic activity of the aboveground part influences the ratio of the root mass and aboveground part. Simply, the aboveground organs will grow as long as the root supplies them with sufficient water and nutrients, and conversely, the root will grow as long as the aboveground organs supply it with sufficient assimilates. As already mentioned, water deficiency cause leaf area and number of leaves decrease in the early phase, while the intensity of photosynthesis tends to remain unchanged. Reducing the leaf area allows less water consumption, but also energy, so more carbohydrates were translocated to the root and allowed it to grow. However, in dry soil, the root tip loses turgor very quickly, so the root grows where the soil is still moist. As water scarcity (drought) progresses, stress most often occurs. Stress caused by the lack of water due to drying out of the upper soil layers so the plants develop deeper roots. The development of deeper roots is the also one of the reaction pathways to the drought. Increased root elongation during drought requires translocation of assimilates from aboveground organs to the root. In the generative phase, a significant outflow is represented by fruits (assimilates are spent on fruit growth), so the roots get less assimilates. Therefore, if the stress of water deficiency occurs in the generative phase of plant development, the effect of enhanced root growth will be less pronounced [10]. In conditions of intense stress (rapid) occurrence as a result of water deficit or the phase where plants have already developed the maximum leaf area, other mechanisms activated to prevent the plant from drying out. One of the most important such mechanisms is the closure of the stoma, which reduces transpiration, that is, water loss. Therefore, the "third line" of drought protection can be considering the

#### *Water Plant and Soil Relation under Stress Situations DOI: http://dx.doi.org/10.5772/intechopen.93528*

It is already clear from this division that the mechanisms and strategies of drought resistance may differ. Water deficiency could be explained as amount of water in a cell tissue that is lacking until optimal hydration. When water deficit gradually occurs, the impact of water scarcity on plant growth and development comes to the fore. The basic acclimatization strategies that occur in conditions of water scarcity are:

• reduction of leaf area,

*Soil Moisture Importance*

**Figure 9.**

*world's arable land.*

Short-term water shortage over a period of several weeks in the surface layer of the soil, which occurs at a critical time for plant development, can cause agronomic drought. The agronomic droughts may lag behind meteorological droughts, depending on the condition of the surface layer soil. High temperatures, low relative

*Current trends of most abundant abiotic stress occurrence in agriculture sector estimated for around 50% of* 

Meteorological drought occurs as a consequence of lack or complete absence of precipitation over a long period of time in a certain area. This deficiency is defined as the deviation of precipitation from normal, that is, from the multi-year average.

Deficit of precipitation over a long period of time affects surface and groundwater supplies: to the flow of water in rivers and streams, level of water in lakes, and level of groundwater. When flows and levels decrease, we talk about hydrological drought. The onset of hydrological drought may lag a few months behind the beginning of the meteorological drought, but also continue after the end of the meteorological droughts. Finally, socio-economic drought could be defined as an event when the need for water is greater than the possibility to provide it with agrotechnical measures. The mentioned concept of drought reflects a strong connection between drought and human activities. The droughts in recent years have had a significant impact on the economies and environment of agricultural production, increasing the vulnerability of society as well as a whole

The lack of water especially in agricultural sector presents important limitation of world food production. The strategies for promoting the mechanisms of plant

a.Desiccation delay is the ability to maintain hydration of the tissue

b.Desiccation tolerance is the retention of cell functions during drought

c.Drought avoidance is the end of the plant vegetation cycle before drought

humidity, and wind amplify the negative consequences agronomic droughts. The agronomic droughts, precipitation deficits are taken into account along with the physical and biological aspects of plants, interactions within the soil-plantatmosphere system and the balance between plants' water needs and available water reserves, which may result in declining yields. Beside agronomical drought, we can

also distinct the meteorological and hydrological drought.

**6.1 Mechanisms of plant adaptation to drought**

Meteorological drought can develop abruptly and stop abruptly.

**98**

ecosystem.

tolerance to drought:

occurs.


Decrement of water plant status affects the turgor pressure, which also decreases. As turgor falls, cell volume decreases, cell contents become more concentrated, and cell membrane becomes less tense and thicker. Cell growth if highly influenced by turgor and consequently declining of turgor will restrict cell growth. In addition to reducing turgor, the lack of water also reduces the elasticity of cell walls, which also affects cell growth. Decreased cell growth results in smaller leaves, that is, reduced leaf area. Reduced leaf area helps conserve water because smaller leaves breathe less (lose water more slowly). Therefore, a plant usually starts to decrease leaves surface and afterwards its number as a result on water stress conditions. The leaves age and fall off faster (there is an increased synthesis of ethylene, which encourages leafs fall).

Beside leaves, plant root is also susceptible to the lack of water. The balance between the uptake of water through the root and the photosynthetic activity of the aboveground part influences the ratio of the root mass and aboveground part. Simply, the aboveground organs will grow as long as the root supplies them with sufficient water and nutrients, and conversely, the root will grow as long as the aboveground organs supply it with sufficient assimilates. As already mentioned, water deficiency cause leaf area and number of leaves decrease in the early phase, while the intensity of photosynthesis tends to remain unchanged. Reducing the leaf area allows less water consumption, but also energy, so more carbohydrates were translocated to the root and allowed it to grow. However, in dry soil, the root tip loses turgor very quickly, so the root grows where the soil is still moist. As water scarcity (drought) progresses, stress most often occurs. Stress caused by the lack of water due to drying out of the upper soil layers so the plants develop deeper roots. The development of deeper roots is the also one of the reaction pathways to the drought. Increased root elongation during drought requires translocation of assimilates from aboveground organs to the root. In the generative phase, a significant outflow is represented by fruits (assimilates are spent on fruit growth), so the roots get less assimilates. Therefore, if the stress of water deficiency occurs in the generative phase of plant development, the effect of enhanced root growth will be less pronounced [10].

In conditions of intense stress (rapid) occurrence as a result of water deficit or the phase where plants have already developed the maximum leaf area, other mechanisms activated to prevent the plant from drying out. One of the most important such mechanisms is the closure of the stoma, which reduces transpiration, that is, water loss. Therefore, the "third line" of drought protection can be considering the

mechanism of stomata closuring. The change of turgor in the guardian cells regulates the opening and closing mechanism of the leaves stomata. The guardian cells are modified cells of the leaf epidermis and that is why they can lose turgor as they loss of water (transpiration) into the atmosphere. Such a way of holding, stomata (due to direct water loss and falling turgor) called hydro-passive stoma detention. The second mechanism called hydro-active stomata closuring occurs in conditions when the whole leaf and/or root is dried. This mechanism is triggered by metabolic processes in the stomata cells. Concentration decrement of osmotic active substances in the stomata results in water release in guardian cells and turgor pressure decrease. Hydroactive detention of the stoma occurs due to a decrease in osmotic potential, which leads to the release of water and a decrease in turgor in the guardian cells. The abscisic acid (ABA) affects the decrement of concentration in osmotic active substance in the stoma. Abscisic acid in very low concentrations is constantly synthesized in mesophilic cells and accumulates in chloroplasts. Under conditions of mild dehydration of mesophylls, two processes activated: (1) part of the abscise acid stored in chloroplasts is released into the apoplast (intercellular spaces) of mesophilic cells and then the transpiration current of water carries ABA to the guardian cells and (2) the synthesis of ABA intensifies and its higher concentrations accumulate in the apoplast of the leaf. This second process (ABA synthesis) prolongs, that is, maintains the process of coupling retention that occurs due to the release of ABA from the chloroplast. In addition, during stress caused by the lack of water, chemical signals (ABA) transmitted from the root to the leaf, leading to the stomata closing. In fact, the conductivity of the shoot indirectly was managed by the soil water status rather than the water status of the leaves, afterward root show high sensitivity respond to lac of soil moisture.

As the amount of water in the soil decreases, the water potential of the soil decreases. In condition when root water potential is more negative then soil water potential plants can receive necessary water. The process of accumulation of solutes (osmotic active substances) in cells presents osmotic adaptation. Thus, the cell reduces the water potential (the water potential becomes more negative) without a significant change in the turgor or volume of the cell. During osmotic adaptation, various solutes accumulate in the cell, mainly sugars, organic acids, amino acids, and inorganic ions (especially K+ ). Ions mainly accumulate in the vacuole because their high concentration in the cytosol can inhibit many enzymes. However, due to the increased concentration of ions in the vacuole, and in the cytoplasm, there must be an increase in the concentration of solutes in order to maintain the balance of water potential within the cell. Dissolved substances that accumulate in the cytoplasm called compatible osmotic active substances and are substances that do not inhibit enzymes. Compatible osmotic active substances were proline as amino acids, and sugar alcohols and glycine betaine as amine. The synthesis of compatible osmotic substances also occurs under conditions caused by increased salinity. The osmotic adjustment takes place slowly over several days. Osmotic adjustment of leaves can provide turgor maintenance with lower water potential compared to leaves that have not undergone osmotic adjustment. The maintenance of turgor enables the normal course of cell growth (cell elongation) and greater stomata conductivity at lower water potential, which leads to the conclusion that the osmotic adaptation is actually a process of acclimatization.

The cuticle is a waxy coat located above the epidermal cells, and its function is to reduce water loss (cuticular transpiration). In conditions of lack of water, plants often synthesize a thicker cuticle. The thicker cuticle also reduces the entry of CO2, but photosynthesis usually remains unchanged because the epidermal cells located below the cuticle do not conduct photosynthesis (CO2 for photosynthesis enters the leaf through the stoma). Cuticular transpiration makes up only 5–10% of the total transpiration, so the thickness of the cuticle is important only in conditions of more severe drought or when the cuticle is damaged (e.g., due to wind). The lack of

**101**

**Figure 10.**

*fibrous root.*

*Water Plant and Soil Relation under Stress Situations DOI: http://dx.doi.org/10.5772/intechopen.93528*

water reduces the intensity of photosynthesis although this process does not react as pronouncedly to lack of water, as is the case with leaf area. The reason for this is that photosynthesis is not as sensitive to turgor decline as cell growth. However, due to the detention of the stoma under drought stress conditions, the entry of CO2 into

Each plant species has its own root growth characteristics, which can be substantially, modify by the plant's environment. In most arable land used in agriculture, the roots occupy the largest volume of soil, although it is dictated by the physical and chemical properties of the soil. The main roots and root hairs are responsible for transporting water through the conductive elements, while the lateral roots play an important role in the absorption of water and minerals [58–60]. The roots are under the extremely high influence of soil factors, its buffering abilities, surrounding environment as one of the most important parameters that reflected in the development of the roots [58]. The soil water and nutrient content, soil type, vegetation, and agrotechnical measures strongly reflect root growth [61–63, 60, 64]. In the early growth phase, roots behave as sink for assimilates which are distributed in vegetative parts of plant until generative phase occurs this role become weaker [61]. Richards [65] found that wheat root shows significant changes if grown in water deficit conditions and also this state reflects root development. Dry soil my intensify growth of root in depth search-

the leaf reduced and the intensity of photosynthesis decreases.

ing for water [66] but elongation of root cells could be restricted [67].

The balance between the aboveground and underground part of the plant is very important in terms of regulating the status of water in the tissue, and it was found that some plants tolerate drought conditions better if the ratio between aboveground and underground plant parts were lower [68]. Under favorable water conditions in the soil and the wheat phase of vegetation, better rooting and increase in root volume can occur [69]. Taylor and Klepper [70] found that minor changes in

water potential did not significantly affect the increase in plant root volume.

A large number of studies regarding the influence of water stress on the plant conclude that not only the root results in certain changes but also certain changes occur in the aboveground part of the plant. Beside arid area also water saturated soil can provide certain changes on root (**Figure 10**) and affect root development as

Salix *sp. root adaptation to water level changing or species tolerant to hypoxic conditions by forming abounded* 

#### *Water Plant and Soil Relation under Stress Situations DOI: http://dx.doi.org/10.5772/intechopen.93528*

*Soil Moisture Importance*

mechanism of stomata closuring. The change of turgor in the guardian cells regulates the opening and closing mechanism of the leaves stomata. The guardian cells are modified cells of the leaf epidermis and that is why they can lose turgor as they loss of water (transpiration) into the atmosphere. Such a way of holding, stomata (due to direct water loss and falling turgor) called hydro-passive stoma detention. The second mechanism called hydro-active stomata closuring occurs in conditions when the whole leaf and/or root is dried. This mechanism is triggered by metabolic processes in the stomata cells. Concentration decrement of osmotic active substances in the stomata results in water release in guardian cells and turgor pressure decrease. Hydroactive detention of the stoma occurs due to a decrease in osmotic potential, which leads to the release of water and a decrease in turgor in the guardian cells. The abscisic acid (ABA) affects the decrement of concentration in osmotic active substance in the stoma. Abscisic acid in very low concentrations is constantly synthesized in mesophilic cells and accumulates in chloroplasts. Under conditions of mild dehydration of mesophylls, two processes activated: (1) part of the abscise acid stored in chloroplasts is released into the apoplast (intercellular spaces) of mesophilic cells and then the transpiration current of water carries ABA to the guardian cells and (2) the synthesis of ABA intensifies and its higher concentrations accumulate in the apoplast of the leaf. This second process (ABA synthesis) prolongs, that is, maintains the process of coupling retention that occurs due to the release of ABA from the chloroplast. In addition, during stress caused by the lack of water, chemical signals (ABA) transmitted from the root to the leaf, leading to the stomata closing. In fact, the conductivity of the shoot indirectly was managed by the soil water status rather than the water status of the leaves, afterward root show high sensitivity respond to lac of soil moisture. As the amount of water in the soil decreases, the water potential of the soil decreases. In condition when root water potential is more negative then soil water potential plants can receive necessary water. The process of accumulation of solutes (osmotic active substances) in cells presents osmotic adaptation. Thus, the cell reduces the water potential (the water potential becomes more negative) without a significant change in the turgor or volume of the cell. During osmotic adaptation, various solutes accumulate in the cell, mainly sugars, organic acids, amino acids, and inorganic ions

). Ions mainly accumulate in the vacuole because their high concentra-

tion in the cytosol can inhibit many enzymes. However, due to the increased concentration of ions in the vacuole, and in the cytoplasm, there must be an increase in the concentration of solutes in order to maintain the balance of water potential within the cell. Dissolved substances that accumulate in the cytoplasm called compatible osmotic active substances and are substances that do not inhibit enzymes. Compatible osmotic active substances were proline as amino acids, and sugar alcohols and glycine betaine as amine. The synthesis of compatible osmotic substances also occurs under conditions caused by increased salinity. The osmotic adjustment takes place slowly over several days. Osmotic adjustment of leaves can provide turgor maintenance with lower water potential compared to leaves that have not undergone osmotic adjustment. The maintenance of turgor enables the normal course of cell growth (cell elongation) and greater stomata conductivity at lower water potential, which leads to the conclusion

The cuticle is a waxy coat located above the epidermal cells, and its function is to reduce water loss (cuticular transpiration). In conditions of lack of water, plants often synthesize a thicker cuticle. The thicker cuticle also reduces the entry of CO2, but photosynthesis usually remains unchanged because the epidermal cells located below the cuticle do not conduct photosynthesis (CO2 for photosynthesis enters the leaf through the stoma). Cuticular transpiration makes up only 5–10% of the total transpiration, so the thickness of the cuticle is important only in conditions of more severe drought or when the cuticle is damaged (e.g., due to wind). The lack of

that the osmotic adaptation is actually a process of acclimatization.

**100**

(especially K+

water reduces the intensity of photosynthesis although this process does not react as pronouncedly to lack of water, as is the case with leaf area. The reason for this is that photosynthesis is not as sensitive to turgor decline as cell growth. However, due to the detention of the stoma under drought stress conditions, the entry of CO2 into the leaf reduced and the intensity of photosynthesis decreases.

Each plant species has its own root growth characteristics, which can be substantially, modify by the plant's environment. In most arable land used in agriculture, the roots occupy the largest volume of soil, although it is dictated by the physical and chemical properties of the soil. The main roots and root hairs are responsible for transporting water through the conductive elements, while the lateral roots play an important role in the absorption of water and minerals [58–60]. The roots are under the extremely high influence of soil factors, its buffering abilities, surrounding environment as one of the most important parameters that reflected in the development of the roots [58]. The soil water and nutrient content, soil type, vegetation, and agrotechnical measures strongly reflect root growth [61–63, 60, 64]. In the early growth phase, roots behave as sink for assimilates which are distributed in vegetative parts of plant until generative phase occurs this role become weaker [61]. Richards [65] found that wheat root shows significant changes if grown in water deficit conditions and also this state reflects root development. Dry soil my intensify growth of root in depth searching for water [66] but elongation of root cells could be restricted [67].

The balance between the aboveground and underground part of the plant is very important in terms of regulating the status of water in the tissue, and it was found that some plants tolerate drought conditions better if the ratio between aboveground and underground plant parts were lower [68]. Under favorable water conditions in the soil and the wheat phase of vegetation, better rooting and increase in root volume can occur [69]. Taylor and Klepper [70] found that minor changes in water potential did not significantly affect the increase in plant root volume.

A large number of studies regarding the influence of water stress on the plant conclude that not only the root results in certain changes but also certain changes occur in the aboveground part of the plant. Beside arid area also water saturated soil can provide certain changes on root (**Figure 10**) and affect root development as

#### **Figure 10.**

Salix *sp. root adaptation to water level changing or species tolerant to hypoxic conditions by forming abounded fibrous root.*

a result of anaerobic conditions [71] even some species have adapted to this condition developing a special root [72]. The reaction of the plant to stressful conditions in most cases was reflected in the unfavorable status of water in plant tissues and organs. The main mechanism of plant cell resistance to stress is to maintain favorable turgor pressure in cells, tissues, and organs (stress is first reflected in important physiological processes in the plant such as photosynthesis, respiration, transpiration, nutrient, and water uptake) [64, 73, 74].

The roots of plants are most often adapted to stressful conditions in a way that changes the structure of the tissue and changes in root volume occur, which was closely related to the reclamation of the osmotic activity of root cells. Cells most often try to protect cells from protoplast dehydration through metabolic exchange of water molecules and some osmotic active substance that result tissue adaptation to newly state that occurred (wall thickness, cell size, osmolite concentration, etc.) [66, 75, 76].
