**3. Water uptake and movement trough the plant**

#### **3.1 Basic process of water uptake**

Plants constantly receive water by root system and excrete water by vegetative plant's parts. The root system consists of primary and secondary roots overgrown with root hairs. Root hairs develop on the root surface. Due to their large number, they significantly increase the root area and thus make it easier for the root to absorb water and minerals from the soil. A close contact of the root surface and soil is necessary for the absorption of water by the root. The root hairs penetrate between the soil particles and immerse themselves in the capillary spaces of the soil where the water is located. Under favorable conditions of growth and development, plants scatter the root network and increase its volume. In restrictive plant growth conditions, plant root growth is often primarily restricted. Roots hairs were the most susceptible to insufficient water and nutrient demands resulting in decay, especially within the herbaceous plants [16]. The pathway plant-soil-atmosphere water movement occurs through different media (cell walls, double phospholipids layer, cytoplasm, etc.), and the transmission mechanism changes depending on the type of medium through which it passes (**Figure 1**).

Basic processes that enable the water uptake and conduction of water in the plants are swelling and osmosis. Both processes were conducted by the reduction

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

the long-distance pathway.

of nutrients or asphyxiation of root.

unpredictably as a response to the suction forces.

of water chemical potential and by the process of diffusion. Diffusion process presents water movement from the medium of high-water potential to the medium of low water potential. In an aqueous solution of a substance, the gradient of the water chemical potential is opposite to the gradient of the electrochemical potential of the solute, so water diffuses in the opposite direction from the solute. During this process, there is no chemical reaction. The diffusion rate presents amount of substance that diffuses in certain period proportional to the concentration gradient. First Fick's law defines diffusion rate of solvent across the membrane. The process of diffusion is important for small molecules in aqueous solution effective on cell level as for entering of solution in root cell or for a stomata transpiration rate. Diffusion is not effective enough to transmit solutes to the too long pathway. The rate of diffusion is rapid over short distance but extremely slow over long distance. The process of diffusion has great importance in receiving water from the soil, the movement of solutes over short distances, and the loss of gaseous water from the vegetative plant's part, but it is not important in the transfer of water over long distances that occur in the main stream. The cohesion-tension theory explains the mechanism of water transport without consuming metabolic energy. Xylem water transport is closely related to cohesion (water molecules bind to each other) and adhesion forces (where water molecules adhere strongly to the conductive elements of the xylem), forming a transpiration column for which upward movement is responsible for the negative hydrostatic pressure [11]. Unlike diffusion driven by a difference in concentration (concentration gradient), mass bulk or mass flow represents water molecule mass flow often driven by differences in pressure (pressure gradient). This mass flow mechanism by xylem elements frequently is used for water transport over

Mass or free flow of water plays a significant role for absorption of nutrients from soil solution of high concentrations even when transpiration is high. Then significant amounts of water move toward root carrying with it dissolved substances (nutrients) which the plant receives. Certainly, if there is not enough water in the soil, there is no flow of nutrients, while too much water can increase leaching

The root hairs penetrate into spaces between soil particles and allow water to enter the apoplasmic space of the root cells. To receive water from the soil, corresponding reduction of water potential between the soil and roots has to occur. The drier the soil, the more negative the water potential becomes, and the potential swelling pressure increases, as moisture is still retained only between the soil capillaries. The water enters root hairs by imbibitions and osmotic transporting into the central cylinder of the root. Further, the plant uses water potential reduction between the soil and the atmosphere to conduct water from the soil through the plant body to the atmosphere without consuming energy. A process leading to water transport in plant cells is osmosis referring to movement of a solvent such water and other substances across a membrane. The membrane for all living cells presents certain barriers: they separate different parts of the cells from each other and greatly impede movement of substances between compartments. The plant cells membranes are semi-permeable, since they are well permeable to water molecules and other smaller particles with a weakly charged charge and quite limited permeability to larger molecules and particles with a pronounced charge [5]. Osmosis similarly to the diffusion and mass flow occurs and it appears

Water is a key factor in initiating the germination of dry seeds when they reach the soil. The water in the seed stimulates the swelling process by imbibitions; hydrolytic enzymes ware activated; and the germination process begins. The entry of water into the seed is wrapped from the hardness of the seed coat, and sometimes

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

*Soil Moisture Importance*

ADP molecule, is a donor of hydrogen in photosynthesis). In addition, as water has a high heat capacity, its presence in the plant ensures that temperature changes occur slowly. The polarity and ability to form hydrogen bonds allows water to participate in a number of interactions. Water molecules were arranged around ions or charged groups of macromolecules and cover their charge. This reduces the interactions between the charged substances and increases their solubility. Therefore, water is the best solvent for ionic substances. Water is in liquid phase and medium in which all enzymatic reactions take place. Various substances can enter into a chemical reaction with each other only if they dissolve in water. The uptake of solutes is possible only from an aqueous solution. As the water content in the plant organism decreases, so does the vital activity. Water regulates turgor pressure and upright visual appearance and cell size. A large number of metabolic functions of water were realized by the processes of uptake and release (transpiration).

Plants constantly receive water by root system and excrete water by vegetative plant's parts. The root system consists of primary and secondary roots overgrown with root hairs. Root hairs develop on the root surface. Due to their large number, they significantly increase the root area and thus make it easier for the root to absorb water and minerals from the soil. A close contact of the root surface and soil is necessary for the absorption of water by the root. The root hairs penetrate between the soil particles and immerse themselves in the capillary spaces of the soil where the water is located. Under favorable conditions of growth and development, plants scatter the root network and increase its volume. In restrictive plant growth conditions, plant root growth is often primarily restricted. Roots hairs were the most susceptible to insufficient water and nutrient demands resulting in decay, especially within the herbaceous plants [16]. The pathway plant-soil-atmosphere water movement occurs through different media (cell walls, double phospholipids layer, cytoplasm, etc.), and the transmission mechanism changes depending on the

Basic processes that enable the water uptake and conduction of water in the plants are swelling and osmosis. Both processes were conducted by the reduction

**3. Water uptake and movement trough the plant**

type of medium through which it passes (**Figure 1**).

*Water movement and different pathway of water uptaking in roots.*

**3.1 Basic process of water uptake**

**76**

**Figure 1.**

of water chemical potential and by the process of diffusion. Diffusion process presents water movement from the medium of high-water potential to the medium of low water potential. In an aqueous solution of a substance, the gradient of the water chemical potential is opposite to the gradient of the electrochemical potential of the solute, so water diffuses in the opposite direction from the solute. During this process, there is no chemical reaction. The diffusion rate presents amount of substance that diffuses in certain period proportional to the concentration gradient. First Fick's law defines diffusion rate of solvent across the membrane. The process of diffusion is important for small molecules in aqueous solution effective on cell level as for entering of solution in root cell or for a stomata transpiration rate. Diffusion is not effective enough to transmit solutes to the too long pathway. The rate of diffusion is rapid over short distance but extremely slow over long distance. The process of diffusion has great importance in receiving water from the soil, the movement of solutes over short distances, and the loss of gaseous water from the vegetative plant's part, but it is not important in the transfer of water over long distances that occur in the main stream. The cohesion-tension theory explains the mechanism of water transport without consuming metabolic energy. Xylem water transport is closely related to cohesion (water molecules bind to each other) and adhesion forces (where water molecules adhere strongly to the conductive elements of the xylem), forming a transpiration column for which upward movement is responsible for the negative hydrostatic pressure [11]. Unlike diffusion driven by a difference in concentration (concentration gradient), mass bulk or mass flow represents water molecule mass flow often driven by differences in pressure (pressure gradient). This mass flow mechanism by xylem elements frequently is used for water transport over the long-distance pathway.

Mass or free flow of water plays a significant role for absorption of nutrients from soil solution of high concentrations even when transpiration is high. Then significant amounts of water move toward root carrying with it dissolved substances (nutrients) which the plant receives. Certainly, if there is not enough water in the soil, there is no flow of nutrients, while too much water can increase leaching of nutrients or asphyxiation of root.

The root hairs penetrate into spaces between soil particles and allow water to enter the apoplasmic space of the root cells. To receive water from the soil, corresponding reduction of water potential between the soil and roots has to occur. The drier the soil, the more negative the water potential becomes, and the potential swelling pressure increases, as moisture is still retained only between the soil capillaries. The water enters root hairs by imbibitions and osmotic transporting into the central cylinder of the root. Further, the plant uses water potential reduction between the soil and the atmosphere to conduct water from the soil through the plant body to the atmosphere without consuming energy. A process leading to water transport in plant cells is osmosis referring to movement of a solvent such water and other substances across a membrane. The membrane for all living cells presents certain barriers: they separate different parts of the cells from each other and greatly impede movement of substances between compartments. The plant cells membranes are semi-permeable, since they are well permeable to water molecules and other smaller particles with a weakly charged charge and quite limited permeability to larger molecules and particles with a pronounced charge [5]. Osmosis similarly to the diffusion and mass flow occurs and it appears unpredictably as a response to the suction forces.

Water is a key factor in initiating the germination of dry seeds when they reach the soil. The water in the seed stimulates the swelling process by imbibitions; hydrolytic enzymes ware activated; and the germination process begins. The entry of water into the seed is wrapped from the hardness of the seed coat, and sometimes

with tightly closed seeds, a negative pressure of −100 MPA [17] can occur when entering the seed coat. Activated hydrolytic enzymes also activate other biochemical processes on which the germination process depends.

Water uptake by seeds is a process called imbibition, after which a certain period causes swelling and rupture of the seed coat. Germinating seeds use reserve nutrients stored in the seed endosperm. This nutrient reserve ensures the growth of the embryo. When the seed absorbs water, hydrolysis enzymes are activated that break down these stored reserve substances into metabolically useful chemicals. After the germ emerges from the seed layer and begins to grow roots and leaves, food supplies are usually depleted; in this case, photosynthesis provides the energy needed for further seedling growth, which now requires a continuous supply of water, nutrients, and light.

The swelling degree depends on the balance between the soil solution water potential and water potential of seeds, described as acting as repulsive and attractive forces between the two charges [18]. Imbibition is a physical process in which water enters the seed coat, where the volume of the seed changes significantly, exceeding its actual seed surface [19].

#### **3.2 Water chemical potential**

The term water potential is the most important factor for understanding the way of water moves in plant cells and through the conducted elements of the plant [20]. The chemical potential represents the water potential when it comes to water. The chemical potential of water or any substance is a measure of the available energy per mole by which that substance will react or move. Because the water molecule is neutral, the electrical potential has no effect on the chemical potential of the water. The chemical potential of water is a relative quantity and expressed as the difference between the potencies of a substance under certain conditions and the potential of that same substance under standard conditions. The unit for chemical potential is the energy per mole of a substance (J mol−1) but for better understanding, we use term water potential. The unit of water potential presents the free energy per unit volume of water solution in relation to the standard state of water, as a result of the combined action of solution concentration, pressure, and gravity in ambient temperature regime [19]. Presented equation express water potential as:

$$\Psi = \mu\_{\text{w1}} - \mu\_{\text{w2}} / \,\text{V}\_{\text{m}} \tag{1}$$

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

potential; Ψg, gravitation potential.

potential of pure water is always equal to zero but water in natural system, in plant cells, or soil pores has always dissolved some solutes as organic molecules, sugars,

This formula describes the energy state of water as the sum of the solute potential, the pressure potential, and the gravitation potential in a mixture of water and other particles in relation to standard conditions. The main factors contributing to

Ψ, water potential; Ψp, pressure potential; Ψs, solute potential; Ψm, matrix

which is important for water distance moving through the vascular tissue.

The pressure potential expresses the effects of pressure within the water potential of solution and refers to the hydrostatic pressure. Water potential has elevated by positive pressure, whereas negative pressure has an opposite effect. Plant cells have solid cell walls and can produce strong positive hydrostatic pressure called turgor. In xylem elements and aplastic space, negative hydrostatic pressure can develop

The solute potential expresses the effects of dissolve solutes within the water potential of solution and refers to the osmotic potential. Dissolved substances dilute water and therefore reduce its free energy. On the contrary, diluting a solution with water leads to a decrease in the concentration potential of dissolved particles and to an increase in the concentration potential of water. The effect of the osmotic potential on the water potential of the solution is negative, which means that the solutes reduce the water concentration and it is potential. The values of the osmotic potential and the osmotic pressure differ only in sign. In other words, the values of the osmotic potential are negative, and the values of the osmotic pressure are positive. The gravity potential (Ψg) represents the gravity effects on water potential of some solution. Gravity causes the water to move downward, and the potential affects the movement of the water depending on the height or transport distance. The effect of gravity on the potential can be neglected in the case of the transfer of solutes between cells or when the substances transported at a shorter height than 5 m.

One of the important components of water potential is potential of matrix (Ψm). This parameter is important for reducing the water potential because of the action of water on a solid surface such as a cell wall or soil particles. Such interactions of water and solid reduce the tendency of water molecules to chemically react or evaporate. In addition to dissolved particles and colloidal dissolved macromolecules, the cell has membrane surfaces and hydrated structural elements that can affect the water potential. For example, the water potential of a cell wall is not equal to zero even when the solution in the cell is pure water. The contribution of the cell wall structure to the water potential consists of a negative pressure component caused by water bound in the capillaries and an osmotic component that can theoretically be included in both the osmotic and turgor potential. For a practical reason, these potentials were often combined into one called the matrix potential. In an adult vacuolated cell, the matrix potential of the protoplasm and cell wall is small compared to the osmotic potential that was often neglected. The more negative the water potential of a cell, the higher its suction force. In water-saturated cells, the water potential is equal to zero, and the turgor potential corresponds to the sum of the osmotic and matrix potentials. The water potential increases with increasing turgor pressure and decreases with increasing osmotic pressure. The values of turgor and osmotic pressure depend on temperature and increase with increasing temperature. Water moves exclusively from the area of higher water potential to the

Ψ=Ψ +Ψ +Ψ +Ψ psmg (2)

enzymes, or other substances making water potential more negative [19].

the water potential can be expressed by the following equation:

Ψ, water potential; μw1, chemical potential of water; μw2, chemical potential of pure water; Vm, molar volume of water.

The molar volume of water presents the molar mass (w) divided by the mass density (ρ) expressed cubic meter per mole (m3 mol−1). The soil particles and plant tissue cells absorb water, and the level of absorbed water depends on different factors. Absorbed water depends on soil pore size, water regime, ambient temperature, and pressure, and working adsorption forces depend on concentration of solutes in water. Uncontrolled movement and movement speed of some solutes in water depends on their concentration. Restricted movement of water molecules reduce chemical potential of water expressed as the Brownian's irregular water movement. A plant cell does not have mechanism of "water pumps" for water potential incensement so the water uses gradient water potential reduction. To determine the value of the water potential, equilibrium methods are most often used in which the plant tissue is brought into balance with solutions of known water potential. Water

*Soil Moisture Importance*

ents, and light.

exceeding its actual seed surface [19].

**3.2 Water chemical potential**

express water potential as:

pure water; Vm, molar volume of water.

density (ρ) expressed cubic meter per mole (m3

with tightly closed seeds, a negative pressure of −100 MPA [17] can occur when entering the seed coat. Activated hydrolytic enzymes also activate other biochemi-

Water uptake by seeds is a process called imbibition, after which a certain period causes swelling and rupture of the seed coat. Germinating seeds use reserve nutrients stored in the seed endosperm. This nutrient reserve ensures the growth of the embryo. When the seed absorbs water, hydrolysis enzymes are activated that break down these stored reserve substances into metabolically useful chemicals. After the germ emerges from the seed layer and begins to grow roots and leaves, food supplies are usually depleted; in this case, photosynthesis provides the energy needed for further seedling growth, which now requires a continuous supply of water, nutri-

The swelling degree depends on the balance between the soil solution water potential and water potential of seeds, described as acting as repulsive and attractive forces between the two charges [18]. Imbibition is a physical process in which water enters the seed coat, where the volume of the seed changes significantly,

The term water potential is the most important factor for understanding the way of water moves in plant cells and through the conducted elements of the plant [20]. The chemical potential represents the water potential when it comes to water. The chemical potential of water or any substance is a measure of the available energy per mole by which that substance will react or move. Because the water molecule is neutral, the electrical potential has no effect on the chemical potential of the water. The chemical potential of water is a relative quantity and expressed as the difference between the potencies of a substance under certain conditions and the potential of that same substance under standard conditions. The unit for chemical potential is the energy per mole of a substance (J mol−1) but for better understanding, we use term water potential. The unit of water potential presents the free energy per unit volume of water solution in relation to the standard state of water, as a result of the combined action of solution concentration, pressure, and gravity in ambient temperature regime [19]. Presented equation

Ψ, water potential; μw1, chemical potential of water; μw2, chemical potential of

The molar volume of water presents the molar mass (w) divided by the mass

tissue cells absorb water, and the level of absorbed water depends on different factors. Absorbed water depends on soil pore size, water regime, ambient temperature, and pressure, and working adsorption forces depend on concentration of solutes in water. Uncontrolled movement and movement speed of some solutes in water depends on their concentration. Restricted movement of water molecules reduce chemical potential of water expressed as the Brownian's irregular water movement. A plant cell does not have mechanism of "water pumps" for water potential incensement so the water uses gradient water potential reduction. To determine the value of the water potential, equilibrium methods are most often used in which the plant tissue is brought into balance with solutions of known water potential. Water

Ψ= − µ µ /V w1 w2 m (1)

mol−1). The soil particles and plant

cal processes on which the germination process depends.

**78**

potential of pure water is always equal to zero but water in natural system, in plant cells, or soil pores has always dissolved some solutes as organic molecules, sugars, enzymes, or other substances making water potential more negative [19].

This formula describes the energy state of water as the sum of the solute potential, the pressure potential, and the gravitation potential in a mixture of water and other particles in relation to standard conditions. The main factors contributing to the water potential can be expressed by the following equation:

$$
\Psi = \Psi\_{\text{p}} + \Psi\_{\text{s}} + \Psi\_{\text{m}} + \Psi\_{\text{g}} \tag{2}
$$

Ψ, water potential; Ψp, pressure potential; Ψs, solute potential; Ψm, matrix potential; Ψg, gravitation potential.

The pressure potential expresses the effects of pressure within the water potential of solution and refers to the hydrostatic pressure. Water potential has elevated by positive pressure, whereas negative pressure has an opposite effect. Plant cells have solid cell walls and can produce strong positive hydrostatic pressure called turgor. In xylem elements and aplastic space, negative hydrostatic pressure can develop which is important for water distance moving through the vascular tissue.

The solute potential expresses the effects of dissolve solutes within the water potential of solution and refers to the osmotic potential. Dissolved substances dilute water and therefore reduce its free energy. On the contrary, diluting a solution with water leads to a decrease in the concentration potential of dissolved particles and to an increase in the concentration potential of water. The effect of the osmotic potential on the water potential of the solution is negative, which means that the solutes reduce the water concentration and it is potential. The values of the osmotic potential and the osmotic pressure differ only in sign. In other words, the values of the osmotic potential are negative, and the values of the osmotic pressure are positive.

The gravity potential (Ψg) represents the gravity effects on water potential of some solution. Gravity causes the water to move downward, and the potential affects the movement of the water depending on the height or transport distance. The effect of gravity on the potential can be neglected in the case of the transfer of solutes between cells or when the substances transported at a shorter height than 5 m.

One of the important components of water potential is potential of matrix (Ψm). This parameter is important for reducing the water potential because of the action of water on a solid surface such as a cell wall or soil particles. Such interactions of water and solid reduce the tendency of water molecules to chemically react or evaporate. In addition to dissolved particles and colloidal dissolved macromolecules, the cell has membrane surfaces and hydrated structural elements that can affect the water potential. For example, the water potential of a cell wall is not equal to zero even when the solution in the cell is pure water. The contribution of the cell wall structure to the water potential consists of a negative pressure component caused by water bound in the capillaries and an osmotic component that can theoretically be included in both the osmotic and turgor potential. For a practical reason, these potentials were often combined into one called the matrix potential. In an adult vacuolated cell, the matrix potential of the protoplasm and cell wall is small compared to the osmotic potential that was often neglected. The more negative the water potential of a cell, the higher its suction force. In water-saturated cells, the water potential is equal to zero, and the turgor potential corresponds to the sum of the osmotic and matrix potentials. The water potential increases with increasing turgor pressure and decreases with increasing osmotic pressure. The values of turgor and osmotic pressure depend on temperature and increase with increasing temperature. Water moves exclusively from the area of higher water potential to the

area of lower water potential, that is, down the chemical gradient. On the contrary, water from the area of lower water potential can be transferred to higher area only with energy consumption (endogenous process). Water will enter the cell until the water potential inside the cell equals that outside the cell. The flow of water into the cell resulting from the water potential gradient causes a hydrostatic pressure (turgor) in the vacuole. This pressure gives the cell tension and strength. Since the cell wall is quite solid but also elastic, small changes in cell volume can cause large changes in turgor pressure.

The turgor pressure is very important for the upright appearance of the plant and the strength of the cells. In conditions when the plant loses water, due to the limited availability of moisture, the turgor pressure decreases, the cell walls relax, and the plants take on a withered appearance. The turgor pressure acts against further osmotic flow of water in the vacuole, as the actual operating pressure for osmotic flow and at disposal, it has only a part of the potential osmotic pressure that was not compensated by turgor and was called the tension or suction force. The positive value of the suction force corresponds to the negative value of the water potential. The difference in water potential between the outer and inner membrane space is the force that allows water to be transported osmotically. The potential osmotic pressure of a cell decreases due to water intake and increases with water excretion. The cell sap contains a relatively high concentration of solutes and entering of water molecule trough the cell wall has controlled by pressure. The turgid cells have a suction force equal to zero, whereas turgor and osmotic pressures were equalized. Plant cells between the saturated state and the wilting state have a suction force corresponding to the negative value of the water potential. When the plant cells are partially dehydrated, the turgor pressure corresponds to zero and the suction force corresponds to the value of the potential osmotic pressure. When the plant cell is saturated, the turgor pressure takes a negative sign, and the suction force corresponds to the sum of the turgor and osmotic pressure. Small changes in Ψs usually accompanied such changes of turgor pressure. The water uptake to the cells leads to positive sign of pressure potential Ψp. Water absorption through the roots is possible only when there is a corresponding drop in water potential. The water potential was significantly affected by humidity, because in conditions of high humidity, the water potential is quite high, while in conditions when the air is quite dry, there are large differences between aboveground plant parts and the atmosphere. The amount of water available to the plant is in balance with the physiological capacity of the plant to absorb water and the environmental conditions that affect the intensity of plant transpiration. The movement of water in the plantenvironment system always takes the place of water and solutes movement from the area of higher potential to the area of lower potential until the concentrations equalize. The water potential in the leaves of plants rating from −10 to −100 bar, while in the atmosphere, in the conditions of relative humidity, up to 50% of the potential reaches up to −1000 bar. A large difference in the water potential gradient of plant leaves and the atmosphere causes transpiration. Negative water potential usually occurs in leaves than the root cells. Similar happens when water enters the root of plants, through the apoplast where the driving force is the difference between the water potential of the xylem and the soil solution. When the water potential of a xylem solution is more negative than the water potential of soil solution, water enters the root and moves all the way to the endoderm. Van den Honert [21] explains the differences in potential that occurs when water enters into the root or exits by transpiration. An increase in the water potential due to an increase in relative humidity of air and a decrease in the water potential of the soil due to desiccation leads to a slower transfer of water from the roots to the aboveground organs of the plant. The lack of water is most reflected in the decline in turgidity of plant

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**3.3 Plant water balance water status**

depends on the rate of water uptake and excretion.

**3.4 Absorption, transport, and transpiration of water**

water potential followed by a difference in pressure (**Figure 2**).

Water movement in the plant occurs predominantly regarding the passive or active transport of osmotic active substances along with water across the membrane. Main trigger for such solutes moving through the plant cells is difference in

The entry of water into the root cells occurs passively, that is, diffusely, and the solution moves freely through the apoplasmic space. Since the water molecule uncharged, it can very easily cross the membrane without hindrance and continue to

cells and thus in the overall appearance of the plant, with the leaves falling down, twisting, decreasing the intensity of photosynthesis, and reducing all other metabolic activities [22]. Any deviation of the available amount of moisture from the optimal in plants results in stress and the plant wilting and earlier the deterioration of plant tissue occurs [23]. In conditions when the plants are short of moisture for a long time, it passes into the phase of permanent or reversible wilting and then dies.

Plant water balance water status in plants is very important for plant growth and development, and metabolic activities, and especially, the lack of water drastically affects the length of the growing season and crop yields [23]. The total ratio between the water absorbed by the root system and the water released by transpiration from aboveground plant part is referred to as water balance. In cases when the transpiration of water exceeds its absorption from the substrate, water deficit occurs. The physiological processes in a plant depend on the amount of available moisture. The lack of water leads to inhibition of growth and photosynthesis. In addition, it acts on cell division and inhibits the synthesis of cell wall components and proteins, causing a stomata closure. Environmental conditions significantly affect the water potential, which is negative in conditions of well-available humidity, and in arid environments, the negative pressure was even more pronounced. Water potential was not only response to the environmental condition but also to the plant genetic characteristics [19]. The water transfer presents passive process so water movement to the plants occurs due to the low water potential of the plant in regard to the water potential of the soil. Since turgor pressure cannot be directly affected, the plant cell can regulate the water balance only by active regulation of the osmotic potential. Osmoregulation is the adaptation of osmotically active substances in the cell to the newly occurring environmental changes that largely control the water potential of the cell. This adjustment can be achieved by receiving/releasing or synthesis/decomposition of osmotically active substances. Under drought stress, plants are able to maintain water adsorption by increasing the cellular solute concentration, a process called osmotic adjustment. Osmotic adjustment occurs when the osmotic potential of a cell changes due to an increase or decrease in the content of osmotically active substances. Many cells respond to water stress by increasing the water potential of the cell. In this way, the decline in turgor can be prevented or minimized. Osmotic adaptation is an important feature of delaying dehydration in water-constrained environments because it maintains cell turgor and physiological processes with the development of water deficit [24]. Osmoregulation and osmotic adjustment are two different mechanisms. Glycophytes and halophytes have the ability to adapt to stress in two ways. One is that under conditions of water deficiency, plants can synthesize organic substances such as organic soluble substances, such as glycine betaine or proline, or high concentrations of inorganic ions [25]. The water balance of a plant

*Soil Moisture Importance*

changes in turgor pressure.

area of lower water potential, that is, down the chemical gradient. On the contrary, water from the area of lower water potential can be transferred to higher area only with energy consumption (endogenous process). Water will enter the cell until the water potential inside the cell equals that outside the cell. The flow of water into the cell resulting from the water potential gradient causes a hydrostatic pressure (turgor) in the vacuole. This pressure gives the cell tension and strength. Since the cell wall is quite solid but also elastic, small changes in cell volume can cause large

The turgor pressure is very important for the upright appearance of the plant and the strength of the cells. In conditions when the plant loses water, due to the limited availability of moisture, the turgor pressure decreases, the cell walls relax, and the plants take on a withered appearance. The turgor pressure acts against further osmotic flow of water in the vacuole, as the actual operating pressure for osmotic flow and at disposal, it has only a part of the potential osmotic pressure that was not compensated by turgor and was called the tension or suction force. The positive value of the suction force corresponds to the negative value of the water potential. The difference in water potential between the outer and inner membrane space is the force that allows water to be transported osmotically. The potential osmotic pressure of a cell decreases due to water intake and increases with water excretion. The cell sap contains a relatively high concentration of solutes and entering of water molecule trough the cell wall has controlled by pressure. The turgid cells have a suction force equal to zero, whereas turgor and osmotic pressures were equalized. Plant cells between the saturated state and the wilting state have a suction force corresponding to the negative value of the water potential. When the plant cells are partially dehydrated, the turgor pressure corresponds to zero and the suction force corresponds to the value of the potential osmotic pressure. When the plant cell is saturated, the turgor pressure takes a negative sign, and the suction force corresponds to the sum of the turgor and osmotic pressure. Small changes in Ψs usually accompanied such changes of turgor pressure. The water uptake to the cells leads to positive sign of pressure potential Ψp. Water absorption through the roots is possible only when there is a corresponding drop in water potential. The water potential was significantly affected by humidity, because in conditions of high humidity, the water potential is quite high, while in conditions when the air is quite dry, there are large differences between aboveground plant parts and the atmosphere. The amount of water available to the plant is in balance with the physiological capacity of the plant to absorb water and the environmental conditions that affect the intensity of plant transpiration. The movement of water in the plantenvironment system always takes the place of water and solutes movement from the area of higher potential to the area of lower potential until the concentrations equalize. The water potential in the leaves of plants rating from −10 to −100 bar, while in the atmosphere, in the conditions of relative humidity, up to 50% of the potential reaches up to −1000 bar. A large difference in the water potential gradient of plant leaves and the atmosphere causes transpiration. Negative water potential usually occurs in leaves than the root cells. Similar happens when water enters the root of plants, through the apoplast where the driving force is the difference between the water potential of the xylem and the soil solution. When the water potential of a xylem solution is more negative than the water potential of soil solution, water enters the root and moves all the way to the endoderm. Van den Honert [21] explains the differences in potential that occurs when water enters into the root or exits by transpiration. An increase in the water potential due to an increase in relative humidity of air and a decrease in the water potential of the soil due to desiccation leads to a slower transfer of water from the roots to the aboveground organs of the plant. The lack of water is most reflected in the decline in turgidity of plant

**80**

cells and thus in the overall appearance of the plant, with the leaves falling down, twisting, decreasing the intensity of photosynthesis, and reducing all other metabolic activities [22]. Any deviation of the available amount of moisture from the optimal in plants results in stress and the plant wilting and earlier the deterioration of plant tissue occurs [23]. In conditions when the plants are short of moisture for a long time, it passes into the phase of permanent or reversible wilting and then dies.

## **3.3 Plant water balance water status**

Plant water balance water status in plants is very important for plant growth and development, and metabolic activities, and especially, the lack of water drastically affects the length of the growing season and crop yields [23]. The total ratio between the water absorbed by the root system and the water released by transpiration from aboveground plant part is referred to as water balance. In cases when the transpiration of water exceeds its absorption from the substrate, water deficit occurs. The physiological processes in a plant depend on the amount of available moisture. The lack of water leads to inhibition of growth and photosynthesis. In addition, it acts on cell division and inhibits the synthesis of cell wall components and proteins, causing a stomata closure. Environmental conditions significantly affect the water potential, which is negative in conditions of well-available humidity, and in arid environments, the negative pressure was even more pronounced. Water potential was not only response to the environmental condition but also to the plant genetic characteristics [19]. The water transfer presents passive process so water movement to the plants occurs due to the low water potential of the plant in regard to the water potential of the soil. Since turgor pressure cannot be directly affected, the plant cell can regulate the water balance only by active regulation of the osmotic potential. Osmoregulation is the adaptation of osmotically active substances in the cell to the newly occurring environmental changes that largely control the water potential of the cell. This adjustment can be achieved by receiving/releasing or synthesis/decomposition of osmotically active substances. Under drought stress, plants are able to maintain water adsorption by increasing the cellular solute concentration, a process called osmotic adjustment. Osmotic adjustment occurs when the osmotic potential of a cell changes due to an increase or decrease in the content of osmotically active substances. Many cells respond to water stress by increasing the water potential of the cell. In this way, the decline in turgor can be prevented or minimized. Osmotic adaptation is an important feature of delaying dehydration in water-constrained environments because it maintains cell turgor and physiological processes with the development of water deficit [24]. Osmoregulation and osmotic adjustment are two different mechanisms. Glycophytes and halophytes have the ability to adapt to stress in two ways. One is that under conditions of water deficiency, plants can synthesize organic substances such as organic soluble substances, such as glycine betaine or proline, or high concentrations of inorganic ions [25]. The water balance of a plant depends on the rate of water uptake and excretion.

#### **3.4 Absorption, transport, and transpiration of water**

Water movement in the plant occurs predominantly regarding the passive or active transport of osmotic active substances along with water across the membrane. Main trigger for such solutes moving through the plant cells is difference in water potential followed by a difference in pressure (**Figure 2**).

The entry of water into the root cells occurs passively, that is, diffusely, and the solution moves freely through the apoplasmic space. Since the water molecule uncharged, it can very easily cross the membrane without hindrance and continue to

**Figure 2.** *Water movement through the plant root and root elements.*

move upward through the conductive elements of the xylem by mass flow. Mass flow is also a passive mode of water transport, which is used for long-distance transport. While diffusion is a way of moving water and solutes over shorter distances which mainly occurs at the entry of water into the root cells and the exit of water through the stoma into the atmosphere, which occurs mainly in nonvascular tissues [26]. In the rhizosphere layer, water generally moves by mass flow to the site of adsorption. However, after contact of water and solutes with the root hairs, the mode of uptake changes significantly as other forces occur that affect the uptake mechanism. Water movement trough the rhizosphere layer depends on the texture and structure of the soil. Since more permeable, sandy soils have weaker buffering capacity, so they tend to dry out quickly, while compacted, clayey soils have very limited capacity to receive and conduct water and nutrients, and transitional soil types are in terms of permeability and moisture retention and nutrients mobility of moderate capacity.

## *3.4.1 Absorption of water*

Absorption of water requires a close contact of the roots of an intact plant and soil particles in the aqueous soil phase. The larger the volume of the roots and the root zone in the rhizosphere layer of the soil the greater the possibility of absorption. Root hair presents a tissue of the rhizoderm (root epidermis) that pronounce the root surface area affecting the root capacity to absorb water and minerals. Water enters the root in the root hair growth zone and in the root tip zone. Older parts of the root are often impermeable to water. Cracks in the root bark, as well as the growth of the secondary (lateral) root, allow water to be received by older parts of the root as well. Plants have relatively low water use efficiency and therefore need to receive large amounts of water. The water potential reduction among soil particles and the root hair occurs in order to root absorb the moisture from the soil. Water potentials in the soil (expressed as the suction tension in the soil) determine the potential imbibitions pressure caused by hydration and capillary forces. Water potential becomes more negative as soil dries out, and the imbibitions pressure increases potential, because water retained only in narrow capillaries, which is also

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conductive cylinder from the root cortex.

difference in gradient of water potential.

*3.4.2 Water transport*

hydrostatic potential.

in absorption die off, resulting in asymmetric root growth.

case with potential osmotic pressure. Root growth usually follows soil moisture sources and growth intensively while those parts that are no longer actively involved

Water can move from the epidermis to the root endoderm in such a way that in apoplastic cells, water passes exclusively through cell walls and intercellular spaces, without passing through the membrane, while in the cell pathway, water passes through protoplasts. The cell pathway has two components: the transmembrane pathway in which water passes from cell to cell through plasma membranes and the symplasmic pathway through which water from cell to cell passes through plasmodesmata [11]. When passing water to the root endoderm, all three types of pathways were usually combined. The root endoderm is single layered and separates the

The movement of water through the apoplast of the endoderm of the cortex blocks the Casparian stripes located in the radial cells of the endoderm. Casparian cells are narrower or wider suberized cells, located around vascular elements of plant tissue. In younger cells, these stripes are similar in thickness to other cell sections, while older cells show these sites as more pronounced. Casparian stripes are barriers, so the water flow and solutes at that point cannot pass through the intercellular space of the root parenchyma, but must take place through the plasma membrane all the way to the endodermal cells—*apoplastic pathway of water movement*. The *symplastic pathway* of water movement takes place from cell to cell through plasmodesmata where water does not cross cell membranes. Unlike the apoplast, the symplast is the living part plants composed of interconnected cytoplasmic cells with the help of plasmodesmata that grow on the cell wall interconnecting cells into a symplast. When water moves through the apoplast and symplast, water does not cross the cell membrane, because this water movement caused by difference in

Water channels as the integrated part of membranes mostly responsible for transcellular transport of water, of which aquaporin's standout, which originate from larger protein families of major intrinsic proteins (MIP) forming pore channels in cell membranes and mediate in many other physiological processes [27]. The transcellular pathway is the movement of water from cell to cell where water crosses cell membranes, entering and getting out of the cells. The transcellular pathway also includes the entry of water into the vacuole, that is, the transport of water through the tonoplast. Because of that, the transcellular water movement was driven by the

Xylem water transport presents the longest path taken by water in a plant (long distance transport). Almost total amount of water moved through the vascular cells was transported by xylem. Compared to transporting water through living cells, the xylem built of dead cell that provides little resistance to water movement. Xylem presents specialized tissue cells for the water and solutes transport. The xylem elements anatomy allows very efficient transport of large amounts of water for long distance. The two types of dead cells that make up the xylem elements: tracheid and trachea. These are cells with lignified, thickened secondary cell walls. Tracheids are spindle cells that communicate with neighboring cells through numerous pores in the walls. Pores are microscopic areas in which there is no secondary wall, and the primary wall is thin and porous. The tracheae were shorter and wider than the tracheid and have perforated walls that form perforated plates at the end. The conduction of water by xylem elements is the result of the action of cohesion and adhesion forces on the conductive wall elements. Within the conductive elements of case with potential osmotic pressure. Root growth usually follows soil moisture sources and growth intensively while those parts that are no longer actively involved in absorption die off, resulting in asymmetric root growth.

## *3.4.2 Water transport*

*Soil Moisture Importance*

move upward through the conductive elements of the xylem by mass flow. Mass flow is also a passive mode of water transport, which is used for long-distance transport. While diffusion is a way of moving water and solutes over shorter distances which mainly occurs at the entry of water into the root cells and the exit of water through the stoma into the atmosphere, which occurs mainly in nonvascular tissues [26]. In the rhizosphere layer, water generally moves by mass flow to the site of adsorption. However, after contact of water and solutes with the root hairs, the mode of uptake changes significantly as other forces occur that affect the uptake mechanism. Water movement trough the rhizosphere layer depends on the texture and structure of the soil. Since more permeable, sandy soils have weaker buffering capacity, so they tend to dry out quickly, while compacted, clayey soils have very limited capacity to receive and conduct water and nutrients, and transitional soil types are in terms of perme-

ability and moisture retention and nutrients mobility of moderate capacity.

Absorption of water requires a close contact of the roots of an intact plant and soil particles in the aqueous soil phase. The larger the volume of the roots and the root zone in the rhizosphere layer of the soil the greater the possibility of absorption. Root hair presents a tissue of the rhizoderm (root epidermis) that pronounce the root surface area affecting the root capacity to absorb water and minerals. Water enters the root in the root hair growth zone and in the root tip zone. Older parts of the root are often impermeable to water. Cracks in the root bark, as well as the growth of the secondary (lateral) root, allow water to be received by older parts of the root as well. Plants have relatively low water use efficiency and therefore need to receive large amounts of water. The water potential reduction among soil particles and the root hair occurs in order to root absorb the moisture from the soil. Water potentials in the soil (expressed as the suction tension in the soil) determine the potential imbibitions pressure caused by hydration and capillary forces. Water potential becomes more negative as soil dries out, and the imbibitions pressure increases potential, because water retained only in narrow capillaries, which is also

**82**

*3.4.1 Absorption of water*

**Figure 2.**

*Water movement through the plant root and root elements.*

Water can move from the epidermis to the root endoderm in such a way that in apoplastic cells, water passes exclusively through cell walls and intercellular spaces, without passing through the membrane, while in the cell pathway, water passes through protoplasts. The cell pathway has two components: the transmembrane pathway in which water passes from cell to cell through plasma membranes and the symplasmic pathway through which water from cell to cell passes through plasmodesmata [11]. When passing water to the root endoderm, all three types of pathways were usually combined. The root endoderm is single layered and separates the conductive cylinder from the root cortex.

The movement of water through the apoplast of the endoderm of the cortex blocks the Casparian stripes located in the radial cells of the endoderm. Casparian cells are narrower or wider suberized cells, located around vascular elements of plant tissue. In younger cells, these stripes are similar in thickness to other cell sections, while older cells show these sites as more pronounced. Casparian stripes are barriers, so the water flow and solutes at that point cannot pass through the intercellular space of the root parenchyma, but must take place through the plasma membrane all the way to the endodermal cells—*apoplastic pathway of water movement*.

The *symplastic pathway* of water movement takes place from cell to cell through plasmodesmata where water does not cross cell membranes. Unlike the apoplast, the symplast is the living part plants composed of interconnected cytoplasmic cells with the help of plasmodesmata that grow on the cell wall interconnecting cells into a symplast. When water moves through the apoplast and symplast, water does not cross the cell membrane, because this water movement caused by difference in hydrostatic potential.

Water channels as the integrated part of membranes mostly responsible for transcellular transport of water, of which aquaporin's standout, which originate from larger protein families of major intrinsic proteins (MIP) forming pore channels in cell membranes and mediate in many other physiological processes [27]. The transcellular pathway is the movement of water from cell to cell where water crosses cell membranes, entering and getting out of the cells. The transcellular pathway also includes the entry of water into the vacuole, that is, the transport of water through the tonoplast. Because of that, the transcellular water movement was driven by the difference in gradient of water potential.

Xylem water transport presents the longest path taken by water in a plant (long distance transport). Almost total amount of water moved through the vascular cells was transported by xylem. Compared to transporting water through living cells, the xylem built of dead cell that provides little resistance to water movement. Xylem presents specialized tissue cells for the water and solutes transport. The xylem elements anatomy allows very efficient transport of large amounts of water for long distance. The two types of dead cells that make up the xylem elements: tracheid and trachea. These are cells with lignified, thickened secondary cell walls. Tracheids are spindle cells that communicate with neighboring cells through numerous pores in the walls. Pores are microscopic areas in which there is no secondary wall, and the primary wall is thin and porous. The tracheae were shorter and wider than the tracheid and have perforated walls that form perforated plates at the end. The conduction of water by xylem elements is the result of the action of cohesion and adhesion forces on the conductive wall elements. Within the conductive elements of the xylem, continuous water columns are formed, which can, due to high tension, cause the interruption and appearance of air bubbles or the so-called embolism. The xylem transport presents of water movement for a long distance pathway from soil to aboveground vegetative plants part under the differences in pressure gradient [28]. The pressure gradient is responsible for the main transpiration flow of water, however, sometimes although less efficient transpiration of water can take over the root pressure in certain condition. In that condition, root pressure mostly show values less than 0.1 MPA [29] that correspond very high humidity as result of the high difference between daily and night temperatures. In such terms, guttation could occur as a result of water transport on leaf edges through specialized pores called hydathode [30]. In addition to guttation, root pressure is also responsible for the appearance of exudates of plant tissue in the case of mechanical injuries or cuts [31]. The amount of exudate that the plant secretes primarily depends on the condition of the plant and environment. Root pressure consumes metabolic energy and can therefore be inhibited by respiratory toxins or low temperatures in the root area. Water movement through the xylem requires the differences in gradient pressure; it can also occur due to the negative pressure (vacuum) that develops by transpiration (loss of water through the coup). The root pressure (0.05–0.5 MPa) cannot develop in conditions of low soil water potential or intensive transpiration and given that the pressure required for long-distance water transport is up to 3 MPa. Due to the loss of water by transpiration, tension (negative hydrostatic pressure) occurs which moves the water upward in xylem. For the transport of water and solutes from the roots to the aboveground parts, the most significant suction force is the aboveground organs of the plant, which create a negative pressure as a result of transpiration and root pressure. The mechanism by which tension drives water by xylem called transpiration suction.

#### *3.4.3 Transpiration of water*

Negative hydrostatic pressure develops on the surface of the cell walls below the stomata due to the loss of water by transpiration. The water movement in vascular elements of plant tissue was explained by transpiration cohesion tension model due to the forces that act based on the existence of soil-plant atmosphere continuum. Transpiration makes releases of water through the stomata on leaf surfaces. The capillary and cohesive forces cause water to enter the root cells from the soil, then go through the xylem elements all the way to the leaves of the plant to make up for the lost water. Xylem is a passive conductive element through which water columns move under pressure. Therefore, on the upper side of the water column, the water is sucked due to transpiration, which allows the entire column to move upward. In order to increase the contact area between water and air, it is necessary to break the hydrogen bonds between water molecules, for which energy must be invested, and this energy represents the surface tension. Surface tension plays an important role in the transport (movement) of water in the soil-plant-atmosphere system. In addition to surface tension, cohesion and adhesion forces are important in water transport. Cohesion is the force by which water molecules are attracted to each other, while adhesion causes water molecules to adhere to another solid (e.g., a cell wall). Cohesion, adhesion, and tension allow the appearance of capillarity, that is, the rise of a column of water through a narrow tube—capillary, where the water level in the capillary is higher compared to the water level in the source that supplies the capillary. Adhesion and surface tension together pull the water column in the capillary allowing it to move upward. The height of the water column depends on its mass, and the water column will rise through the capillary until the mass of the

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mass and the adhesion and surface tension will be higher.

apoplastic pathway during transpiration probably dominates [32].

the epidermis of the leaf, a process that is called transpiration.

The actual flow rate of the xylem content is difficult to determine because the substances traveling through the xylem were constantly alternating with the environment of the conductive elements. The rate of transpiration flow increases with increasing transpiration rate until rupture control begins to operate, and there are no difficulties in water supply. In this case, short-term fluctuations in transpiration can be manifested as changes in the speed of transpiration flow. The speed of the transpiration flow shows the daily rhythm: in the morning, the transpiration begins and the water movement starts, and in the evening when the stoma close, the transpiration flow is interrupted. This process does not require energy and it is a passive way of water flow. The plants transpire most of the water over the stoma and release the water in the form of water vapor. Some of the moisture can be

Xylem elements are structurally adapted to large changes in pressure. Pressure changes are dependent on temperature oscillations and, as mentioned earlier, can cause bubbles to appear in the conductive elements—embolism or cavitation. Such bubbles only briefly interfere with transpiration flow, and this problem easily overcomes, thanks to the numerous pores in the walls of the trachea and tracheid. Such occurrences of bubbles in the conductive elements interfere with the normal transpiration flow and the established pressure, so they can often affect the photosynthetic activity and other physiological processes of the intact plant [34]. Embolic or cavitation is often occurred in very high tree that can develop high tension needed for transpiration flow. The cell wall balances the volume changes without major change in water potential that occurs because of water loss by transpiration. Changes in water potential are related to changes in volume in cells that are mainly the result of transpiration. Differences in hydrostatic potential (Ψp) result in changes in cellular water potential (Ψw) with respect to the cell wall strength. On the way out of the leaf to the atmosphere, water passes from the xylem to the cellular walls of mesophilic cells and from there evaporates into intercellular spaces of leaves. From the leaves, water released by diffusion in the form of water vapor through the small openings of the dental apparatus (stomata), which are located in

water column equals the action of surface tension and adhesion forces. The narrower the capillary, the higher the water column in it will be because it will have less

The leaf cell wall system acts as a network of microscopic pores filled with water that adheres to cellulose microfibrils of the walls. Leaf mesophilic cells are located below the stomata, and in conditions when the stomas are open, they are in direct contact with the surrounding atmosphere. As water evaporates into the surrounding atmosphere, the surface of the water retreats into the interspaces between the cells where curves of the contact surface between air and water are created. Because of the surface water tension, menisci cause tension, that is, negative hydrostatic pressure. As water evaporates more and more, the menisci become deeper (more curved) and the tension increases (more and more negative hydrostatic pressure). By moving water from the root to the stem through the xylem, water enters the leaves via the petiole. The petiole xylem redistributes water to the edge of the main leaf vessel, which then branches into progressively smaller veins and is incorporated into the leaf mesophyll. Since different plant species have different anatomical leaf structure and thus the arrangement of veins on the leaf for dicotyledonous plants, it is considered that most of the water used for transpiration of this plant is stored in smaller veins [32, 33]. After the water leaves the xylem, it moves through the bundle cells that surround the veins. It is not yet clear which exact path of water follows after exiting the xylem through bundle cells and enters into mesophilic cells, but the

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

*Soil Moisture Importance*

tion suction.

*3.4.3 Transpiration of water*

the xylem, continuous water columns are formed, which can, due to high tension, cause the interruption and appearance of air bubbles or the so-called embolism. The xylem transport presents of water movement for a long distance pathway from soil to aboveground vegetative plants part under the differences in pressure gradient [28]. The pressure gradient is responsible for the main transpiration flow of water, however, sometimes although less efficient transpiration of water can take over the root pressure in certain condition. In that condition, root pressure mostly show values less than 0.1 MPA [29] that correspond very high humidity as result of the high difference between daily and night temperatures. In such terms, guttation could occur as a result of water transport on leaf edges through specialized pores called hydathode [30]. In addition to guttation, root pressure is also responsible for the appearance of exudates of plant tissue in the case of mechanical injuries or cuts [31]. The amount of exudate that the plant secretes primarily depends on the condition of the plant and environment. Root pressure consumes metabolic energy and can therefore be inhibited by respiratory toxins or low temperatures in the root area. Water movement through the xylem requires the differences in gradient pressure; it can also occur due to the negative pressure (vacuum) that develops by transpiration (loss of water through the coup). The root pressure (0.05–0.5 MPa) cannot develop in conditions of low soil water potential or intensive transpiration and given that the pressure required for long-distance water transport is up to 3 MPa. Due to the loss of water by transpiration, tension (negative hydrostatic pressure) occurs which moves the water upward in xylem. For the transport of water and solutes from the roots to the aboveground parts, the most significant suction force is the aboveground organs of the plant, which create a negative pressure as a result of transpiration and root pressure. The mechanism by which tension drives water by xylem called transpira-

Negative hydrostatic pressure develops on the surface of the cell walls below the stomata due to the loss of water by transpiration. The water movement in vascular elements of plant tissue was explained by transpiration cohesion tension model due to the forces that act based on the existence of soil-plant atmosphere continuum. Transpiration makes releases of water through the stomata on leaf surfaces. The capillary and cohesive forces cause water to enter the root cells from the soil, then go through the xylem elements all the way to the leaves of the plant to make up for the lost water. Xylem is a passive conductive element through which water columns move under pressure. Therefore, on the upper side of the water column, the water is sucked due to transpiration, which allows the entire column to move upward. In order to increase the contact area between water and air, it is necessary to break the hydrogen bonds between water molecules, for which energy must be invested, and this energy represents the surface tension. Surface tension plays an important role in the transport (movement) of water in the soil-plant-atmosphere system. In addition to surface tension, cohesion and adhesion forces are important in water transport. Cohesion is the force by which water molecules are attracted to each other, while adhesion causes water molecules to adhere to another solid (e.g., a cell wall). Cohesion, adhesion, and tension allow the appearance of capillarity, that is, the rise of a column of water through a narrow tube—capillary, where the water level in the capillary is higher compared to the water level in the source that supplies the capillary. Adhesion and surface tension together pull the water column in the capillary allowing it to move upward. The height of the water column depends on its mass, and the water column will rise through the capillary until the mass of the

**84**

water column equals the action of surface tension and adhesion forces. The narrower the capillary, the higher the water column in it will be because it will have less mass and the adhesion and surface tension will be higher.

The leaf cell wall system acts as a network of microscopic pores filled with water that adheres to cellulose microfibrils of the walls. Leaf mesophilic cells are located below the stomata, and in conditions when the stomas are open, they are in direct contact with the surrounding atmosphere. As water evaporates into the surrounding atmosphere, the surface of the water retreats into the interspaces between the cells where curves of the contact surface between air and water are created. Because of the surface water tension, menisci cause tension, that is, negative hydrostatic pressure. As water evaporates more and more, the menisci become deeper (more curved) and the tension increases (more and more negative hydrostatic pressure).

By moving water from the root to the stem through the xylem, water enters the leaves via the petiole. The petiole xylem redistributes water to the edge of the main leaf vessel, which then branches into progressively smaller veins and is incorporated into the leaf mesophyll. Since different plant species have different anatomical leaf structure and thus the arrangement of veins on the leaf for dicotyledonous plants, it is considered that most of the water used for transpiration of this plant is stored in smaller veins [32, 33]. After the water leaves the xylem, it moves through the bundle cells that surround the veins. It is not yet clear which exact path of water follows after exiting the xylem through bundle cells and enters into mesophilic cells, but the apoplastic pathway during transpiration probably dominates [32].

Xylem elements are structurally adapted to large changes in pressure. Pressure changes are dependent on temperature oscillations and, as mentioned earlier, can cause bubbles to appear in the conductive elements—embolism or cavitation. Such bubbles only briefly interfere with transpiration flow, and this problem easily overcomes, thanks to the numerous pores in the walls of the trachea and tracheid. Such occurrences of bubbles in the conductive elements interfere with the normal transpiration flow and the established pressure, so they can often affect the photosynthetic activity and other physiological processes of the intact plant [34]. Embolic or cavitation is often occurred in very high tree that can develop high tension needed for transpiration flow. The cell wall balances the volume changes without major change in water potential that occurs because of water loss by transpiration. Changes in water potential are related to changes in volume in cells that are mainly the result of transpiration. Differences in hydrostatic potential (Ψp) result in changes in cellular water potential (Ψw) with respect to the cell wall strength. On the way out of the leaf to the atmosphere, water passes from the xylem to the cellular walls of mesophilic cells and from there evaporates into intercellular spaces of leaves. From the leaves, water released by diffusion in the form of water vapor through the small openings of the dental apparatus (stomata), which are located in the epidermis of the leaf, a process that is called transpiration.

The actual flow rate of the xylem content is difficult to determine because the substances traveling through the xylem were constantly alternating with the environment of the conductive elements. The rate of transpiration flow increases with increasing transpiration rate until rupture control begins to operate, and there are no difficulties in water supply. In this case, short-term fluctuations in transpiration can be manifested as changes in the speed of transpiration flow. The speed of the transpiration flow shows the daily rhythm: in the morning, the transpiration begins and the water movement starts, and in the evening when the stoma close, the transpiration flow is interrupted. This process does not require energy and it is a passive way of water flow. The plants transpire most of the water over the stoma and release the water in the form of water vapor. Some of the moisture can be

evaporated by plants through lenticels or cuticles, although it is a very small amount of moisture. The intensity of transpiration is related to the size of the leaf area, the number and size of stoma, the appearance of the leaf surface, and of course, the environmental conditions in which the plant is located. If the plant suffers more damage during intensive transpiration, it must compensate for the water by receiving it from the soil. The numerous open stomata allow the exchange of O2 and CO2 gases and the evaporation of water. If the air immediately around the leaf is dry, water vapor molecules move from saturated air to the unsaturated external atmosphere according to the law of diffusion. The function of the stoma is to facilitate the excretion of water vapor by opening it, and on the other hand, to make it difficult for stomata transpiration by closing it with the insufficient water supply [35]. The mechanism of opening and closing of the stomata works based on the water and osmotic potential of the gate cells. In order to act by opening of stomata, the gate cells after the water entering should have water potential lower than the water potential of the surrounding cells. The water potential of the gate cells largely depends on the osmotic potential. Since gate cells contain chloroplasts, they also show photosynthetic activity. The leaves under daily conditions where maximum transpiration occurs release 50–70% water vapor. In the light period, intensity of photosynthesis may decrease for 50% or more due to the limited water supply [36, 37]. Since the gate cells are photosynthetically active, this enable them the accumulation of sugars, which reflected the osmotic potential, and the regulation of turgor pressure, responding on the mechanism of closing and opening the stomata [38]. Water has important role in the mechanism of opening and closing of stomata. When the plants are well supplied with water, the guard cells are turgescent and the stomates are open, while in conditions of water deficit, the guard cells lose turgor and the stomata are closed. Model of opening and closing of stomata would be used in genetic engineering for producing of species with reduced water requirements and better production rate [35]. The physiological activity of plants is significantly disrupted by interfering with the process of photosynthesis either through a process of reduced transpiration or altered gas uptake and release [39]. This is one of the reasons of balance maintaining between process of photosynthesis and transpiration. Concept soil-plant-atmosphere continuum is based on the decrement of tension of sap flow through the vessels, and transpiration flux is proportional to the pressure gradient in leaves [40]. Transpiration into cormophytes mostly shows

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and in Calvin process, starch compounds were created.

**4. Plant adaptation to the soil moisture regime**

a characteristic diurnal rhythm. Time control of the opening and closing of the stomata serves to maximize photosynthesis and minimize transpiration. At night when there is no photosynthesis, there is no need for CO2 absorption either, the stomates are closed and unnecessary water loss is prevented. While in the morning, when the water supply is abundant and the sun's radiation is conducive to strong photosynthetic activity, CO2 requirements are pronounced and stomates are open. Some succulent's plant have crassulacean acid metabolism (CAM) that enables plant to keep stoma open during the night and uptaking the CO2 and water making the acidification process through the malic acid building up in vacuole (**Figure 3**). During the day, stomas are close and transpiration as well as CO2 fixation stopped,

Soil moisture is primarily important for water circulation in continuum soilplant-atmosphere system. The importance of moisture is especially emphasized for the life of terrestrial plants, which are tied to the soil by their roots. Plant roots from the soil absorb water and solutes, so that they can grow and develop. The water that the plants absorb passes through the conductive elements of the plant, reaches the vegetative parts, and then comes out as water vapor trough the stomata. We say for this process that the plant transpires. The needs of plants for water vary, but the fact remains that water in agricultural production is one of the main limiting resources in gaining the optimal yields, with the implementation of regular agrotechnical measures. Soil is supplied with moisture through precipitation; however, soil moisture and moisture retention in soil pores and the pathways by which it reaches plants are different depending on the buffering capabilities of the soil. It is clear that some plants remain viable even after a long drought, because their roots manage to find moisture in the deeper layers of the soil. There are also those plants that, due to their anatomical structure, have adapted to life in arid environments. This mode of survival allows them to have large water storage capacities in mesophilic leaf cells as they have succulent leaves. Their leaves transpire in very limited quantities of water, because otherwise they would die very quickly. There are thousands of different species of plants that are adapted to living in desert conditions. Among them are common plants that are classified as succulents. The term succulent can be explained from different perspectives but is most commonly used in terms of a plant that has specialized tissues for water storage, resulting in their special morphological characteristics: thick, fleshy stems, leaves, and/or roots. Sometimes the leaves are transformed into thorns, with the photosynthetic function taken over by a thickened green stem, in other cases by geophytes that have most of their thickened water storage tissue underground, and the third is large trees that store water in huge swollen trunks. Of course, there is a continuum among plants from those that have almost no water storage tissues to those that possess highly developed tissues intended for that purpose so it is difficult, or even impossible and inaccurate, to speak of plants that are and are not succulents. It might be more accurate to use the term "plants with pronounced succulent characteristics," but for simplicity, the term "succulent" is used. Terrestrial plants usually adapt to moisture conditions and soil type by developing roots that provide them with water and nutrients. There are plants that grow on soil with very little water supply and where the plant faces drought for most of the year. In such plants, the roots are often adapted in such a way that they exceed the volume of the aboveground part of the plant. In desire, they stretched and branched to the extent that there was favorable moisture content in the soil, from which it draws strength for its growth. Often such plants show both

**Figure 3.** *CAM—Crassulacean acid metabolism in plant cells of arid area.*

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

*Soil Moisture Importance*

evaporated by plants through lenticels or cuticles, although it is a very small amount of moisture. The intensity of transpiration is related to the size of the leaf area, the number and size of stoma, the appearance of the leaf surface, and of course, the environmental conditions in which the plant is located. If the plant suffers more damage during intensive transpiration, it must compensate for the water by receiving it from the soil. The numerous open stomata allow the exchange of O2 and CO2 gases and the evaporation of water. If the air immediately around the leaf is dry, water vapor molecules move from saturated air to the unsaturated external atmosphere according to the law of diffusion. The function of the stoma is to facilitate the excretion of water vapor by opening it, and on the other hand, to make it difficult for stomata transpiration by closing it with the insufficient water supply [35]. The mechanism of opening and closing of the stomata works based on the water and osmotic potential of the gate cells. In order to act by opening of stomata, the gate cells after the water entering should have water potential lower than the water potential of the surrounding cells. The water potential of the gate cells largely depends on the osmotic potential. Since gate cells contain chloroplasts, they also show photosynthetic activity. The leaves under daily conditions where maximum transpiration occurs release 50–70% water vapor. In the light period, intensity of photosynthesis may decrease for 50% or more due to the limited water supply [36, 37]. Since the gate cells are photosynthetically active, this enable them the accumulation of sugars, which reflected the osmotic potential, and the regulation of turgor pressure, responding on the mechanism of closing and opening the stomata [38]. Water has important role in the mechanism of opening and closing of stomata. When the plants are well supplied with water, the guard cells are turgescent and the stomates are open, while in conditions of water deficit, the guard cells lose turgor and the stomata are closed. Model of opening and closing of stomata would be used in genetic engineering for producing of species with reduced water requirements and better production rate [35]. The physiological activity of plants is significantly disrupted by interfering with the process of photosynthesis either through a process of reduced transpiration or altered gas uptake and release [39]. This is one of the reasons of balance maintaining between process of photosynthesis and transpiration. Concept soil-plant-atmosphere continuum is based on the decrement of tension of sap flow through the vessels, and transpiration flux is proportional to the pressure gradient in leaves [40]. Transpiration into cormophytes mostly shows

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**Figure 3.**

*CAM—Crassulacean acid metabolism in plant cells of arid area.*

a characteristic diurnal rhythm. Time control of the opening and closing of the stomata serves to maximize photosynthesis and minimize transpiration. At night when there is no photosynthesis, there is no need for CO2 absorption either, the stomates are closed and unnecessary water loss is prevented. While in the morning, when the water supply is abundant and the sun's radiation is conducive to strong photosynthetic activity, CO2 requirements are pronounced and stomates are open. Some succulent's plant have crassulacean acid metabolism (CAM) that enables plant to keep stoma open during the night and uptaking the CO2 and water making the acidification process through the malic acid building up in vacuole (**Figure 3**). During the day, stomas are close and transpiration as well as CO2 fixation stopped, and in Calvin process, starch compounds were created.
