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Main page > TAMOP 4.2.5 Book Database > Books > Applied Sciences > Agriculture
Plant Physiology
Ördög Vince, Molnár Zoltán (2011)
Debreceni Egyetem, Nyugat-Magyarországi Egyetem, Pannon Egyetem
Beágyazás
Chapter 2. Water and nutrients in plant
Table of Contents
Water balance of plantWater potentialAbsorption by rootsTransport through the xylemTranspirationPlant water statusInfluence of extreme water supplyNutrient supply of plantEssential nutrientsNutrient uptakeSolute transportNutritional deficiencies
Water balance of plant
Water in plant life
Water plays a crucial role in the life of plant. It is the most abundant constituents of most organisms. Water typically accounts for more than 70 percent by weight of non-woody plant parts. The water content of plants is in a continual state of flux. The constant flow of water through plants is a matter of considerable significance to their growth and survival. The uptake of water by cells generates a pressure known as turgor. Photosynthesis requires that plants draw carbon dioxide from the atmosphere, and at the same time exposes them to water loss. To prevent leaf desiccation, water must be absorbed by the roots, and transported through the plant body. Balancing the uptake, transport, and loss of water represents an important challenge for land plants. The thermal properties of water contribute to temperature regulation, helping to ensure that plants do not cool down or heat up too rapidly. Water has excellent solvent properties. Many of the biochemical reactions occur in water and water is itself either a reactant or a product in a large number of those reactions.
The practice of crop irrigation reflects the fact that water is a key resource limiting agricultural productivity. Water availability likewise limits the productivity of natural ecosystems (Figure 1.1). Plants use water in huge amounts, but only small part of that remains in the plant to supply growth. About 97% of water taken up by plants is lost to the atmosphere, 2% is used for volume increase or cell expansion, and 1% for metabolic processes, predominantly photosynthesis. Water loss to the atmosphere appears to be an inevitable consequence of carrying out photosynthesis. The uptake of CO2 is coupled to the loss of water (Figure 1.2). Because the driving gradient for water loss from leaves is much larger than that for CO2 uptake, as many as 400 water molecules are lost for every CO2 molecule gained.
Figure 1.1 Productivity of various ecosystems as a function of annual precipitation (source: Taiz L., Zeiger E., 2010)
Figure 1.2 Water pathway through the leaf (source: Taiz L., Zeiger E., 2010)
Water potential
The structure and properties of water
Water consists of an oxygen atom covalently bonded to two hydrogen atoms (Figure 1.3). The oxygen atom carries a partial negative charge, and a corresponding partial positive charge is shared between the two hydrogen atoms. This asymmetric electron distribution makes water a polar molecule. However, the partial charges are equal, and the water remains a neutral molecule. There is a strong electrical attraction between adjacent water molecules or between water and other polar molecules, which is called hydrogen bonding. The hydrogen bonding ability of water and its polar structure make it a particularly good solvent for ionic substances and for molecules such as sugars and proteins. The hydration shells that form around biologically important macromolecules are often referred to as bound water. Bound water prevents protein molecules from approaching close enough to form aggregates large enough to precipitate.
Figure 1.3 A) Structure of a water molecule B) Hydrogen bonds among water molecules (source: Hopkins W.G., Hüner N.P.A., 2009)
The extensive hydrogen bonding between water molecules results in water having both a high specific heat capacity and a high latent heat of vaporization. Because of its highly ordered structure, liquid water also has a high thermal conductivity. This means that it rapidly conducts heat away from the point of application. The combination of high specific heat and thermal conductivity enables water to absorb and redistribute large amounts of heat energy without correspondingly large increases in temperature. The heat of biochemical reactions may be quickly dissipated throughout the cell. Compared with other liquids, water requires a relatively large heat input to raise its temperature. This is important for plants, because it helps buffer temperature fluctuations. The latent heat of vaporization decreases as temperature increases, reaching a minimum at the boiling point (100°C). For water at 25°C, the heat of vaporization is 44kJ mol-1 – the highest value known for any liquid.
The excellent solvent properties of water are due to the highly polar character of the water molecule. The polarity of molecules can be measured by a quantity known as thedielectric constant. Water has one of the highest dielectric constant, which is as high as 78.4. The dielectric constant of benzene and hexane is 2.3 and 1.9, respectively. Water is thus an excellent solvent for charged ions or molecules, which dissolve very poorly in non-polar organic liquids.
The extensive hydrogen bonding in water gives a new property known as cohesion, the mutual attraction between molecules. A related property, called adhesion, is the attraction of water to a solid phase, such as cell wall. The water molecules are highly cohesive. One consequence of cohesion is that water has exceptionally high surface tension, which is the energy required to increase the surface area of a gas-liquid interface. Surface tension and adhesion at the evaporative surfaces in leaves generate the physical forces that pull water through the plant’s vascular system. Cohesion, adhesion and surface tension give rise to a phenomenon known as capillarity. These combined properties of water help to explain why water rises in capillary tubes and are exceptionally important in maintaining the continuity of water columns in plants.
Hydrogen bonding gives water a high tensile strength, defined as the maximum force per unit area that a continuous column of water can withstand before breaking. Water can resist pressures more negative than -20 MPa, where the negative sign indicates tension, as opposed to compression. Pressure is measured in units called pascals (Pa), or more conveniently, megapascals (MPa). One MPa equals approximately 9.9 atmospheres.
Water movement by diffusion, osmosis and bulk flow
Movement of substances from one region to another is commonly referred to as translocation. Mechanisms for translocation may be classified as either active or passive. It is sometimes difficult to distinguish between active and passive transport, but the translocation of water is clearly a passive process. Passive movement of most substances can be accounted for by bulk flow or diffusion. The diffusion of water across a selectively permeable barrier is known as osmosis, which must also be taken into account.
Bulk flow accounts for some water movement in plants through the xylem tissues of plants. Movement of materials by bulk flow (or mass flow) is pressure driven. Bulk flow occurs when an external force, such as gravity or pressure, is applied. As a result, all of the molecules of the substance move in mass. Bulk flow is pressure-driven, diffusion is driven principally by concentration differences.
The molecules in a solution are not static, they are in continuous motion. Diffusion results in the net movement of molecules from regions of high concentration to regions of low concentration. This tendency for a system to evolve toward and even distribution of molecules can be understood as a consequence of the second law of thermodynamics, which tells us that spontaneous processes evolve in the direction of increasing entropy or disorder. Diffusion represents the natural tendency of systems to move toward the lowest possible energy state. Fick’s first law describes the process of diffusion, which is most effective over short distances. Diffusion in solutions can be effective within cellular dimensions but is far too slow to be effective over long distances. The average time required for a glucose molecule to diffuse across a cell with a diameter of 50 µm is 2.5 s. However, the average time needed for the same glucose molecule to diffuse a distance of 1 m in water is approximately 32 years.
The net movement of water across a selectively permeable barrier is referred to as osmosis. Membranes of plant cells are selectively permeable. The diffusion of water directly across the lipid bilayer is facilitated by aquaporins, which are integral membrane proteins that form water-selective channels across membrane. In osmosis the maximization of entropy is realized by the volume of solvent diffusing through the membrane to dilute the solute. Osmosis can be easily demonstrated using a device known as an osmometer, constructed by closing off the open end of a thistle tube with a selectively permeable membrane (Figure 1.4). If the tube is filled with a sugar solution and inverted in a volume of pure water, the volume of solution in the tube will increase over time. The increase in the volume of the solution will continue until the hydrostatic pressure developed in the tube is sufficient to balance the force driving the water into the solution.
Figure 1.4 A demonstration of hydrostatic pressure (source: Hopkins W.G., Hüner N.P.A., 2009)
The concept of water potential
All living things, including plants, require a continuous input of free energy to maintain and repair their highly organised structures, as well as to grow and reproduce. Chemical potential is a quantitative expression of the free energy associated with a substance. The chemical potential of the water represents the free energy associated with wa
Ugrás a tartalomhozLoginInformationContactMagyarA- A A+Front pageNewsBrowsingMain page > TAMOP 4.2.5 Book Database > Books > Applied Sciences > AgriculturePlant PhysiologyÖrdög Vince, Molnár Zoltán (2011)Debreceni Egyetem, Nyugat-Magyarországi Egyetem, Pannon EgyetemBeágyazásChapter 2. Water and nutrients in plantTable of ContentsWater balance of plantWater potentialAbsorption by rootsTransport through the xylemTranspirationPlant water statusInfluence of extreme water supplyNutrient supply of plantEssential nutrientsNutrient uptakeSolute transportNutritional deficienciesWater balance of plantWater in plant lifeWater plays a crucial role in the life of plant. It is the most abundant constituents of most organisms. Water typically accounts for more than 70 percent by weight of non-woody plant parts. The water content of plants is in a continual state of flux. The constant flow of water through plants is a matter of considerable significance to their growth and survival. The uptake of water by cells generates a pressure known as turgor. Photosynthesis requires that plants draw carbon dioxide from the atmosphere, and at the same time exposes them to water loss. To prevent leaf desiccation, water must be absorbed by the roots, and transported through the plant body. Balancing the uptake, transport, and loss of water represents an important challenge for land plants. The thermal properties of water contribute to temperature regulation, helping to ensure that plants do not cool down or heat up too rapidly. Water has excellent solvent properties. Many of the biochemical reactions occur in water and water is itself either a reactant or a product in a large number of those reactions.The practice of crop irrigation reflects the fact that water is a key resource limiting agricultural productivity. Water availability likewise limits the productivity of natural ecosystems (Figure 1.1). Plants use water in huge amounts, but only small part of that remains in the plant to supply growth. About 97% of water taken up by plants is lost to the atmosphere, 2% is used for volume increase or cell expansion, and 1% for metabolic processes, predominantly photosynthesis. Water loss to the atmosphere appears to be an inevitable consequence of carrying out photosynthesis. The uptake of CO2 is coupled to the loss of water (Figure 1.2). Because the driving gradient for water loss from leaves is much larger than that for CO2 uptake, as many as 400 water molecules are lost for every CO2 molecule gained.Figure 1.1 Productivity of various ecosystems as a function of annual precipitation (source: Taiz L., Zeiger E., 2010)Figure 1.2 Water pathway through the leaf (source: Taiz L., Zeiger E., 2010)Water potentialThe structure and properties of waterWater consists of an oxygen atom covalently bonded to two hydrogen atoms (Figure 1.3). The oxygen atom carries a partial negative charge, and a corresponding partial positive charge is shared between the two hydrogen atoms. This asymmetric electron distribution makes water a polar molecule. However, the partial charges are equal, and the water remains a neutral molecule. There is a strong electrical attraction between adjacent water molecules or between water and other polar molecules, which is called hydrogen bonding. The hydrogen bonding ability of water and its polar structure make it a particularly good solvent for ionic substances and for molecules such as sugars and proteins. The hydration shells that form around biologically important macromolecules are often referred to as bound water. Bound water prevents protein molecules from approaching close enough to form aggregates large enough to precipitate.Figure 1.3 A) Structure of a water molecule B) Hydrogen bonds among water molecules (source: Hopkins W.G., Hüner N.P.A., 2009)The extensive hydrogen bonding between water molecules results in water having both a high specific heat capacity and a high latent heat of vaporization. Because of its highly ordered structure, liquid water also has a high thermal conductivity. This means that it rapidly conducts heat away from the point of application. The combination of high specific heat and thermal conductivity enables water to absorb and redistribute large amounts of heat energy without correspondingly large increases in temperature. The heat of biochemical reactions may be quickly dissipated throughout the cell. Compared with other liquids, water requires a relatively large heat input to raise its temperature. This is important for plants, because it helps buffer temperature fluctuations. The latent heat of vaporization decreases as temperature increases, reaching a minimum at the boiling point (100°C). For water at 25°C, the heat of vaporization is 44kJ mol-1 – the highest value known for any liquid.
The excellent solvent properties of water are due to the highly polar character of the water molecule. The polarity of molecules can be measured by a quantity known as thedielectric constant. Water has one of the highest dielectric constant, which is as high as 78.4. The dielectric constant of benzene and hexane is 2.3 and 1.9, respectively. Water is thus an excellent solvent for charged ions or molecules, which dissolve very poorly in non-polar organic liquids.
The extensive hydrogen bonding in water gives a new property known as cohesion, the mutual attraction between molecules. A related property, called adhesion, is the attraction of water to a solid phase, such as cell wall. The water molecules are highly cohesive. One consequence of cohesion is that water has exceptionally high surface tension, which is the energy required to increase the surface area of a gas-liquid interface. Surface tension and adhesion at the evaporative surfaces in leaves generate the physical forces that pull water through the plant’s vascular system. Cohesion, adhesion and surface tension give rise to a phenomenon known as capillarity. These combined properties of water help to explain why water rises in capillary tubes and are exceptionally important in maintaining the continuity of water columns in plants.
Hydrogen bonding gives water a high tensile strength, defined as the maximum force per unit area that a continuous column of water can withstand before breaking. Water can resist pressures more negative than -20 MPa, where the negative sign indicates tension, as opposed to compression. Pressure is measured in units called pascals (Pa), or more conveniently, megapascals (MPa). One MPa equals approximately 9.9 atmospheres.
Water movement by diffusion, osmosis and bulk flow
Movement of substances from one region to another is commonly referred to as translocation. Mechanisms for translocation may be classified as either active or passive. It is sometimes difficult to distinguish between active and passive transport, but the translocation of water is clearly a passive process. Passive movement of most substances can be accounted for by bulk flow or diffusion. The diffusion of water across a selectively permeable barrier is known as osmosis, which must also be taken into account.
Bulk flow accounts for some water movement in plants through the xylem tissues of plants. Movement of materials by bulk flow (or mass flow) is pressure driven. Bulk flow occurs when an external force, such as gravity or pressure, is applied. As a result, all of the molecules of the substance move in mass. Bulk flow is pressure-driven, diffusion is driven principally by concentration differences.
The molecules in a solution are not static, they are in continuous motion. Diffusion results in the net movement of molecules from regions of high concentration to regions of low concentration. This tendency for a system to evolve toward and even distribution of molecules can be understood as a consequence of the second law of thermodynamics, which tells us that spontaneous processes evolve in the direction of increasing entropy or disorder. Diffusion represents the natural tendency of systems to move toward the lowest possible energy state. Fick’s first law describes the process of diffusion, which is most effective over short distances. Diffusion in solutions can be effective within cellular dimensions but is far too slow to be effective over long distances. The average time required for a glucose molecule to diffuse across a cell with a diameter of 50 µm is 2.5 s. However, the average time needed for the same glucose molecule to diffuse a distance of 1 m in water is approximately 32 years.
The net movement of water across a selectively permeable barrier is referred to as osmosis. Membranes of plant cells are selectively permeable. The diffusion of water directly across the lipid bilayer is facilitated by aquaporins, which are integral membrane proteins that form water-selective channels across membrane. In osmosis the maximization of entropy is realized by the volume of solvent diffusing through the membrane to dilute the solute. Osmosis can be easily demonstrated using a device known as an osmometer, constructed by closing off the open end of a thistle tube with a selectively permeable membrane (Figure 1.4). If the tube is filled with a sugar solution and inverted in a volume of pure water, the volume of solution in the tube will increase over time. The increase in the volume of the solution will continue until the hydrostatic pressure developed in the tube is sufficient to balance the force driving the water into the solution.
Figure 1.4 A demonstration of hydrostatic pressure (source: Hopkins W.G., Hüner N.P.A., 2009)
The concept of water potential
All living things, including plants, require a continuous input of free energy to maintain and repair their highly organised structures, as well as to grow and reproduce. Chemical potential is a quantitative expression of the free energy associated with a substance. The chemical potential of the water represents the free energy associated with wa
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