{"id":1992,"date":"2017-01-31T18:50:22","date_gmt":"2017-01-31T18:50:22","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/wm-biology2\/?post_type=chapter&#038;p=1992"},"modified":"2024-04-25T18:57:55","modified_gmt":"2024-04-25T18:57:55","slug":"water-potential","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/wm-biology2\/chapter\/water-potential\/","title":{"raw":"Water Potential","rendered":"Water Potential"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Outcomes<\/h3>\r\n<ul>\r\n \t<li>Describe how water potential influences how water is transported in plants<\/li>\r\n<\/ul>\r\n<\/div>\r\nPlants are phenomenal hydraulic engineers. Using only the basic laws of physics and the simple manipulation of potential energy, plants can move water to the top of a 116-meter-tall tree (Figure 1a). Plants can also use hydraulics to generate enough force to split rocks and buckle sidewalks (Figure 1b). Plants achieve this because of water potential.\r\n\r\n[caption id=\"attachment_1997\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-1997\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31184659\/Figure_30_05_01-1024x536.jpg\" alt=\" Photo (a) shows the brown trunk of a tall sequoia tree in a forest. Photo (b) shows a grey tree trunk growing between a road and a sidewalk. The roots have started to lift up and crack the concrete slabs of the sidewalk.\" width=\"1024\" height=\"536\" \/> Figure 1.\u00a0With heights nearing 116 meters, (a) coastal redwoods (<em>Sequoia sempervirens<\/em>) are the tallest trees in the world. Plant roots can easily generate enough force to (b) buckle and break concrete sidewalks, much to the dismay of homeowners and city maintenance departments. (credit a: modification of work by Bernt Rostad; credit b: modification of work by Pedestrians Educating Drivers on Safety, Inc.)[\/caption]\r\n\r\n<strong>Water potential<\/strong> is a measure of the potential energy in water. Plant physiologists are not interested in the energy in any one particular aqueous system, but are very interested in water movement between two systems. In practical terms, therefore, water potential is the difference in potential energy between a given water sample and pure water (at atmospheric pressure and ambient temperature). Water potential is denoted by the Greek letter \u03c8 (psi) and is expressed in units of pressure (pressure is a form of energy) called <strong>megapascals<\/strong> (MPa). The potential of pure water (\u03a8<sub>w<\/sub><sup>pure H<sub>2<\/sub>O<\/sup>) is, by convenience of definition, designated a value of zero (even though pure water contains plenty of potential energy, that energy is ignored). Water potential values for the water in a plant root, stem, or leaf are therefore expressed relative to \u03a8<sub>w<\/sub><sup>pure H<sub>2<\/sub>O<\/sup>.\r\n\r\nThe water potential in plant solutions is influenced by solute concentration, pressure, gravity, and factors called matrix effects. Water potential can be broken down into its individual components using the following equation:\r\n<p style=\"text-align: center;\">\u03a8<sub>system<\/sub>\u00a0=\u00a0\u03a8<sub>total<\/sub>\u00a0=\u00a0\u03a8<sub>s<\/sub>\u00a0+\u00a0\u03a8<sub>p<\/sub>\u00a0+\u00a0\u03a8<sub>g<\/sub>\u00a0+\u00a0\u03a8<sub>m<\/sub><\/p>\r\nwhere \u03a8<sub>s<\/sub>, \u03a8<sub>p<\/sub>, \u03a8<sub>g<\/sub>, and \u03a8m refer to the solute, pressure, gravity, and matric potentials, respectively. \u201cSystem\u201d can refer to the water potential of the soil water (\u03a8<sup>soil<\/sup>), root water (\u03a8<sup>root<\/sup>), stem water (\u03a8<sup>stem<\/sup>), leaf water (\u03a8<sup>leaf<\/sup>) or the water in the atmosphere (\u03a8<sup>atmosphere<\/sup>): whichever aqueous system is under consideration. As the individual components change, they raise or lower the total water potential of a system. When this happens, water moves to equilibrate, moving from the system or compartment with a higher water potential to the system or compartment with a lower water potential. This brings the difference in water potential between the two systems (\u0394\u03a8) back to zero (\u0394\u03a8 = 0). Therefore, for water to move through the plant from the soil to the air (a process called transpiration),\u03a8<sup>soil<\/sup>\u00a0must be &gt;\u03a8<sup>root<\/sup>\u00a0&gt;\u03a8<sup>stem<\/sup>\u00a0&gt;\u03a8<sup>leaf<\/sup>&gt;\u03a8<sup>atmosphere<\/sup>.\r\n\r\nWater only moves in response to \u0394\u03a8, not in response to the individual components. However, because the individual components influence the total \u03a8<sub>system<\/sub>, by manipulating the individual components (especially \u03a8<sub>s<\/sub>), a plant can control water movement.\r\n<h2>Solute Potential<\/h2>\r\nSolute potential (\u03a8<sub>s<\/sub>), also called osmotic potential, is negative in a plant cell and zero in distilled water. Typical values for cell cytoplasm are \u20130.5 to \u20131.0 MPa. Solutes reduce water potential (resulting in a negative \u03a8<sub>w<\/sub>) by consuming some of the potential energy available in the water. Solute molecules can dissolve in water because water molecules can bind to them via hydrogen bonds; a hydrophobic molecule like oil, which cannot bind to water, cannot go into solution. The energy in the hydrogen bonds between solute molecules and water is no longer available to do work in the system because it is tied up in the bond. In other words, the amount of available potential energy is reduced when solutes are added to an aqueous system. Thus, \u03a8<sub>s<\/sub>\u00a0decreases with increasing solute concentration. Because \u03a8<sub>s<\/sub>\u00a0is one of the four components of \u03a8<sub>system<\/sub>\u00a0or \u03a8<sub>total<\/sub>, a decrease in \u03a8<sub>s<\/sub>\u00a0will cause a decrease in \u03a8<sub>total<\/sub>. The internal water potential of a plant cell is more negative than pure water because of the cytoplasm\u2019s high solute content (Figure 2). Because of this difference in water potential water will move from the soil into a plant\u2019s root cells via the process of osmosis. This is why solute potential is sometimes called osmotic potential.\r\n\r\nPlant cells can metabolically manipulate \u03a8<sub>s<\/sub>\u00a0(and by extension,\u03a8<sub>total<\/sub>) by adding or removing solute molecules. Therefore, plants have control over \u03a8<sub>total<\/sub>\u00a0via their ability to exert metabolic control over \u03a8<sub>s<\/sub>.\r\n\r\n[caption id=\"attachment_1998\" align=\"aligncenter\" width=\"600\"]<img class=\" wp-image-1998\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31184842\/Figure_30_05_02.png\" alt=\" Illustration shows a U-shaped tube holding pure water. A semipermeable membrane, which allows water but not solutes to pass, separates the two sides of the tube. The water level on each side of the tube is the same. Beneath this tube are three more tubes, also divided by semipermeable membranes. In the first tube, solute has been added to the right side. Adding solute to the right side lowers psi-s, causing water to move to the right side of the tube. As a result, the water level is higher on the right side. The second tube has pure water on both sides of the membrane. Positive pressure is applied to the left side. Applying positive pressure to the left side causes psi-p to increase. As a results, water moves to the right so that the water level is higher on the right than on the left. The third tube also has pure water, but this time negative pressure is applied to the left side. Applying negative pressure lowers psi-p, causing water to move to the left side of the tube. As a result, the water level is higher on the left.\" width=\"600\" height=\"539\" \/> Figure 2. A semipermeable membrane between two aqueous systems[\/caption]\r\n\r\nIn Figure 2, water will move from a region of higher to lower water potential until equilibrium is reached. Solutes (\u03a8<sub>s<\/sub>), pressure (\u03a8<sub>p<\/sub>), and gravity (\u03a8<sub>g<\/sub>) influence total water potential for each side of the tube (\u03a8<sub>total<\/sub> <sup>right or left<\/sup>), and therefore, the difference between \u03a8<sub>total<\/sub>\u00a0on each side (\u0394\u03a8). (\u03a8<sub>m<\/sub>, the potential due to interaction of water with solid substrates, is ignored in this example because glass is not especially hydrophilic). Water moves in response to the difference in water potential between two systems (the left and right sides of the tube).\r\n<div class=\"textbox exercises\">\r\n<h3>Practice Question<\/h3>\r\nPositive water potential is placed on the left side of the tube by increasing \u03a8<sub>p<\/sub> such that the water level rises on the right side. Could you equalize the water level on each side of the tube by adding solute, and if so, how?\r\n\r\n[practice-area rows=\"2\"][\/practice-area]\r\n[reveal-answer q=\"229403\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"229403\"]Yes, you can equalize the water level by adding the solute to the left side of the tube such that water moves toward the left until the water levels are equal.[\/hidden-answer]\r\n\r\n<\/div>\r\n<h2>Pressure Potential<\/h2>\r\nPressure potential (\u03a8<sub>p<\/sub>), also called turgor potential, may be positive or negative (Figure 2). Because pressure is an expression of energy, the higher the pressure, the more potential energy in a system, and vice versa. Therefore, a positive \u03a8<sub>p<\/sub> (compression) increases \u03a8<sub>total<\/sub>, and a negative \u03a8<sub>p<\/sub> (tension) decreases \u03a8<sub>total<\/sub>. Positive pressure inside cells is contained by the cell wall, producing turgor pressure. Pressure potentials are typically around 0.6\u20130.8 MPa, but can reach as high as 1.5 MPa in a well-watered plant. A \u03a8<sub>p<\/sub> of 1.5 MPa equates to 210 pounds per square inch (1.5 MPa x 140 lb in<sup>-2<\/sup> MPa<sup>-1<\/sup> = 210 lb\/in<sup>-2<\/sup>). As a comparison, most automobile tires are kept at a pressure of 30\u201334 psi. An example of the effect of turgor pressure is the wilting of leaves and their restoration after the plant has been watered (Figure 3). Water is lost from the leaves via transpiration (approaching \u03a8<sub>p<\/sub> = 0 MPa at the wilting point) and restored by uptake via the roots.\r\n\r\nA plant can manipulate \u03a8<sub>p<\/sub>\u00a0via its ability to manipulate \u03a8<sub>s<\/sub>\u00a0and by the process of osmosis. If a plant cell increases the cytoplasmic solute concentration, \u03a8s will decline, \u03a8<sub>total<\/sub>\u00a0will decline, the \u0394\u03a8 between the cell and the surrounding tissue will decline, water will move into the cell by osmosis, and \u03a8<sub>p<\/sub>\u00a0will increase. \u03a8<sub>p<\/sub>\u00a0is also under indirect plant control via the opening and closing of stomata. Stomatal openings allow water to evaporate from the leaf, reducing \u03a8<sub>p<\/sub>\u00a0and \u03a8<sub>total<\/sub>\u00a0of the leaf and increasing ii between the water in the leaf and the petiole, thereby allowing water to flow from the petiole into the leaf.\r\n\r\n[caption id=\"attachment_1999\" align=\"aligncenter\" width=\"800\"]<img class=\"size-full wp-image-1999\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31184942\/Figure_30_05_03.jpg\" alt=\" Left photo shows a wilted plant with wilted leaves. Right photo shows a healthy plant.\" width=\"800\" height=\"395\" \/> Figure 3. When (a) total water potential (\u03a8total) is lower outside the cells than inside, water moves out of the cells and the plant wilts. When (b) the total water potential is higher outside the plant cells than inside, water moves into the cells, resulting in turgor pressure (\u03a8<sub>p<\/sub>) and keeping the plant erect. (credit: modification of work by Victor M. Vicente Selvas)[\/caption]\r\n<h2>Gravity Potential<\/h2>\r\nGravity potential (\u03a8<sub>g<\/sub>) is always negative to zero in a plant with no height. It always removes or consumes potential energy from the system. The force of gravity pulls water downwards to the soil, reducing the total amount of potential energy in the water in the plant (\u03a8<sub>total<\/sub>). The taller the plant, the taller the water column, and the more influential \u03a8<sub>g<\/sub>\u00a0becomes. On a cellular scale and in short plants, this effect is negligible and easily ignored. However, over the height of a tall tree like a giant coastal redwood, the gravitational pull of \u20130.1 MPa m<sup>-1<\/sup> is equivalent to an extra 1 MPa of resistance that must be overcome for water to reach the leaves of the tallest trees. Plants are unable to manipulate \u03a8<sub>g<\/sub>.\r\n<h2>Matric Potential<\/h2>\r\nMatric potential (\u03a8<sub>m<\/sub>) is always negative to zero. In a dry system, it can be as low as \u20132 MPa in a dry seed, and it is zero in a water-saturated system. The binding of water to a matrix always removes or consumes potential energy from the system. \u03a8<sub>m<\/sub> is similar to solute potential because it involves tying up the energy in an aqueous system by forming hydrogen bonds between the water and some other component. However, in solute potential, the other components are soluble, hydrophilic solute molecules, whereas in \u03a8<sub>m<\/sub>, the other components are insoluble, hydrophilic molecules of the plant cell wall. Every plant cell has a cellulosic cell wall and the cellulose in the cell walls is hydrophilic, producing a matrix for adhesion of water: hence the name matric potential. \u03a8<sub>m<\/sub> is very large (negative) in dry tissues such as seeds or drought-affected soils. However, it quickly goes to zero as the seed takes up water or the soil hydrates. \u03a8<sub>m<\/sub> cannot be manipulated by the plant and is typically ignored in well-watered roots, stems, and leaves.\r\n<div class=\"textbox tryit\">\r\n<h3>Try It<\/h3>\r\nhttps:\/\/assess.lumenlearning.com\/practice\/367ab8d9-06bd-4bed-8a17-77c98fc1ba54\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Outcomes<\/h3>\n<ul>\n<li>Describe how water potential influences how water is transported in plants<\/li>\n<\/ul>\n<\/div>\n<p>Plants are phenomenal hydraulic engineers. Using only the basic laws of physics and the simple manipulation of potential energy, plants can move water to the top of a 116-meter-tall tree (Figure 1a). Plants can also use hydraulics to generate enough force to split rocks and buckle sidewalks (Figure 1b). Plants achieve this because of water potential.<\/p>\n<div id=\"attachment_1997\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1997\" class=\"size-large wp-image-1997\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31184659\/Figure_30_05_01-1024x536.jpg\" alt=\"Photo (a) shows the brown trunk of a tall sequoia tree in a forest. Photo (b) shows a grey tree trunk growing between a road and a sidewalk. The roots have started to lift up and crack the concrete slabs of the sidewalk.\" width=\"1024\" height=\"536\" \/><\/p>\n<p id=\"caption-attachment-1997\" class=\"wp-caption-text\">Figure 1.\u00a0With heights nearing 116 meters, (a) coastal redwoods (<em>Sequoia sempervirens<\/em>) are the tallest trees in the world. Plant roots can easily generate enough force to (b) buckle and break concrete sidewalks, much to the dismay of homeowners and city maintenance departments. (credit a: modification of work by Bernt Rostad; credit b: modification of work by Pedestrians Educating Drivers on Safety, Inc.)<\/p>\n<\/div>\n<p><strong>Water potential<\/strong> is a measure of the potential energy in water. Plant physiologists are not interested in the energy in any one particular aqueous system, but are very interested in water movement between two systems. In practical terms, therefore, water potential is the difference in potential energy between a given water sample and pure water (at atmospheric pressure and ambient temperature). Water potential is denoted by the Greek letter \u03c8 (psi) and is expressed in units of pressure (pressure is a form of energy) called <strong>megapascals<\/strong> (MPa). The potential of pure water (\u03a8<sub>w<\/sub><sup>pure H<sub>2<\/sub>O<\/sup>) is, by convenience of definition, designated a value of zero (even though pure water contains plenty of potential energy, that energy is ignored). Water potential values for the water in a plant root, stem, or leaf are therefore expressed relative to \u03a8<sub>w<\/sub><sup>pure H<sub>2<\/sub>O<\/sup>.<\/p>\n<p>The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and factors called matrix effects. Water potential can be broken down into its individual components using the following equation:<\/p>\n<p style=\"text-align: center;\">\u03a8<sub>system<\/sub>\u00a0=\u00a0\u03a8<sub>total<\/sub>\u00a0=\u00a0\u03a8<sub>s<\/sub>\u00a0+\u00a0\u03a8<sub>p<\/sub>\u00a0+\u00a0\u03a8<sub>g<\/sub>\u00a0+\u00a0\u03a8<sub>m<\/sub><\/p>\n<p>where \u03a8<sub>s<\/sub>, \u03a8<sub>p<\/sub>, \u03a8<sub>g<\/sub>, and \u03a8m refer to the solute, pressure, gravity, and matric potentials, respectively. \u201cSystem\u201d can refer to the water potential of the soil water (\u03a8<sup>soil<\/sup>), root water (\u03a8<sup>root<\/sup>), stem water (\u03a8<sup>stem<\/sup>), leaf water (\u03a8<sup>leaf<\/sup>) or the water in the atmosphere (\u03a8<sup>atmosphere<\/sup>): whichever aqueous system is under consideration. As the individual components change, they raise or lower the total water potential of a system. When this happens, water moves to equilibrate, moving from the system or compartment with a higher water potential to the system or compartment with a lower water potential. This brings the difference in water potential between the two systems (\u0394\u03a8) back to zero (\u0394\u03a8 = 0). Therefore, for water to move through the plant from the soil to the air (a process called transpiration),\u03a8<sup>soil<\/sup>\u00a0must be &gt;\u03a8<sup>root<\/sup>\u00a0&gt;\u03a8<sup>stem<\/sup>\u00a0&gt;\u03a8<sup>leaf<\/sup>&gt;\u03a8<sup>atmosphere<\/sup>.<\/p>\n<p>Water only moves in response to \u0394\u03a8, not in response to the individual components. However, because the individual components influence the total \u03a8<sub>system<\/sub>, by manipulating the individual components (especially \u03a8<sub>s<\/sub>), a plant can control water movement.<\/p>\n<h2>Solute Potential<\/h2>\n<p>Solute potential (\u03a8<sub>s<\/sub>), also called osmotic potential, is negative in a plant cell and zero in distilled water. Typical values for cell cytoplasm are \u20130.5 to \u20131.0 MPa. Solutes reduce water potential (resulting in a negative \u03a8<sub>w<\/sub>) by consuming some of the potential energy available in the water. Solute molecules can dissolve in water because water molecules can bind to them via hydrogen bonds; a hydrophobic molecule like oil, which cannot bind to water, cannot go into solution. The energy in the hydrogen bonds between solute molecules and water is no longer available to do work in the system because it is tied up in the bond. In other words, the amount of available potential energy is reduced when solutes are added to an aqueous system. Thus, \u03a8<sub>s<\/sub>\u00a0decreases with increasing solute concentration. Because \u03a8<sub>s<\/sub>\u00a0is one of the four components of \u03a8<sub>system<\/sub>\u00a0or \u03a8<sub>total<\/sub>, a decrease in \u03a8<sub>s<\/sub>\u00a0will cause a decrease in \u03a8<sub>total<\/sub>. The internal water potential of a plant cell is more negative than pure water because of the cytoplasm\u2019s high solute content (Figure 2). Because of this difference in water potential water will move from the soil into a plant\u2019s root cells via the process of osmosis. This is why solute potential is sometimes called osmotic potential.<\/p>\n<p>Plant cells can metabolically manipulate \u03a8<sub>s<\/sub>\u00a0(and by extension,\u03a8<sub>total<\/sub>) by adding or removing solute molecules. Therefore, plants have control over \u03a8<sub>total<\/sub>\u00a0via their ability to exert metabolic control over \u03a8<sub>s<\/sub>.<\/p>\n<div id=\"attachment_1998\" style=\"width: 610px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1998\" class=\"wp-image-1998\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31184842\/Figure_30_05_02.png\" alt=\"Illustration shows a U-shaped tube holding pure water. A semipermeable membrane, which allows water but not solutes to pass, separates the two sides of the tube. The water level on each side of the tube is the same. Beneath this tube are three more tubes, also divided by semipermeable membranes. In the first tube, solute has been added to the right side. Adding solute to the right side lowers psi-s, causing water to move to the right side of the tube. As a result, the water level is higher on the right side. The second tube has pure water on both sides of the membrane. Positive pressure is applied to the left side. Applying positive pressure to the left side causes psi-p to increase. As a results, water moves to the right so that the water level is higher on the right than on the left. The third tube also has pure water, but this time negative pressure is applied to the left side. Applying negative pressure lowers psi-p, causing water to move to the left side of the tube. As a result, the water level is higher on the left.\" width=\"600\" height=\"539\" \/><\/p>\n<p id=\"caption-attachment-1998\" class=\"wp-caption-text\">Figure 2. A semipermeable membrane between two aqueous systems<\/p>\n<\/div>\n<p>In Figure 2, water will move from a region of higher to lower water potential until equilibrium is reached. Solutes (\u03a8<sub>s<\/sub>), pressure (\u03a8<sub>p<\/sub>), and gravity (\u03a8<sub>g<\/sub>) influence total water potential for each side of the tube (\u03a8<sub>total<\/sub> <sup>right or left<\/sup>), and therefore, the difference between \u03a8<sub>total<\/sub>\u00a0on each side (\u0394\u03a8). (\u03a8<sub>m<\/sub>, the potential due to interaction of water with solid substrates, is ignored in this example because glass is not especially hydrophilic). Water moves in response to the difference in water potential between two systems (the left and right sides of the tube).<\/p>\n<div class=\"textbox exercises\">\n<h3>Practice Question<\/h3>\n<p>Positive water potential is placed on the left side of the tube by increasing \u03a8<sub>p<\/sub> such that the water level rises on the right side. Could you equalize the water level on each side of the tube by adding solute, and if so, how?<\/p>\n<p><textarea aria-label=\"Your Answer\" rows=\"2\"><\/textarea><\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q229403\">Show Answer<\/span><\/p>\n<div id=\"q229403\" class=\"hidden-answer\" style=\"display: none\">Yes, you can equalize the water level by adding the solute to the left side of the tube such that water moves toward the left until the water levels are equal.<\/div>\n<\/div>\n<\/div>\n<h2>Pressure Potential<\/h2>\n<p>Pressure potential (\u03a8<sub>p<\/sub>), also called turgor potential, may be positive or negative (Figure 2). Because pressure is an expression of energy, the higher the pressure, the more potential energy in a system, and vice versa. Therefore, a positive \u03a8<sub>p<\/sub> (compression) increases \u03a8<sub>total<\/sub>, and a negative \u03a8<sub>p<\/sub> (tension) decreases \u03a8<sub>total<\/sub>. Positive pressure inside cells is contained by the cell wall, producing turgor pressure. Pressure potentials are typically around 0.6\u20130.8 MPa, but can reach as high as 1.5 MPa in a well-watered plant. A \u03a8<sub>p<\/sub> of 1.5 MPa equates to 210 pounds per square inch (1.5 MPa x 140 lb in<sup>-2<\/sup> MPa<sup>-1<\/sup> = 210 lb\/in<sup>-2<\/sup>). As a comparison, most automobile tires are kept at a pressure of 30\u201334 psi. An example of the effect of turgor pressure is the wilting of leaves and their restoration after the plant has been watered (Figure 3). Water is lost from the leaves via transpiration (approaching \u03a8<sub>p<\/sub> = 0 MPa at the wilting point) and restored by uptake via the roots.<\/p>\n<p>A plant can manipulate \u03a8<sub>p<\/sub>\u00a0via its ability to manipulate \u03a8<sub>s<\/sub>\u00a0and by the process of osmosis. If a plant cell increases the cytoplasmic solute concentration, \u03a8s will decline, \u03a8<sub>total<\/sub>\u00a0will decline, the \u0394\u03a8 between the cell and the surrounding tissue will decline, water will move into the cell by osmosis, and \u03a8<sub>p<\/sub>\u00a0will increase. \u03a8<sub>p<\/sub>\u00a0is also under indirect plant control via the opening and closing of stomata. Stomatal openings allow water to evaporate from the leaf, reducing \u03a8<sub>p<\/sub>\u00a0and \u03a8<sub>total<\/sub>\u00a0of the leaf and increasing ii between the water in the leaf and the petiole, thereby allowing water to flow from the petiole into the leaf.<\/p>\n<div id=\"attachment_1999\" style=\"width: 810px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1999\" class=\"size-full wp-image-1999\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31184942\/Figure_30_05_03.jpg\" alt=\"Left photo shows a wilted plant with wilted leaves. Right photo shows a healthy plant.\" width=\"800\" height=\"395\" \/><\/p>\n<p id=\"caption-attachment-1999\" class=\"wp-caption-text\">Figure 3. When (a) total water potential (\u03a8total) is lower outside the cells than inside, water moves out of the cells and the plant wilts. When (b) the total water potential is higher outside the plant cells than inside, water moves into the cells, resulting in turgor pressure (\u03a8<sub>p<\/sub>) and keeping the plant erect. (credit: modification of work by Victor M. Vicente Selvas)<\/p>\n<\/div>\n<h2>Gravity Potential<\/h2>\n<p>Gravity potential (\u03a8<sub>g<\/sub>) is always negative to zero in a plant with no height. It always removes or consumes potential energy from the system. The force of gravity pulls water downwards to the soil, reducing the total amount of potential energy in the water in the plant (\u03a8<sub>total<\/sub>). The taller the plant, the taller the water column, and the more influential \u03a8<sub>g<\/sub>\u00a0becomes. On a cellular scale and in short plants, this effect is negligible and easily ignored. However, over the height of a tall tree like a giant coastal redwood, the gravitational pull of \u20130.1 MPa m<sup>-1<\/sup> is equivalent to an extra 1 MPa of resistance that must be overcome for water to reach the leaves of the tallest trees. Plants are unable to manipulate \u03a8<sub>g<\/sub>.<\/p>\n<h2>Matric Potential<\/h2>\n<p>Matric potential (\u03a8<sub>m<\/sub>) is always negative to zero. In a dry system, it can be as low as \u20132 MPa in a dry seed, and it is zero in a water-saturated system. The binding of water to a matrix always removes or consumes potential energy from the system. \u03a8<sub>m<\/sub> is similar to solute potential because it involves tying up the energy in an aqueous system by forming hydrogen bonds between the water and some other component. However, in solute potential, the other components are soluble, hydrophilic solute molecules, whereas in \u03a8<sub>m<\/sub>, the other components are insoluble, hydrophilic molecules of the plant cell wall. Every plant cell has a cellulosic cell wall and the cellulose in the cell walls is hydrophilic, producing a matrix for adhesion of water: hence the name matric potential. \u03a8<sub>m<\/sub> is very large (negative) in dry tissues such as seeds or drought-affected soils. However, it quickly goes to zero as the seed takes up water or the soil hydrates. \u03a8<sub>m<\/sub> cannot be manipulated by the plant and is typically ignored in well-watered roots, stems, and leaves.<\/p>\n<div class=\"textbox tryit\">\n<h3>Try It<\/h3>\n<p>\t<iframe id=\"assessment_practice_367ab8d9-06bd-4bed-8a17-77c98fc1ba54\" class=\"resizable\" src=\"https:\/\/assess.lumenlearning.com\/practice\/367ab8d9-06bd-4bed-8a17-77c98fc1ba54?iframe_resize_id=assessment_practice_id_367ab8d9-06bd-4bed-8a17-77c98fc1ba54\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:300px;\"><br \/>\n\t<\/iframe>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-1992\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Biology 2e. <strong>Provided by<\/strong>: OpenStax. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\">http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Access for free at https:\/\/openstax.org\/books\/biology-2e\/pages\/1-introduction<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":17,"menu_order":9,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Biology 2e\",\"author\":\"\",\"organization\":\"OpenStax\",\"url\":\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Access for free at https:\/\/openstax.org\/books\/biology-2e\/pages\/1-introduction\"}]","CANDELA_OUTCOMES_GUID":"122e0155-ced6-480a-99a6-82467ecbf55f, 29477490-5b41-41c0-8eeb-7943e75e5466","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-1992","chapter","type-chapter","status-publish","hentry"],"part":145,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/1992","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":10,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/1992\/revisions"}],"predecessor-version":[{"id":8372,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/1992\/revisions\/8372"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/parts\/145"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/1992\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/media?parent=1992"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapter-type?post=1992"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/contributor?post=1992"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/license?post=1992"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}