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Plant-Water Relations - Accessscience from Mcgraw-Hill Education Plant-water relations - AccessScience from McGraw-Hill Education http://www.accessscience.com/content/plant-water-relations/525300 (http://www.accessscience.com/) Article by: Meyer, Bernard S. Department of Botany, Ohio State University, Columbus, Ohio. Publication year: 2015 DOI: http://dx.doi.org/10.1036/1097-8542.525300 (http://dx.doi.org/10.1036/1097-8542.525300) Content Cellular water relations Significance of transpiration Root growth and water absorption Stomatal mechanism Water Translocation Mechanisms of water absorption Transpiration Process Water-conductive tissues Environment and absorption Stomatal transpiration Root pressure Wilting Environmental factors Water cohesion and ascent of sap Internal redistributions of water Daily periodicity of transpiration Water Absorption Drought resistance Magnitude of transpiration Soil water conditions Bibliography The relationships between plants and water, including the hydration of plant cells and the transport of water within a plant. Water is the most abundant constituent of all physiologically active plant cells. Leaves, for example, have water contents that lie mostly within a range of 55–85% of their fresh weight. Other relatively succulent parts of plants contain approximately the same proportion of water, and even such largely nonliving tissues as wood may be 30–60% water on a fresh-weight basis. The smallest water contents in living parts of plants occur mostly in dormant structures, such as mature seeds and spores. The great bulk of the water in any plant constitutes a unit system. This water is not in a static condition. Rather it is part of a hydrodynamic system, which in terrestrial plants involves absorption of water from the soil, its translocation throughout the plant, and its loss to the environment, principally in the process known as transpiration. See also: Plant (/content/plant /522400); Water (/content/water/738300) Cellular water relations The typical mature, vacuolate plant cell constitutes a tiny osmotic system, and this idea is central to any concept of cellular water dynamics. Although the cell walls of most living plant cells are quite freely permeable to water and solutes, the cytoplasmic layer that lines the cell wall is more permeable to some substances than to others. This property of differential permeability appears to reside principally in the layer of cytoplasm adjacent to the cell wall (plasma membrane, or plasmalemma) and in the layer in contact with the vacuole (vacuolar membrane, or tonoplast). This cytoplasmic system of membranes is usually relatively permeable to water, to dissolved gases, and to certain dissolved organic components. It is often much less permeable to sugars and mineral salts. The permeability of the cytoplasmic membranes is quite variable, however, and under certain metabolic conditions solutes that ordinarily penetrate through these membranes slowly or not at all may pass into or out of cells rapidly. See also: Cell (biology) (/content/cell-biology/116000); Plant cell (/content/plant­ cell/522600) Osmotic and turgor pressures If a plant cell in a flaccid condition—one in which the cell sap exerts no pressure against the encompassing cytoplasm and cell wall—is immersed in pure water, inward osmosis of water into the cell sap ensues. Osmosis may be defined as the movement of solvent molecules, which in living organisms are always water, across a membrane that is more permeable to 1 of 18 11/7/2016 7:17 AM Plant-water relations - AccessScience from McGraw-Hill Education http://www.accessscience.com/content/plant-water-relations/525300 the solvent than to the solutes dissolved in it. The driving force in osmosis is the difference in free energies of the water on the two sides of the membrane. Pure water, as a result of its intrinsic properties, possesses free energy. Since there is no certain way of measuring the free energy of water, it is arbitrarily given a value of zero when exposed only to atmospheric pressure. The free energy of water is influenced by temperature, but this discussion will be restricted to isothermal systems. This free energy has been designated water potential and may be expressed in either energy or pressure units. It is more meaningful to use pressure units in considerations of the water potential of plants. Inward osmosis of water takes place under the conditions specified above because the water potential of the cell sap is less than that of the surrounding pure water by the amount of its osmotic pressure. Introduction of solutes always lowers the potential of water by the amount of the resulting osmotic pressure. If the osmotic pressure of the cell sap is 15 atm (1.5 megapascals), then the water potential of the cell sap is 15 atm (1.5 MPa) less than that of pure water at the same temperature and under the same pressure. The gain of water by the cell as a result of inward osmosis results in the exertion of a turgor pressure against the protoplasm, which in turn is transmitted to the cell wall. This pressure also prevails throughout the mass of solution within the cell. If the cell wall is elastic, some expansion in the volume of the cell occurs as a result of this pressure, although in many kinds of cells this is relatively small. Because of the solutes invariably present, the cell sap possesses an osmotic pressure. The osmotic pressures of most plant cell saps lie within the range of 5–40 atm (0.5–4 MPa), although values as high as 200 atm (20 MPa) have been found in some halophytes (plants that can tolerate high-solute media). The osmotic pressures of the cells of a given plant tissue vary considerably with environmental conditions and intrinsic metabolic activities. More or less regular daily or seasonal variations occur in the magnitude of cell sap osmotic pressures in the cells of many tissues. It is the osmotic pressure of the cell sap, coupled with the differential permeability of the cytoplasmic membranes and the relative inelasticity of the cell walls, which permits the development of the more or less turgid condition characteristic of most plant cells. With continued osmosis of water into the cell, its turgor pressure gradually increases until it is equal to the final osmotic pressure of the sap. Subjection of the water in the cell sap to pressure increases its water potential by the amount of the imposed pressure. In the example given above, disregarding the usually small amount of sap dilution as a result of cell expansion, the water potential of the cell sap is reduced 15 atm (1.5 MPa) because of the presence of solutes (the osmotic pressure is the index of this lowering of water potential) and raised 15 atm (1.5 MPa) as a result of turgor pressure when maximum turgor is reached. Hence, when a dynamic equilibrium is attained, the water potential of the cell sap is zero and thus is equal to that of the surrounding water, a necessary condition if equilibrium is to be achieved. If the same cell in a flaccid condition is immersed in a solution with an osmotic pressure of 6 atm (0.6 MPa), inward osmosis of water occurs, but does not continue as long as when the cell is immersed in pure water. Disregarding sap dilution, a dynamic equilibrium will be attained under these circumstances when the turgor pressure of the cell sap has reached 9 atm (0.9 MPa), because at this point the water potential in the cell sap and the water potential in the surrounding solution will be equal. Since the water potential of the cell sap was originally diminished 15 atm (1.5 MPa) because of the presence of solutes and then raised 9 atm (0.9 MPa) because of turgor pressure, the net value of the water potential in the cell at equilibrium is −6 atm (−0.6 MPa). This is also the value of the water potential in the surrounding solution, as indexed by its osmotic pressure. At dynamic equilibrium the number of water molecules entering the cell and the number leaving per unit of time will be equal. See also: Osmoregulatory mechanisms (/content/osmoregulatory-mechanisms/478300) Water potential As the examples just given indicate, the effective physical quantity controlling the direction of osmotic movement of water 2 of 18 11/7/2016 7:17 AM Plant-water relations - AccessScience from McGraw-Hill Education http://www.accessscience.com/content/plant-water-relations/525300 from cell to cell in plants or between a cell and an external solution is the water potential (WP) of the water. In most plant cells this quantity is equal to the osmotic pressure (OP) of the water less the turgor pressure (TP) to which it is subjected. Since the osmotic pressure is the index of the amount by which the water potential of a solution is less than that of pure water under the same conditions, it must be treated as a negative quantity. Turgor pressures usually have a positive value, but if water passes into a state of tension, they are negative. Examples of the occurrence of water under tension in plants will be discussed later in this article. In an unconfined solution the water potential is equal to the osmotic pressure, since there is no turgor pressure. In a fully turgid plant cell the water potential is zero, since the turgor pressure is equal to the osmotic pressure. In a fully flaccid plant cell the turgor pressure is zero and the water potential is equal to the osmotic pressure. The interrelationships among these quantities may be expressed: WP = (−OP) + TP. Since the osmotic pressure must be treated as a negative quantity and since turgor pressures rarely exceed osmotic pressures, the water potentials in plant cells are negative in value. The only well-known exception to this is root pressure, discussed later, during which small positive water potentials can be generated in the water-conductive tissues.
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