A plant can manipulate Ψ p via its ability to manipulate Ψ s and by the process of osmosis. A Ψ p of 1.5 MPa equates to 210 pounds per square inch (psi) for a comparison, most automobile tires are kept at a pressure of 30-34 psi. Pressure potentials can reach as high as 1.5 MPa in a well-watered plant. Positive pressure inside cells is contained by the rigid cell wall, producing turgor pressure. Positive pressure (compression) increases Ψ p, and negative pressure (vacuum) decreases Ψ p. Pressure potential ( Ψ p), also called turgor potential, may be positive or negative. This is why solute potential is sometimes called osmotic potential. Plant cells can metabolically manipulate Ψ s by adding or removing solute molecules. Because of this difference in water potential, water will move from the soil into a plant’s root cells via the process of osmosis. The internal water potential of a plant cell is more negative than pure water because of the cytoplasm’s high solute content. Solute potential (Ψ s ), also called osmotic potential, is negative in a plant cell and zero in distilled water, because solutes reduce water potential to a negative Ψ s.Let’s consider solute and pressure potential in the context of plant cells: In order for water to move through the plant from the soil to the air (a process called transpiration), Ψ soil must be > Ψ root > Ψ stem > Ψ leaf > Ψ atmosphere. At equilibrium, there is no difference in water potential on either side of the system (the difference in water potentials is zero). Water always moves from a region of high water potential to an area of low water potential, until it equilibrates the water potential of the system. Addition of pressure will increase the water potential, and removal of pressure (creation of a vacuum) will decrease the water potential. Addition of more solutes will decrease the water potential, and removal of solutes will increase the water potential. Where Ψs = solute potential, and Ψp = pressure potential. The water potential measurement combines the effects of solute concentration (s) and pressure (p): Water potential values for the water in a plant root, stem, or leaf are expressed relative to Ψ pure H2O. The potential of pure water (Ψ pure H2O) is designated a value of zero (even though pure water contains plenty of potential energy, that energy is ignored). Water potential is denoted by the Greek letter Ψ ( psi) and is expressed in units of pressure (pressure is a form of energy) called megapascals (MPa). Water potential can be defined as the difference in potential energy between any given water sample and pure water (at atmospheric pressure and ambient temperature). Water potential is a measure of the potential energy in water, specifically, water movement between two systems. (credit a: modification of work by Bernt Rostad credit b: modification of work by Pedestrians Educating Drivers on Safety, Inc.) Image credit: OpenStax Biology Plant roots can easily generate enough force to (b) buckle and break concrete sidewalks, much to the dismay of homeowners and city maintenance departments. With heights nearing 116 meters, (a) coastal redwoods (Sequoia sempervirens) are the tallest trees in the world. Plants achieve this because of water potential. Plants can also use hydraulics to generate enough force to split rocks and buckle sidewalks. 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. Plants are phenomenal hydraulic engineers. To understand how these processes work, we must first understand the energetics of water potential. Water potential, evapotranspiration, and stomatal regulation influence how water and nutrients are transported in plants. The phloem and xylem are the main tissues responsible for this movement. The structure of plant roots, stems, and leaves facilitates the transport of water, nutrients, and photosynthates throughout the plant. The information below was adapted from OpenStax Biology 30.5 Explain the three hypotheses explaining water movement in plant xylem, and recognize which hypothesis explains the heights of plants beyond a few meters.Identify and describe the three pathways water and minerals can take from the root hair to the vascular tissue.Describe the effects of different environmental or soil conditions on the typical water potential gradient in plants.Explain water potential and predict movement of water in plants by applying the principles of water potential.
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