1 / 38

Plant Water Relations

Plant Water Relations. Prof. Dr. Muhammad Ashraf. What are Water Relations?. A field of study in which one can observe plant and environmental interactions with respect to water OR

janetkirkman
Download Presentation

Plant Water Relations

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Plant Water Relations Prof. Dr. Muhammad Ashraf

  2. What are Water Relations? • A field of study in which one can observe plant and environmental interactions with respect to water OR • Study of all mechanisms related to uptake of water from soil by plants, its translocation from root to shoot and evaporation through stomata • Movement of water and other substances from soil to plant roots across membranes, throughout the plant and between the plant and its environment (Salisbury, 1992)

  3. Water relations of a single cell

  4. Nature of cellular water or distribution of water in cells • Water is continuously present throughout the plant body • Some water is held in the micro-capillaries • Water exists in two systems • Apoplast • symplast

  5. Cell wall water (matric water) Apoplastic water • 5-40% of cell water occurs in the walls depending on the age, thickness and composition of the walls • 50% of cell wall volume is water • In thick leaves, walls are thick – low water is held in the walls by matric forces including H-bonding to various constituents • The wall contains cellulose microfibrils, pectic substances, proteins and OH and COOH groups which absorb water by H-bonding. Water is also held in inter-micro-fibrillar spaces (matric water or imbibed water)

  6. Water in the Cytoplasm • In meristematic tissues, vacuole is small so most water occurs in the cytoplasm. However, in mature cells cytoplasm is as a thin layer and consists of 5 to 10% of the cell water

  7. Vacuole • 50-80% or more of the cell water occurs in the vacuoles. Cell sap consists of 2% solid and 98% water. This water contains solutes mainly sugars, salts and sometimes organic acids. OP = -1.0 to -3.0 MPa. • In leaves of Eucalyptus 50% water in vacuole • In wheat roots 80% or more in vacuoles

  8. Cell Membrane • Water is held by means of dipolar and H-bond. • The inner membrane spaces within proteins and lipids are occupied by water molecules • Water is very dense so that they form semi-crystalline structure. • Membrane surface is also covered by one molecular thick layer of water • Water moves to the region of low water potential or low energy

  9. Water relations of plant

  10. HISTORY of Plant Water Relations • Tang and Wang (1941) first used the term “water potential” to explain cell water relations • Then it was used by Owen (1952) to explain DPD (diffusion pressure deficit) which is equivalent to water potential DPD = OP -TP • Slatyer from Australia and Taylor from Utah (1960) recognized this term

  11. Acceptance of the water potential concept was slow because of the confusion regarding terminology, the lack of convenient methods for measuring it, and the inadequate training of plant physiologists in physical chemistry (Kramer 1995). • As a result, plant water status seldom was measured during the second quarter of the 20th century. • Development of thermocouple psychrometers (Monteith and Owen, 1958; Richards and Ogata, 1958; Spanner, 1951) and pressure equilibration by Scholander and his colleagues (1964,1965) made measurement of water potential relatively easy, and they are the measurements used most often today to characterize plant water status.

  12. Chemical Potential • Chemical potential (a thermodynamic term) is the amount of energy per mole of a substance to do work • Chemical potential of water is water potential and it is a measure of the free energy per unit volume available for reaction or movement • a quantity that determines the transport of matter from one phase to another: a component will flow from one phase to another when the chemical potential of the component is greater in the first phase than in the second. • Chemical potential depends upon concentration, pressure, electric potential and gravity. e.g. molecules at high temp move toward low temp regime.

  13. Different Definitions of Water Potential • Water potential is the chemical potential (Free energy) of water in a system expressed in units of pressure and compared to water potential of pure water i.e. 0 • Free energy of water (water potential) in plants relates to creating and breaking molecular bonds, moving ions through the cellular organelles or moving water from soil to root and leaf through different cells and xylem • Chemical Potential of water divided by partial molal volume of water • It is the difference between matrically bound, pressurised or osmotically held water and pure water

  14. Chemical Potential and Water Potential Ψw = DPD = OP - TP Ψw = μw - μºw = RT ln e/ºe VwVw μw = Chemical potential of water μºw = Chemical potential of pure water R = 0.00831 kg MPa mol-1 K-1 or 0.00831 kJ mol-1 K-1 T = Absolute temperature (K) K = 273 + ºC e = vapor pressure of water in a system eº = vapor pressure of pure water Vw =partial molal volume of water (e.g. volume of 1 mole of water is 18 cm3 mol-1)

  15. Factors affecting Chemical potential • Chemical activity (Solute conc.) • temperature contributes to chemical activity (molecules with high temp will move toward low temp regime) • electrical potential – only important for charged substances • pressure – elastic cell walls allow plant cells to develop significant hydrostatic pressure • gravitational pull – only applicable in tall trees

  16. Units of Water Potential • 1 MPa = 10 bars • 1 bar = 0.987 atm = 106 dynes cm-2 = 106 ergs cm-3 • 1 MPa = 1 kJ kg-1 = 1 J g-1

  17. w plant = s + p + m - 0.8 MPa = - 0.9 + 0.3 - 0.2 w soil = s + m - 0.6 MPa = - 0.2 +- 0.4 Water Potential-Example

  18. -3 -2 -1 0 1 2 3 Water Potential

  19. w = 0 MPa w≈ -6 MPa Pure Water Desert soils Plant/Cell in good condition w = 0 to -1 MPa Plant/Cell under mild water stress w = -1 to -2 MPa Plant/Cell under water stress w < -2 MPa Water Potential-Magnitude

  20. -2 MPa -30 MPa -0.3 MPa -1 MPa Soil Root Stem Leaf Air Water Potential-Flux Water will flow from sites of high w (close to zero) to sites of low w (more negative):

  21. -2 MPa -1 MPa 0 MPa Cell growth Protein synthesis Stomatal opening Photosynthesis Respiration Pro/sugar accum. Transport

  22. Photosynthesis Respiration Enlargement Water Deficit-Effects 75 % of maximum 50 25 -1.6 -1.2 -0.8 -0.4 Water Potential (MPa)

  23. Leaf Stem Root Water Deficit-Effects 0.9 Rate of elongation (um s-1) 0.6 0.3 -1.6 -1.2 -0.8 -0.4 Water Potential (MPa) Westgate and Boyer (1985)

  24. Components of Water Potential • The components of water potential are osmotic potential, turgor potential, matric potential, and gravitational potential Ψw = Ψs + Ψp + Ψm+ Ψg

  25. Values of Water Potential • Water potential of pure water at standard atmospheric conditions is “0” (the maximum) • Water potential of a system is always negative • More solute concentration, more negative will be the water potential • It is zero when cell is fully turgid • Water potential becomes positive when pressurized or compressed

  26. Water Potential Solution A Unconfined system Ψw = Ψs + Ψp -10 = -10 + 0 -10 = -10 Ψw = -10 Solution A Confined system Ψw = Ψs + Ψp -30 = -30 + 0 (Initial stage) -10 = -30 + 20 -10 = -10 Ψw = -10

  27. Osmotic Potential • Osmotic Pressure is a pressure that a solution develops to increase its chemical potential to that of pure water or it is a hydrostatic pressure when applied to a solution prevents the influx of water • Is based on concentration of solutes in water • Is potential developed by solutes in a system with which influx of water occurs • Is always negative • Higher solute concentration, more negative will be the value of osmotic potential • Is denoted by ψs • An isolated solution has no osmotic pressure but it does have an osmotic potential.

  28. Van’t Hoff Equation Ψs = -miRT = -CiRT = -nRT/V • i = ionization constant • C = concentration • m = molality • n = number of solutes • R = Gas constant • T = Absolute temperature

  29. Effect of Temperature on Osmotic Potentials of Same Solution Ψs = -miRT • Osmotic potential of 1 molal glucose solution at 30 oC • Ψs = - (1.0 mol kg-1) 1.0 x (0.00831 kg MPa mol-1 K-1) x (273+30) K = -2.518 MPa at 30 oC • Osmotic potential of 1 molal glucose solution at 0 oC • Ψs = - (1.0 mol kg-1) 1.0 x (0.00831 kg MPa mol-1 K-1) x (273+0) K = -2.269 MPa at 0 oC

  30. Turgor Pressure • Turgor pressure is produced by the diffusion of water into protoplasts enclosed in walls which resist expansion • Turgor pressure is hydrostatic pressure of water that is exerted on the liquid by the walls of a turgid cell (pressure per unit area of liquid) • Is denoted by ψP • Is zero in open vessel • Is –ve in xylem of transpiring plant while it is positive in guttating plants • Is zero when cell is flaccid

  31. Turgor pressure pushes the plasma membrane against the cell wall of plant, bacteria, and fungi cells as well as those of protist cells which have cell walls. This pressure, turgidity, is caused by the osmotic flow of water from area of low solute concentration outside of the cell into the cell's vacuole, which has a higher solute concentration. Healthy plant cells are turgid and plants rely on turgidity to maintain rigidity.

  32. Matric Potential • Matric potential is due to the adhesive characteristics of water when in contact with surface or large macromolecules • Potential developed due to water held in microcapillaries or bound on surfaces of cell walls • Matric water increases as the cell water decreases • Is negligible at high tissue hydration • If tissue hydration is low (60%), it should be considered (Nobel et al., 1992)

  33. Gravitational Potential • It is the potential that is developed due to gravitational pull • It is also negligible in crop plants while in tall trees it influences the water potential

  34. Soil water potential • ψT = ψm + ψs + ψp + ψz • Ψm, matric potential resulting from the combined effects of capillarity and adsorptive forces within the soil matrix (Value –ve) • Ψs, Solute potential resulting from solutes present in water (Value –ve) • Ψp, pressure potential i.e. hydrostatic pressure exerted by unsupported (free) water that tends to saturate the soil. Soil ψp is always positive below a water table, or zero at or above the water table. • Ψz, gravitational potential is simply the vertical distance from a reference level to the point of interest • If we consider no effect of gravity while considering soil water potential at the same point so the overall equation will be Ψw = ψm + ψs + ψp

  35. Soil water potential is measured by Psychrometer • Soil matric potential by Tensiometer • Soil pressure potential by Piezometer • Soil solute potential by EC or Osmometer

More Related