Components of Plant Water Relations

Title: Procedures for measurement of components of water relations (water, osmotic and turgor potentials)

What are Water Relations?

A field of study in which plant and environmental interactions with respect to water are observed OR study of all mechanisms related to uptake of water from soil by plants, its translocation from root to shoot and  evaporation through stomata OR 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)

What is Water Potential?

  1. 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
  2. It is symbolized as Ψ and expressed as MPa (1 MPa = 10 Bars)
  3. It actually quantifies the tendency of water to move from one area to another due to osmosis (Ψs), mechanical pressure (Ψp), gravity(Ψg), or matrix effects such as capillary action (Ψm)
  4. Thus, it is the difference between matrically bound (Ψm), pressurised (Ψp) or osmotically held (Ψs) water and pure water

Ψw = Ψs + Ψp + Ψg + Ψm

Where,

  1. Osmotic/solute  potential (Ψs)
  2. Turgor potential (Ψp)
  3. Matric potential (Ψm)
  4. Gravitational potential (Ψg)

1. Osmotic/Solute Potential: “Osmotic Pressure is a pressure that a solution develops to increase its chemical potential compared to that of pure water” OR “It is  hydrostatic pressure when applied to a solution prevents the influx of water towards solution side”

  1. Is based on concentration of solutes in water
  2. Is potential developed by solutes in a system with which influx of water occurs
  3. Is always negative
  4. Higher solute concentration, more negative  will be the value of osmotic potential
  5. Is denoted by ψs

2. 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)

  1. Is denoted by ψP
  2. It is usually positive
  3. Is zero in open vessel
  4. Is –ve in xylem of transpiring plant while it is positive in guttating plants
  5. Is zero when cell is flaccid

3. 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. Is negligible at high tissue hydration while it increases as the cell water decreases and below (60%), it should be considered while calculating water potential (Nobel et al., 1992)

4. 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.

 

Methodologies/Protocols  for the Measurement of different components of water potential

Water potential can be measured by

  1. Volume Method (measures change in volume)
  2. Gravimetric method (measures change in mass)
  3. Falling drop method or Chardakov’s  or Dye Method (based on change in density)
  4. Pressure Chamber or Pressure bomb (measures tension)

Osmotic potential can be measured by

  1. Vapor pressure method (Thermocouple Psychrometer – measures humidity)
  2. Plasmolytic method
  3. Cryoscopic method (Freezing Point Depression)

Pressure potential can be measured by

  1. Pressure probe
  2. Indirect method (By using values of water and osmotic potentials)

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