Plant water relations and water potential: Diffusion, Osmosis and Imbibition

Plant physiology and Water relations:

• The functional study of live processes is termed as physiology.
• Plant physiology deals with water relations (such as diffusion, osmosis, absorption, transpiration, and ascent of sap), photosynthesis, respiration, photorespiration, growth hormones, movements and locomotion, vernalization and seed germination.
• Plant absorbs water and soluble mineral salts from soil by means of root system.
• The unicellular hairs present on the roots facilitate the absorption.
• Plant cells comprises of cell wall and protoplast.
• The term protoplast is usually used to refer collectively to the plasma membrane and protoplasm.
• Normally, the plant cell comprises of three components:
  • Vacuole
  • Protoplasm
  • Cell wall
• These compartments are separated from each other by plasma membranes i.e., the tonoplast is found between the vacuole and the protoplasm, whereas plasma lemma is found between the protoplasm and the cell wall.
• The protoplasm of one cell is connected to the other cell by plasmodesmata.
• The plasma membrane is selectively permeable in nature i.e. it permits some materials to pass through and not others.
• The water is the most essential factor for the vital functioning of the plants.
• The plants fail to survive in its absence.
• The plant cells become turgid by water and it gives temporary mechanical support to the young plants.
• The small portion of water absorbed by water is retained in plant cells whereas most portion of water goes out of the surface of plants by a vital process termed as transpiration.

Availability of water in the soil:

• There are about three forms of water in the soil.
  1. Free water: Due to gravitation, some amount of water just after the rains along with some minerals goes in the lower strata of the earth. This is termed as gravitational or free water and cannot be utilized by the plants.
  2. Hygroscopic water: Besides free water, every particle of soil captures some imbibed water in it. The imbibition force is greater and hence the plants cannot absorb from the soil. This is termed as hygroscopic water which also cannot be absorbed from soil by the plants.
  3. Capillary water: Each soil particle is surrounded by a loose film of water, the capillary force attracted this film to the soil and hence this water film is termed as capillary water. Only capillary water can be absorbed by the plants. In this water, mineral salts are also found and are hence absorbed by plants along with water. If soil lacks the capillary water, the plants starts to wither and finally dies.

Water potential:

• The potential energy possessed by water is termed as water potential.
• The tendency of water to leave the system is referred to as water potential.
• The term is often used to explain the direction in which the flow of water will take place from one cell to another, or from one part of the plant to another.
• Water always flows from a region of higher water potential to the region of lower water potential.
• Osmotic movement of water involves the particular work done.
• The major driving factor behind this movement is the difference between free energies of water on two sides of the selectively permeable membrane.
• The free energy molecule for water is termed as water potential (ψ_w)
• Water potential is measured in terms of pressure.
• The unit is Pascal, Pa.
• The water potential of pure water is zero at atmospheric pressure.
• Hence, all the solutions at atmospheric pressure have lower water potential than water, that means they have a negative value.
• Water potential ψ_w is expressed as the difference between the potential of a solution in a given state and the potential of the same solution in a standard state.
• The water potential is lowered by the addition of solutes.
• Since the water potential of pure water is 0, all other water potential values will be negative.
• Hence, the movement of water occurs in osmotic or other systems from a region of higher water potential (i.e. less negative) to a region of lower water potential (i.e. more negative).
• Water potential of any solution is affected by three factors:
  • Concentration
  • Pressure
  • Gravity
  • This can be represented by the equation as:
  • ψ_w = ψ_s + ψ_p + ψ_g
  • ψ_s = effect of solutes (i.e. solute potential or osmotic potential). Solutes in a cell decrease the free energy of water, or the water potential.
  • ψ_p = effect of pressure (i.e. pressure potential or hydrostatic pressure). The positive hydrostatic pressure is termed as turgor pressure.
  • ψ_g  = effect of gravity (i.e. gravity potential). This term refers to the effect of gravity on water potential. It relies on the height of the water. If the vertical height is less than five meters, the ψ_g is negligible.
• In case of plant cell, only ψ_s and ψ_p are essential and considered i.e., ψ_w = ψ_s + ψ_p
• According to the equation, when water moves into the cell from outside, the hydrostatic pressure, i.e. pressure potential (ψ_p) increases, that results in an increased water potential (ψ_w) of the cell, and the difference between the inside and outside ψ_w is decreased.
• On the other hand, the concentration of solute when increases in the cell, the solute potential (ψ_s) is lowered, and thus, water potential (ψ_w) is reduced.
• Thus, due to water potential gradient the water from outsideflows inside the cell.
• The water moves out of the cell if a pressure is applied on the cell.
• The external pressure raises the water potential (ψ_w ) of the cell, and hence the difference in water potential inside and outside will be such that water will expel out.
• Hence, the two basic factors that affect the water potential are amount of solute and external pressure.

1. Diffusion:

• The diffusion is a passive movement of individual molecules in all directions from a region of higher concentration to a region of lower concentration.
• The flow of molecules is directly proportional to the difference in concentration.
• Diffusion is more faster in gases than in liquids.
• In case of diffusion, the movement is random and is independent of each other.
• It occurs along concentration gradient and kinetic energy and converts solvent into solution.
• The imaginary potential of solute particles to diffuse from its higher concentration to lower concentration is termed as diffusion pressure (DP).
• Its value is always greater for a pure solvent than its solution.
•  If sugar solution is made in water, then diffusion pressure is lower than that of water.
• The rate of diffusion is directly proportional to temperature, kinetic energy, and diffusion pressure gradient.
• The rate of diffusion in inversely proportional to density of medium, humidity, size of solute, molecular weight of solute and distance between diffusing particles.
• Examples of diffusion are loss of water vapour from leaves to the atmosphere, and supply of carbon dioxide from atmosphere to the leaves for photosynthesis.
• Every liquid has a fixed diffusion pressure. A pure solvent has maximum diffusion pressure.
• When some amount of solute is added to it, it’s diffusion pressure decreases. This deficit in diffusion pressure of the solution due to the addition of solute is termed as Diffusion pressure deficit (DPD).

2. Osmosis:

• It is a special type of diffusion.
• It involves the movement of water from its low concentrations to high concentrations through semi permeable membrane.
• When concentrated and dilute solutions are separated by semi permeable membrane, the solvent moves from a concentrated solution to dilute solution. This process is termed as osmosis.
• Osmosis involves movement of solvent from higher water potential to lower or from high free energy to lower free energy.
• Osmosis does not take place in isotonic solutions.
• Osmosis is feasible only in living cells.
• Osmosis continues till the equilibrium occurs between hydrostatic pressure and osmotic pressure.
• Osmosis can be either endosmosis or exo-osmosis.
• During endosmosis, the movement of solvent takes place into the cells from the surrounding, and the cell becomes turgid.
• During exo-osmosis, the movement of solvent takes place towards the surrounding from the cell, and the cell becomes flaccid.
• Osmotic pressure (OP) or Solute potential (ψ_s):
  • The pressure developed in a solution when it is separated from it’s pure solvent by means of semipermeable membrane is termed as osmotic pressure (OP).
  • The OP of pure solvent is regarded as 0, so OP for solution is always positive.
  • Osmotic pressure is directly proportional to diffusion pressure deficit and concentration of solution.
• Turgor pressure (ψ_p):
  • The hydrostatic pressure that is generated when solvent particles enter the cell and the cell membrane pushes the cell wall is termed as turgor pressure (TP).
  • Turgor pressure is only applicable for osmotic solution.
  • Flaccid cell has zero turgor pressure, turgid cell has maximum OP, TP near to minimum.
  • In case of plasmolyzed cells, negative TP is believed to be present.
  • Turgor pressure maintains turgidity and growth of cell.
  • It provides necessary energy to plumule to come out and aids in penetration of radicle into soil during germination of seeds.
• Wall pressure (ψ_w ):
  • When the cell wall becomes slightly rigid, it exerts an equal and opposite pressure in a turgid cell and is termed as wall pressure.
  • In case of turgid cells, it is equal to turgor pressure.
• Diffusion pressure deficit (DPD):
  • The difference between the diffusion pressure of the pure solvent and its solution is termed as diffusion pressure deficit.
  • DPD is the osmotic parameter that is reason for the entry of water into the plant cell.
  • The direction of osmosis is determined by DPD.
  • It is also termed as suction pressure.
  • The difference between OP and TP is known as DPD.
  • DPD = OP-TP
  • Normally, OP is greater than the turgor pressure.

Experiments demonstrating osmosis

• Egg membrane experiment:
  • An egg is taken and a hole is formed on the pointed side of the egg.
  • Then all the white and yellow part of the egg is taken out through the hole.
  • Now, the empty egg is placed in the dilute solution of hydrochloric acid (HCl), such that the hard shell of the egg made up of calcium carbonate is dissolved in it.
  • The membrane of the egg, present just beneath the cell remains intact.
  • Now, this egg membrane is thoroughly washed with tap water, then a glass tube is inserted in it and tied tightly with a piece of thread, so that the end of the glass tube remains inside the egg membrane.
  • Then, the concentrated solution of sugar is poured in it with the help of glass tube by thistle funnel.
  • The sugar solution is filled up in such a quantity, so that after completely filling the egg membrane sac, it also reaches to a small height in the glass tube.
  • The sugar solution filled egg membrane sac is then hanged in a beaker that is filled up with water, so that the level of the inner and outer liquid is same.
  • This apparatus is left for some time in the same position.
  • After some moments it is to be noted that the level of the liquid inside the glass tube has risen up enough from the level of the water of the beaker.
  • Water will continue to move across the membrane, resulting in the rise of solution in the funnel until an equilibrium is reached. Thus, osmosis is demonstrated.
  • From upper part of funnel, pressure can be applied to the solution to prevent movement of water into it through egg membrane.
  • The pressure required to halt the movement of water totally is termed as osmotic pressure.
  • This can be termed as osmotic potential or solute potential.
  • Osmotic pressure and osmotic potential are numerically equal but osmotic potential has a negative sign.
  • If one reverses the experiment by filling up the water in the egg membrane sac and the sugar solution in the beaker then the water moves from inner side to outer side.
  • This process is termed as reverse osmosis.
• Potato osmoscope experiment:
  • One peeled potato tuber is taken and made flat at its one end.
  • A deep and broad cavity is made on the other end of it, so that the bed of the cavity may reach up to flat surface, and the outer walls of the cavity becomes somewhat thin.
  • The cavity is filled up with concentrated sugar solution.
  •  The potato is placed in the beaker containing water, such that the level of the outer and inner liquids may remain same.
  • After a short time, it is noted that the level of the liquid filled in the cavity of the potato osmoscope goes higher due to endosmosis.
  • The density of the sugar solution, filled up inside the potato cavity is higher than the water of the beaker.
  • Here, the tissues of the tuber acts as a semipermeable membrane and hence the water moves from outside to inside.
  • Thus, osmosis is demonstrated.

Plasmolysis:

• When protoplast of plant cell is placed in a hypertonic solution, the shrinkage of protoplast takes place due to exo-osmosis. It is termed as plasmolysis.
• The cell soon reaches to its minimum volume after the shrinkage and if the exo-osmosis continues further, the shrinkage stops and protoplasmic membrane begins to recede or contract from the corners first. This stage is termed as incipient plasmolysis.
• If exo-osmosis continues further, there is further contraction of protoplasmic membrane, thus reaching the complete plasmolysis stage.
• In this stage, cell sap, protoplasm and nucleus etc. are wholly contracted in the centre of the cell leaving away the cell wall.
• The sugar or salt solution fills the in-between part of the cell.
• If the plasmolysed cells are placed in water, because of the endosmosis of water, there is recovery to the native condition, and this process is called deplasmolysis.
• Plasmolysis assists to determine nature of membrane, OP of cell, living or dead nature of cells.

3. Imbibition:

• It is a type of diffusion where movement of water takes place along a diffusion gradient.
• Because of this process specific dried and half-dried matters absorb water.
• Substances such as fibres, wood pieces, proteins and sponges are termed as adsorbants.
• An adsorbant is needed for the imbibition to take place.
• The cell wall and protoplasm also absorb water by the process of imbibition.
• The two essential conditions for imbibition to take place are:
• Water potential gradient between surface of adsorbant and the liquid imbibed.
• compatibility between the adsorbant and the imbibed liquid.
• Heat is released during imbibition process i.e. imbibition is an exothermic process.
• Imbibition plays an important role for dry seeds before germination.
• During germination of seeds, water enters the seed first due to imbibition pressure or matrix potential (DPD=IP).

Plant water relations and water potential: Diffusion, Osmosis and Imbibition