Imbibition is a type of diffusion by which movement of water takes place along a diffusion gradient. Due to this process certain dried and half-dried matters absorb water. Such substances (e.g., fibres, wood pieces, proteins and sponges) are called adsorbants. An adsorbant is required for the imbibition to occur. The cell wall and the protoplasm also absorb water by the process of imbibition.
The absorption by imbibitions is possible only in those conditions, where there is a great affinity between adsorbant and water, otherwise this is not possible.
For example, cotton fibres absorb water by this process, whereas rubber sheet does not, as there is no affinity between rubber and water. Dry plant material or seeds act as adsorbant to imbibe water and swell. The swollen seeds produce a large pressure developed by imbibition.
This pressure forces the seedlings to emerge above the ground through the soil surface. Here, with the result of imbibition pressure, the seed coat bursts and seedling comes out of ground. This pressure is called imbibition pressure or matric potential (i.e., water potential of the matrix) in the context of plant water relations.
For imbibition, two conditions are important. They are:
(i) Water potential gradient between surface of adsorbant and the liquid imbibed.
(ii) Affinity between the adsorbant and the imbibed liquid.
Imbibition plays an important role in life of plants:
(i) The first step in the absorption of water by the roots of higher plants is the imbibition of water by the cell walls of root hairs.
(ii) Imbibition of water is essential for dry seeds prior to germination.
Now, imbibition pressure is called matric potential (Ψw). The matric potential in an imbibant results primarily from adsorptive forces which bind water molecules to micelles or molecules of the adsorbant (imbibant) and is analogous to the osmotic potential of a solution.
In a solution, the greater the amount of water present in proportion to a given amount of solute, the less negative (or higher) is the osmotic potential. Similarly, in an imbibant the greater the amount of water present in proportion to a given amount of imbibant, the less negative is the matric potential. With reference to pure water, the values of matric potentials are always negative.
The water potential of an imbibant is equal to its matric potential (always negative) plus any turgor or other pressure (pressure potential) which may be imposed upon the imbibant.
Ψw = Ψm + Ψs
If the imbibant is unconfined no turgor or such pressure is involved and hence the above equation is simplified as:
Ψw = Ψm