Imbibition in Plants: Meaning and Factors

In this article we will discuss about the meaning and factors of imbibition.

Meaning of Imbibition:

Place a few grams of air-dried seeds in a flask with water to which a capillary tube is attached.

Initially, the water level increases but as the imbibition continues; this rise is followed by a fall. Water molecules adsorbed by the colloidal particles surface are held relatively tight and consequently are packed together. As a result the volume of the system decreases.

The uptake or absorption of liquids by the solid particles of a substance without forming a solution is called imbibition.

Here, the net movement of water is along a diffusion gradient. Imbibition is the capacity of a gel or any other colloidal material to take up relatively large quantities of water and swell e.g. absorption of water by cell wall, swelling of seed coats, starch, glue, cellulose, agar, gelatin, swelling of doors, and wood work during the rainy season.

As a result of imbibition the volume of imbibant increases even though the total volume of the system (volume of water, in which imbibant is submerged + volume of the imbibant) is always less after imbibition than initially.

There is always a rise in temperature as a result of imbibition since some of the kinetic energy of the molecules is lost.

During the swelling of an imbibant, pressure of enormous magnitude develops. In practice, this is used in the breaking up of rocks by pouring water on the wooden wedges driven into them and also in pulling apart of the bones of a skull.

A substance imbibes only those liquids for which its molecules have affinity e.g., dry plant materials imbibe water but not ether. Similarly, rubber imbibes ether but not water.

Imbibition pressure (IP) is analogous to osmotic potential and represents the potential maximum pressure that a substance develops when immersed in pure water.

The actual pressure which develops following water imbibition by a substance is considered as pressure potential. Thus Ψ = IP_r + Ψ – TP.

However, no pressure potential develops in an unconfined imbibant and, accordingly, the above expression is stated as follows:

Ψ = Ψ_r = IP

The imbibition pressure (IP) of air-dried seeds of Xanthium is nearly 1000 atm. If these seeds are immersed in pure water, the Ψ of water in the dry seeds would be nearly 1000 atm. at equilibrium.

After imbibition stops, the Ψ of the external and the internal water is nil. Temperature and osmotic potential of the substance affect the rate and extent of imbibition.

An increase in temperature increases the rate of imbibition but not the amount of water imbibed.

Both the amount of water imbibed and the rate of imbibition are affected by the osmotic potential of the liquid to be imbibed.

Factors Affecting Imbibition:

Imbibition is influenced by several factors and some of these are discussed below:

i. Temperature:

With an increase in temperature the rate of imbibition increases. Perhaps the increased temperature increases the kinetic energy of the system. The situation is reverse at low temperature.

ii. Texture of imbibant:

The cohesion of molecules of the imbibant largely affects the amount of water imbibed. Thus a closely packed imbibant imbibes less amount of water compared to loosely packed one.

iii. Pressure:

It may be remembered that imbibition pressure comprises hundreds of atmospheres. Thus colloidal particles can imbibe water against lot of pressure.

iv. pH:

Imbibition is also dependent upon the acidity and alkalinity or pH of the medium. Thus cellulose which is negatively charged colloid imbibes maximum in alkaline medium while it will absorb least in the acidic medium.

In case of positively charged colloids, a reverse situation exists. However, proteins being amphoteric are exceptions.

v. Electrolytes:

Inorganic solutes also influence imbibition by neutralising the charge of imbibing colloids and production of osmotic pressure.

Different ions have variable power to reduce imbibition. Accordingly some of the electrolytes are most effective in influencing imbibition than the others.

It is a common experience that during the rainy season, wooden frames of the doors and windows imbibe water and expand. Similarly when the seeds are sown watering is immediately seen to affect imbibition. This affects the seed germination and emergence of radicle.

The seed coat is made up of cellulose which imbibes enormous amount of water and facilitates the breakage of seed coat and the embryo is unaffected because it is made up of proteins, lipids and starch.

In the young cells, the wall is made up of hydrophillic colloids and thus water absorption is brought about through imbibition. These colloids also provide protection to the seeds against desiccation in a dry environment and freezing temperature.

Some of the lower plants like mosses and Selaginella remain green even after low level of precipitation. These plants possess hydrophilic colloids which help absorbance of water from slight precipitation.