What Is Imbibition In Plants?
It is a special type of diffusion in which phenomena of adsorption instead of absorption are involved. As with osmosis, it may be considered a special type of diffusion since the net movement of water is along a diffusion gradient.
In the case of imbibition, however, an adsorbent is involved. If dry plant material is placed in water, a noticeable swelling takes place and sometimes amounts to a considerable increase in volume.
Prerequisite For Imbibition
Two conditions appear to be prerequisite for imbibition is:
- A water potential gradient must exist between the surface of the adsorbent and the liquid imbibed, and
- A certain affinity must exist between components of the adsorbent and the imbibed substance.
The Imbibition Process
When dry plant material is placed in pure water, a steep water potential gradient is established, and water moved rapidly to the surface of the adsorbent.
Water Potential Of Dry Seeds
Very negative water potentials can exist in dry plant materials and pure water. For example, water potentials of -900 bars have been recorded in some dry seeds.
As water continues to be adsorbed, the water potential becomes less negative until it finally equals that of the external water.
At this point, equilibrium is established, imbibition stops, and water moves to and from the adsorbent in equal quantity. An adsorbent does not necessarily imbibe all kinds of liquids.
For example, dry plant materials immersed in ether do not swell appreciably. Rubber, however, does imbibe ether and will swell appreciably if submerged in it. But rubber will not imbibe water.
The obvious implication is that certain attractive forces must exist between components of the imbibant and the imbibed substance.
Importance Of Imbibition
Plant cells possess a considerable amount of carbohydrates, in the form of cellulose and starch, to which water is strongly attracted.
The adsorption of water to the surfaces of hydrophilic colloids is of major importance to the imbibition process.
Imbibition In Seeds
Seeds, which are particularly high in colloidal material, are very good adsorbents. Water is brought into the germinating seed largely through this process.
Factors Affecting Imbibition Process
The rate and extent of imbibition are affected primarily by temperature and by the osmotic potential of the substance to be imbibed.
Temperature does not affect the amount of water taken up by the adsorbent, but it does have a definite effect on the imbibition; an increase in temperature causes an increase in the rate of imbibition.
Osmotic Potential Of The Substance
Both the amount of water imbibed and of imbibition is affected by the osmotic potential of the substance to be imbibition.
The addition of a solute to pure water has a more negative water potential. This addition has the effect of altering the water potential gradient between the solution water and the adsorbent.
Water Potential Gradient
The water potential gradient is less steep than it would be if the same adsorbent were submerged in pure water.
Similarly, a decrease in the water potential gradient will bring about a decrease in the rate at which water is imbibed and thus the amount of water taken up.