Imbibition : Plant-Water Relations
Imbibition is the absorption of water by the solid particles of an adsorbant without forming a solution . The particles which absorb water are the imbibants. The pressure which is created is called imbibition pressure. The liquid (usually water) which is imbibed is known as imbibate. Plant imbibants are hydrophilic (water loving or readily absorbing moisture) colloids.
They have a negative water potential. As a result when they come in contact with water, a steep water potential gradient is established. Water diffuses rapidly into the adsorbant. The process will continue till it establishes an equilibrium.
A dry piece of wood when placed in water, swells and the volume increases. Seeds swell when soaked in water. Wooden doors and windows absorb water in humid rainy season and swell. All these are the examples of imbibition.
Characteristics of imbibition
- Adsorption: Imbibant substance is an adsorbent. Imbibants holds imbibate (water) by adsorption due to the great force of attraction between the two.
- Water Potential: Imbibants have a high negative water potential.
- Increase in volume: The dry seeds of pea are negative to water potential. A steep water potential gradient is established when they are soaked in water. As a result the seeds swell.
- Heat of wetting: During imbibition, temperature increases. The water molecules possess kinetic energy. They get tightly absorbed . As a result they become immobile and lose most of their kinetic movement. The kinetic energy is released in the form of heat called heat of wetting.
Imbibition of water increases the volume of the imbibant, which results in imbibitional pressure. It develops due to the matric potential of the imbibant, hence called matric potential. It is similar to turgor pressure.
Diffusion pressure deficit=Imbibition pressure-Turgor pressure
However, turgor pressure develops only when imbibant is confined.
Therefore, DPD=IP for unconfined imbibant.
Imbibition capacity varies in different imbibants. It is maximum in phycocolloids followed by proteins, starch and cellulose. Lignin is hydrophobic. It, therefore, does not show imbibition of water.
Conditions necessary for imbibition are as follows:
Diffusion pressure gradient:
A diffusion pressure gradient must exist between the imbibant and the substance being imbibed. Diffusion pressure of dry substance is zero.
A diffusion pressure gradient is established as soon as such a substance is immersed in pure water which has the highest diffusion pressure. Water enters the substance (i.e., imbibant) rapidly. The pressure continues till the diffusion pressure of both, the water and the imbibant becomes equal.
A certain degree of affinity exists between the imbibant and the substance being imbibed. For example, proteins, carbohydrates and other colloidal substances have a strong attraction for water molecules.
In germinating seeds, the amount of colloidal substances is very high and, therefore, water enters as a result of imbibition. Hence, the storage materials in germinating seeds swell causing the seed coat to rupture.
Factors affecting the rate of imbibition
Effect of concentration of solute in the medium:
Presence of solutes in water affects the rate of imbibition. Thus the rate of imbibition is more in pure water than in a solution. Therefore the rate of imbibition decreases with increase in the concentration of solute in the medium. If a solution contains a very high concentration of solutes then imbibition totally ceases.
Effect of pH:
Hydrogen ion concentration or pH of the medium largely affects the rate of imbibition. 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 takes place. However, proteins being amphoteric (able to react both as a base and as an acid) are exceptions.
Nature of imbibant:
Different types of substances have different imbibing capacities. Protiens have a very high imbibing capacity. Starch has less imbibing capacity and cellulose is the weakest imbiber. That is why proteinaceous pea seeds swell more on imbibition than starchy wheat seeds.
Surface area of the imbibant:
Imbibition will be greater when the surface area of the imbibant is larger.
The rate of imbibition increases with the increase in temperature. Perhaps the increased temperature increases the kinetic energy of the system. Thus at low temperatures the rate of imbibition decreases.
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 as compared to loosely packed one.
Importance of imbibition
- Imbibition is the initial step of germination of seeds. The water is first imbibed by the seed coat than by other tissues of embryo and endosperm.
- Breaking of seed coat in germinating seed is due to greater imbibitional swelling of the seed kernel (starch and protein) as compared to seed coverings (cellulose).
- Seedling is able to come out of the soil due to development of imbibition pressure.
- Imbibition pressure developed during germination of seeds and spores can even break concrete pavements and asphalt roads.
- Absorption of water by young cells is mostly due to imbibition.
- Imbibition is dominant in the initial stage of water absorption by root.
- Imbibition helps in the movement of water into ovules that are ripening into seeds .
- The swelling of wood due to imbibition causes jamming of wooden frames during rain.
- Fruits retain water due to imbibition.
- Imbibition force is useful in adhering water to the walls of the xylem elements.
Difference between imbibition and osmosis
|1. Imbibition involves the absorption of solvent or water by a solid substance.||1. It involves the movement of water or solvent from its higher chemical potential to its lower chemical potential.|
|2. Imbibition does not produce a solution.||2. Osmosis usually takes place in solution separated by a semipermeable membrane.|
|3. It does not require a semipermeable membrane .||3. A semipermeable membrane is essential for the operation of osmosis.|
|4. Imbibition produces heat.||4. Osmosis does not produce heat.|
|5. Imbibition can develop a very high pressure (up to 1000 atm) known as imbibition pressure.||5. Osmosis develops a comparatively low pressure (up to 100 amp) known as osmotic pressure.|
|6. Imbibition requires the presence of colloidal particles.||6. Osmosis usually requires the presence of solute particles.|