Plant Water Retention
Plant Water Retention
Water is the most important constituent of plants and is essential for the maintenance of life, growth and development. Plants lose huge amount of water through transpiration. They have to replenish this lost water to prevent wilting. Water is mainly absorbed by the roots of the plants from the soil, than it moves upward to different parts and is lost from the aerial parts, especially through the leaves. Before taking up the absorption and movement of water in plants, it is worthwhile to understand the phenomenon of imbibition, diffusion and osmosis involved in the water uptake and its movement in the plants.
The process of adsorption of water by solid particles of a substance without forming a solution is called 'imbibition'. It is a type of diffusion by which movement of water take place along a diffusion gradient. The solid particles which adsorb water or any other liquid are called imbibants. The liquid which is imbibed is known as imbibate. Cellulose, pectic substances, protoplasmic protein and other organic compound in plant cells show great power of imbibition.
- Characteristics of imbibition : The phenomenon of imbibition has three important characteristics:
- Volume change : During the process of imbibition, imbibants increase in volume. It has been observed that there is an actual compression of water. This is due to arrangement of water molecules on surface of imbibant and occupy less volume than the same molecules do when are in free stage in the normal liquid. During the process of imbibition affinity develops between the adsorbant and liquid imbibed. A sort of water potential gradient is established between the surface of adsorbant and the liquid imbibed.
- Production of heat : As the water molecules are adsorbed on the surface of the imbibant, their kinetic energy is released in the form of heat which increase the temperature of the medium. It is called heat of wetting (or heat of hydration). e. g., during kneading, the flour of wheat gives a warm feeling due to imbibition of water and consequent release of heat.
- Development of imbibitional pressure : If the imbibing substance (the imbibant) is confined in a limited space, during imbibition it exerts considerable pressure. The bursting of seed coats of germinating seeds is the result of imbibition pressure developed within the seeds as they soak the water. Imbibition pressure can be defined as the maximum pressure that an imbibant will develop when it is completely soaked in pure water. Imbibition pressure is also called as the matrix potential because it exists due to the presence of hydrophilic substances in the cell which include organic colloids and cell wall.
e.g. If a dry piece of wood is placed in water, it swells and increases in its volume. Similarly, if dry gum or pieces of agar agar are placed in water, they swell and their volume increases. Wooden doors and windows adsorb water in humid rainy season and increase in their volume so that they are hard to open or close, in gram and wheat the volume increase by adsorption of water, in plant systems are adsorption of water by cell wall.
Resurrection plants of Selaginella, lichens, velamen roots and dry seeds remain air dry for considerable periods because they can absorb water from the slight downpour by the process of imbibition.
- Factors influencing the rate of imbibition
- Nature of imbibant : Proteins are the strongest imbibants of water, starch less strong, cellulose being the weakest. That is why proteinaceous pea seeds swell more than the starchy wheat seeds. During seed germination seed coat rupture first because it is made up of cellulose (weak imbibant) and kernel is made up of protein, fat and starch (strong imbibant).
- Surface area of imbibant : If more surface area of the imbibant is exposed and is in contact with liquid, the imbibition will be more.
- Temperature : Increase in temperature causes an increase in the rate of imbibition.
- Degree of dryness of imbibant : If the imbibant is dry it will imbibe more water than a relatively wet imbibant.
- Concentration of solutes : Increase in the concentration of solutes in the medium decreases imbibition due to a decrease in the diffusion pressure gradient between the imbibant and the liquid being imbibed. It is due to the fact that imbibition is only a special type of diffusion accompanied by capillary action. If some solute is added into the liquid which is being imbibed, its diffusion pressure decreases and the process of imbibition slows down.
- pH of imbibant : Proteins, being amphoteric in nature, imbibe least in neutral medium. Towards highly acidic or highly alkaline pH, the imbibition increases till a maximum is reached, there after it starts slowing down.
- Significance of imbibition
- The water is first imbibed by walls of root hairs and then absorbed and helps in rupturing of seed coat (made up of cellulose).
- Water is absorbed by germinating seeds through the process of imbibition.
- Germinating seeds can break the concrete pavements and roads etc.
- The water moves into ovules which are ripening into seeds by the process of imbibition.
- It is very significant property of hydrophilic surfaces.
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