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The process involving the loss of water from the aerial parts of the plants (especially from leaves) in the form of water vapour is called transpiration. Water is a necessity in all forms of life and plants, it is the roots that perform the function of acquiring water from the soil. However, the plants for growth and metabolism use a very small percentage of that water. The remaining amount of water, which is almost 95-99%, is lost via transpiration and guttation. The driving force that acts in favour of transpiration is the difference in water vapour between the leaf air spaces and the external air. The higher is this difference in vapour pressure, the more is the rate of transpiration. Transpiration can be divided into three types depending upon its location:
Cuticular Transpiration: Cuticle is the waxy layer that covers the epidermis of leaves and herbaceous stems. It is a polymer made of cutin, which is its chief constituent, and wax. The polymer is composed of long-chain epoxy fatty acids, attached via ester linkages. Even though the primary function of the cuticle remains prevention of transpiration, some transpiration does take place through it, which is about 5-10% of the total transpiration that takes place in a plant.
Lenticular Transpiration: The openings in barks and stems that allow the gaseous exchange between the inner living cells of the plants and the atmosphere are termed as lenticels. The loss of water in the form of water vapour from lenticels is called lenticular transpiration. Only 1-5% of the total transpiration takes place through lenticels.
Stomatal Transpiration: Stomatal transpiration accounts for approximately 90% of the total transpiration from plants, which is the highest among the three types. Stomata are specialized structures located on the epidermis of plants for the regulation of gaseous exchange between the plant and its surroundings. The opening and closing of stomata are regulated by turgor pressure.
Various factors have been known to determine the rate of transpiration, some of them are light, temperature, humidity, and even the surface of the leaf from which transpiration is occurring. Even though leaves are the most common sites of transpiration, this phenomena of water loss can also occur from stems and flowers, as well. This biological process is witnessed in all higher plants and trees.
Transpiration pull is a physiological process can be defined as a force that works against the direction of gravity in plants due to the constant process of transpiration in the plant body. This force helps in the movement of water as well as the minerals dissolved in it to the upper parts of the plants. This movement of water takes place through the xylem, a dead tissue that is found throughout the length of plants. Transpiration pull can alternatively be described as a suctional force that pulls up the groundwater in an upwards direction. The plant, for photosynthesis, utilizes a very small percentage of that water and remaining is transpired into the atmosphere via water vapours.
In the process of transpiration, the water molecules from the soil combine, owing to their cohesive force, to form a column in the xylem. An adhesive force also comes in to play that acts between the water molecules and the xylem vessel. The pressure that is created by the transpiration pull generates a force on the combined water molecules and aids in their movement in an upward direction into the leaves, stems and other green parts of the plant that is capable of performing photosynthesis. This theory explaining this physiological process is termed as the cohesion-tension theory.
The mechanism underlying this biological phenomenon is based on the upward movement of water, which starts from the tip of the root, in the soil and ends in the aerial parts of the plant body. This movement of the water and the minerals dissolved in it through the xylem tissue is called the ascent of sap. During the process of transpiration in form of water vapour into the atmosphere, a negative hydrostatic pressure is also created in the mesophyll cells of leaves to favour the draw of water from the roots to the veins of the leaves.
As mentioned previously, there can be several factors affecting the rate of transpiration. These factors can be external; for example, environmental conditions or can also be controlled by the plants (internal) by adjusting the size of the stomatal apertures. For environmental influences, the rate of transpiration can be altered by the evaporative demand of the atmosphere surrounding the site of transpiration, like boundary layer conductance, temperature, humidity, wind, and incident sunlight. The level of soil, water and temperature of the soil can also affect stomatal opening and closing, and hence on the transpiration rates. The percentage of water loss from transpiration also depends on the size of the plant or its leafiness.
Transpiration rates are also enhanced in plants with young shoots. Transpiration, though accounts for a large amount of water loss from the plant body, aids in keeping the plant cool by evaporation, since the evaporating water vapour carries away some of the heat energy owing to its large amount of latent heat of vaporization, which is approximately 2260 kJ per litre. During transpiration, molecules of water get evaporated from the stomata. As a result of this, the concentration of water is lowered in the plant’s mesophyll cells resulting in the reduction of the cells’ sap of mesophyll compared to that in the xylem vessels. This causes the upward force that pulls the water from the root to the mesophyll cells by creating a negative pressure in xylem vessels that aids in pulling off the water from the soil via the roots.
Transpiration pulls in plants consequences from the excretion or evaporation of water that is lost from the surface mesophyll cells present in the leaves. This process aids the proper and uninterrupted flow of water and prevents the plant from creating an embolism. That is why, even though the plant loses water via this physiological process, it is also necessary for the plants' metabolism, hence designating the process of transpiration as a 'necessary evil'.