Content Strategy Manager
Transpiration Pull is the biological force generated by plants to draw the water upwards from roots to leaves through xylem tissues. This water thus transported from roots to leaves helps in the process of photosynthesis. The extra water is excreted out to the atmosphere by the leaves in the form of water vapours through stomatal openings. Transpirational Pull is mainly seen in higher plants and trees as their stems are surrounded by fine xylem tubes.
The following article discusses the phenomena of transpiration and transpiration pull which is generated in the plants because of it and why it is essential for the survival of plants.
| Table of Contents |
Key Terms: Photosynthesis, Transpiration, Nutrition, Metabolism, Vaporisation, Xylem, Water, Vessel, Stomata, Root, Stem, Leaves
What is Transpiration?
[Click Here for Sample Questions]
The approach which involves the loss of water from the aerial portion of the plants in the form of water vapours is termed transpiration. Water is the fundamental necessity in all forms of life and plants. The roots of the plants serve the function of fetching water from the soil. However, the plants for growth and metabolism utilise a very little portion of that water. The remaining portion of water, which is nearly 95-99%, is lost via transpiration and guttation.
- The driving force which acts in favour of transpiration is the difference in water vapour between the leaf air spaces and the external air. The greater is this difference in vapour pressure, the more is the rate of transpiration.
- Transpiration can also be termed the process in which the moisture and other gaseous wastes of the plants are excreted through the stomata of its leaf, lenticels of its stem, and fruit.
- In this procedure, the water immersed by the root tips is emitted out into the atmosphere by the leaves and stems of the respective plants to keep the plants cool and to allow the root to absorb more water and other important nutrients from the soil.
Transpiration
Read More: Difference between Xylem and Phloem
Types of Transpiration
[Click Here for Sample Questions]
There exist three types of Transpiration, depending upon its location, namely,
- Cuticular Transpiration
- Lenticular Transpiration
- Stomatal Transpiration
Cuticular Transpiration: The epidermis of the plant leaves and herbaceous stems is covered by the waxy layer named Cuticle. It is a polymer made up of cutin, which is its principal component, and wax. The polymer is made of long-chain epoxy fatty acids, which are attached via ester linkages. Even though the primary function of the cuticle remains the 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 lenticels are the openings in the barks and stems of the plant that allow the gaseous exchange between the inner living cells of the plant and the atmosphere. The loss of water in the form of water vapour is termed lenticular transpiration. Nearly about 1-5% of the whole transpiration occurs through lenticels.
Stomatal Transpiration: The specialised structures located on the epidermis of the plants and are responsible for the gaseous exchange between the plant and its surroundings is termed the Stomata. The opening and the closing of the stomata are regulated by the turgor pressure. Approximately 90% of the total transpiration is done by the stomata transpiration which is the highest amongst the three types of transpiration.
Read More: Plant Cell
Transpiration Pull
[Click Here for Sample Questions]
The diverse living world surrounding us is divided into two major groups - plants and animals. The plants provide us with our primary sources for nutrition and keep the atmosphere balanced by taking up carbon dioxide during photosynthesis, discharging oxygen in exchange for it. The physiological process which can be determined as the force functioning against the direction of gravity in plants due to the continuous processes of transpiration in the plant body is referred to as the transpiration pull. It can also be characterised as the pulling forces produced inside the xylem tissue supporting the upward movement of water into the xylem vessels.
The transpiration pull can also be explained as a suction force used to drag the water from the roots in an upward direction to the leaves of the plant. The quantity of water received by the plant leaves is utilised for photosynthesis and the excess portion of water is discharged into the surrounding atmosphere in the form of vapours through the opening in the leaves understood as stomata.
Transpiration Pull
Cohesion Hypothesis
[Click Here for Sample Questions]
Alternatively known as Transpiration, the emphasis on cell sap or vascular sap in living vascular Plants was effectively clarified by the hypothesis of Cohesion-Tension by Two botanists- Dixon and Joly in 1894 and after that by Askenasy in 1895. The theory emphasises the statement that “the increase of water in trees is mainly because of the Transpirational Pull executed because of constant segments of water in the Xylem vessels that go through the whole length of the Plant (from roots to leaf)”. Sap ascends in plants through intermolecular collaborations. Otto Renner, another Botanist in 1911 effectively showed the relevance of Cohesion theory through his tests, resulting from solid proofs in support of the theory made.
On a molecular basis, it is assumed those cohesive, as well as adhesive properties of water and its special exchange with Xylem’s wall, give a push to the strong connection needed to transport and pull water against the gravitational forces, up high. This hypothesis was not challenged and was constant in botanical history, but at present, the scenario is changed and is not applicable to all species of plants & trees.
Phenomenon of Transpiration Pull
[Click Here for Sample Questions]
The phenomenon of transpiration pull is explained below:
- In the transpiration procedure, the molecules of water from the soil combine, owing to their cohesive force, to construct a column in the xylem.
- An adhesive force also comes into play that operates between the water molecules and the xylem vessel.
- A force is generated on the combined water molecules as a result of the pressure developed by the transpiration pull and aids in their movement in an upward direction into the leaves, stems, and other green parts of the plants that are capable of executing photosynthesis. This approach describing the physiological operation is referred to as the cohesion-tension theory.
- The mechanism underlying the biological phenomenon relies upon the upward movement of water, which initiates from the root’s tip in the soil and terminates in the aerial regions of the plant body.
- The movement of the water and the minerals dissolved in it through the xylem tissue is termed the rise of sap.
- During the transpiration procedure in the form of water vapour into the environment, an opposing hydrostatic pressure to favour the pull of water from the roots to the veins of the leaves is also created in the mesophyll cells of the leaves.
Role of Transpiration Pull in Plants
[Click Here for Sample Questions]
The roles of transpiration pull in plants are:
- Although, transpiration accounts for a huge amount of water loss from the plant body, benefits in keeping the plant cool by evaporation since the water vapours formed during evaporation carry away some amount of the heat energy owing to its large quantity of latent heat of evaporation, which is approximately 2260 kl per litre.
- During the procedure of transpiration, the water molecules get evaporated from the stomata of the plant.
- In this process, the water concentration is reduced in the mesophyll cells, resulting in the lowering of the cells sap of mesophyll compared to that of the xylem vessels.
- As a result, the upward movement of water commences from the root to the mesophyll cells to pull the water from the soil by the generation of an opposing pressure in xylem vessels.
- Due to the evaporation or excretion of water from the cell’s surface in the plant leaves, transpiration pull in the plants takes place. The above technique helps in the proper flow of water and rescues the plant from an embolism.
Read More: Difference between Evaporation and Transpiration
Things to Remember
- The tiny droplets formed on the surface & margin of the leaf are the vapours excreted from the plant leaves.
- The approach involving the loss of water and various other gaseous wastes from the stomata of the leaf, lenticels of the stem, and fruits is determined as transpiration.
- There are mainly 3 types of transpiration: Cuticular, Lenticular, and Stomatal transpiration.
- The evaporation of excretion of water from the cell's surface in the leaves results in the transpiration pull.
- The transpiration pull is the biological approach where the pulling force in the xylem tissue of plants aids in dragging the water in an upward direction into the xylem vessels.
- Loss of water takes place in the form of vapours during the process of transpiration pull.
- The water molecules get evaporated from the stomata of the plant leaves during the transpiration procedure.
Sample Questions
Ques. What are the factors which impact the speed of transpiration in plants? (4 marks)
Ans. Factors that impact the transpiration in plants are mentioned below:
- Light: Plants develop faster in light than at night. This means light encourages the opening of the stomata. Hence light helps in accelerating transpiration by warming the leaf since evaporations take place then.
- Soil Humidity: The presence of less humidity/water in soil interrupts the transpiration in a plant as the plant absorbs the water through its roots when it fails to do so decrease in turgor happens resulting in the closure of stomata and leaving the plant vulnerable.
- Temperature: The evaporation process speeds up during high temperatures resulting in increased speed of transpiration.
- Wind: In the absence of breeze, the air containing vapours surrounding a leaf starts to become more humid which causes a low speed of transpiration. The humid air is carried by the wind and replaced with dry air.
Ques. How effective is transpiration pull in plants? And what could be the possible threats of transpiration? (4 marks)
Ans. The transpiration pull is the biological procedure through which the plants pull water upwards. Due to the pulling action generated, water rises high in cases of tall trees. The force generated by the transpiration can lift the water over 130 metre high. Transpiration pull creates negative strain (pressure) identical to – 2 MPa at the leaf surface and other various threats to the plant mainly:
- Reduced Growth: Transpiration reduces water inside the plant which leads to slow growth of the plant.
- Wilting: Loss of turgidity during midday is normal, due to which the water evaporation paces. And higher the rate of evaporation higher would be the transpiration reducing photosynthesis & other metabolic processes.
- Abscisic Acid: Low water existence releases abscisic acid in a plant. This stops various plant processes and promotes the fall of leaves, flowers, or fruits.
Ques. Which process is responsible for creating a pulling force which pulls the water or minerals solution in the plant? (2 marks)
Ans. In plants, the transpiration process is responsible for creating a suction pressure that pulls up the water from the xylem of the plant roots to its stem and further to its leaves. Water is fundamental for plants however just a limited quantity of water taken up by the roots is utilised for development and metabolism. The leftover 97 - 99.5% is lost by happening and guttation. There are three types of transpiration like Cuticular transpiration, Stomatal transpiration, Bark Transpiration & Lenticular Transpiration.
Ques. What is capillary action in plants? Give an example. (2 marks)
Ans. The capillary action in plants is a process where the water moves upward from the roots and stems to the remaining parts of the plants. This action of the capillary is observed in the plants during the upward movement of water through the xylem tissues. The particles of the water (the fluid) are drawn to the atoms within the stem (the strong). This attraction is utilised to support the water up from the ground level and scatter it all within the plant. Examples for capillary action daily can be seen like, water movement upwards in straw against gravity, tear moving through tear ducts of a body, etc.
Ques. What are the major forces that are responsible for the movement of water from the soil? (3 marks)
Ans. Two major forces move the liquid/water through the soil pores. The forces are as follows:
- Gravity: The gravity force is the most crucial in saturated soils as it causes the downward force on the water. When the soil is near saturation, the large pores of the soil tend to fill; the water begins to move rapidly.
- Adhesion: It is useful, and the most important force that moves the water in the unsaturated soils, where most crops are grown. Adhesion & cohesion causes water molecules to hang together moving water in the particle surfaces and through the little pores.
Ques. Differentiate between the transpiration pull and the root pressure. (4 marks)
Ans. Mentioned below are the differences:
- The key difference between the root pressure and the transpiration pull is that the root pressure is the osmotic pressure which is developed due to the movement of water from soil solution to root cells.
- The transpiration pull is negative pressure, which develops at the plant’s top due to the water evaporation from the surfaces of the mesophyll cells.
- Transpiration is the main participant responsible for the movement of water from the ground whereas root pressure is a partial contributor to the increase in water & minerals within the plant.
- The process of root pressure takes place in the morning when the stomata are not open. On the other hand, transpiration pull occurs during noon when water tends to evaporate.
Ques. What is the upward movement of water? (2 marks)
Ans. In plants, the upward movement of water and minerals from the root to the tip through the xylem tissue of plants is referred to as the ascent of sap. It is called the ‘ascent of sap’ as when the water is transported all along with the height of the stem, it also contains various organic and inorganic components or impurities in other words the upward movement of water mixed with minerals from roots to water part of the plant is known as Ascent of sap. To support the statement various theories were made namely:
- Vital Theory
- Root-pressure Theory
- Physical Force Theory
Ques. Explain the Cohesion-Tension theory of water. (3 marks)
Ans. Cohesion-tension theory of water was developed around 1894. This theory states that water molecules contain a strong mutual force of attraction which is known as Cohesive force to which the molecules cannot be easily separated from each other. The cohesive force along with adhesive force forms an unbreakable continuous water column in the xylem. It makes negative strain (pressure) identical to – 2 MPa at the leaf surface. Water from the roots is eventually pulled up by this strain. Negative water potential brings water from the dirt into the root hairs, then, at that point, into the root xylem. Cohesion and adhesion draw water up the xylem. Transpiration moves water from the leaf via the stoma. Water potential decreases from the root cells to the stem to the most elevated leaves, lastly to the atmosphere.
Check-Out:
Comments