Transpiration in plants is a process of water loss (in the form of water vapours) into the atmosphere, from the leaf surface and stems. A plant undergoes transpiration primarily to access carbon-dioxide needed for photosynthesis and to cool itself. As the plants are immobile, they manage their water needs by the two fundamental processes like water absorption and transpiration.
During water absorption, a plant takes up the water from the root system via xylem. In contrast, transpiration in plant expels excessive water from the leaf surface or stems into the atmosphere via specialized parts. Stomata, lenticels are the specialized parts of the plant that allow the flow of transpiration or water loss. Potometer and other devices like lysimeter, porometer etc. are the equipment which can measure the rate of transpiration in plants.
Content: Transpiration in Plants
- Definition of Transpiration in Plants
- Transpiration Rate
- Types of Transpiration
- Factors Affecting Transpiration in Plants
- Transpiration Experiment
Definition of Transpiration in Plants
Transpiration in plants can define as the natural process, where a plant releases excessive water into the atmosphere in the form of water vapour like a human body does. A human releases its excessive water by the process refers as “Sweating” whereas, in plants, it commonly refers to as “Transpiration”. The plant uses around 5% of the water to fulfil its basic needs like to prepare food and some uses to perform cellular activities. Thus, a plant releases much water or transpires more because transpiration helps in:
- Maintaining the turgidity of the plant cells.
- Ingress CO2 for the photosynthesis.
- It also increases the nutrient uptake by a plant.
- Transpiration cools the plants, by the exothermic release of energy in the form of vapours from the leaf cell into the atmosphere.
A transpiration rate can define as the driving forces of the environment like humidity, light, temperature etc. to expel water out of the plant resistances like stomata, cuticle, lenticels etc.
A driving force is a potential difference between the soil and atmosphere surrounding the plant. It can define as the difference in potential of the leaf to uptake water from the roots via xylem and the potential of atmosphere to uptake the evaporated water from the leaf cell. Thus, water diffuses out of the leaf from the high water concentration to the low concentration region. Drier is the air surrounding the leaves, and higher will be the driving force of water to expel out of the leaves.
There are Stomata, cuticle and lenticel resistances in a plant which restricts the water movement out of the leaf into the atmosphere. Thus, a transpiration rate strongly depends upon the driving forces of the environment and the resistances of a plant. Transpiration rate can represent as the given equation:
Transpiration rate = Water potential of the leaf – Water potential of the atmosphere / Resistance
Through this equation, we can predict the ease of transpiration by knowing the value of driving force and resistance. Higher is the driving force; faster will be the transpiration rate. Greater is the value of resistance; slower will be the transpiration rate.
The unit of transpiration rate is the moles of water lost in per leaf area per time (mol/cm2/s).
Types of Transpiration
A majority (90%) of transpiration takes place via leaf surface, and a process refers as “Foliar transpiration”. Sometimes, it occurs via stems, and a process termed as “Cauline transpiration”. There are four types of transpiration, like:
Lenticular transpiration: It can define as another type of the water loss from the pores of older or woody stems refers as “Lenticels”. It accounts for 0.1% of the total water loss.
Cuticular transpiration: It is a type of transpiration where a water loss is through the cuticles found on the surface of the leaf. It accounts for 5-10% of the total water loss. The cuticle is the waxy coating or a fatty substance which covers the epidermis layer of the plant.
Stomatal transpiration: It can define as the diffusion of water from the stomatal pore of the lower epidermis layer of the leaf. It accounts for 85-90% of the total water loss.
Bark transpiration: Sometimes transpiration occurs through the corky or hard covering of stem refers as “Bark” which has a high rate of transpiration than the lenticular type due to its large surface area.
A mechanism of transpiration involves water absorption and water expulsion. A whole mechanism of transpiration involves:
- Root hairs are in direct contact with the soil, which uptakes the water and mineral salts present in the ground.
- Then, the water absorbed by the roots moves up to the different parts of the plant via xylem by capillary action.
- Xylem transports the water to the mesophyll cells.
- The mesophyll cells of a plant comprise of large intercellular spaces where the water is stored.
- At the time of transpiration, the water from the intercellular spaces diffuses into the sub-stomatal cavity in the form of water vapours.
- Finally, the water vapours escape into the atmosphere by the opening of the guard cell.
Factors Affecting Transpiration in Plants
It includes the stomatal, cuticle and the boundary layer resistance which decides the rate of transpiration.
Stomata: These are the pores found on the leaf surface, which are surrounded by guard cells, promotes the process of gaseous exchange. The guard cell allows the exchange of gases by its opening and closing activity and controls the water flow during transpiration. Higher the number of leaf, higher the number of stomata and thus, higher will be the rate of transpiration.
Cuticle: It is the waxy layer present above the epidermis layer of leaf and acts as a barrier to the water movement out of leaves. Cuticle being waxy, will show repulsion of water and attracted by the water vapours. Thus, a cuticle is lipophilic or hydrophobic, and the movement of water through this is difficult.
The thickness of the cuticle decides the transpiration flow, i.e. more is the thickness of cuticle, slower will be the transpiration rate. Deciduous plants have much thicker cuticles.
Boundary layer: It is the thin layer of stagnant air surrounding the leaf surface, which varies with different plant species. At the time of transpiration, water vapours should diffuse from the boundary layer to the atmosphere.
With the increase in the thickness of the boundary layer, there will be a decrease in the transpiration rate. The plants, those having hairy structures on their surface or large leaf size, possesses a more substantial boundary layer.
It includes the environmental factors like relative humidity, light, temperature etc. which can affect the driving force of the water loss.
Relative humidity: It is the amount of water vapour contained in the atmosphere. As the relative humidity reduces, the moisture in the atmosphere also decreases, and more significantly transpiration will occur. Therefore, a drier atmosphere promotes the water movement out of the plant by increasing the driving force.
Temperature: The environmental temperature can affect the magnitude of the driving force of water moving out of a plant. With the increasing temperature, the water holding capacity of that air also increases. Therefore, dry or warmer air will increase the driving force for water loss whereas cooler air will decrease the driving force for transpiration.
Soil water: Soil also provides the source of water to carry out the transpiration process in a plant. If a Plant contains a sufficient amount of soil moisture then it will transpire at higher rates as the soil provides the water to move through the plant.
Light – Stomata tends to open in the presence of light allow the entry of available carbon dioxide for photosynthesis. In the absence of light, most plants have a closed stoma. Therefore, light is needed for the transpiration process to trigger the stomata for the gaseous exchange between plant and environment.
Wind: It can affect the rate of transpiration by eliminating or reducing the boundary layer surrounding the leaf surface. Wind increases the transpiration or water loss from the leaf surface to the atmosphere. As it reduces the boundary layer the movement of water to condense with the atmosphere becomes shorter.
We can understand the concept of transpiration by performing a simple experiment by taking a well-potted plant. To prevent evaporation of water cover the plant with a polythene sheath and tie it with a rope.
After some time, the water released by the plant will appear on the top inner surface of the polythene sheath. A film of water droplets condenses on the polythene sheath represents the transpiration activity of a plant, where the water releases in the form of vapours.
Transpiration is an important process, where a plant releases out the excess water by the various plant resistances like Stomata, lenticels etc. through the driving environmental forces. Therefore, transpiration is an expulsion of water from the plant (via leaves and stems) in the form of vapours which condenses in the atmosphere.