The term “transpiration” refers to the process of removing surplus water from the plant’s body. The most prevalent reason is water evaporation from the surface of the leaves. During transpiration, water molecules in plant tissues are removed from the aerial portions of the plants. Only a small amount of the water absorbed by plants is used for growth and development. The remainder is expelled through transpiration. 

Types of transpiration:

Transpiration is of 3 types:

Stomatal transpiration:

It’s the process of water evaporating from a plant’s stomata. This is how the majority of the water from the plants gets out. When the stomata open, the water at the surface of the leaves evaporates as vapour. 

Lenticular transpiration:

The microscopic gaps in the bark of branches and twigs that enable light to enter through are known as lenticels. The process of water evaporating from a plant’s lenticels is known as lenticular transpiration. Lenticels aren’t present in all plants. Only a little amount of water is lost by lenticels. 

Cuticular transpiration:

It’s the process of water evaporating off a plant’s cuticle. The cuticle is a waxy layer that covers the plant’s leaves. Cuticular transpiration is responsible for about 5-10% of the water lost by the leaves. More water is transpired through the cuticles during dry conditions because the stomata are closed. 

Stomatal transpiration:

Stomatal transpiration is the process of sweating through the stomata. Stomatal transpiration accounts for between 50–and 97% of total transpiration. Gaseous exchange is the purpose of stomata. It also allows for the most transpiration. 

Structure of stomata:

1. Stomata are found primarily in the bottom epidermis of leaves. 
2. These small pores can be seen on the epidermal surface of leaves, immature stems, and some fruits.
3. Guard cells in the shape of kidneys surround the stomatal pores.
4. Because these guard cells are much smaller than the other epidermal cells, even a small change in turgor pressure can affect them.
5. Guard cells have thick walls in the direction of the pore and thin walls in the opposite direction.
6. Each guard cell is made up of cytoplasm, nucleus, and photosynthesis chloroplasts.
7. The guard cell walls’ cellulose microfibrils are oriented radially rather than laterally. 
8. The walls of the guard cells have a unique elasticity.
9. The adjacent cell walls of two guard cells surrounding a pore are free and unattached. They can expand laterally during stomatal opening because of these unique features. 
10. The primary function of stomata is to allow CO2 to enter the plant during photosynthesis. When CO2 enters the stomatal pore during photosynthesis, which occurs during the day, water is lost from the stomatal hole.
11. As a result, transpiration can be considered the price that plants pay for photosynthesis. 

Stomatal transpiration mechanism:

The following steps are involved in stomatal transpiration:

1. Osmotic diffusion transports water from the leaf’s xylem to the intercellular space above the stomata. 
2. The stomata open and close. 
3. Through stomata, water is transported from the intercellular space to the external environment. 

Sugar concentration theory about mechanism of stomatal transpiration:

This is an old notion that states that guard cells undertake photosynthesis and create sugar (glucose) throughout the day. As a result, the water potential in the guard cells decreases, allowing water from neighbouring epidermal cells to enter. This causes the cells to become turgid, causing them to protrude outward due to their thinner outer walls, causing the pore to open.

Guard cells lose their water content due to exosmosis throughout the night, when no photosynthesis occurs, resulting in no turgidity of guard cells, which become flaccid, culminating in stomata closure. 

Factors affecting stomatal transpiration:

1. Humidity in the Atmosphere– When the outside air is humid, the rate of transpiration slows down. 
2. Temperature– As the temperature rises, so does transpiration. 
3. Transpiration increases in bright light and reduces in dim light. 
4. The rate of transpiration is increased when the wind velocity is high. 
5. Atmospheric Pressure– As atmospheric pressure drops, the rate of transpiration increases. 
6. Soil Water– As the amount of water in the soil diminishes, the rate of transpiration drops. 

Conclusion:

Transpiration is the loss of water from stomata and lenticels in the form of vapour. When water replaces the gaps left by evaporation, the pulling force causes sap to rise, leaves to freeze on the surface, and minerals dissolved in water to enter the plant body.