Transpiration

Introduction

Transpiration is a process in which plants release the water absorbed by their roots. The water is released in vapour form via stomata, i.e., the minute openings present in the leaves. These openings remain open during the day and shut during the night. 

Factors affecting the transpiration rate

Some factors are responsible for enhancing or decreasing the rate of transpiration. These are:

• An increase in temperature enhances the transpiration rate
• If the surface area of the leaf is large, the rate of transpiration increases
• An increase in the humidity level decreases the rate of transpiration
• An increase in wind speed enhances the rate of transpiration.

Purpose of Transpiration

As per the transpiration definition, it is the loss of water via evaporation from a plant’s or tree’s aerial portion. Transpiration meets the following purposes:

• Transpiration builds the ‘transpiration pull,’ which stimulates absorption and regulates the transport of water and minerals in the plants
• It supplies mineral-rich water to the leaves, which is essential for the process of photosynthesis
• It encourages the transport of essential minerals and nutrients from the roots to the various parts of the plant
• During extreme temperatures or heat, transpiration cools the leaves and assists in balancing their size and shape

As per the transpiration definition, the extra amount of water taken by roots is released during transpiration. The plant utilises only 2 to 5% of the water for conducting its metabolic activities and other processes. 

Types of transpiration 

The three types of transpiration are:

1. Cuticular Transpiration: Around 10% of the transpiration happens through the outer surface of the epidermis. This process is known as cuticular transpiration as the outer wall consists of cuticles that influence water diffusion. 

The rate of diffusion is inversely proportional to the amount of water and cuticle thickness. An increase in cuticular transpiration reduces transpiration that happens through stomata. 

2. Lenticular Transpiration: Lenticles are the minute pores present in the epidermis of the stem after they get matured. Lenticles are also found in the roots and fruits of some plants. Lenticular Transpiration is the water loss that happens in the form of vapours through lenticels. It contributes only up to 0.1% to the total water lost due to evaporation. 

3. Stomatal Transpiration: Stomatal transpiration is the process in which plants give out water through stomata. It accounts for 90% of the transpiration and the moisture that the leaves lose, and it is also known as foliar transpiration. 

Strange facts about Stomata 

• Fact 1: Submerged plant species, fungi, and algae do not have stomata
• Fact 2: In monocotyledonous plants, stomata are evenly distributed and are found on both the lower and upper surfaces of the leaves
• Fact 3: In dicotyledonous plants, stomata are not distributed evenly as they are mostly found on the lower surface as compared to the upper surface

Transpiration Water Cycle 

During transpiration, the water absorbed by the roots is released back into the atmosphere through stomata. The water vapours are released on a large scale as around 95% to 98% of the water absorbed by the plants is given back through transpiration. 

These water vapours contribute to cloud formation, and when the temperature increases, the water vapours condense and precipitate in the form of rainfall. Some of this rain enters the water channels located at different levels beneath the earth’s surface. Thus, the water absorbed by the roots is returned back to the soil, and this completes the transpiration water cycle. 

According to some studies, water vapour released due to transpiration contributes up to 10% of the moisture in the atmosphere. The water vapours released by the surface waters of oceans, rivers, seas, and other water bodies account for the remaining 90% of the atmospheric moisture. 

Transpiration and Groundwater 

The connection between transpiration and groundwater has been established through multiple studies. It is observed that generally, plants that grow much above the water table become dependent on precipitation for getting their regular supply of water. 

However, in areas where there is scanty rainfall, much of the water gets evaporated from the soil’s surface only. Therefore, very little water reaches the roots, which, too, is transpired back to the atmosphere. Due to this continuous cycle of transpiration, the plants or trees in arid regions become dry and suffer from the withering of branches and leaves. 

On the contrary, plants located near the water table have easy access to groundwater. It is because their roots reach the saturated area beneath the water table and absorb water rapidly through roots. However, as only 2 to 5% of this water is actually required for the growth and development of plants, the remaining water is transpired. The continuous cycle of absorption through roots and transpiration via leaves results in a quick loss in the water table. Therefore, we can conclude that the plants near the water table, like those located near lakes, rivers, oceans, etc., transpire water directly from groundwater. 

Conclusion

In this topic we learned about transpiration definition, various factors affecting transpiration, process of transpiration, it’s types which are: Cuticular transpiration, Lenticular transpiration and Stomatal transpiration. We also learned about the transpiration water cycle in which the water absorbed by the roots is released back into the atmosphere through stomata. At last we learned about the transpiration and groundwater .