Transpiration

Like all other life processes (eating, digestion, growth, etc.) excretion is also an important life process. Plants are no different, they also have to get rid of the excess water from their body. Roots in plants absorb water from the ground which is transported to all the plant parts for various activities like preparing food, growth, and metabolism. But all the water that roots absorb is not used up by the plants. The excess water absorbed by the roots is eliminated from the plant body by the process of transpiration.

Thus, transpiration can be defined as the process by which the plant body releases water in the form of vapors through its aerial parts. Transpiration also refers to the evaporation of water from the plant part. It is an important process in plants that facilitates other processes and keeps a balance of water and nutrients. It has various advantages for the plant body, which we will follow in detail.

The Process of Transpiration

Absorption of water and transpiration is a cyclic process. Roots absorb water from the soil in order to transport it to the plant parts for photosynthesis. The upward movement of water against the gravitational pull is caused by the transpiration pull. Two types of forces, cohesive and adhesive force work together to bring about this upward pull. During the gaseous exchange or from barks, stems, or other parts through pores, the excess water absorbed by roots and which remain unused is lost to the atmosphere in the form of water vapors. 

Investigating Transpiration

Certain factors like 'mass decrement' (due to water loss) and by 'knowing the volume of water absorbed by the plant', we can calculate and investigate about it's rate of transpiration :

• Decrease in Mass - The decrease in mass of the leaf can be used to determine it's transpiration rate and the type of transpiration. 

• Absorption of Water - The volume of water absorbed by the leaf can also determine the transpiration ratio. This volume can be measured by a 'potometer'. In this study, the leafy shoot is taken in use rather than just a single leaf. The shoot is kept in the container filled with water and a capillary tube is also connected. With the stem of the leafy shoot, a rubber tubing is connected. Now, the scale is used to measure the distance traveled in the capillary tube by bubbles. Here, the faster the bubble moves, the greater is the rate of transpiration. 

Rate of Transpiration

It is the rate ratio of the mass of water transpired to the matter product (dry matter). It is also known as the transpiration ratio. For most of the crops, the transpiration ratio is between 200 and 1000. This defines that 'for every kg of dry matter, crop plants transpire 200-1000 kg of water'. The following factors affect the rate of transpiration :

• Temperature - If temperature is increased, evaporation and diffusion are faster. 

• Humidity - If the leaf is surrounded by moist air (due to increase in humidity), the rate of diffusion of water vapor decreases.

• Wind Speed - The moving wind removes humidity. This results in the increase in the rate of diffusion of water vapor. 

• Light Intensity - If light intensity increases, more carbon dioxide enters into the leaf due to wide opening of stomatal pores. This helps in making photosynthesis easy. 

Types of Transpiration

Transpiration in plants occurs only by three special structures. Depending upon the means by which transpiration is taking place it is divided into three types.

• Stomatal Transpiration - Stomatal transpiration is one form of foliar transpiration. When transpiration takes place through leaves, it is foliar transpiration. Plants leaves have microscopic openings on the epidermis which are meant for gaseous exchange and transpiration. These openings are stomata. In Stomatal transpiration, water in the surface of the leaves is evaporated into the atmosphere when the stomatal opening opens. The stomatal opening opens when light falls on it. Stomatal transpiration accounts for 85%- 90% of the total water loss in plants.

• Cuticular Transpiration - As the name suggests the type of transpiration that takes place through cuticles is cuticular transpiration. This is also a kind of foliar transpiration. Cuticles are waxy covering on the epidermis of the leaves. The cuticular layer is impermeable to water and permeable to water vapor. Hence, water in the form of vapor is lost through this layer. Cuticular transpiration occurs in plants with a thick cuticle layer and a lesser number of stomata. It also occurs in plants when the stomatal opening does not open under certain conditions. Cuticular transpiration accounts for around 5%- 10% of the total water loss.

• Lenticular Transpiration - Lenticular transpiration refers to water loss in the form of vapors through the lenticels of the plant. Lenticels are a porous tissue found on the barks of woody stems and roots of dicots. The pores in the tissue act as a pathway for gaseous exchange and transpiration. Water loss by lenticular transpiration is very less.

Mechanism of Stomatal Transpiration

The surface of mesophyll cells in leaves release a thin layer of water. This thin water layer then evaporates. The water vapor formed after evaporation saturates in the intracellular spaces, then diffuses into the connecting intracellular spaces, and finally reaches the sub stomal space. When the stomata open for gaseous exchange, it evaporates from these openings. Also, the cell sap of cells in leaves exert pressure on the outward of the cell wall. This pressure forces some water out of the cell wall into the intracellular space. This water also evaporates and the water vapors diffuse into the sub stomal space, which finally diffuses out by stomata.

Too much transpiration may lead to dehydration and damage to the plant. Hence stomata are provided with guard cells that regulate the entire process. When water enters the guard cells, they swell and open and when the water leaves the stomata they relax and close.

How does the Structure of Plants affect Transpiration?

The amount of water lost by plants over a period of time refers to the rate of transpiration. Transpiration rate is affected by a series of environmental and other morphological factors.

• Leaves and their orientation- Number of leaves, the size and orientation of leaves, and the structural form of the leaves affect the rate of water loss.

• The number of stomata present affects the transpiration rate as most of the transpiration occurs through the stomata.

• Presence of cuticles and lenticels

Effect of Pesticides on Transpiration Rate

Pesticides or Herbicides have a huge effect on the transpiration in plants. Sometimes, pesticides interfere with important plant processes like 'photosynthesis' and 'transpiration'. When the photosynthetic activities slow down, it also affects the transpiration rate of the plant and causes an imbalance or damage to the plant. 

Environmental Factors Affecting Transpiration

Rate of transpiration greatly depends on environmental factors such as:

• Availability of Light

Stomata only open in the presence of light. Therefore, during the daytime, the transpiration is maximum and at night it is less.

• Relative Humidity

Relative humidity means the amount of water vapor present in the atmosphere expressed as a percentage of the number of water vapors needed for saturation at that same temperature. The relative humidity of the atmosphere is inversely proportional to the rate of transpiration.

• Temperature

High temperature decreases relative humidity, which forces the stomata to open even in the dark, thus increasing the rate of transpiration.

• Wind Speed

Still, air lowers the rate of transpiration. In such atmospheric conditions, water vapors accumulate around the transpiring organs and reduce the rate of diffusion.

• Water Availability

During scarcity of water, roots absorb less water from the soil. This results in the closing of the stomata and even wilting. Thus, decreasing the transpiration rate.

Significance of Transpiration

Although transpiration is an unavoidable phenomenon that takes place during the gaseous exchange in plants, it’s necessary and has its own set of benefits for the plant.

• Transpiration pull creates a pressure gradient that helps draw water and minerals up to the plant body from the roots.

• Transpiration helps keep the water balance in the plant body through the loss of water. It gets rid of the excess water absorbed by roots and returns it back to the atmosphere.

• It maintains heat balancells ce in the plant body and acts as a natural cooling system.

• It helps maintain osmosis and keeps them rigid.

• It promotes the proper growth of plants and cell division.