Article:

In this article, we will discuss transpiration in plants and the role of stomata and guard cells in transpiration. Factors affecting transpiration and the types of transpiration are also discussed.

Transpiration:

Transpiration is the process of losing plants' moisture from roots to small pores on the underside of leaves, where it converts into vapor and is released into the atmosphere. Transpiration moves water and nutrients to the parts of the plant where they are required for growth and photosynthesis. This process also maintains cell turgidity. Water in root hairs enters through a phenomenon called osmosis.

Osmosis:

Osmosis is the process by which molecules are diffused and tend to pass through a semipermeable membrane. Water will be passed through the cells of plants or between the cells within the cell wall. Xylem are tube-like networks of cells and water is distributed in plants through it. This process is important in biological systems as it allows water and nutrients to pass into and out of cells. Plant tissue serves for the transportation of minerals in plants through diffusion.

Diffusion:

Diffusion is the process by which molecules move from a region of high concentration to an area of low concentration. Diffusion is a fundamental process in various biological and chemical systems, including exchanging gases in the environment and moving nutrients into the plant cells.

Transpiration strains:

The upward movement of water molecules from roots to leaves is known as transpiration strains.

Cohesion:

Cohesion is the property between the like molecules e.g. attraction between the water molecules. This property is responsible for the high surface tension of water and allows water to form droplets and maintain a continuous column in narrow tubes. In plants, cohesion helps water move from the roots to the leaves up through the xylem vessels. As water is loosed from outer leaves, tissue water moves in the water-deficient cells from adjacent cells.

Adhesion:

Adhesion is the tendency of unlike molecules to cling to one another. In water, adhesion refers to the attraction between water molecules and the plant tissue. This property allows water to "stick" to surfaces, such as the walls of the xylem, due to hydrogen bonding and other intermolecular forces aiding in water movement from the roots to the leaves. The cohesion tension theory suggests that water rises in plants due to its adhesion to capillary walls and cohesion end to itself.

Stomata:

Stomata are small openings or pores found on the surfaces of leaves located on the lower epidermis of plants. Stomata allow water molecules to evaporate. They are surrounded by specialized cells known as guard cells. Guard cell control opening and closing of the stomata. Stomata keep water molecules from evaporating and this evaporation occurs when the moisture of the surrounding air is less moist than the mesophyll. The spongy mesophyll allows for the exchange of gasses in photosynthesis and transpiration.

Turgor Pressure:

Turgor pressure is the measure of cell firmness. When the central vacuole is full of water, the cell is said to be turgid, and this turgor pressure provides support to the plant. The water loss by the turgor pressure in plants is associated with plasmolysis. Plasmolysis is the process in which a plant protoplasm is shrinkage from the cell wall. This causes the cell membrane to separate from the cell wall. Plasmolysis occurs when the external solution has a higher solute concentration than the cytoplasm of the cell, leading to the loss of turgor pressure and causing the cell to wilt.

Factor effecting Transpiration:

Factors that affect transpiration include environmental conditions such as temperature, humidity, wind, and light intensity. Additionally, the size and number of stomata, as well as the presence of a waxy cuticle on the leaves, can also impact the rate of transpiration. Other factors include the water availability in the soil, there are three main types of transpiration:

Transpiration = Driving force/ resistance

Types of Transpiration:

1. Stomatal transpiration: This is the most common type of transpiration and it occurs through the stomata. This transpiration is 99 percent in plants.

2. Cuticular transpiration: This type of transpiration occurs through the cuticle. This transpiration is 0.3 to 0.10 percent. On the outer surface of leaves and stems there is a waxy layer which is called the cuticle.

3. Lenticular transpiration: Transpiration occurs through lenticels is called lenticular transpiration. This transpiration is 0.1 percent in plants. Lenticels are present in the bark of woody stems and roots and these are small openings in plants.

Environmental factor affecting stomatal aperture:

     1. Carbon dioxide:

High carbon dioxide concentration stomata will open. Low carbon dioxide concentration stomata will close.

2. Temperature:

Temperature 20 to 30 ^C affecting stomata aperture.

3. Light:

Red and blue light is more effective for stomata.

4.       4. Water content:

When water is in more amount stomata open and when water is in less amount stomata will close.

5. Hormones:

When cytokinin is released opens stomata while ABA inhibits stomatal opening.

Guard Cells:

Guard cells are specialized cells and are present within the stomata. They have no plasmodesmata thus ions move through diffusion. These kidney-shaped cells play a crucial role in regulating gas exchange and transpiration. When the guard cells take in water, they become turgid and swell, causing the stomatal pores to open. This allows for the exchange of gases, such as carbon dioxide and oxygen, essential for photosynthesis and respiration. Additionally, the open stomata enable the release of water vapor during transpiration. When the guard cells lose water, the stomatal pore closes, and the guard cells become flaccid, lowering water loss from the plant and preventing more transpiration, particularly in dry or hot conditions. The opening and closing of the stomata by the guard cells are controlled by various factors, including light intensity, humidity, and the plant's water status.

• Theory related to the regulation of transpiration:
• K+ ions move inward in the guard cell and solute potential decreases in surroundings. Water enters the guard cells and the guard cell becomes turgid then the stomata is open.
•  K+ ions move outward then solute potential increases in surroundings. Water moves outward and the stomata are close.