Transpiration in Plants

Transpiration is the process by which plants lose water in the form of water vapour from their aerial parts, especially through leaves. Despite large amounts of water being absorbed from the soil, only a small fraction is utilized for metabolic processes, while the excess is lost through transpiration. There are three types of transpiration:

• Stomatal Transpiration: The majority of transpiration occurs through stomata, small pores found primarily on the underside of the leaf. In monocots like grasses, stomata are distributed on both sides of the leaf, and in aquatic plants with floating leaves, they are mostly on the upper surface.
• Cuticular Transpiration: The cuticle, a waxy layer covering the leaf surface, is mostly impervious to water. However, a small amount of water can still be lost through it, contributing up to 10% of total transpiration.
• Lenticular Transpiration: Water can also be lost through lenticels, small pores in the bark of woody stems, which is known as lenticular transpiration.

Mechanism of Stomatal Transpiration

The process of stomatal transpiration occurs in three steps:

1. Osmotic Diffusion of Water: Water from the xylem moves through mesophyll cells into the intercellular spaces above the stomata. This movement occurs by osmotic diffusion, where water moves from regions of high water potential (in the xylem) to regions of low water potential (in the mesophyll cells).

2. Opening and Closing of Stomata (Stomatal Movement): The guard cells surrounding each stoma regulate its opening and closing. When the guard cells absorb water (due to osmotic pressure), they swell and increase in turgor pressure. This causes the stomata to open. Conversely, when the osmotic pressure of the guard cells decreases (due to the loss of water), they become flaccid, and the stomatal pores close.

3. Diffusion of Water Vapour: Once the stomata open, water vapour from the intercellular spaces diffuses into the atmosphere, where the water concentration is lower. This diffusion is driven by the difference in water vapour concentration between the inside of the leaf and the surrounding air.

Mechanisms Controlling Stomatal Movement

There are several mechanisms involved in regulating the movement of stomata:

1. Hydrolysis of Starch into Sugars in Guard Cells: During the day, starch in guard cells is converted into glucose-1-phosphate, which increases the osmotic pressure of the guard cells. This causes water to enter the guard cells, making them turgid and opening the stomata. At night, the reverse process occurs, and the stomata close.

2. Synthesis of Sugars or Organic Acids in Guard Cells: During daylight, photosynthesis in guard cells leads to the formation of soluble sugars that increase the osmotic pressure, causing the stomata to open. Additionally, the formation of organic acids like malic acid can lead to the active pumping of K+ ions into guard cells, further increasing osmotic pressure.

3. ATP-Driven Proton (H+) – Potassium (K+) Exchange Pump: The active transport of K+ ions into the guard cells, facilitated by ATP, results in a decrease in the water potential of the guard cells. This draws water into the guard cells, causing them to swell and the stomata to open. In the dark, this process is reversed, and the stomata close.

Significance of Transpiration

Transpiration is a crucial physiological process for plants, involving the loss of water vapor primarily through stomata on the leaves. While it may seem wasteful as plants lose a large amount of water, it plays several vital roles in plant health and functioning.

Advantages of Transpiration

• Role in the Movement of Water: Transpiration aids in the upward movement of water, facilitating the ascent of sap in plants. As water evaporates from the leaf surfaces, it creates a negative pressure that helps draw water from the roots through the plant.

• Role in Absorption and Translocation of Mineral Salts:
  While absorption of water and mineral salts occurs independently, transpiration helps in the translocation of absorbed nutrients. Water moves through the xylem, facilitating the distribution of essential mineral salts throughout the plant.

• Role in Temperature Regulation:
  Transpiration plays a critical role in regulating plant temperature. The evaporation of water from plant surfaces helps cool the plant, preventing it from overheating due to excess light absorption. This cooling effect ensures that the plant maintains a suitable temperature for its metabolic processes.

Transpiration as a Necessary Evil

Despite its many benefits, transpiration also has its disadvantages:

1. Water Deficit in Dry Conditions: When transpiration rates are high and water availability in the soil is low, plants can experience water deficits. This may impair their metabolic processes, affecting growth and productivity.
2. Adaptations in Xerophytes: Plants adapted to dry conditions, such as xerophytes, often develop structural modifications to reduce transpiration, such as thicker cuticles or fewer stomata.
3. Deciduous Trees Shedding Leaves: Some deciduous trees shed their leaves during autumn to reduce water loss. This adaptation helps conserve water during dry or cold seasons.

Despite these challenges, plants cannot avoid transpiration due to their internal structure, particularly the stomata, which are essential for gas exchange (e.g., oxygen, carbon dioxide) and photosynthesis. These functions require the opening of stomata, which consequently leads to water loss through transpiration.

Factors Affecting Transpiration Rate

External Factors

1. Atmospheric Humidity: High humidity in the atmosphere reduces the rate of transpiration since the air is already saturated with moisture, impeding the diffusion of water vapor from the plant to the surroundings.
2. Temperature: An increase in temperature accelerates transpiration by lowering relative humidity and opening the stomata, both of which facilitate faster evaporation of water.
3. Wind: Gentle winds increase the rate of transpiration by removing the moist air near the plant, promoting the diffusion of water vapor. Strong winds can hinder transpiration by preventing the diffusion of water vapor from the leaf surface or causing stomatal closure.
4. Light: Light triggers stomatal opening, increasing transpiration. In contrast, darkness causes stomatal closure, reducing transpiration.
5. Soil Water Availability: If soil water is insufficient for absorption, transpiration rates decrease as there is not enough water to replace that lost.
6. CO₂ Concentration: Elevated CO₂ concentrations lead to partial stomatal closure, reducing transpiration rates.

Internal Factors

• Internal Water Conditions: The plant’s water status is crucial for transpiration. If water is not readily available in the soil, transpiration may decrease, and prolonged high transpiration can lead to water deficits within the plant.
• Structural Features: The number, size, position, and activity of stomata directly impact transpiration. Xerophytes often have structural modifications like sunken stomata, smaller leaves, or thick cuticles that reduce transpiration. In dark conditions, stomata close, preventing transpiration.

Antitranspirants

Certain substances, known as antitranspirants, can reduce transpiration by affecting the stomata or forming a physical barrier on leaf surfaces:

1. Plastic Films, Silicone Oils, Low Viscosity Waxes: These substances create a film over the leaves that allows gas exchange but reduces water loss.
2. Phenyl Mercuric Acetate: This fungicide, when applied in low concentrations, causes partial stomatal closure, reducing transpiration for a period without damaging the plant.
3. Abscisic Acid (ABA): A plant hormone, ABA induces stomatal closure and decreases transpiration.
4. CO₂: Increased CO₂ concentrations (up to a certain level) induce stomatal closure, slowing down transpiration.

Guttation

Guttation is a phenomenon where water drops are exuded from the uninjured margins of the leaves, typically at the tips where the main veins end. This occurs in certain plants, such as garden nasturtium, tomato, and colocasia. Guttation usually happens early in the morning, when the rate of water absorption and root pressure are high, but the transpiration rate is low.

Mechanism of Guttation

• Hydathodes (Water Stomata): Guttation is facilitated by special types of stomata called hydathodes, located at the margins of leaves. These hydathodes remain open and are involved in the release of water.
• Process: Beneath the hydathodes, there is a cavity called epithem that contains loose tissue. This tissue is closely associated with the ends of the vascular elements (xylem) in the veins of the leaves. The high root pressure forces water from the xylem into the epithem. When this cavity fills up with a watery solution, the water begins to ooze out through the water pores of the hydathodes, resulting in guttation.

Difference Between Transpiration and Guttation