Plant Physiology: Transpiration

1. Concept and Importance of Transpiration

Transpiration is the process by which water vapor is released from the aerial parts of plants, primarily through small pores called stomata on the leaves. It is essentially the evaporation of water from plant leaves, but it also occurs from stems and flowers. This process is a vital component of the water cycle and plays several critical roles in plant life.

1.1. Process of Transpiration

The process of transpiration can be broken down into several steps:

1. Water Absorption: Water is absorbed by the roots from the soil and transported upwards through the xylem vessels.
2. Movement to Leaves: The water travels through the stem and into the leaves, reaching the mesophyll cells.
3. Evaporation: Water evaporates from the surfaces of the mesophyll cells into the air spaces within the leaf.
4. Diffusion through Stomata: The water vapor then diffuses out of the leaf through the stomata into the atmosphere.

1.2. Significance of Transpiration

Transpiration is not merely a loss of water; it is a crucial physiological process with several benefits for the plant:

• Creation of Transpirational Pull: The evaporation of water from the leaves creates a negative pressure (tension) in the xylem, which pulls water upwards from the roots. This is the primary force driving the ascent of sap.
• Transport of Minerals: The continuous stream of water moving through the plant (transpiration stream) carries dissolved mineral nutrients from the soil to all parts of the plant, especially the leaves where photosynthesis occurs.
• Cooling Effect: As water evaporates from the leaf surface, it absorbs latent heat from the leaf, thereby cooling the plant. This is particularly important in hot environments, preventing the plant from overheating.
• Maintenance of Turgor Pressure: Transpiration helps in maintaining the turgidity of plant cells, which is essential for keeping the plant erect, for cell enlargement, and for the opening and closing of stomata.

2. Ganong’s Potometer and its Limitations

A Ganong’s potometer is a simple apparatus used to measure the rate of water uptake by a leafy shoot, which is assumed to be nearly equal to the rate of transpiration. It works on the principle that as water is transpired from the leaves, it is replaced by water drawn from the potometer, causing an air bubble in a capillary tube to move.

2.1. How it Works

1. A leafy shoot is cut under water to prevent air bubbles from entering the xylem.
2. The cut end is inserted into a rubber stopper, which is then fitted into the potometer, ensuring an airtight seal.
3. The apparatus is filled with water, and an air bubble is introduced into the capillary tube.
4. As the plant transpires, water is drawn from the capillary tube, and the movement of the air bubble is measured over time.

2.2. Limitations of Ganong’s Potometer

Despite its usefulness, the Ganong’s potometer has several limitations:

• Measures Water Uptake, Not Transpiration Directly: It measures the rate of water uptake, which may not always be exactly equal to the rate of transpiration, as some water might be used for photosynthesis or retained by the plant cells.
• Airtight Seal is Crucial: Any leakage in the apparatus will lead to inaccurate readings.
• Cutting the Shoot: Cutting the shoot can cause damage to the xylem vessels, potentially affecting water conduction.
• Environmental Factors: The rate of water uptake is highly sensitive to environmental factors, making it challenging to maintain constant conditions during the experiment.
• Air Bubble Issues: The air bubble can get stuck or dissolve in the water, affecting the accuracy of the measurement.

3. Factors Affecting the Rate of Transpiration

The rate of transpiration is influenced by both environmental and plant factors:

3.1. Environmental Factors

• Light Intensity: Increased light intensity generally increases the rate of transpiration because it promotes stomatal opening (for photosynthesis) and raises the leaf temperature, leading to faster evaporation.
• Temperature: Higher temperatures increase the kinetic energy of water molecules, leading to a faster rate of evaporation and diffusion of water vapor from the leaf.
• Humidity: High atmospheric humidity reduces the water potential gradient between the leaf and the atmosphere, thereby decreasing the rate of transpiration. Conversely, low humidity increases the rate.
• Wind Speed: Increased wind speed removes the humid air layer surrounding the leaf, maintaining a steep water potential gradient and thus increasing the rate of transpiration.
• Atmospheric Pressure: Lower atmospheric pressure can slightly increase transpiration as it facilitates the diffusion of water vapor.
• Availability of Soil Water: If the soil water is scarce, the plant may reduce its transpiration rate by closing stomata to conserve water.

3.2. Plant Factors

• Stomatal Density and Distribution: Plants with a higher density of stomata or stomata on both surfaces of the leaf tend to transpire more.
• Leaf Area: Larger leaf areas generally lead to higher transpiration rates.
• Cuticle Thickness: A thicker waxy cuticle on the leaf surface reduces cuticular transpiration.
• Root-to-Shoot Ratio: A larger root system relative to the shoot can support a higher transpiration rate.
• Leaf Orientation: Leaves oriented to avoid direct sunlight may transpire less.

4. Experiments on Transpiration

Several experiments can demonstrate the process and effects of transpiration:

4.1. Loss in Weight of a Potted Plant or a Leafy Shoot

Aim: To demonstrate that plants lose water through transpiration.

Procedure:

1. Take a potted plant and cover the pot and soil with a plastic sheet to prevent evaporation from the soil surface.
2. Weigh the potted plant and record its initial weight.
3. Place the plant in a well-lit area for several hours.
4. Weigh the plant again after a few hours.

Observation: The weight of the potted plant will decrease, indicating a loss of water due to transpiration.

Variation (Leafy Shoot in a Test Tube):

1. Take a leafy shoot and place its cut end in a test tube filled with water. Add a layer of oil on top of the water to prevent evaporation from the water surface.
2. Mark the initial water level in the test tube.
3. Place the setup in a well-lit area.

Observation: The water level in the test tube will decrease over time, demonstrating water loss from the leafy shoot through transpiration.

4.2. Use of Cobalt Chloride Paper

Aim: To demonstrate unequal rates of transpiration in a dorsiventral leaf (e.g., a dicot leaf).

Procedure:

1. Take two dry cobalt chloride papers (blue in color).
2. Place one paper on the upper surface and the other on the lower surface of a dorsiventral leaf.
3. Secure the papers with clips or glass slides to ensure good contact with the leaf surfaces.
4. Observe the color change over time.

Observation: The cobalt chloride paper on the lower surface of the leaf will turn pink much faster than the one on the upper surface. This is because the lower surface of a dorsiventral leaf typically has a higher density of stomata, leading to a higher rate of transpiration from that surface.

5. Mechanism of Stomatal Transpiration: Potassium Ion Exchange Theory

The opening and closing of stomata, which regulate the rate of transpiration, are primarily controlled by the turgor pressure of the guard cells. The Potassium Ion Exchange Theory (also known as the K+ pump theory) explains the mechanism of stomatal movement.

5.1. Stomatal Opening

1. Light Stimulus: In the presence of light, guard cells become photosynthetically active, producing ATP.
2. K+ Influx: ATP powers proton pumps (H+-ATPases) in the guard cell membrane, which pump H+ ions out of the guard cells. This creates an electrochemical gradient.
3. Membrane Potential: The outward movement of H+ ions hyperpolarizes the guard cell membrane, making the inside more negative.
4. K+ Channel Activation: This negative membrane potential activates voltage-gated K+ channels, leading to a rapid influx of K+ ions into the guard cells from the surrounding epidermal cells.
5. Water Potential Decrease: The accumulation of K+ ions (and sometimes Cl- ions) inside the guard cells lowers their water potential.
6. Water Influx: Water then moves by osmosis from the surrounding epidermal cells into the guard cells, increasing their turgor pressure.
7. Stomatal Opening: As the guard cells become turgid, their inner walls (adjacent to the stomatal pore) are thicker and less elastic than their outer walls. This differential thickening causes the guard cells to bow outwards, opening the stomatal pore.

5.2. Stomatal Closing

1. Absence of Light/Stress: In the dark or under conditions of water stress (e.g., drought), abscisic acid (ABA) is produced.
2. K+ Efflux: ABA triggers the efflux of K+ ions (and Cl- ions) from the guard cells.
3. Water Potential Increase: The loss of solutes increases the water potential inside the guard cells.
4. Water Efflux: Water moves by osmosis out of the guard cells into the surrounding epidermal cells, causing the guard cells to lose turgor.
5. Stomatal Closing: As the guard cells become flaccid, they straighten and move closer together, closing the stomatal pore.

6. Adaptations in Plants to Reduce Transpiration

Plants have evolved various adaptations to minimize water loss through transpiration, especially in arid or dry environments:

• Thick Cuticle: A thick, waxy layer on the leaf surface reduces water evaporation from the epidermis.
• Sunken Stomata: Stomata located in pits or depressions on the leaf surface create a humid microenvironment, reducing the water potential gradient and thus transpiration (e.g., Nerium).
• Hairy Leaves (Trichomes): Hairs on the leaf surface trap a layer of moist air, reducing air movement and transpiration.
• Rolled Leaves: Some plants roll their leaves to reduce the exposed surface area and create a humid environment around the stomata (e.g., Marram grass).
• Reduced Leaf Surface Area: Small leaves, needle-like leaves (e.g., conifers), or spines (e.g., cacti) reduce the surface area available for transpiration.
• Succulence: Fleshy stems or leaves store water, allowing plants to survive periods of drought.
• Crassulacean Acid Metabolism (CAM) Photosynthesis: CAM plants open their stomata at night to take in CO2 and close them during the day, significantly reducing water loss.
• Deep Root Systems: Allow plants to access water from deeper soil layers.
• Shedding Leaves: Deciduous plants shed their leaves during dry seasons to avoid water loss.

7. Guttation and Bleeding

These are two phenomena related to water movement in plants but are distinct from transpiration.

7.1. Guttation

Guttation is the exudation of drops of xylem sap from the tips or edges of leaves of some vascular plants, especially in conditions of high humidity and low transpiration (e.g., at night). It occurs when root pressure is high, and transpiration is low, leading to an accumulation of water in the xylem that is forced out through specialized pores called hydathodes.

• Key Difference from Dew: Guttation droplets are secreted by the plant, while dew forms from condensation of atmospheric water vapor.
• Composition: Guttation fluid contains dissolved minerals and sugars, unlike pure water from transpiration.

7.2. Bleeding

Bleeding (or weeping) is the exudation of xylem sap from a cut or injured part of a plant. This phenomenon is also driven by root pressure. When a stem is cut, the positive pressure from the roots forces the sap out of the severed xylem vessels. This is commonly observed in grapevines or maple trees when tapped for syrup.

• Significance: While not a normal physiological process, bleeding demonstrates the presence of root pressure and the continuous movement of sap within the plant.