Transpiration and Guttation

Transpirational pull

Transpiration is the process where water evaporates from the surface of the plant, primarily through the stomata (tiny pores) in the leaves.

1. As water vapor is lost from the leaf, it creates a tension or negative pressure in the leaf's xylem. This negative pressure, known as transpirational pull, draws water up from the xylem in the stem and roots.
2. The continuous loss of water through transpiration results in a column of water being pulled upward, from the roots through the stem and up to the leaves. This is called the transpirational stream.

Capillarity

Capillarity refers to the ability of water to rise in narrow tubes, such as the xylem vessels, due to the forces of cohesion and adhesion.

1. Cohesion is the attraction between water molecules. This force causes water molecules to stick together, forming a continuous column of water.
2. Adhesion is the attraction between water molecules and the walls of the xylem vessels. This allows the water to "stick" to the inside of the xylem walls and helps it move upward.
3. The narrow lumen of the xylem vessels enhances capillarity, allowing the water to rise through the plant's vascular system.

Root pressure

Root pressure is generated by the active transport of minerals from the root cells into the xylem. As the concentration of minerals in the xylem increases, water moves into the xylem via osmosis, increasing the pressure inside the xylem.

1. This osmotic pressure can push water up through the plant, particularly in smaller plants or during periods of low transpiration.
2. Root pressure is sometimes visible when a plant is cut, and water or sap oozes from the cut stem, showing that the water is actively being pushed upward from the roots.

Stump oozing sap

Transpiration and its role in water movement

Transpiration is essential for the overall movement of water through the plant. It occurs primarily through the stomata in the leaves:

1. As water evaporates from the leaf's surface, it creates a concentration gradient where the concentration of water vapor in the leaf is higher than in the surrounding air.
2. This difference in concentration causes the water vapor to diffuse out of the stomata and into the atmosphere.
3. As water evaporates, it creates a pull (transpirational pull) that draws more water up from the roots through the plant's vascular system.

Movement of water through leaves

Guttation

In addition to transpiration, guttation occurs when plants lose water as liquid droplets through specialized glands located at the leaf margins, particularly at night or in humid conditions. Unlike transpiration, which involves the loss of water vapor, guttation involves the excretion of liquid water. Guttation occurs mainly when transpiration is low, and root pressure is high.

Types of transpiration

1. Stomatal transpiration: Water loss through stomata on leaves, accounting for about 90% of water loss in plants.
2. Cuticular transpiration: Water loss through the cuticle (waxy layer) covering the leaves, less significant than stomatal transpiration.
3. Lenticular transpiration: Water loss through lenticels (pores) on the bark of stems or roots, mainly in woody plants.

Factors affecting the rate of transpiration

Plant features

1. Size of leaves: Larger leaves lose more water due to more stomata.
2. Root system: Extensive roots absorb more water, increasing water loss through transpiration.
3. Leaf cuticle: Thicker cuticle reduces water loss; thinner cuticle increases it.
4. Number of stomata: More stomata result in faster transpiration.
5. Position of stomata: Stomata on the upper leaf surface lose more water than those on the lower surface.
6. Size of substomatal air spaces: Larger air spaces promote faster transpiration.
7. Sunken stomata: Sunken stomata reduce transpiration by limiting exposure to air.
8. Epidermal hairs: Traps water vapor, reducing transpiration.

Environmental factors

1. Temperature: Higher temperatures increase transpiration by opening stomata.
2. Relative humidity: Higher humidity reduces transpiration as the air is already saturated with moisture.
3. Wind and air movement: Wind increases transpiration by replacing moist air with dry air around the leaf.
4. Soil moisture: Less soil moisture reduces transpiration and leads to water stress.
5. Light: Increased light intensity opens stomata and boosts transpiration.
6. Atmospheric pressure: Low atmospheric pressure at high altitudes increases transpiration.

Significance of transpiration

1. Maintains transpirational pull: Helps in continuous movement of water and nutrients from roots to leaves.
2. Loss of excess water: Prevents water accumulation and maintains internal water balance.
3. Cooling the plant: Helps cool the plant by releasing water vapor during evaporation.
4. Absorption and distribution of water and minerals: Facilitates the absorption and transport of water and minerals necessary for plant growth.