8.1 Transport in Plants: Transpiration Pull

This section explains how water is moved through a plant via transpiration pull, a crucial process for delivering water and nutrients from the roots to the leaves.

The Water Cycle in Plants

Water uptake by the roots and its subsequent movement through the plant is a complex process driven primarily by transpiration pull. This involves several key steps:

1. Water is absorbed by the roots through osmosis.
2. The water moves upwards through the xylem vessels in the roots, stems, and leaves.
3. Transpiration, the evaporation of water from the leaves, creates a negative pressure (tension) in the xylem.
4. This tension pulls water up the xylem from the roots, continuing the water column.

Transpiration: The Driving Force

Transpiration is the process by which water evaporates from the surface of leaves, primarily through the stomata. Stomata are small pores on the leaf surface that allow for gas exchange (carbon dioxide intake and oxygen release). The rate of transpiration is influenced by several factors:

• Light intensity: Higher light intensity generally increases the rate of transpiration as stomata open wider.
• Temperature: Higher temperatures increase the rate of evaporation, leading to higher transpiration rates.
• Humidity: Lower humidity outside the leaf increases the rate of transpiration as the water potential gradient is greater.
• Wind: Wind removes water vapor from the leaf surface, increasing the rate of transpiration.
• Soil water availability: If the soil is dry, the rate of transpiration will be limited by the plant's ability to absorb water.

The Cohesion-Tension Theory

The movement of water up the xylem is best explained by the cohesion-tension theory. This theory combines the properties of water (cohesion and adhesion) with the process of transpiration.

Cohesion is the attraction between water molecules due to hydrogen bonding. This creates a continuous column of water within the xylem. Adhesion is the attraction between water molecules and the xylem walls, which helps the water column to climb upwards against gravity.

When water evaporates from the leaf (transpiration), it creates a negative pressure or tension in the xylem. This tension pulls the water column upwards from the roots. The cohesion between water molecules ensures that the water column remains continuous, even as it is pulled upwards.

Diagram of Transpiration Pull

Factors Affecting Transpiration Rate

Factor	Effect on Transpiration Rate
Light Intensity	Increases
Temperature	Increases
Humidity	Decreases
Wind	Increases
Soil Water Availability	Decreases (if water is limited)

Conclusion

Transpiration pull is the primary mechanism by which water is transported upwards through the xylem in plants. The process is driven by the evaporation of water from the leaves and relies on the cohesive properties of water to maintain a continuous water column. Understanding transpiration pull is essential for understanding how plants function and adapt to different environmental conditions.