IGCSE Biology 0610 - 8.1 Water Transport in Plants

Objective: Describe how water vapour loss is related to leaf structure.

Introduction

Plants require water for various processes such as photosynthesis, nutrient transport, and maintaining turgor pressure. Water is absorbed by the roots and transported upwards through the xylem to the leaves. However, plants also lose water through transpiration, which is the loss of water in the form of water vapour from the leaves. The structure of the leaf plays a crucial role in regulating the rate of water vapour loss.

Water Vapour Loss and Leaf Structure

The rate of water vapour loss from a leaf is influenced by several structural features. These features can either increase or decrease the rate of transpiration.

Leaf Surface Area

The surface area of the leaf is a major factor affecting water vapour loss. A larger surface area allows for more water to evaporate.

• Leaf Shape: Different leaf shapes have different surface areas. For example, broad leaves generally have a larger surface area than needle-like leaves.
• Leaf Size: Larger leaves have a greater surface area compared to smaller leaves.
• Leaf Arrangement: The arrangement of leaves on a stem can also affect the overall surface area exposed to the air.

Stomata

Stomata are tiny pores on the leaf surface that are responsible for the majority of water vapour loss during transpiration. Each stoma is flanked by two guard cells.

Guard Cells: Guard cells regulate the opening and closing of the stomata. When guard cells are turgid (full of water), they bow outwards, opening the stoma. When guard cells lose water, they become flaccid, and the stoma closes.

Factors Affecting Stoma Opening and Closing:

• Light: Light generally stimulates the opening of stomata.
• Carbon Dioxide Concentration: Low carbon dioxide concentrations inside the leaf stimulate stomatal opening.
• Water Availability: When water is scarce, the plant hormone abscisic acid (ABA) is produced, which causes the guard cells to lose water and close the stomata.
• Temperature: Higher temperatures generally increase the rate of transpiration, leading to stomatal opening.
• Humidity: Low humidity increases the rate of transpiration, leading to stomatal opening.

Cuticle

The cuticle is a waxy layer that covers the outer surface of the leaf. It helps to reduce water loss by creating a waterproof barrier.

Thickness of Cuticle: A thicker cuticle reduces water loss, while a thinner cuticle allows for more water vapour to escape.

Pectical and Hairs

Pectical (small hairs) on the leaf surface can reduce water loss by creating a boundary layer of still air around the leaf, which reduces the rate of water vapour diffusion.

Table Summarizing Leaf Structure and Water Vapour Loss

Leaf Structure Feature	Effect on Water Vapour Loss
Leaf Surface Area	Larger surface area = higher water vapour loss
Stomata	Open stomata = higher water vapour loss; Closed stomata = lower water vapour loss
Cuticle	Thicker cuticle = lower water vapour loss; Thinner cuticle = higher water vapour loss
Pectical (Hairs)	Creates a boundary layer, reducing water vapour loss

Conclusion

The structure of the leaf is intricately linked to the process of transpiration. Adaptations in leaf surface area, stomata, cuticle, and other features allow plants to regulate water loss and maintain a balance between water uptake and water loss. Understanding these relationships is essential for comprehending how plants survive in different environments.