Gas Exchange in Woody Plants

Plants, like all living organisms, require oxygen for respiration and need to expel carbon dioxide. While most people are familiar with stomata—tiny pores on leaves that facilitate gas exchange—another crucial structure plays a significant role, particularly in woody plants: lenticels. These small, often visible structures on bark and stems ensure a continuous supply of oxygen to internal tissues. Without lenticels, trees and shrubs would struggle to survive.

What Are Lenticels?

Lenticels are spongy, porous tissues found on the surface of woody stems, branches, and sometimes even fruits. These structures act as breathing pores, allowing for gas exchange between the plant’s internal tissues and the external environment.

Unlike stomata, which open and close based on environmental conditions, lenticels always remain open. This ensures a constant flow of oxygen, particularly crucial in thick, woody plants where the outer bark might otherwise block gas diffusion.

Where Are Lenticels Found?

Lenticels are most commonly observed on:

• Woody stems and branches (e.g., trees like cherry, birch, and apple)
• Roots undergoing secondary growth
• Fruits such as apples, pears, and mangoes, where they appear as small dots on the skin

Structure and Appearance of Lenticels

Lenticels are small, oval, round, or elongated structures on the surface of plant stems and fruits. Their structure can vary depending on the plant species, but they generally consist of loosely packed cells with intercellular spaces, allowing gas movement.

Key Structural Features

1. Periderm Formation: Lenticels develop in the periderm, replacing the epidermis as plants grow.
2. Loosely arranged parenchyma cells: These allow gases to diffuse in and out freely.
3. Shape and Size Variation: Some plants have raised, conspicuous lenticels, while others have flattened, barely visible ones.

For instance, cherry trees have prominent, elongated lenticels, while apple fruits display small, dot-like lenticels that change color as they mature.

How Do Lenticels Function?

Lenticels play a vital role in facilitating respiration and preventing oxygen deficiency in woody plants. Their primary functions include:

1. Gas Exchange

Lenticels allow oxygen to enter and carbon dioxide to exit, ensuring proper respiration in internal tissues. This is essential for energy production in cells, keeping the plant alive.

2. Facilitating Water Vapor Release

Plants lose a small amount of water vapor through lenticels, a process known as lenticellular transpiration. Though it contributes less to overall water loss compared to stomata, it still plays a role in regulating moisture.

3. Adapting to Environmental Conditions

• In waterlogged soils, where roots struggle to get oxygen, lenticels help in aerating submerged parts.
• In dry conditions, they assist in regulating internal moisture levels.

Unlike stomata, lenticels do not close and remain open throughout the plant’s life, making them a permanent gateway for respiration.

How Lenticels Develop

The development of lenticels starts during primary growth, under the influence of stomatal complexes. As plants undergo secondary growth, the epidermis is replaced by the periderm, and lenticels form to maintain gas exchange.

Lenticel Formation Process

1. Initiation: Lenticels form beneath the stomata, ensuring continued gas exchange.
2. Cell Differentiation: Special parenchyma cells develop loose structures, enabling permeability.
3. Expansion: As the stem thickens, lenticels grow in size and may become more pronounced.

Some trees develop new lenticels annually, while others retain the same lenticels throughout their lifespan.

Lenticels vs. Stomata: Key Differences

While stomata dominate in herbaceous plants, lenticels are crucial for woody plants, ensuring continuous respiration even when stomata are absent or inactive.

Examples of Lenticels in Nature

1. Cherry Bark (Prunus spp.): displays prominent, horizontal lenticels.
2. Birch trees (Betula spp.): are notable for their distinctive, elongated lenticels.
3. Apples and Pears (Malus & Pyrus spp.): Lenticels on fruit skins help in gas exchange and serve as maturity indicators.
4. Mangoes (Mangifera indica): Small lenticels on mango skin regulate internal gas balance.

The Role of Lenticels in Agriculture and Horticulture

Lenticels are more than just tiny pores; they influence:

• Fruit Ripening: In apples and pears, darkened lenticels indicate ripeness.
• Pathogen Entry Points: Bacteria and fungi can invade fruits through lenticels, leading to spoilage.
• Waterlogging tolerance: Some plants, like mangroves, have specialized lenticels that help them survive in flooded environments.

Farmers and horticulturists often monitor lettuce health to assess plant well-being.

Common Issues with Lenticels

1. Lenticel Breakdown in Fruits

• Caused by high humidity and poor storage conditions.
• Leads to spotting and decay in apples and pears.

2. Fungal and Bacterial Infections

• Lenticels serve as an entry point for pathogens.
• Proper sanitation and controlled humidity can prevent infections.

3. Environmental Stress

• Pollution and climate stress can cause lenticel hypertrophy (abnormal swelling).
• Some plants develop cracked lenticels due to extreme weather fluctuations.

Understanding these issues helps in improving post-harvest handling and plant care.

Conclusion

Lenticels are essential gateways for gas exchange in woody plants, ensuring continuous respiration and adaptation to environmental conditions. Unlike stomata, they remain open at all times, playing a crucial role in plant survival.

From tree bark to fruit skins, lenticels influence plant health, agriculture, and even fruit ripening. Their ability to regulate oxygen intake and moisture loss makes them indispensable in plant physiology.