Question

What is apical dominance? How do auxin and cytokinin interact in determining the growth of lateral buds??

Short answer

Apical dominance is a phenomenon in plant growth where the apical bud inhibits the growth of lateral buds, allowing the plant to focus on upward growth. The plant hormones auxin and cytokinin play crucial roles in this process. Auxin is produced in the shoot apical meristem and inhibits lateral bud growth, while cytokinin is produced in the root tips and promotes lateral bud growth. The interaction between these hormones and their relative levels determine the growth patterns of lateral buds. In the release of apical dominance, auxin levels decrease when the apical bud is removed, allowing cytokinin levels to stimulate lateral bud growth and branch development.

Step-by-step solution

Understanding Apical Dominance

Apical dominance is a phenomenon in plant growth where the main shoot, or apical bud, inhibits the growth of lateral buds. This mechanism helps the plant to primarily focus on upward growth and conserve energy. The key hormones involved in this process are auxin and cytokinin.

Role of Auxin in Apical Dominance

Auxin is a plant hormone that is primarily produced in the shoot apical meristem, which is the tip of the growing shoot. This hormone has several functions in the plant, one of which is promoting cell elongation and thus the upward growth of the plant. Auxin moves downwards from the apical meristem with a higher concentration at the apex and reducing as we move downwards. In the context of apical dominance, auxin inhibits the growth of lateral buds, thus suppressing their development and allowing the apical bud to grow more. So, when the auxin level is high, lateral bud growth is suppressed.

Role of Cytokinin in Apical Dominance

Cytokinin is another plant hormone that plays a crucial role in promoting cell division and growth. Unlike auxin, cytokinin is predominantly produced in the root tips and moves upwards in the plant. Cytokinin has an opposite effect on lateral buds compared to auxin, i.e., it promotes the growth of lateral buds. Therefore, when cytokinin levels are high, the growth of lateral buds is stimulated.

Interaction between Auxin and Cytokinin in Determining Lateral Bud Growth

Both auxin and cytokinin have opposing effects on the growth of lateral buds, making their interaction in the plant important for controlling lateral bud growth. A high level of auxin inhibits lateral bud growth, whereas a high level of cytokinin stimulates it. In most plants, the apical bud produces large amounts of auxin that moves downwards and leads to apical dominance and the inhibition of lateral bud growth. However, if the apical bud is removed (e.g., by pruning), the auxin level in the plant decreases, and the balance between auxin and cytokinin shifts. In this case, the cytokinin levels become more significant, promoting lateral bud growth and leading to the development of branches. This phenomenon is known as the release of apical dominance. In conclusion, apical dominance in plants involves the regulation of lateral bud growth by the hormones auxin and cytokinin. Auxin, produced in the apical bud, suppresses lateral bud growth, whereas cytokinin, produced in the roots, promotes it. The interaction between these two hormones and their relative levels in the plant determine the overall growth patterns of lateral buds.

Key concepts

Auxin

Auxin is a fundamental plant hormone that plays a critical role in regulating various growth processes in plants. It is primarily synthesized in the shoot apical meristem, which is the very tip of the plant's growing shoot.
One of auxin's main functions is to promote cell elongation, encouraging the plant to grow upwards and towards light, a process known as phototropism. This characteristic is essential for ensuring that the plant competes successfully for sunlight and continues healthy development.
Auxin travels from the apical part of the plant downwards through transport mechanisms, establishing a concentration gradient. This gradient is crucial because a higher amount of auxin at the top suppresses the growth of side (lateral) buds. Here’s how it affects lateral bud growth:

• A high concentration of auxin inhibits the development of lateral buds.
• This action helps the plant to direct its energy towards vertical growth initially.

By inhibiting lateral branches, auxin ensures that the plant's resources are primarily directed towards making the main stem taller and stronger.

Cytokinin

Cytokinin is another essential plant hormone that complements the growth functions of auxin. Unlike auxin, cytokinin is majorly produced in the root tips and is known for enhancing cell division and growth in plants.
Cytokinin has a stimulating effect on lateral bud growth. It moves upwards through the plant, from the roots towards the shoots, providing signals to encourage the development of new branches. The role of cytokinin in lateral bud growth includes:

• Promotion of lateral bud development when present in higher concentrations.
• Encouraging the plant to grow more horizontally by branching out.

During instances where the level of cytokinin overtakes that of auxin, possibly due to events like pruning, lateral buds are stimulated to grow and form new shoots. This phenomenon supports the plant's adaptability and ability to occupy more space efficiently.

Lateral Bud Growth

Lateral bud growth is vital for plant branching and overall architecture. This process is chiefly influenced by the balance between auxin and cytokinin. Lateral buds are embryonic shoots located at the leaf axils, which have the potential to form branches.
The growth and development of lateral buds are regulated by:

• The concentration gradient of auxin decreasing from the top of the plant to the bottom.
• The amount of cytokinin moving from roots towards the shoot tips.

An elevated amount of auxin inhibits lateral bud growth, keeping the plant's structure streamlined for vertical growth. Conversely, when cytokinin levels become more pronounced, lateral buds are stimulated to grow.
The balance of these hormones can shift due to external factors such as pruning, where removing the apical bud reduces auxin influence and allows cytokinin to promote branching. This interaction highlights the plant's ability to adapt and modify growth patterns in response to environmental changes.

Plant Hormones

Plant hormones, also known as phytohormones, are critical in adapting and regulating plant growth and development. These chemical messengers influence various physiological processes, such as flowering, fruit development, and response to environmental stimuli.
Major types of plant hormones include:

• Auxin: Promotes cell elongation, apical dominance, and root development.
• Cytokinin: Stimulates cell division and lateral bud growth.
• Other hormones like gibberellins, ethylene, and abscisic acid which are involved in other vital growth processes.

The interplay between different hormones determines plant growth patterns and responses. In terms of apical dominance, auxin and cytokinin play opposing yet complementary roles, regulating whether a plant will primarily grow upwards or increase its branching.
Understanding the intricate balance of these hormones helps to explain how plants maintain a competitive advantage in their environment and optimize their architecture for survival.