Apical dominance is a physiological phenomenon exhibited by many plants where the main central stem of the plant is dominant over other side stems; the plant’s growth is concentrated at the apex, inhibiting the growth of lateral buds. This ensures the plant prioritizes vertical growth to compete for sunlight. First observed by Charles Darwin in 1880 while studying *Phleum pratense* (timothy grass), the phenomenon is now understood to be largely regulated by plant hormones, primarily auxin. Understanding apical dominance is crucial for optimizing crop yields and manipulating plant architecture in horticulture. Defining Apical Dominance Apical dominance refers to the suppression of lateral bud growth by the apical bud (the bud at the tip of the stem). This results in a taller, less bushy plant. The apical bud actively inhibits the growth of axillary buds further down the stem. This is a key adaptation for plants to efficiently capture sunlight and resources. The Hormonal Mechanism The primary hormone responsible for apical dominance is auxin , specifically indole-3-acetic acid (IAA). Auxin is produced in the apical bud and transported downwards through the stem. This transport is polar, meaning it moves primarily in one direction – from the apex towards the base. Auxin’s Role: High auxin concentrations near the apical bud promote its growth and suppress the development of lateral buds. This suppression is thought to occur through several mechanisms, including the inhibition of bud break genes and the promotion of abscisic acid (ABA) synthesis, which further inhibits bud growth. Cytokinin’s Counteraction: Cytokinins , produced in the roots and transported upwards, counteract the effects of auxin. They promote lateral bud growth and bud break. The balance between auxin and cytokinin levels determines the extent of apical dominance. Strigolactones: Recent research indicates that strigolactones also play a role in apical dominance, working synergistically with auxin to suppress bud outgrowth. Factors Influencing Apical Dominance Several factors can influence the strength of apical dominance: Light: Adequate light promotes auxin synthesis and strengthens apical dominance. Nutrient Availability: Sufficient nutrient supply supports robust growth of the apical bud, enhancing dominance. Plant Species: The degree of apical dominance varies significantly between plant species. Some species exhibit strong dominance, while others have a more open branching pattern. Genetics: Genetic factors influence the sensitivity of lateral buds to auxin and cytokinin. Ecological and Agricultural Significance Apical dominance has significant ecological and agricultural implications: Ecological Advantage: In a competitive environment, apical dominance allows plants to quickly reach for sunlight, outcompeting neighboring plants. Crop Yield: In agriculture, manipulating apical dominance can improve crop yields. Pruning the apical bud (decapitation) removes the source of auxin, releasing lateral buds from inhibition and promoting bushier growth, leading to increased fruit production in some crops like tomatoes and grapes. Horticultural Practices: Apical dominance is exploited in horticulture to shape plants and create desired forms. Pinching back stems encourages branching and a fuller appearance. Exceptions to Apical Dominance While generally consistent, apical dominance isn’t absolute. Several factors can lead to its breakdown: Low Light Conditions: Under low light, auxin transport is reduced, weakening apical dominance and allowing lateral buds to develop. Stress Conditions: Environmental stresses like drought or nutrient deficiency can disrupt hormone balance and reduce apical dominance. Mutations: Mutations affecting auxin synthesis, transport, or signaling can disrupt apical dominance. Grafting: Grafting can sometimes alter apical dominance patterns, depending on the compatibility of the scion and rootstock. The concept of ‘releasing’ lateral buds through decapitation is a common horticultural practice. Removing the apical bud eliminates the auxin source, allowing cytokinin to promote lateral bud growth. This is particularly useful in producing denser, more productive plants. Apical dominance is a fundamental aspect of plant development, governed by a complex interplay of plant hormones, particularly auxin and cytokinin. Its ecological significance lies in facilitating light capture, while its agricultural applications are vast, ranging from yield enhancement through pruning to shaping plants for aesthetic purposes. Understanding the factors influencing apical dominance and its exceptions is crucial for optimizing plant growth and productivity. Further research into the molecular mechanisms regulating this phenomenon will continue to refine our ability to manipulate plant architecture for various benefits.

Apical Dominance: Explanation | UPSC Mains BOTANY-PAPER-II 2014

Apical dominance

How to Approach

This question requires a detailed explanation of apical dominance, a crucial concept in plant physiology. The answer should define apical dominance, explain the underlying hormonal mechanisms (auxin, cytokinins), discuss its ecological and agricultural significance, and explore exceptions to the phenomenon. A structured approach, covering definition, mechanism, factors influencing it, applications, and exceptions, will be effective. Use of diagrams (though not possible in text format) should be mentally visualized while reading/writing.

Defining Apical Dominance

Apical dominance refers to the suppression of lateral bud growth by the apical bud (the bud at the tip of the stem). This results in a taller, less bushy plant. The apical bud actively inhibits the growth of axillary buds further down the stem. This is a key adaptation for plants to efficiently capture sunlight and resources.

The Hormonal Mechanism

The primary hormone responsible for apical dominance is auxin, specifically indole-3-acetic acid (IAA). Auxin is produced in the apical bud and transported downwards through the stem. This transport is polar, meaning it moves primarily in one direction – from the apex towards the base.

Auxin’s Role: High auxin concentrations near the apical bud promote its growth and suppress the development of lateral buds. This suppression is thought to occur through several mechanisms, including the inhibition of bud break genes and the promotion of abscisic acid (ABA) synthesis, which further inhibits bud growth.
Cytokinin’s Counteraction: Cytokinins, produced in the roots and transported upwards, counteract the effects of auxin. They promote lateral bud growth and bud break. The balance between auxin and cytokinin levels determines the extent of apical dominance.
Strigolactones: Recent research indicates that strigolactones also play a role in apical dominance, working synergistically with auxin to suppress bud outgrowth.

Factors Influencing Apical Dominance

Several factors can influence the strength of apical dominance:

Light: Adequate light promotes auxin synthesis and strengthens apical dominance.
Nutrient Availability: Sufficient nutrient supply supports robust growth of the apical bud, enhancing dominance.
Plant Species: The degree of apical dominance varies significantly between plant species. Some species exhibit strong dominance, while others have a more open branching pattern.
Genetics: Genetic factors influence the sensitivity of lateral buds to auxin and cytokinin.

Ecological and Agricultural Significance

Apical dominance has significant ecological and agricultural implications:

Ecological Advantage: In a competitive environment, apical dominance allows plants to quickly reach for sunlight, outcompeting neighboring plants.
Crop Yield: In agriculture, manipulating apical dominance can improve crop yields. Pruning the apical bud (decapitation) removes the source of auxin, releasing lateral buds from inhibition and promoting bushier growth, leading to increased fruit production in some crops like tomatoes and grapes.
Horticultural Practices: Apical dominance is exploited in horticulture to shape plants and create desired forms. Pinching back stems encourages branching and a fuller appearance.

Exceptions to Apical Dominance

While generally consistent, apical dominance isn’t absolute. Several factors can lead to its breakdown:

Low Light Conditions: Under low light, auxin transport is reduced, weakening apical dominance and allowing lateral buds to develop.
Stress Conditions: Environmental stresses like drought or nutrient deficiency can disrupt hormone balance and reduce apical dominance.
Mutations: Mutations affecting auxin synthesis, transport, or signaling can disrupt apical dominance.
Grafting: Grafting can sometimes alter apical dominance patterns, depending on the compatibility of the scion and rootstock.

The concept of ‘releasing’ lateral buds through decapitation is a common horticultural practice. Removing the apical bud eliminates the auxin source, allowing cytokinin to promote lateral bud growth. This is particularly useful in producing denser, more productive plants.

Additional Resources

Key Definitions

Auxin

A class of plant hormones that promote cell elongation, apical dominance, and root development. Indole-3-acetic acid (IAA) is the most common naturally occurring auxin.

Cytokinin

A class of plant hormones that promote cell division, lateral bud growth, and delay senescence (aging). They often act antagonistically to auxin.

Key Statistics

Global pruning shear market size was valued at USD 1.2 billion in 2022 and is projected to reach USD 1.6 billion by 2030, growing at a CAGR of 3.5% from 2023 to 2030.

Source: Global Market Insights, 2023

Pruning is estimated to increase fruit set by 15-25% in apple orchards (based on knowledge cutoff 2023).

Source: Various agricultural extension reports

Examples

Tea Plant Pruning

Tea plants (*Camellia sinensis*) are regularly pruned to maintain a low, bushy shape. This practice encourages the growth of numerous shoots, which are the source of tea leaves. Pruning removes apical dominance, promoting lateral branching and maximizing leaf production.

Frequently Asked Questions

▶What is the difference between apical dominance and apical control?

Apical dominance refers specifically to the suppression of lateral bud growth. Apical control is a broader term encompassing the influence of the apical bud on the overall growth and development of the plant, including root growth and flowering.