Question

Apical dominance in dicot plants is due to the presence of more _______ in the apical bud than in the lateral ones. (a) auxins (c) gibberellins (b) cytokinins (d) ethylene

Short answer

Apical dominance in dicot plants is due to the presence of more auxins in the apical bud than in the lateral ones, hence the answer is (a) auxins.

Step-by-step solution

Understanding Apical Dominance

Apical dominance is the phenomenon where the main, central stem of the plant is dominant over (grows more strongly than) other side stems. This is due to the concentration of certain hormones which inhibit the growth of lateral buds.

Identifying the Responsible Hormone

Among the options given, we must identify the hormone known to maintain apical dominance by inhibiting the growth of lateral buds.

Recalling Plant Hormone Functions

From plant physiology, we know that auxins are a class of plant hormones responsible for various growth processes, including the promotion of cell elongation in shoots and the inhibition of lateral bud growth, thereby maintaining apical dominance.

Choosing the Correct Answer

Given that auxins are the hormones responsible for inhibiting the growth of lateral buds and maintain apical dominance, we select option (a) auxins as the correct answer.

Key concepts

Auxins

Auxins are a crucial group of plant hormones that play a pivotal role in the regulation of plant growth and development. Primarily produced in the shoot tips, auxins facilitate cell elongation which contributes to the plant's structured downward growth. One of the most significant functions of auxins is their contribution to the phenomenon of apical dominance.

This dominance occurs when the apical bud's high concentration of auxins suppresses the growth of lateral (side) buds. This concentration gradient ensures the plant's resources are devoted mainly to the vertical growth of the main stem, which can be quite beneficial for the plant's access to sunlight when taller vegetation is nearby. However, this can be a limitation for the expansion or bushiness of the plant.

When the apical bud is removed, the lower concentration of auxins allows lateral buds to break dormancy and grow. This is a practice often used in horticulture to encourage fuller plants. In the context of the textbook exercise, recognizing the role of auxins in apical dominance helps students to identify the correct hormonal factor influencing growth patterns in dicot plants.

Plant Hormones

Plant hormones, also called phytohormones, are chemicals that regulate many aspects of plant growth and development, similar to hormones in animals. Besides auxins, there are several other groups including cytokinins, gibberellins, ethylene, and abscisic acid, each with distinct functions.

• Cytokinins are primarily involved in cell division in plant roots and shoots, and they work in opposition to auxins to promote the growth of lateral buds.
• Gibberellins are important for stem elongation, seed germination, and flowering.
• Ethylene is a gas hormone that affects fruit ripening and response to stress.
• Abscisic acid primarily functions in seed dormancy and the closing of stomata during water stress.

Understanding the balance and interaction between these hormones is crucial for students to comprehend complex growth behaviors in plants. For instance, auxins and cytokinins must be in the right balance for a plant to maintain proper shoot and root development.

Dicot Plant Growth

Dicot plant growth refers specifically to the growth patterns and structures seen in dicotyledonous plants. These plants have two cotyledons, or seed leaves, and often exhibit a broadleaf. Growth in dicot plants is characterized by a branching vascular system and the development of a taproot system that anchors the plant and absorbs nutrients and water from the soil.

In terms of growth habit, many dicots feature a central main stem, from which lateral branches emerge. This pattern is influenced by plant hormones and results in the typical dome or pyramidal shape seen in many dicot species. The role of auxins in promoting apical dominance is a key factor in this growth structure, as it encourages the plant to grow primarily upward and outward at the tip, while restraining lateral branching.

Understanding dicot growth helps students studying botany or agriculture to not only identify these plants but also to engage in practices like pruning or training plants, optimizing their growth and fruit production based on an understanding of plant hormones and their control over plant architecture.