Problem 12 - Biology Today and Tomorrow with Physiology

Question

Which of the following statements is false? a. Auxin and gibberellin promote stem elongation. b. Cytokinin promotes cell division in lateral buds. c. Ethylene delays fruit ripening and abscission.

Step-by-Step Solution

Answer

Statement c is false; ethylene promotes fruit ripening and abscission.

Step 1: Understanding Plant Hormones Roles

First, let's look at each hormone's role: auxin and gibberellin indeed promote stem elongation (so statement a is true). Cytokinin is involved in promoting cell division, especially in lateral buds (so statement b is also true). Ethylene, however, is known for promoting fruit ripening and abscission, contrary to statement c.

Step 2: Identifying the False Statement

Based on our understanding of the hormones, we match each statement with its corresponding hormonal function. We find that statement c is incorrect because ethylene does not delay fruit ripening and abscission, it actually promotes them.

Key Concepts

Auxin

Auxin is a crucial plant hormone that plays a significant role in the growth and development of plants. It is primarily known for promoting stem elongation, which is essential for plants to grow towards sunlight. By doing so, auxin ensures that they photosynthesize efficiently.
When auxin is distributed unevenly throughout the plant, it can cause certain parts of the plant to grow faster than others. This results in bending towards the light, a phenomenon known as phototropism. In addition to its role in stem elongation, auxins also help in regulating other growth processes such as:

• Root growth and initiation
• Fruit development
• Suppressing lateral bud growth when concentrations are high

The synthesis of auxin mainly occurs at the tips of shoots and roots and is transported throughout the plant, influencing growth and development wherever it goes.

Gibberellin

Gibberellin is another important plant hormone that, like auxin, is known for promoting stem elongation. However, gibberellins are distinct in that they enhance growth by stimulating cell division and elongation, leading plants to achieve greater heights. While promoting stem growth, gibberellins have a range of other functions as well:

• Breaking seed dormancy, which helps seeds germinate
• Encouraging flowering processes in some species
• Contributing to leaf and fruit senescence reduction

This hormone is produced in various parts of the plant, including seeds, young leaves, and flowers. Gibberellins not only aid in normal growth but also in overcoming growth limitations, allowing plants to flourish even in challenging conditions.

Cytokinin

Cytokinins are plant hormones that primarily promote cell division and are crucial for the growth and branching of plants. They are especially influential in the growth of lateral buds, meaning they help plants grow in width or bushiness.
Cytokinins work by promoting cell differentiation and division, which is vital for the development of plant tissues. Their impact on plant physiology includes:

• Delaying leaf senescence, which keeps leaves green and productive longer
• Promoting the development of chloroplasts, essential for photosynthesis
• Interacting with auxins to regulate root and shoot growth

Produced mainly in the roots, cytokinins travel upwards to exert their effect on various parts of the plant, showcasing their adaptability and essential roles in plant growth strategies.

Ethylene

Ethylene is a unique gaseous plant hormone that plays a pivotal role in various growth and developmental processes. Contrary to its portrayal in the original exercise, ethylene actually accelerates fruit ripening and abscission. This means it helps fruits mature more quickly and facilitates the dropping of leaves and fruits from the plant.
Apart from ripening, ethylene mediates several other critical processes, such as:

• Response to mechanical stress by triggering certain defense mechanisms
• Influencing flower development and senescence
• Regulating gene expression related to growth and stress responses

Ethylene's ability to influence such varied processes highlights its significance in adapting plants to their environment. Its effects can be widespread as it diffuses easily through the air, affecting nearby plants as well. This hormone exemplifies how plants coordinate growth and environmental responsiveness through internal signaling pathways.