Question

Hormone involved in phototropism is (a) \(\mathrm{IAA}\) (b) gibberellin (c) kinetin (d) \(2,4-D\).

Short answer

The hormone involved in phototropism is (a) \(\mathrm{IAA}\).

Step-by-step solution

Understand phototropism

Phototropism is the growth of a plant in the direction of its light source. This growth pattern is caused by a specific hormone that triggers the elongation of cells on the plant's shaded side, causing the plant to bend towards the light.

Identify the hormone responsible for phototropism

Among the options given, the hormone responsible for phototropism is Indole Acetic Acid (IAA), which is also known as Auxin.

Select the correct answer

The hormone IAA is the one involved in phototropism, therefore the correct answer is option (a) \(\mathrm{IAA}\).

Key concepts

Auxin

Auxins are a class of plant hormones that play a crucial role in coordinating various growth and behavioral processes in the plant's life cycle. One of their most notable functions is influencing phototropism, the directed growth of plant organs toward or away from light, which is vital for maximizing photosynthesis. Auxins help to ensure that the plant grows in the most advantageous direction for absorbing light energy.

Auxin typically accumulates on the shaded side of the stem, promoting cell elongation. This asymmetric growth causes the stem to bend towards the light. The most well-known natural auxin is Indole Acetic Acid (IAA), which serves as a signaling molecule, triggering diverse developmental processes.

Indole Acetic Acid (IAA)

Indole Acetic Acid (IAA) is the most abundant naturally occurring auxin found in plants. Its primary role is to stimulate cell elongation, which is a fundamental part of plant growth and development. IAA orchestrates the expansion of cells, particularly in young shoots, by softening the cell walls, which in turn allows the cells to take in more water and expand.

This hormone's distribution within a plant is not uniform; it is transported from cell to cell and its concentration can vary depending on environmental stimuli such as light. In the phenomenon of phototropism, IAA is unevenly distributed in response to light, creating a gradient that causes the plant to grow towards the light source.

Plant Hormones

Plant hormones, also known as phytohormones, are chemicals that regulate plant growth, development, and responses to stimuli. Apart from auxins like IAA, plants produce several other types of hormones, including cytokinins, gibberellins, ethylene, and abscisic acid. Each of these hormones has specific functions and they often interact with one another to form a complex web of regulation.

For instance, cytokinins promote cell division and are generally found in greater concentrations in areas of rapid growth such as fruits and flowers. Gibberellins aid in seed germination, stem growth, and fruit development. Ethylene is involved in fruit ripening and the abscission of leaves. Abscisic acid, on the other hand, plays a pivotal role in stomatal closure and stress responses. The balance of these hormones ensures proper plant development and adaptability to environmental conditions.

Plant Growth and Development

Plant growth and development encompasses a range of processes that are controlled by the plant hormones mentioned above. Growth involves both cell division and cell expansion, processes that lead to an increase in the size and mass of the plant. Development, however, includes the formation of leaves, flowers, and fruit, and the overall architectural structure of the plant.

Throughout the life cycle of a plant, from germination to senescence, plant hormones tightly control these processes. Environmental signals such as light, gravity, and touch can trigger hormonal responses that dictate how a plant should grow. This adaptation is crucial for survival, enabling plants to either exploit environmental resources or defend themselves against stressors. Understanding the mechanisms behind plant growth and development not only enlightens us about the fascinating world of botany but also equips us with knowledge that can enhance agricultural practices and solve food security issues.