Question

How many of the following are phytohormones? \(\mathrm{ABA}, \mathrm{C}_{2} \mathrm{H}_{4}, \mathrm{IAA}, \mathrm{GA}_{3}, \mathrm{ABA}\) (a) 2 (b) 3 (c) 4 (d) 5

Short answer

The correct answer is (c) 4. The phytohormones present are Abscisic Acid (ABA), Ethylene (C2H4), Indole Acetic Acid (IAA), and Gibberellin (GA3).

Step-by-step solution

Identify Phytohormones

To solve this exercise, it's crucial to recognize which of these chemical compounds are known as phytohormones. \(\mathrm{ABA}\) stands for Abscisic Acid, \(\mathrm{C}_{2}\mathrm{H}_{4}\) is Ethylene, \(\mathrm{IAA}\) is Indole Acetic Acid, and \(\mathrm{GA}_{3}\) refers to Gibberellin. All of them are phytohormones.

Count the Phytohormones

Now, count the number of phytohormones in the list. It's important to note that \(\mathrm{ABA}\) appears twice. However, it is the same phytohormone, so it is only counted once. Thus, the total number of phytohormones is 4.

Key concepts

Abscisic Acid

Abscisic Acid, often abbreviated as ABA, is a vital plant hormone that plays multiple roles in the regulation and adaptation of plants to their environment. It is best known for its role in helping plants respond to stress, particularly drought.
Abscisic Acid functions by closing stomata, the tiny openings on leaves, to minimize water loss during dry conditions. This feature is crucial because it helps plants preserve essential water resources.
Additionally, ABA is involved in seed dormancy, ensuring that seeds do not germinate under unfavorable conditions. It acts as a signal that delays growth when the environment is not suitable for young seedlings. This mechanism ensures that seeds wait for optimal conditions before sprouting.

Ethylene

Ethylene is a simple hydrocarbon gas that functions as a powerful plant hormone. Despite its simple structure, C extsubscript{2}H extsubscript{4}, it has significant effects on plant growth and development.
It is most famous for its role in fruit ripening. When fruits start to ripen, ethylene production increases, triggering the breakdown of cell walls and the conversion of starches to sugars, thereby softening and sweetening the fruit.
Ethylene also plays a significant role in leaf abscission, which refers to the shedding of leaves, flowers, or even fruit. It helps plants naturally detach these parts once they have completed their lifecycle, ensuring energy conservation and preparing the plant for new growth.

Indole Acetic Acid

Indole Acetic Acid (IAA) is a prominent auxin, a group of hormones significant for plant growth and behavior. IAA is essential in regulating plant development, affecting cellular elongation and division.
This hormone is central to phototropism and gravitropism—the way plants grow in response to light and gravity. When plants grow towards the light, IAA plays an integral part by concentrating on the darker side of the plant, causing cells to elongate and the plant to bend towards the light.
Furthermore, IAA influences root formation, shaping how roots establish and extend within the soil. Its presence encourages the creation of root systems that improve access to nutrients and water, crucial for sustaining healthy plant growth.

Gibberellin

Gibberellins are a group of plant hormones, with GA extsubscript{3} being one of many. They are key players in promoting growth and overcoming dormancy in various parts of a plant’s life cycle.
These hormones influence several developmental processes, including stem elongation, seed germination, and flowering. Gibberellins break seed dormancy, stimulating germination and encouraging the plant to grow when conditions are suitable.
Gibberellins are also used in agriculture to induce or delay flowering and for creating seedless fruits. This adaptability showcases their importance as tools in both natural and managed plant development. By accelerating cell division and elongation, gibberellins help plants reach their optimal growth height.