Question

Which plant hormone is responsible for cell elongation?

• A. Cytokinin
• B. Gibberellin
• C. Auxin
• D. Abscisic acid

Hint:

Associate each major plant hormone with its primary functions:
- Auxins: Cell elongation, tropisms.
- Cytokinins: Cell division.
- Gibberellins: Stem elongation (especially in dwarfism), germination.
- Abscisic acid: Stress response, dormancy.
- Ethylene: Fruit ripening, senescence.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks to identify the specific plant hormone that plays a primary role in promoting cell elongation.

Step 3: Detailed Explanation:
- Auxin: Auxins are a class of plant hormones that are crucial for cell elongation (especially in stems and coleoptiles), apical dominance, tropisms (phototropism and gravitropism), and fruit development. They promote the loosening of cell walls, allowing cells to expand.
- Cytokinin: Cytokinins primarily promote cell division and differentiation, and delay senescence.
- Gibberellin: Gibberellins promote stem elongation (especially in dwarf plants), seed germination, and flowering. While they promote elongation, auxins are more directly linked to the initial cell wall plasticity for elongation.
- Abscisic acid (ABA): ABA is a stress hormone that primarily inhibits growth, promotes dormancy, and plays a role in stomatal closure.
Among the given options, auxin is most directly responsible for cell elongation.

Step 4: Final Answer:
Auxin is the plant hormone responsible for cell elongation.

Answer 2

Looking at how each of these hormones was discovered and what it primarily does in the plant body helps pin down which one truly drives cell elongation.

1. Cytokinin: Cytokinins mainly promote cell division (mitosis) and differentiation, and they delay the ageing (senescence) of plant parts. Their main action is on splitting cells, not stretching them.
2. Gibberellin: Gibberellins are well known for causing stem elongation, especially dramatically in genetically dwarf plants, and they also help break seed dormancy. However, this elongation effect works largely by promoting cell division along with some elongation, rather than being the hormone most directly responsible for loosening the cell wall to let a cell stretch.
3. Auxin: Auxin is the hormone most directly linked to cell elongation. It activates proton pumps in the cell membrane that push hydrogen ions into the cell wall space, lowering the local pH; this activates wall-loosening enzymes called expansins, which loosen the bonds between cellulose fibres. With a loosened wall, the cell's own turgor pressure can then stretch it. This mechanism is the basis of phototropism and gravitropism as well, both of which occur because auxin builds up unevenly, causing one side of a stem or root to elongate faster than the other.
4. Abscisic acid: This is often called the stress hormone, and it generally inhibits growth, promotes dormancy in seeds and buds, and triggers stomatal closure to conserve water. It works in the opposite direction of growth-promoting hormones.

Of the four, only auxin's mechanism is specifically about loosening the cell wall so that the cell itself can elongate.

Therefore, the correct answer is Auxin.