Impact of Abscisic Acid on Growth

Abscisic acid (ABA) is a plant hormone that plays a crucial role in various physiological processes, including growth and development. While auxin promotes growth, ABA acts as a growth inhibitor by counteracting the effects of auxin. Therefore, the correct answer to the question is option 'C' - Abscisic acid.

Explanation:

1. Role of Auxin in Growth
Auxin is a plant hormone responsible for promoting cell elongation, cell division, and differentiation. It controls various aspects of plant growth, such as root development, stem elongation, and apical dominance.

2. Role of Abscisic Acid in Growth
Abscisic acid (ABA) is a plant hormone that functions in response to stress conditions, such as drought, high salinity, and cold temperatures. ABA helps plants adapt to these adverse environmental conditions by inhibiting growth and promoting stress tolerance.

3. Reversing the Effects of Auxin
When plants are exposed to stress, such as drought, the levels of ABA increase. ABA acts by inhibiting the growth-promoting effects of auxin. It does so by reducing the transport of auxin to the elongation zones of the plant, thereby reversing the effects of auxin on cell elongation and growth.

4. Examples of ABA's Impact on Growth
- In the case of drought stress, ABA accumulates in the leaves, triggering the closure of stomata. Stomatal closure reduces water loss through transpiration, but it also limits the entry of CO2 required for photosynthesis. This leads to a decrease in cell division and growth.
- ABA also affects seed germination and dormancy. It inhibits seed germination under unfavorable conditions, ensuring that seeds remain dormant until suitable conditions for growth are present.

Conclusion
In summary, Abscisic acid (ABA) acts as a growth inhibitor by reversing the effects of auxin. It plays a crucial role in plant adaptation to stress conditions and regulates various aspects of growth and development.

C