The primary and secondary growth of a shoot inhibited by

Options

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

Answer

### Step-by-Step Solution:

1. **Understanding Plant Growth**:
- Plant growth occurs in two main ways: primary growth and secondary growth. Primary growth refers to the elongation of the plant, while secondary growth involves the thickening of the plant.

2. **Identifying Plant Hormones**:
- Several plant hormones influence growth, including auxin, gibberellin, cytokinin, and abscisic acid. Each hormone has specific roles in plant development.

3. **Role of Abscisic Acid**:
- Abscisic acid (ABA) is known as a growth inhibitor hormone. Its primary function is to inhibit growth in various parts of the plant, including roots, shoots, and leaves.

4. **Effect on Growth**:
- The concentration of abscisic acid increases during stress conditions, leading to the inhibition of both primary and secondary growth of shoots. This makes it a crucial hormone in managing plant responses to environmental stress.

5. **Comparison with Other Hormones**:
- Auxin promotes root and shoot elongation.
- Gibberellin promotes stem elongation and flowering.
- Cytokinin promotes cell division and delays aging (senescence).
- In contrast, abscisic acid inhibits growth.

6. **Conclusion**:
- Based on the roles of these hormones, the correct answer to the question is **abscisic acid**, as it inhibits both primary and secondary growth of shoots.