Which of the following is both a growth promoter as well as a growth inhibitor?

Options

A. Auxin
B. Gibberellic acid
C. ABA
D. Ethylene

Answer

To determine which of the following options is both a growth promoter and a growth inhibitor, we need to analyze the roles of the given plant growth regulators: auxins, gibberellins, abscisic acid (ABA), and ethylene.

### Step-by-Step Solution:

1. **Understanding Plant Growth Regulators**:
- Plant growth regulators (PGRs) are chemical substances that regulate the growth and functioning of plants. The major types include auxins, gibberellins, cytokinins, ethylene, and abscisic acid.

2. **Identifying Growth Promoters**:
- Auxins, gibberellins, and cytokinins are primarily involved in promoting plant growth. They facilitate processes such as cell elongation, division, and differentiation.

3. **Identifying Growth Inhibitors**:
- Abscisic acid (ABA) is primarily known as a growth inhibitor. It plays a role in processes such as seed dormancy and stress responses, inhibiting growth under certain conditions.

4. **Analyzing Ethylene**:
- Ethylene is unique among the PGRs because it functions both as a growth promoter and a growth inhibitor.
- As a growth promoter, ethylene initiates flowering, promotes fruit ripening, and breaks dormancy in buds.
- As a growth inhibitor, it retards mitotic cell divisions in root and shoot meristems and inhibits the growth of auxiliary buds.

5. **Evaluating the Options**:
- Auxin: Growth promoter only (incorrect).
- Gibberellins: Growth promoter only (incorrect).
- Abscisic acid: Growth inhibitor only (incorrect).
- Ethylene: Both a growth promoter and a growth inhibitor (correct).

### Conclusion:
The correct answer is **ethylene**, as it is the only plant growth regulator that acts as both a growth promoter and a growth inhibitor.

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