Callus formation is promoted by

Options

A. Proper light and subculuring
B. Darkness and subculturing
C. Excess of NAA
D. Absence of salts.

Answer

### Step-by-Step Solution:

1. **Understanding Callus Formation**:
- Callus is an unorganized mass of cells that forms when plant tissues are cultured in vitro. It consists of meristematic tissue, which is capable of active cell division.

2. **Factors Promoting Callus Formation**:
- Callus formation can be influenced by various environmental conditions and substances. The options provided in the question include:
- a. Proper light and subculturing
- b. Darkness and subculturing
- c. Excess of NAA (Naphthalene Acetic Acid)
- d. Absence of salts

3. **Role of Darkness**:
- Darkness is known to promote callus formation because it prevents photosynthesis. In the absence of light, the plant cells do not produce phenolic compounds, which can inhibit callus growth.

4. **Role of Subculturing**:
- Subculturing involves transferring a few cells from the parent culture to a new medium. This process helps in increasing the growth of the callus by providing fresh nutrients and stimulating further cell division.

5. **Evaluating the Options**:
- Option a (Proper light and subculturing) is less favorable because light can inhibit callus formation due to phenolic accumulation.
- Option b (Darkness and subculturing) is favorable as both factors contribute positively to callus growth.
- Option c (Excess of NAA) may promote growth but is not as significant as the combination of darkness and subculturing.
- Option d (Absence of salts) does not directly promote callus formation.

6. **Conclusion**:
- The most effective conditions for promoting callus formation are darkness and subculturing. Therefore, the correct answer is **option B: darkness and subculturing**.