A : The translocation in phloem is bidirectional .
R : The source and sink relationship is variable .

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion, then mark (1)
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2)
C. Assertion is true statement but Reason is false, then mark (3)
D. If both Assertion and Reason are false statements , then mark (4)

Answer

**Step-by-Step Text Solution:**

1. **Understanding the Assertion (A):** The assertion states that "the translocation in phloem is bidirectional." This means that the movement of substances (mainly sugars) through the phloem can occur in two directions, from source to sink and vice versa.

2. **Understanding the Reason (R):** The reason provided is that "the source and sink relationship is variable." This indicates that the roles of source and sink can change depending on the plant's conditions, such as the season.

3. **Defining Source and Sink:**
- **Source:** In plants, a source is typically a part where substances are produced or absorbed. For instance, leaves are considered sources during photosynthesis because they produce glucose.
- **Sink:** A sink is a part of the plant where these substances are utilized or stored. This includes growing points like root apical meristems, shoot apical meristems, and young buds.

4. **Seasonal Variation in Source and Sink:**
- During favorable conditions (like spring or summer), leaves act as sources, producing food through photosynthesis.
- During unfavorable conditions (like winter), storage organs (like roots or tubers) can act as sources by providing stored food to support growth in other parts of the plant.

5. **Conclusion:** Since the source and sink roles can change based on environmental conditions, the translocation of substances in phloem is indeed bidirectional. Therefore, both the assertion and the reason are true, and the reason correctly explains the assertion.

6. **Final Answer:** Both the assertion (A) and the reason (R) are true, and the reason is the correct explanation of the assertion.

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