Assertion :- Phloem translocation is bidirectional.
Reason:- Source-sink relationship is variable according to season and need of plant.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false

Answer

**Step-by-Step Solution:**

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

2. **Understanding the Reason**: The reason provided is that "Source-sink relationship is variable according to season and need of the plant." This indicates that the parts of the plant that produce sugars (sources) and the parts that require sugars (sinks) can change depending on various factors such as the season or the plant's current needs.

3. **Explaining Phloem Translocation**: Phloem translocation is primarily based on the pressure flow hypothesis, which explains how sugars are transported from sources (like leaves where photosynthesis occurs) to sinks (like roots or fruits where sugars are utilized or stored).

4. **Source and Sink Dynamics**: In a plant, the source is typically a leaf where photosynthesis occurs, producing sugars in the form of sucrose. The sink can be any part of the plant that requires these sugars, such as roots, fruits, or growing shoots. The relationship between source and sink can change; for example, during the growing season, the leaves may act as sources, while during fruiting, fruits become the primary sinks.

5. **Conclusion**: Since the source-sink relationship can change, phloem translocation is indeed bidirectional. Therefore, both the assertion and the reason are correct, and the reason accurately explains the assertion.

**Final Answer**: Both the assertion and reason are correct, and the reason is the correct explanation of the assertion.

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