The specific relationship between a legume and its mutualistic Rhizobium strain probably depends on

Options

A. each legume having a chemical dialogue with a fungus.
B. each Rhizobium strain having a form of nitrogenase that works only in the appropriate legume host.
C. each legume being found where the soil has only the Rhizobium specific to that legume.
D. specific recognition between the chemical signals and signal receptors of the Rhizobium strain and legume species.

Answer

### Step-by-Step Solution:

1. **Understanding the Relationship**: Recognize that the relationship between legumes and Rhizobium is mutualistic, meaning both organisms benefit from the interaction. Rhizobium bacteria help legumes fix atmospheric nitrogen, which is essential for plant growth.

2. **Evaluating the Options**: The question presents four options regarding the specific relationship between legumes and Rhizobium strains. We need to analyze each option to determine which one accurately describes the nature of this relationship.

3. **Analyzing Option A**: The first option states that each legume has a chemical dialogue with the fungus. This is incorrect because the relationship is specifically between legumes and Rhizobium bacteria, not fungi.

4. **Analyzing Option B**: The second option suggests that Rhizobium strains have a specific form of nitrogenase that works only in the appropriate legume. This is also incorrect because nitrogenase enzymes are not specific to particular legumes; they can function across different species.

5. **Analyzing Option C**: The third option claims that each legume is found where the soil has only the Rhizobium specific to that legume. This is misleading as Rhizobium can be found in various environments and is not exclusive to specific legumes.

6. **Analyzing Option D**: The fourth option states that the specific relationship depends on the recognition between chemical signals and signal receptors of the Rhizobium strain and the legume species. This is correct because the mutualistic relationship is initiated through chemical signaling, where Rhizobium releases signals that are recognized by specific receptors on the legume.

7. **Conclusion**: Based on the analysis, the correct answer is Option D, which highlights the importance of chemical signaling in establishing the mutualistic relationship between legumes and Rhizobium.