Which of the following structures or compartments is not part of the plant's apoplast?

Options

A. Lumen of a xylem vessel
B. Lumen of a sieve tube
C. Cell wall of a mesophyll cell
D. Cell walls of root hairs

Answer

To determine which structure or compartment is not part of the plant's apoplast, we need to understand the definitions and characteristics of the apoplast and the other pathways of transport in plants.

### Step-by-Step Solution:

1. **Understand the Apoplast Pathway**:
- The apoplast pathway involves the movement of water and minerals through non-living components of the plant, such as cell walls and intercellular spaces. This pathway does not involve living cell membranes.

2. **Identify the Options**:
- The options provided are:
- Option 1: Lumen of a xylem vessel
- Option 2: Lumen of a sieve tube
- Option 3: Cell wall of a mesophyll cell
- Option 4: Cell walls of root hairs

3. **Analyze Each Option**:
- **Option 1: Lumen of a xylem vessel**:
- The xylem vessel is a non-living component. Therefore, it is part of the apoplast.
- **Option 2: Lumen of a sieve tube**:
- Sieve tubes are living components of the phloem. The movement of substances in sieve tubes occurs through the symplast pathway, not the apoplast.
- **Option 3: Cell wall of a mesophyll cell**:
- The cell wall is a non-living component. Thus, it is part of the apoplast.
- **Option 4: Cell walls of root hairs**:
- The cell walls of root hairs are also non-living components, making them part of the apoplast.

4. **Conclusion**:
- The only structure that is not part of the apoplast is the **lumen of a sieve tube** because it is associated with the symplast pathway due to being a living component.

### Final Answer:
The structure that is not part of the plant's apoplast is **Option 2: Lumen of a sieve tube**.