`phi_(p)` becomes negative in transpiring xylem tissue when

Options

A. The water column inside the xylem is subjected to negative pressure or tension
B. The water column inside the xylem is subjected to push
C. The water column inside the xylem is subjected to positive hydrostatic pressure
D. The water column in the xylem is broken due to embolism

Answer

To answer the question "φp becomes negative in transpiring xylem tissue when," we need to analyze the conditions under which the pressure potential (φp) in the xylem becomes negative.

### Step-by-Step Solution:

1. **Understanding Transpiration**:
- Transpiration is the process by which water evaporates from the leaves of a plant through small openings called stomata. This loss of water creates a demand for more water from the roots.

**Hint**: Remember that transpiration is primarily driven by evaporation from leaf surfaces.

2. **Formation of Negative Pressure**:
- As water evaporates from the leaves, it creates a tension or negative pressure in the xylem. This negative pressure is essential for pulling water upward from the roots to the leaves.

**Hint**: Think about how pulling on a straw creates a vacuum that draws liquid upward.

3. **Role of Water Column**:
- The water column in the xylem is subjected to this negative pressure or tension due to the continuous loss of water from the leaves. This phenomenon is often referred to as the "transpiration pull."

**Hint**: Consider how the cohesive properties of water molecules help maintain the integrity of the water column under tension.

4. **Evaluating the Options**:
- **Option A**: The water column inside the xylem is subjected to negative pressure or tension.
- This is correct because the evaporation of water creates a negative pressure that pulls water upwards.
- **Option B**: The water column inside the xylem is subjected to push.
- This is incorrect because the movement of water is due to pulling, not pushing.
- **Option C**: The water column inside the xylem is subjected to positive hydrostatic pressure.
- This is incorrect as the pressure is negative due to the transpiration pull.
- **Option D**: The water column in the xylem is broken due to embolism.
- This is incorrect because embolism refers to the breaking of the water column, which does not contribute to negative pressure.

5. **Conclusion**:
- The correct answer is that φp becomes negative in transpiring xylem tissue when the water column inside the xylem is subjected to negative pressure or tension.

### Final Answer:
φp becomes negative in transpiring xylem tissue when the water column inside the xylem is subjected to negative pressure or tension.