Which causes the negative pressure potential in xylem?

Answer

### Step-by-Step Solution:

1. **Understanding Pressure Potential**:
- Pressure potential in plants can be either positive or negative. In xylem, it is typically negative. This is important to understand as it sets the stage for how water movement occurs in plants.

2. **Role of Transpiration**:
- Transpiration is the process where water evaporates from the surface of leaves. This loss of water creates a negative pressure in the xylem vessels.

3. **Effect on Guard Cells**:
- The evaporation of water causes guard cells and other epidermal cells to lose water, leading to a state known as flaccidity. This flaccidity is crucial as it contributes to the overall tension in the xylem.

4. **Water Uptake from Xylem**:
- As the guard cells and epidermal cells become flaccid, they draw water from the xylem. This movement of water creates a tension or negative pressure in the xylem vessels.

5. **Tension Creation**:
- The negative pressure created by the water being pulled up from the xylem extends from the surface of the leaf down to the roots. This tension is essential for the upward movement of water through the plant.

6. **Conclusion**:
- Therefore, the negative pressure potential in xylem is primarily caused by the process of transpiration, which leads to the flaccidity of cells and the subsequent uptake of water from the xylem.