Explain how this pressure potential can be negative?
Pressure potential is a component of water potential in a cell. Pressure potential is due to the hydrostatic pressure exerted on the water present in a cell.
Pressure potential increases as water enters a cell. As water passes through the cell wall, it increases the total amount of water present inside the cell, which exerts an outward pressure that is opposed by the structural rigidity of the cell wall.
By creating this pressure, the plant can maintain turgor, which allows the plant to keep its rigidity.
Without turgor, plants will lose structure and wilt.
The pressure potential in a plant cell is usually positive. In plasmolysed cell, pressure potential is almost zero.
Negative pressure potentials occur when water is pulled through an open system like xylem vessel. Withstanding negative pressure potentials is an important adaptation of xylem. This negative pressure potential pulls water upward constantly.
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