B3.2.17 Generation of root pressure in xylem vessels by active transport of mineral ions (HL) Notes

Generation of Root Pressure in Xylem Vessels by Active Transport of Mineral Ions

1. Root pressure is a positive pressure potential generated within the roots of plants, which helps move water up the xylem when transpiration alone is insufficient.
2. Unlike the tension created by transpiration pull, root pressure is a pushing force that moves water upward from the roots into the xylem vessels.
3. It becomes particularly important when transpiration is minimal or absent, such as during high humidity, at night, or in early spring before leaves develop.
4. The mechanism behind root pressure involves the active transport of mineral ions into the xylem.
5. This creates an osmotic gradient, driving water movement from surrounding root cells into the xylem and increasing pressure inside the vessels.

Role of Active Transport in Root Pressure Generation

1. Root pressure is a positive pressure potential created when water is pushed up through the plant.
2. This occurs when the pressure in the xylem vessels increases, forcing water to move into the plant, even when transpiration (the primary mechanism of water transport) is not active.

Step-by-Step Process of Root Pressure Generation

1. Active Transport of Mineral Ions into Root Cells

1. Root pressure begins when minerals such as potassium (K⁺), calcium (Ca²⁺), and other ions are actively transported into the root cells from the surrounding soil.
2. This occurs via ion channels and pumps that move the ions against their concentration gradient, a process requiring energy in the form of ATP.
3. The energy is used to pump these ions into the xylem vessels of the root.

2. Increase in Solute Concentration

1. As the mineral ions accumulate in the xylem vessels, the solute concentration increases in the xylem relative to the surrounding root cells.
2. This results in a lower water potential in the xylem vessels, making them more concentrated than the surrounding cells.

3. Water Movement by Osmosis

1. The higher concentration of solutes in the xylem vessels creates a gradient that causes water to move into the xylem by osmosis.
2. As water enters the xylem, it generates a positive pressure in the root xylem, which pushes the water upward into the plant’s stem and potentially into the leaves.

4. Positive Root Pressure

1. The resulting pressure, known as root pressure, is responsible for moving water upwards, often contributing to the movement of water into the stems and even into the leaves under certain conditions.
2. Root pressure is particularly important when transpiration is minimal, such as during the night, high humidity, or in early spring before leaves have emerged.