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.

When Is Root Pressure Important?

1. Root pressure plays a vital role under specific conditions:
   1. High Humidity: When the air is saturated with moisture, transpiration slows or stops. Root pressure ensures that water and nutrients still move upward to support the plant.
   2. Nighttime: At night, stomata (pores on leaves) close, halting transpiration. Root pressure helps maintain water movement in the absence of transpiration pull.
   3. Spring Rehydration: In deciduous trees, xylem vessels often become air-filled during winter dormancy. Root pressure helps refill these vessels with sap in spring before the leaves develop and transpiration resumes.

Active Transport in the Xylem

1. In the xylem, active transport plays a pivotal role in the movement of minerals from the soil into the plant.
2. The mineral ions are transported via root cell membranes using ATP-driven pumps such as the H⁺-ATPase pump, which moves protons out of the root cells, allowing other ions (e.g., K⁺, Ca²⁺, Mg²⁺) to enter through ion channels.
3. This active process ensures that minerals are available in high enough concentrations in the root xylem to facilitate water movement via osmosis.
4. Xylem parenchyma cells and xylem vessels work together to transport these minerals efficiently.

Active Transport in the Phloem

1. In the phloem, active transport is essential for the loading of sugars (primarily sucrose) from photosynthesizing leaves into the sieve tubes of the phloem.
2. This process also requires ATP.

Sucrose Loading

1. Sucrose is actively transported from source cells (like leaf mesophyll cells) into the phloem sieve tubes using a proton-sucrose co-transporter that moves sucrose against its concentration gradient.
2. This creates a high solute concentration in the phloem, which draws water into the phloem via osmosis, creating turgor pressure that pushes the sap (containing sugars and water) through the phloem to sink tissues (such as roots, flowers, and fruits).

Role of Active Transport in Phloem Function

1. Active transport in the phloem helps establish the pressure gradients that drive bulk flow of nutrients and sugars from sources (e.g., leaves) to sinks (e.g., roots, flowers).
2. This process is essential for plant growth, development, and energy storage.