The Apoplast Pathway

The apoplast is the system of adjacent cell walls which is continuous throughout the plant. When water moving through spaces in the cell walls reaches the endodermis, its progress is stopped by the casparian strips, a band of suberin and lignin bordering four sides of root endodermal cells. Therefore water and solutes particularly salts in the form of ions must pass through the cell surface and into the cytoplasm of the cells of the endodermis. In this way the cells of the endodermis can control and regulate the movement of solutes through the xylem.

The apoplast pathway is one of the two main pathways for water movement in plant roots. In this pathway, water moves through the cell walls and intercellular spaces of the root cortex without crossing any cell membranes. The apoplast pathway is driven by the water potential gradient, which is determined by the concentration of solutes and the pressure potential. Water moves from an area of high water potential to an area of low water potential until equilibrium is reached.

The apoplast pathway is important for the uptake of water and minerals from the soil. It allows water to move rapidly through the root system, but it does not provide any selectivity or regulation of solute uptake. Most water and minerals taken up by the root enter through the apoplast pathway and then cross the plasma membranes of root cells to enter the symplast pathway, which is the other major pathway for water movement in plants.

The apoplast pathway can be blocked by suberin, a waxy substance that is present in the Casparian strip, a specialized cell wall layer in the root endodermis. The Casparian strip prevents the unregulated movement of water and solutes into the xylem, which helps to maintain proper water and nutrient balance in the plant.