Apoplastia

Apoplastia refers to the movement of water and dissolved substances through the cell walls and intercellular spaces of plant tissues. This pathway is outside the cell membrane, hence the term "apoplast," meaning "away from the protoplast" or cell. It is one of the two main routes for water transport in plants, the other being the symplast pathway, which moves through the cytoplasm of cells connected by plasmodesmata.

The apoplast pathway is a continuous system of non-living elements, including cell walls, xylem vessels, and tracheids. Water enters the apoplast from the soil through the epidermis and moves across the cortex, endodermis, and pericycle. The cell walls of these tissues are porous and readily allow water to pass through. Intercellular spaces, which are air-filled gaps between plant cells, also form part of the apoplast and facilitate rapid water movement.

This pathway plays a significant role in the initial uptake of water by plant roots and its subsequent upward movement towards the leaves. The apoplast offers less resistance to water flow compared to the symplast, making it a faster route. However, at the endodermis, the Casparian strip, a band of suberin in the cell walls of endodermal cells, blocks the apoplastic movement of water and solutes. This forces water and dissolved nutrients to enter the symplast, allowing the plant to selectively absorb or exclude certain substances before they reach the vascular tissues. Therefore, the apoplast pathway is crucial for bulk flow and efficient water transport but is ultimately regulated at the endodermis.