apoplastic

The apoplast refers to the external network of plant extracellular spaces, encompassing the cell walls, the spaces between cells, and the lumens of non-living tissues such as xylem vessels. It lies outside the plasma membranes of living cells, forming a continuous pathway through which water and dissolved minerals can move. This contrasts with the symplast, which comprises the interconnected cytoplasm of living cells via plasmodesmata.

In plant transport, the apoplast provides a rapid, largely non-selective route for water and solutes to move toward vascular tissues. Water movement in the apoplast occurs by bulk flow driven by hydrostatic and osmotic gradients, and solutes can diffuse along the pathway. The apoplast works in concert with the symplast, which involves passage through cell membranes and cytoplasm, enabling selective uptake and regulation.

A key regulatory feature is the Casparian strip in root endodermis, a lignin-rich barrier that blocks apoplastic flow into the stele. This forces solutes to cross a plasma membrane to enter the vascular system, enabling selective uptake and preventing unregulated entry of ions. In some roots, apoplastic bypass flow can occur where the barrier is incomplete, influencing nutrient acquisition and salt tolerance.

Beyond transport, the apoplast is relevant to phloem loading and unloading in some species and to plant defense, where apoplastic enzymes, antimicrobial compounds, and physical barriers contribute to resistance against pathogens. Understanding the apoplast is essential for studying plant water relations, nutrient transport, and interactions with the environment and microbes.