The Apoplastic Pathway

The apoplastic pathway or also knowned as a non-living pathways provides a direction toward the vascular stele through free spaces and cell walls of the epidermis and cortex. In addition,the apoplastic route allows direct access to the xylem and phloem along the margins of secondary roots. Secondary roots formed from the pericycle, a cell layer just inside the endodermis. The endodermis is characterized by the Casparian strip, a band of cell wall material deposited in the radial and transverse walls of the endodermis to move in the symplast in order to enter the vascular system.

Since the secondary roots grow through the endodermis, a direct pathway to the xylem and phloem is available that bypasses the Casparian strip and allows herbicides to enter the vascular system without moving into the symplast or also called as a non-living pathways. This one of the two main pathways for water transport in plants, the other being symplastic pathway. In apoplastic transport, water and minerals flow in an upward direction via the apoplast to the xylem in the root.[7] The concentration of solutes transported in aboveground organs is established through a combination of import from the xylem, absorption by cells, and export by the phloem. Transport velocity is higher in the apoplast than the symplast. This method of transport also accounts for a higher proportion of water transport in plant tissues than does symplastic transport.

The apoplastic pathway is also involved in passive exclusion. Some of the ions that enter through the roots do not make it to the xylem. The ions are excluded by the plasma membranes of the endodermal cells.Water and soil solutes might move through the cell walls of the cortical cells until the endodermis, where they would cross the plasma membrane of the endodermal cells. Water would cross via aquaporins, and solutes via ion channels or transporters. This is possible in roots without a suberised hypodermis and when the endodermis is in the primary stage of development.Entry of anions to deep layers of the cortex is likely to be restricted by charge repulsion from dissociated, negative carboxyl groups in cell walls. In general, cations also pass through cell walls more readily than anions, particularly if many of the carboxyl groups in cell walls are not occupied by Ca2+ ions. Nonetheless, apoplastic flow of water through roots can sustain large ion fluxes during periods of high transpiration.

It is likely that the bulk of the soil water taken up by the plant moves in the apoplast across the cortex, but that solutes are taken up by epidermal or outer cortical cells and then move in the symplasm across the cortex.

When the endodermis is suberised, water and solutes cannot enter from the apoplast and instead are taken up into the adjacent cortical cells and move via plasmodesmata into the endodermis.