Pathways for Root Absorption

Some herbicides will be root absorbed and will tend to remain in membranes and lipid bodies of the in epidermis,  while herbicides with some water solubility can move by three major pathways toward the plant’s vascular system (Figure 2).

1. The apoplastic (non-living) pathway provides a route toward the vascular stele through free spaces and cell walls of the epidermis and cortex. An additional apoplastic route that allows direct access to the xylem and phloem is along the margins of secondary roots. Secondary roots develop from the pericycle, a cell layer just inside the endodermis. The endodermis is characterized by the Casparian strip, a suberized layer that forces all herbicides to move in the symplast in order to enter the vascular system. Since 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 (living tissue).  
2. The symplastic (living) route to the vascular stele involves cell to cell transport by plasmodesmata. Plasmodesmata are channels of cytoplasm lined by plasma membrane that transverse cell walls. These channels allow herbicides to move from cell to cell without passing through the cell wall. Plasmodesmata represent direct cytoplasmic connections from one cell to the next.  
3. The trans-membrane route involves movement across cells and cell walls combining both symplastic and apoplastic movement.

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Figure 2. Cross-section of plant root showing the various structures and expanded view of the Casparian strip and vascular stele.

Herbicides that move by the symplastic and transmembrane pathway are already moving in such a way that the Casparian strip represents no significant barrier to movement into the vascular system. Herbicides moving in the apoplast; however, are forced by the Casparian strip to crossthe plasma membrane and enter the cytoplasm of endodermal cells. Once inside the endodermis, these herbicides must cross the plasma membrane a second time to reach the vascular stele. This process can restrict herbicide movement to the vascular system and subsequently translocation to the shoot.