Pathways of water movements in roots: Apoplast and Symplast pathways
Water from the soil comes into the plant mostly through root hairs in the piliferous layer of the root. The root hairs are tubular extensions of epidermal cells and greatly increase surface area for absorption of water from soil.
Normally the soil solution has a higher water potential than cells of the piliferous layer. Water, therefore, enters the root from the soil by osmosis. A water potential exists across the root from higher potential in the piliferous layer to lower potential in the cells adjacent to the xylem.
Thus a gradient is maintained and flow of water across the root from epidermis to the xylem takes place in a down gradient of water potential via cortex, endodermis, pericycle and xylem parenchyma.
Water absorbing organs of plants:
In plants water is absorbed through roots . Under certain conditions, water is absorbed by shoot also. Some plants also absorb water through their leaves. The leaves of coast redwood (Sequoia sempervirens) absorb water from the fog in which they are frequently bathed along the California coast.
Certain epiphytes, for example, orchids, absorb moisture from the atmosphere by their special modified hanging roots.
Root hairs are the main water-absorbing organ of plants. The root hairs are responsible for absorption of water and mineral nutrients from the soil. The root bears unicellular hairs, with the help of which it can absorb water from the soil. It usually possesses a protective covering at the tip, known as root cap. They develop in the region of maturation and their number varies from plant to plant.
Root hair cell:
Cell wall of root hair consists of two distinct layers. The outer layer and the inner layer. The outer layer is composed of pectin and the inner layer is made up of cellulose. Both the layers are hydrophilic (water loving) in nature. Cell wall is permeable to both solute and solvent.
The cell wall surrounds plasma membrane and thin layer of cytoplasm. Plasma membrane along with cytoplasm acts as selectively permeable membrane. The cytoplasm encloses a central vacuole that contains cell sap. Nucleus of the cell is generally present at the tip.
Apoplast and Symplast Pathways of water:
There are three pathways of water passage from root hairs to xylem inside the root-
- Apoplastic pathway (cell walls and intercellular spaces)
- Transmembrane pathway (by crossing the plasma membranes) and
- Symplastic pathway (through plasmodesmata)
Munch (1932), a German physiologist introduced the concept of apoplast and symplast which is helpful in understanding the pathway of water movement through the root.
The interconnecting cell walls and intercellular spaces, including the water-filled nonliving xylem elements i.e., tracheids and vessels form one pathway, called the apoplast.
The other pathway is the living protoplast of the cell lying within the boundary of the selectively permeable membrane, called the symplast.
Apoplast is discontinuous and is separated into two regions. One region comprises the cortex and tissues external to the endodermis. The other region consists of the tissues of the stele i.e., tracheids and vessels which lie inner to the endodermis. This discontinuity of apoplast is due to endodermis.
Here water passes from root hair to xylem through the walls of intervening cells without crossing any membrane or cytoplasm. The pathway provides the least resistance to movement of water. However, it is interrupted by the presence of impermeable lignosuberin casparian strips in the walls of endodermal cells.
The Casparian strips in the endodermal cell wall prevent the passage of water from outside to inside or back, unless it passes through the cell membrane and cytoplasm of the endodermal cells. These strips contain waterproof substance called suberin which prevents apoplastic movement of water.
In this pathway, water and dissolved minerals never move through a cell’s plasmamembrane but instead travel through the porous cell walls that surround the plant cell.
The symplast forms a continuous system. The cytoplasmic strands, called plasmodesmata connects the protoplast of the cortex, endodermis and vascular tissue from cell to cell.
Transmembrane Pathway or Non-vacuolar Symplast Pathways:
In this pathway water passes from cell to cell through their protoplasm. It does not enter the cell vacuoles. Plasmodesmata connects the cytoplasm of the adjacent cells . Water moves through water channels present in the plant cell plasmamembrane from one cell to the next until it reaches the xylem.
Vacuolar Symplast Pathway:
In this pathway each root cell functions as tiny osmotic system. The root hair cell first absorbs water from soil through osmosis. Then it passes into vacuoles of adjacent cortical cells through osmosis. The process continues till water reaches the xylem parenchyma cells.
Apoplastic water is in constant equilibrium with water in the symplast and cell vacuoles. Exchange of water constantly takes place across both the cell and the vacuolar membranes. In fact, flow of water through cortex involves both the pathways.