Plants absorb water from the soil by the root hairs. They absorb mainly the capillary water of the soil along which dissolved nutrients are also absorbed. The root wall is water permeable since it comprises pectic substances and cellulose which are water loving i.e. hydrophilic. Roots are always growing towards water. So, the root hair zone is in contact with water for continuous water absorption.
Absorption of water is referred to the taking in of water from the soil in plants.
The path of water absorption:
Water is absorbed through the root hair cells from the soil. From the root hair, water moves into the cortical cells, then into endodermis.
The endodermal cells present opposite to the root hair are modified specially to transport water. These cells are also known as passage cells which lack casparian thickenings due to which they are permeable to water. Other endodermal cells with casparian thickenings are impermeable to water. Water from the passage cells passes into the pericycle, then into xylem cells through which water moves up through the stem up to the leaves. The root hair, passage cells and xylem cells are all present in a line to facilitate easy movement of water from soil to the leaves.
Inside the root, there are three pathways for water to enter from root hairs to xylem:
- Apoplast pathway
- Symplast pathway
- Trans-membrane pathway
Apoplast pathway: The transport of water in this channel occurs through the cell wall, with no participation of membranes. The apoplast route transports the majority of the water. The cortex of the root does not obstruct such water movement.
Symplast pathway: The plasmodesmata (connecting structures between two adjacent cells) allow water molecules to pass from one cell to the next. The plasmodesmata of all cells form a cytoplasmic network. Symplast pathway is slower than the apoplast.
Trans-membrane pathway: The transport of water in this channel occurs by both apoplast and symplast pathways. It is also called the vacuolar pathway as even vacuoles take part in this pathway.
Ascent of sap:
The upward transportation of water and minerals from the xylem cells of root to the leaves is called the ascent of sap. Sap is an aqueous solution of water, dissolved minerals and some organic solutes present in the xylem. Hence, this sap is also called xylem sap. Ascent of sap occurs due to transpiration pull from the top of the plant and cohesion-adhesion properties of water. The pulling force developed in the water of xylem due to the transpiration is called transpiration pull.
Root pressure:
Root pressure is a force or pressure generated in the roots that helps in the upward movement of fluids and other ions from the soil into the vascular tissue of the plant – Xylem. The impacts of root pressure can only be visible at night and early in the morning because the rate of evaporation is very low.
Guttation and bleeding are caused by root pressure.
Bleeding in plants: Root pressure causes sap to flow from stumps and other wounds in some tree species, a phenomenon that occurs only under certain conditions and at specific times of the year. This phenomenon is called bleeding in plants.
Guttation: Guttation is the loss of water or xylem sap from the uninjured margins of the leaves which occurs mostly during night as the stomata of most of the plants are closed during night. It occurs through a special type of stomata called hydathodes or water stomata present at the margins of some plant leaves and the fluid that is coming out of hydathodes is called as guttation fluid.
Mechanism of guttation:
Water will enter the roots of the plants when the soil moisture is high, because the water potential of the roots is lower than that of the soil solution. Water will collect in the plant, resulting in a minor root pressure as the stomata are closed during night. Some water is forced to leak through particular leaf tip or edge structures, hydathodes or water glands, generating drops as a result of the root pressure. Root pressure, not transpiration pull, is the driving force behind this flow.
Guttation can be seen in most grasses and in dicotyledonous plants like tomato, colocasia etc.