Ascent of Sap - Transport of Water

Plants absorb water from the soil through the roots. Thus, water has to be transported upwards to the other parts of the plant. This upward movement of water is called ascent of sap. It is called the sap as it contains many dissolved minerals. Ascent of sap involves root pressure and transpiration pull.

Root Pressure

Roots absorb water from the soil by osmosis or diffusion as the water is in higher concentration in the soil than in the roots. The absorption may take place into the intercellular spaces of the root cells or the root hairs. Root hairs are epidermal outgrowths. Once the water is inside the roots, it travels either through the intercellular spaces or from cell to cell and ultimately enters the xylem. Thus, the xylem in the root develops a positive water potential and the water is pushed up the tubes formed by the xylem elements. The pressure with which the water is pushed up by the xylem of the roots is called the root pressure.

The Pathway of Water from the Soil into the XylemRoot pressure explains the transport of water to the leaves and other parts in the case of short plants like the herbaceous plants. However, in tall trees the root pressure (about 1 atmosphere) is not sufficient to send the water up to the leaves. In these trees, transpiration pull transports water.

Transpiration Pull

The three types of transpiration together evaporate almost 99% of the water absorbed by the plant. Most of the transpiration is from the leaves through the stomata. The cells around the stomata absorb water from the neighbouring cells. Ultimately water is drawn from the xylem in the leaves. This results in a negative water potential in the upper portions of the xylem tube. Negative water potential means low pressure. Low pressure draws the water up from the lower regions - first the stem and then the roots of the xylem tube that have positive water potential and therefore, high pressure. The water moves up the plant as a result of the potential gradient. This gradient has developed due to transpiration. So, the force with which the water is pulled up the xylem is called the transpiration pull.

Wilting and the Transpiration Stream

The transpiration pull results in a continuous stream of water called the transpiration stream. It is a continuous stream of water extending from the xylem of the leaves to the xylem of the roots. In fact, transpiration pull can occur only when there is a continuous column of water. This continuity is maintained by the cohesive and adhesive properties of water. Adhesion causes the water molecules to adhere to the xylem walls and because of cohesion, the water molecules remain together and move up as a stream. Any break in this column, makes ascent of sap difficult. When a stem is cut, the water column moves away from the cut end of the xylem making conduction of water from the cut end difficult.

During day time when transpiration is more, there is hardly any root pressure as water is being continuously drawn from the roots.