Root pressure and transpirational pull enables the upward movement of water and minerals in plants. The mineral ions from the soil are pushed into the root vascular tissues by diffusion and as a result, the pressure inside the xylem increases. This positive pressure is known as root pressure.
Root pressure can be observed in a plant by a cut-stem experiment. In the early morning hours of a humid day, if a soft stem is cut horizontally near its base with a blade, drops of solution ooze from the cut stem. This oozing is caused by positive root pressure. If a rubber tube is fixed to this cut stem, we can measure the rate of exudation and the composition of exudates.
The impact of root pressure is best witnessed during the night and early in the morning, when evaporation is slow. At this time, the excess water collects as droplets around the special openings of the veins called hydathodes near the tip of grass blades. This water loss in its liquid form is called guttation. However, root pressure itself does not account for the majority of water transport.
Most of the water movement in tall trees takes place due to transpirational pull. The driving force behind this transpirational pull is transpiration from the leaves. Unlike guttation, where water is lost in liquid form, water is lost as vapour during transpiration. Transpiration can be witnessed by closing a healthy plant inside a polythene bag. The water loss due to transpiration ‘pulls’ the water upwards in a plant stem. Did you know that the transpirational pull generated by transpiration is strong enough to cause water to move upwards even in tall trees by as much as 15 metres per hour.
Water transport is also aided by cohesion and surface tension. Cohesion is the mutual attraction between water molecules and surface tension is the force acting on a water molecule travelling upwards in the xylem.
Since cohesion-tension helps the transpirational pull, it is also known as the cohesion-tension-transpirational pull model of water transport.