Water Movement in a Plant (Root Pressure and Transpirational Pull)- Science Queue

Why do plants need water?

Plants require water for survival, just like other living entities. We all are well aware that the plant uses water to perform photosynthesis in order to make food. But, along with this, water movement also facilities other processes like mineral transport, temperature maintenance and so on.

How do plants transport water?

Plants absorb water from the soil via roots, from where it travels through the stem and is later distributed to the other plant part, mainly the leaves. Transportation and distribution of the mineral nutrients in the entire plant body depend upon water movement. Thus, we can say that the water carries the necessary minerals back and forth between the roots and leaves.

Plants bear unique transport systems for the employment of water movement. This system involves specialised vascular tissues known as ‘Xylem’ that acts as pipelines for the conduction of water molecules.

Xylem is long thin tubes that extend from the root region to the shoot region. Their main function is to move water molecules upwards from the root to the upper parts of a plant. This upward movement of the water and minerals through the xylem vessel is referred to as the ascent of sap. There are many theories and hypotheses describing the intake of sap as well as the ascent of sap via the xylem column.

This post discusses about root pressure, transpiration, transpiration pull, guttation, cohesion-tension theory etc., in brief.

Soil -> Roots -> Stems -> Leaves

Long Distance Water Transport

The plants are always in desperate need of water, especially during the daytime. This is because, on a warm sunny day, a leaf can evaporate around 90-95 % of its water content per hour via transpiration, implying that most water uptake from the roots is lost.

Thus, the plant has to compensate for the water deficiency by transporting water from the roots to the leaves. For the short-heightened plants, this process is quite easier as the plant can use a simple capillary action mechanism to transport water. But, the problem arises with the tall trees above several metres’ heights.

Long trees have developed unique systems and mechanisms rapidly transporting the water even to heights above 100 m. This water transport may take place at the velocity of 16 m/hr, i.e., 4 mm/sec, by the xylems having a width of 100 µm. The velocity of the ascent of sap decreases up to 1-6 m/hr with small xylem having a 25-75 µm width.

Some most common phenomena related to water transport are transpirational pull, root pressure theory, cohesion tension theory etc.
Let’s know more about them.

Movement of water up a plant

As we all know, a strong gravitational force acts upon any entity on earth, pulling it downwards. Now imagine how the plants transport the water to several metres high, overcoming the gravitational pull. They use no electric pump to push the water upwards; still, the plants carry it against gravity.

There are many related theories, concepts and hypotheses to answer this question. Earlier it was believed that only a single mechanism works in lifting the water. Still, with recent studies, it is clear that there is a mutual contribution of many phenomena for this purpose.

For instance, three main processes work for the movement of water from root to shoot part via xylem:

Root Pressure: Forces or pushes the water molecules absorbed from the soil up in the xylem column.
Capillary Action: Draws the water through the xylem
CohesionTension Transpirational Pull: Applies suction force that pulls the water molecule up the xylem

Root Pressure

The concept that root pressure is used in the water transport mechanism in plants was postulated by Priestley.

To examine the presence of root pressure, you can simply perform an experiment. Cut the stem of any plant from a few inches above the ground. You will see xylem sap exudating out via the cut end. The phenomenon behind this is the root pressure.

The magnitude of root pressure is low, ranging from 3 to 5 atm. Yet this force is sufficient enough to lift the water in small-heightened plants, especially the monocot and herbaceous ones. However, in taller trees, it works in collaboration with other phenomena like transpirational pull to transport the water to the entire plant body.

Definition of Root Pressure

Water, along with ions and minerals, actively moves into the xylem of roots following its gradient potential, which elevates the pressure inside the xylem. This positive hydrostatic pressure generated inside the roots is referred to as root pressure.

The root pressure imparts a modest push to the water in the overall water transport process. It also re-establishes the chain of water molecules that often breaks due to the extreme tension created by the transpirational pull. As this pressure is not very strong, hence it cannot transport the water upto a very long distance. The water molecules pushed up in the xylem by the root pressure require a pull to suck them up. This requirement is fulfilled by the transpirational pull.

Features of Root Pressure

The root pressure relies on the osmotic pressure that is present in the root cell membrane.
The rate of transpiration is quite low in the early morning and nighttime because of the absence of sunlight. For this reason, the effects of root pressure are mainly visible during dawn and night.
The most important function of the root pressure is to channelise regular motions of the water molecules within the xylem that are susceptible to the transpirational pull.

“Guttation” is the process when the plant leaves discharge the excessive water in its liquid phase through hydathodes.

Transpirational Pull

The concept of transpirational pull was presented by Dixon and Jolly. The movement of water is not just required for photosynthesis but also for the transportation of minerals across the plant. Trees that are metres long need the minerals up at that very height which is not possible without external aids like a pump. Yet, the plants make the movement of the water molecules possible with the help of a phenomenon called transpirational pull.

What is Transpirational Pull?

We can define transpirational pull as a biological process that generates a force to pull up the ascent of sap upwards from the xylem vessel. The long and developed trees possess many xylems columns to facilitate this process. The transpirational pull generates a negative pressure that pulls the molecules up.

The water molecules travelling within the xylem do not travel alone. In fact, they adhere to each other as well as to the walls of the xylem ad move as an entire column.

Many other phenomena like adhesion, cohesion, tensile strength, surface tension etc. drive the transpirational pull

Cohesion: It infers to the bonding or interaction between the like-like molecules. Here, it indicates the interaction between water molecules to each other via a hydrogen bond.
Adhesion: It infers to the boding of unlike molecules. Here, in this scenario, when the water molecules get attached to the xylem cell walls, it indicates adhesion.
Surface Tension: It occurs as the H-bonding existing between the water molecules is much stronger at the air-water interface in comparison to the H-bonding in the water.
Tensile Strength: There is a little bit of flexibility between hydrogen bonding of the water molecules. This flexibility helps to avoid the strain or jerk in the bond so as to prevent the breakage of it. It enables the combined movement of many water molecules together.

Cohesion – Tension Theory

Polar water molecules develop an unbroken column of water by cohering to each other (cohesion) as well as by adhering to the walls of the xylem (adhesion). These water columns are initially pushed up by roots pressure.

Later they travel from roots to shoots due to the concentration gradient and water potential of the soil and that of air outside the stomata. This implies that the difference between the water potential of soil and air produces a suction force to pull the water from bottom to top.

Thus, this cycle continues as long as the column remains unbroken and the water keeps on moving by the transpirational pull. We refer to this as the cohesion-tension Transpirational pull model.