The theory of sap ascent explained by root pressure

Root Pressure Theory Of Ascent Of Sap

Take a deep dive into the process of sap ascent with this comprehensive exploration. By delving into this article, you will gain knowledge on (A) the route through which sap ascends, (B) the mechanism behind its ascent, and various theories surrounding this phenomenon.

Sap Ascension Theory: Understanding Root Pressure

After the roots absorb water, it is transported throughout the plant and any excess is lost through transpiration. To reach the uppermost parts of the plant, water needs to move upwards through the stem. This upward movement of water is known as Ascent of Sap.

The process of sap movement upwards can be examined in two ways:

Path of Sap Ascent: The Root Pressure Theory

It is widely accepted that the upward movement of sap occurs within the xylem.

(i) Take a stem from a balsam plant, which has a semi-transparent appearance. Cut the stem underwater to prevent air bubbles from entering through the cut end. Place the stem in a beaker filled with water containing eosin, a dye. After some time, observe colored lines moving upwards in the stem. If you cut sections of the stem at this point, only the xylem elements will appear to be filled with colored water.

When a leafy branch is cut underwater and placed in a beaker filled with water, the outer layer of bark is removed from the stem. After some time, it can be seen that the leaves above the removed bark remain fresh and green. This is due to the continuous supply of water to the upper part of the branch through its xylem tissue.

(B) Mechanism of Sap Upward Movement

The process of sap rising in small trees and herbaceous plants is easily explained, but it becomes more challenging to understand how this occurs in tall trees like Australian Eucalyptus or large conifers such as Sequoias. These towering trees can reach heights of 300-400 feet, making the ascent of sap a complex phenomenon. While there is still much to learn about this mechanism, several theories have been proposed to shed light on the process.

Theory of Sap Ascent: Root Pressure

Advocates of vital theories believe that the upward movement of sap is regulated by the living processes occurring within the stem.

There are two commonly proposed theories, but they lack strong evidence to support their claims.

The notion of this theory appeared to be only speculative and was later disproven by the experiments conducted by Strasburger in 1891 and 1893. His findings revealed that the upward movement of sap persists even in stems where living cells have been destroyed due to the absorption of toxins.

In his experiment, Bose utilized an electric probe connected to a galvanometer. As the electric probe needle was inserted gradually into the stem, the galvanometer needle displayed slight oscillations. However, when the probe reached the innermost layer of cortex, the galvanometer needle exhibited intense oscillations. Bose attributed this phenomenon to the pulsating activity of these cells.

Root Pressure Theory: The Mechanism of Sap Ascent

However, the force known as root pressure, which occurs in the xylem of plant roots, is not considered to be a significant factor in the upward movement of sap. This is due to several reasons.

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The root pressure is quite low, measuring around 2 atmospheres.

(ii) In the absence of root pressure, the flow of sap still continues. For instance, if a leafy twig is cut underwater and placed in a beaker filled with water, it remains fresh and green for a considerable amount of time.

In gymnosperms, the occurrence of root pressure has been seldom witnessed.

Theory of Sap Ascent through Root Pressure

Different physical forces could play a role in the upward movement of sap.

(i) The root pressure theory does not have an effect on the water that is already present in the xylem of roots.

(ii) Even if the root pressure is functioning properly, it has limitations and cannot push water higher than 34 feet.

Sachs (1878) believed that the movement of sap could occur through the absorption of water by the xylem walls. However, it is now widely accepted that this imbibitional force plays a negligible role in the ascent of sap, as it primarily occurs through the interior space within the xylem elements rather than across their walls.

A leafy branch is submerged in water and its cut end is coated with melted paraffin wax for a period of time. A small portion of the stem near the cut end is removed to reveal the cell walls. The branch is then placed in a beaker filled with water. After some time, the branch starts to droop as the lumens of xylem elements become blocked by the wax coating.

In plants, the xylem vessels are arranged in a stacked manner, creating a continuous channel similar to long capillary tubes. It was previously believed that water moves upwards in these capillary-like channels due to capillary force, just like how sap ascends in the xylem.

During the spring season, when new leaves are growing and there is a greater need for water, the wood contains wider elements. Conversely, in autumn when water supply decreases, the wood consists of narrower elements. This contradicts the principle of capillarity.

(iv) Gymnosperms typically lack vessels in their xylem and do not possess continuous channels formed by other xylem elements.

Transpiration Pull and Water Cohesion Theory

The theory of root pressure as the mechanism for the ascent of sap was initially suggested by Dixon and Joly in 1894. It gained significant support and further development from Dixon in 1914 and 1924. This theory has garnered widespread acceptance among researchers.

The ability of water molecules to stick together and adhere to the walls of xylem vessels allows for the formation of a continuous column of water.

(ii) The water column experiences a pull due to transpiration.

Water molecules are held together by hydrogen bonds, which form when a hydrogen atom is positioned between two electronegative atoms. In the case of water, the positively charged hydrogen atoms of one molecule are connected to the negatively charged oxygen atoms of other molecules through these hydrogen bonds.

Despite the weak nature of hydrogen bonds, which only contain about 5k. cal. energy, their abundance in water leads to a strong cohesive force between water molecules. This force allows them to form a continuous column within the xylem and prevents it from being easily disrupted by gravity or internal tissue obstructions during the upward movement of water.

The ability of water to stick to the walls of xylem, known as its adhesive properties, helps maintain a continuous flow of water in the xylem.

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During the process of transpiration, water is lost from the leaves through small openings called stomata. This water comes from both the intercellular spaces and mesophyll cells in the leaf. The mesophyll cells obtain this water from the xylem within the leaf.

As a result of these processes, water in the xylem vessels of the leaves experiences tension. This tension is then transmitted downwards through the xylem tissues of the petiole and stem, ultimately reaching the roots. Consequently, water is drawn upwards as an uninterrupted column to reach the transpiring surfaces located at the top of plants.

Some researchers argue that the main criticism of this theory is that air bubbles in the conducting channels can disrupt the flow of water. However, others have countered this by stating that there are either no air bubbles present or, if they do exist, they will not interrupt the continuous movement of water through other parts of the xylem (as shown in Figure 6.2).

Explanation of the root pressure theory

When there are more dissolved substances inside the root cells, water molecules tend to move towards areas with higher concentration through osmosis. As a result, water enters the root cells from the soil. This influx of water creates an increase in pressure within these cells, known as root pressure.

Lack of wide acceptance for root pressure theory in sap ascent

Root pressure theory cannot account for the upward movement of sap beyond a height of 10 meters. The intensity of root pressure is higher in the early morning compared to the afternoon. Root pressure does not occur during spring.


– Root pressure theory does not explain sap ascent beyond 10 meters.

– The intensity of root pressure is greater in the morning than in the afternoon.

– Root pressure does not occur during spring.

The two theories explaining the physical forces behind sap ascent

According to the imbibition theory, sap ascends through plant tissues due to the process of imbibition. Imbibition occurs when water molecules are absorbed by cell walls and intercellular spaces within a plant. This absorption creates a pressure gradient that helps push sap upwards.

The cohesion of water and transpiration pull theory is considered one of the most widely accepted explanations for sap ascent. It proposes that as water evaporates from leaves during transpiration, it creates tension or negative pressure in xylem vessels. This tension pulls up more water molecules from roots to replace those lost through evaporation.

In contrast, the atmospheric theory suggests that changes in atmospheric pressure play a role in driving sap upwards. When air pressure decreases above ground level due to factors like wind or temperature changes, it can create suction forces that help lift sap towards higher regions within a plant.

The capillary force theory posits that tiny tubes called capillaries present in xylem vessels contribute to upward movement by utilizing capillary action. Capillary action occurs when liquid rises against gravity within narrow tubes due to adhesive and cohesive forces between liquid molecules and tube surfaces.

The primary role of root pressure in sap ascent

The root pressure theory plays a significant role in the upward movement of sap in plants. One of its key contributions is the restoration of continuous chains of water molecules within xylem vessels, which tend to break due to the immense tension caused by transpiration.

When plants undergo transpiration, water is lost through small openings called stomata on their leaves. This loss creates a negative pressure or tension that pulls water upwards from the roots towards the leaves. However, this tension can be so strong that it causes air bubbles to form and disrupts the flow of water within the xylem vessels.

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To counteract this disruption, root pressure comes into play. Root pressure refers to the active pumping of ions into the xylem cells by root cells, creating a higher solute concentration inside these cells compared to surrounding soil particles. As a result, water moves osmotically from areas with lower solute concentration (soil) towards areas with higher solute concentration (xylem), generating positive pressure within the roots.

This positive pressure helps restore any broken chains of water molecules in the xylem vessels and ensures an uninterrupted flow from roots to shoots. By maintaining continuous columns of water throughout plant tissues, root pressure aids in efficient nutrient transport and provides structural support for upright growth.

Physiology of sap ascent

In plants, root pressure plays a crucial role in driving the movement of sap upwards. Root pressure occurs due to osmosis, where solutes present in the root cells create a higher concentration inside compared to outside. As a result, water molecules move into these cells through osmosis, increasing their internal pressure. This build-up of pressure forces water and dissolved nutrients up through the xylem vessels towards other parts of the plant.

Root pressure alone cannot account for long-distance transportation within tall trees or large plants; therefore, additional mechanisms come into play. One such mechanism is capillarity or cohesion-tension theory. According to this theory, transpiration (the loss of water vapor from leaves) creates negative tension or suction force within xylem vessels located in aerial parts like stems and leaves. This negative tension pulls up more water from roots due to cohesive forces between water molecules.

Furthermore, environmental factors such as temperature fluctuations can influence root pressure and consequently affect sap ascent rates in plants. For instance, warmer temperatures generally enhance metabolic activities in roots leading to increased solute accumulation inside cells resulting in greater root pressures.

The significance of the root pressure theory

1. Water absorption: Plants absorb water through their roots from the soil. The presence of dissolved minerals in the soil helps create a concentration gradient, meaning there are more minerals inside the roots than in surrounding areas.

2. Osmosis: Root cells have semi-permeable membranes that allow only certain substances, like water, to pass through easily. Due to osmosis, where molecules move from an area of higher concentration to lower concentration, water moves into the root cells because they have higher mineral concentrations.

3. Root pressure: As more and more water enters the root cells through osmosis, it creates internal pressure within them called root pressure. This force pushes up on both liquid and dissolved nutrients present in xylem vessels (tiny tubes responsible for transporting fluids) towards other parts of the plant.

4. Capillary action: Once pushed by root pressure into xylem vessels located within stems and leaves, capillary action takes over – this is when liquids can flow against gravity due to adhesive forces between liquid molecules and vessel walls.

5. Transpiration pull: Another significant factor aiding upward movement is transpiration pull or tension created by evaporation at leaf surfaces during transpiration (the release of excess moisture). When one molecule evaporates from a leaf surface via tiny pores called stomata, it pulls another molecule behind it due to cohesive forces between liquid molecules.

6.Water column continuity: All these processes work together to maintain continuity within xylem vessels, creating a continuous column of water and dissolved minerals from roots to leaves. Root pressure ensures that this column remains intact and provides a constant supply of water for photosynthesis, nutrient transport, and other essential plant functions.