Long-Distance Transport Of Water Notes

Long-Distance Transport Of Water

Water acts as the transporting medium in plants. Plants with well-developed vascular systems and root systems conduct water from the soil to the aerial parts (mainly leaves) of plants. Water is absorbed from soil by root hairs. The absorbed water is then transported upward through the xylem vessel to the leaves and other parts of plants.

Movement of water and minerals occurs over several meters, which cannot be achieved by diffusion. Long-distance transport takes place by means of mass flow or bulk flow. This process involves the movement of substances in bulk from one region to another due to differences in pressure between the two regions.

This pressure difference can be achieved either by a positive hydrostatic pressure gradient (for example a garden hose) or by a negative hydrostatic pressure gradient (for example suction by a straw). The bulk movement of substances in plants through conducting or vascular tissue is called translocation.

” mechanism of water transport through xylem”

Long-distance transport of water in advanced plants, mainly vascular plants includes—

1. Absorption of water and water-soluble minerals by root hairs and transport of water and water-soluble minerals from root hairs to xylem vessels.
2. Upward movement of water and water-soluble minerals through xylem vessels by the process of the ascent of sap.

Water Absorption By Plants

Definition: The process by which plants obtain water and water-soluble minerals by root hairs from the soil for the proper functioning of their various physiological processes like nutrition, growth, etc., is called absorption.

The main source of soil water is rain. Some percentage of water received on earth during rainfall evaporates and most of the water flows away to other regions known as run-way water. The rest of the water is absorbed by soil particles under the action of gravitational force. The water which remains within the soil is divided into four categories.

These are as follows

Gravitational water: Most of the soil water percolates under the ground through the spaces between soil particles, under the action of gravitation force. This is called gravitational water. Gravitational water is beyond the reach of plants. So, it has no importance in their life.

Capillary water: Water molecules downwards, under the ground, by the action of gravitation force. During this descent, a few of them get trapped in spaces between and around soil particles due to the surface tension property of the soil particles. This water is known as capillary water. Most plants absorb this capillary water by endosmosis. Capillary water is essential for many physiological functions of plants.

Hygroscopic water: Capillary water attached to superficial soil particles evaporates. The remaining water persists as a thin film, bound with soil particles. This thin layer of water is known as hygroscopic water. Except for some grass and herbs, this water is not usable for most plants.

Chemically combined water: A small portion of water remains bound to soil particles through chemical reactions. This water is called chemically combined water. Roots of plants cannot absorb this water. Hence, it is not important to them.

Water-absorbing parts of plants

In plants, the absorption of water and minerals is accomplished by different organs. However, root is the main organ for absorption in most of the plants. Terrestrial plants mostly absorb water from the soil through root hairs. Root hairs are thin-walled slender extensions of root epidermal cells that greatly increase the surface area for absorption.

Water absorbing system

The root is the major point of entrance of water and minerals in higher plants, except in submerged plants. The area of young roots, where most absorption takes place, is the root hair zone. The root hairs lack cuticle and provide a large surface area. They are extensions of the epidermal cells.

They have sticky walls by which they adhere tightly to soil particles. Root hairs remain in contact with soil water. Water enters into the root hairs of the epiblema. From there, water reaches up to the endodermis through the cortex by cell-to-cell osmosis. The path followed is

Soil —> Root Hair —> Epiblema of root —> Pericycle —> Endodermis of root —>Xylem

They take water from the intervening spaces mainly by osmosis. Water movement across the regions of roots takes place in three probable systems or pathways

1. Apoplastic pathway,
2. Symplastic pathway,
3. Transmembrane pathway.

Munch (1930) first gave the idea about apoplastic and symplastic pathways.

Apoplastic pathway: In this pathway, the movement of water occurs exclusively through the dead cell wall, forming a continuous channel, without the involvement of any living membrane. The apoplast does not provide any barrier to water molecules, the movement depends purely on gradient and occurs through mass flow. In the plant body, a major portion of water is transported through the apoplastic pathway.

Symplastic pathway: In this pathway, the movement of water molecules takes place from one cell to another, through plasmodesmata.

The walls of some endodermal cells contain depositions of lignin and suberin. These depositions are known as Casparian strips. The Casparian strips, separate the cortex and the endodermis.

Suberin, present in these strips, blocks water and solute molecules from passing through the cell wall of the endodermis. In this case, water movement takes place through the endodermal cells.

” transport plants”

In some plants, some specialized endodermal cells take part in the transport of water. These endodermal cells are located at the opposite of xylem vessels. They are called passage cells. These cells are poorly supervised.

This forces the water to go through the cell membranes of different cells

Transmembrane pathway: In this pathway, water and water-soluble minerals from root hair cells move to the protoplast of living parenchymatous cells of the epidermis and cortex through the plasma membrane. The water molecules cross the vacuolar membrane, i.e.tonoplast and then move via symplast.

Mycorrhizal Absorption

Mycorrhiza is a symbiotic association between a fungus and a seeded plant. The fungus is known as mycorrhizal fungus. The fungal mycelia form a network around the young root or they penetrate the cortical cells of the root.

Hyphal growth increases the root surface area which assures increased absorption of water and minerals from soil. The fungus provides minerals and water to the roots. The roots, in turn, provide sugars and N-containing compounds to the fungus.

Some plants have an obligate association with the mycorrhizae. For example, association with mycorrhizal fungi is essential for the germination of Pinus, seeds as well as the establishment of young Pinus seedlings in soil.

There are two mechanisms regarding the absorption of water by root hairs

1. Passive absorption, and
2. Active absorption.

Passive absorption: In this process, the absorption of water does not require any expenditure of energy (breakdown of ATP). About 98% of total water uptake by a plant occurs by this process. In this process, water is absorbed through roots. The root cells act as a physical absorbing system.

In this type of absorption, the force originates due to transpiration. Transpiration creates a lower water potential (or tension) in the mesophyll cells of leaves, thus water is drawn upward from the xylem.

As the water column is continuous from leaves to roots through the xylem, the tension is transmitted to the xylem in roots and ultimately in the root hairs. This develops a water potential gradient between root hair and soil solution and helps in the movement of water across the root.

During passive absorption, the transport of water mainly occurs through the apoplast, but it endodermis through the symplast.

Active absorption: This is absorbed due to the osmotic pressure of the roots with the help of energy. A root hair cell functions as an osmotic system. Water is absorbed by root hair due to the difference in water potential between soil water and cell sap.

Thus, water moves from the region of high water potential towards the lower water potential. Water continues to enter root hair cells as long as its water potential remains lower. This type of absorption involves symplast.

Theories regarding the absorption of water by plants

Regarding the mechanism of water absorption, there are two theories—osmotic and non-osmotic theory.

Osmotic Theory: According to this theory, the transportation of water takes place by osmosis along a gradient of decreasing water potential (Ψ_w) or increasing solute potential ( ψ_s), from the soil solution to the root xylem. The cell sap of root hair possesses a higher osmotic potential (ψ_s) than that of the soil solution.

The difference in the osmotic potential of the two systems causes the water to enter the cells of root hair. Entry of water decreases the protoplasmic concentration of the cell which causes water to move to the next cellular layer and ultimately to endodermis.

So, in this way, water moves from the soil to the root xylem and finally to the endodermis, by cell-to-cell osmosis. Exudation of sap from the cut end of a plant and guttation are some important examples of the osmotic absorption of water from the soil.

Non-osmotic theory: According to this theory, absorption of water can also occur by expending energy (provided by respiration). Active water absorption needs metabolic energy, hence it is restricted to living cells.

Water Movement Up A Plantascent Of Sap

Plantascent Of Sap Definition: The process by which the xylem vessel of the root and shoot of terrestrial plants transport water and water-soluble minerals or xylem sap, against gravity, to leaves and other aerial parts of plants, is known as the ascent of sap.

The ascent of sap in plants occurs in xylem tissue. Tracheids and vessels or tracheae of xylem tissue are the cells involved in the ascent of sap.

Tracheids are long, dead cells, with tapered ends. The end walls of matured tracheal cells are perforated. These are known as perforation plates. Tracheids and tracheae have thick and lignified cell walls.

Ornamentation like pits or secondary thickenings are also seen on the walls. Minerals are transported upward mainly by xylem vessels. But solutes are transported laterally to the phloem by cell-to-cell diffusion. Hoagland and Stout (1939), used radioactive potassium (K²⁴) to prove this.

Experiments to show the path of water transportation

The path of water transportation can be shown through some simple experiments.

Eosin Dye Test

Eosin Dye Test Ingredients: Large glass jar, Peperomia sp. plant with intact root, eosin dye, cork with a central hole, mustard oil, glass marker.

Eosin Dye Test Procedure:

1. A Peperomia sp. plant with intact root is collected.
2. 2/3^rdparts of a clean glass jar is filled with water and a few drops of eosin are added to it. The water turns red.
3. The level of water is marked by a glass marker.
4. The root portion of the plant is inserted through the central hole of the cork. It is placed in such a way that the root must submerge in water.
5. A few drops of mustard oil are added to the surface of the water to prevent evaporation.
6. The mouth of the glass jar is sealed by placing the plant-containing cork.
7. The arrangement is kept under direct sunlight for some hours.

Eosin Dye Test Observation: The stem of Peperomia sp. is translucent. After a few hours, it is observed that the stem and leaves become reddish and the level of water in the jar has gone down. If transverse sections are cut from the shoot and are observed under a microscope, only the wall of the xylem vessels will show red coloration.

Inference and explanation: The above experiment proves that the roots of plants absorb water and transport it in an upward direction. The absorbed water reaches the leaves through the xylem.

Eosin Dye Test Ringing experiment

Ingredients: A freshly collected Impatiens balsamina plant with intact root system, knife, beaker, water, and clamp.

transport of water class 10

Eosin Dye Test Procedure:

1. A section of the stem of the Impatiens balsamina plant is girdled, i.e., of a strip of bark (consisting of phloem and other tissues but not xylem)
2. The roots of this plant are submerged in water taken in a beaker. The plant is kept erect with a damp.
3. This arrangement is kept under light for 2-3 days.

Eosin Dye Test Observation: After 2-3 days, it is observed that the j leaves of the plant are still fresh and turgid. Leaves are F not showing signs of wilting.

Inference and explanation: It happens due to the upward movement of water and water-soluble minerals from roots to leaves, through xylem vessels. Lateral tissues (tissue present in the removed part) are not involved in the upward transport of water.

Theories On The Ascent Of sap

The water column in the stem of plants can rise up to a height of j 10m (34 ft), under standard atmospheric pressure j (1 atm). However, in some plant groups, most of the I members are more than 10m in height.

For example, the height of Eucalyptus and Sequoia sp., are more than 90m. Sometimes the height can be up to 120m. In such plants, the ascent of sap cannot be explained by root pressure only.

Various theories have been proposed from time to time to explain the mechanism of the ascent of sap in plants.

The two main theories are

1. Root pressure theory and
2. Cohesion-tension or Transpiration pull theory.

Root Pressure Theory

This theory was proposed by Priestley (1916). According to this theory, the absorbed water in roots generates hydrostatic pressure which extends up to passage cells of the endodermis.

Root pressure: When water is transported through the various cellular layers, from root hair to epiblema of roots, the cells become turgid and flaccid, alternatively. The hydrostatic pressure that develops in those cells, forces the sap into the lumen of the xylem. This pressure is known as root pressure. Stephan Hales (1727) first used the term ‘root pressure’.

Experiment to demonstrate root pressure

Root pressure Materials required: A potted tomato plant, knife, rubber band, a glass tube, one manometer, clamp, and standing water.

Root pressure Procedure:

1. A potted tomato plant is taken. It is watered properly,
2. The shoot of the plant is cut transversely at a height of 20 cm from the ground.
3. A glass tube and a manometer are tied up at the cut end with the rubber band.
4. The manometer is kept upright by means of a clamp and stand.

Root pressure Observation: After a few hours, it is observed that the glass tube has been filled with water, and the mercury in the manometer has risen.

Root pressure Inference and explanation: Due to root pressure, water conducted from the root through the shoot, enters the glass tube through the cut shoot end. This pressure, in turn, causes the mercury in the manometer to rise.

Role of root pressure in water transport: Water and minerals absorbed by root hair reach the endodermis. However, this sap cannot penetrate the wall of the xylem vessel by simple endosmosis. This is because most of the cells of the xylem are dead. The sap is actually pumped into the xylem. This happens with the help of the root pressure.

Absorption of water by root hairs causes a decrease in the concentration of cell sap. However, the concentration of cell sap is greater in adjacent parenchyma cells of the living cortex. So, water is transported by cell-to-cell osmosis to parenchyma cells. This increases turgidity in the cells.

Again, when water leaves those turgid cells and moves to adjacent cells, the former cell becomes flaccid. This alternate turgid and flaccid condition the of cells generates a hydrostatic pressure that originates in root hair and extends up to endodermal cells.

However, this hydrostatic pressure is not enough for the movement of water across endodermal cells having Casparian strips. Passage cells in endodermis, remain associated with the xylem vessels. These cells exert a huge pressure (root pressure) on the water stream to push water into the xylem lumen.

” xylem transport in plants”

Root pressure conducts water and minerals to the tracheids and tracheae through these passage cells. Endodermis acts as a water dam. As a result, water cannot revert back once it crosses the endodermal layer. The upward movement of water, through the xylem, is made possible by root pressure.

Guttation: In herbaceous plants (like grasses) and plants with serrated leaves of warmer climates droplets of water have been observed on the tip of the leaves. These small droplets can be seen at regular intervals, late at night, and during early morning in the spring season. Earlier these droplets were well known as condensed atmospheric moisture or dew drops.

However, after experiments, it has been found that these water droplets contain dissolved enzymes, proteins, organic acid minerals, and waste products of plants. Moreover, these droplets can be distinguished by the fact that guttation occurs only at the tips and margins of the leaf lamina but not over the whole surface of a leaf.

Through vigorous further research, it has been found that the margin of the leaves of these plants contains stomata-like openings. These are called hydathodes through which waste substances of plants come out.

This process of eliminating waste as fluid is called exudation or guttation. However, this process of exudation occurs only in herbs, it has never been observed in trees.

Guttation Definition: The process by which herbaceous plants of tropical areas expel water, containing dissolved substances such as enzymes, proteins, organic acids, carbohydrates, minerals, nitrogenous substances etc., through openings at the leaf margins called hydathodes, at night and early morning mainly in the spring season, is called guttation.

Structure of hydathode: Hydathodes are special structures present on leaf margins where veins and venules get terminated. The terminal pore of each hydathode is surrounded by two guard cells. Unlike stomata, guard cells always remain open.

A water cavity is found within the opening of the hydathode. Each hydathode contains a tissue of small, thin-walled clusters of parenchyma cells with extensive intercellular spaces, called epithem. Beneath the epithem, vascular bundle, and specially terminated ends of tracheids are present. On both of the lateral sides of tracheids chloroplast containing parenchyma, i.e., chlorenchyma tissues are present.

2. Conditions and reasons for guttation:

• It generally occurs in the spring season, either at night or in the early morning, in herbs,
• In these plants, the rate of water absorption is more than the rate of transpiration. Thus, excess root pressure is the main cause of exudation,
• Guttation is prevented by water deficiency, water logging conditions, and excessive mineral deficiency in the soil.

3. Process of guttation: Due to excessive root pressure in a vascular bundle, excess water, and minerals are transported through tracheids to the water cavity of hydathodes. At last, these water and minerals ooze out on the leaf surface in the form of droplets.

However, the eliminated water is not pure. It contains dissolved enzymes, proteins, organic acids, minerals, excretory substances, etc. A spot or impression is left by the salt on the leaf surface after the evaporation of these water droplets.

4. Importance of guttation:

1. Some herbs absorb more water than that being lost by of transpiration. So, guttation helps to remove the excess water from the plant body,
2. Guttation also removes excretory substances from the plant body.

Bleeding: In plants growing in well-watered soil, the injured parts exude water with dissolved organic and inorganic substances i.e., sap. This is known as bleeding, Exudation of latex from laticiferous ducts in Euphorbia sp., Morns sp., etc., are examples of bleeding.

Contribution of root pressure: Root pressure provides a gentle push in the process of water transport in plants this helps to re-establish the continuous water column in xylems which often break under the tension created by transpiration.

Contribution of root pressure: Root pressure provides a gentle push in the process of water transport in plants this helps to re-establish the continuous water column in xylems which often break under the tension created by transpiration.

Drawbacks of root pressure theory: Kramer, Unger, Renner, Dixon, and Jolly strongly opposed the root pressure theory. They pointed out some drawbacks in this theory.

Those are—

1. The magnitude of root pressure is very low which may not be applicable for tall trees. Root pressures developed in herbaceous plants are sufficient to move water to the top but not in gymnosperms with lofty habits. So, this theory is not applicable to tall trees.
2. Root pressure is negligible during summer when transpiration is very high.
3. It is very less or absent in gymnosperms.
4. It is absent in plants growing in cold or drought conditions and in poorly aerated soil. But the ascent of sap is still normal in those plants. Hence root pressure cannot explain the ascent of sap.

Cohesion-tension theory or Transpiration pull theory

According to many scientists, living cells have no role in ascent of sap. Rather, some physical processes are responsible for the ascent of sap.

In 1894 Henry H. Dixon, an Irish botanist, and John Jolly, a physicist developed the idea of this specific theory of ascent of sap. This has been the most accepted theory until today.

It states that these two physical forces are responsible for the ascent of sap.

1. Cohesion and adhesion forces, and
2. Transpiration pull

Cohesion and adhesion forces: The force of attraction between two similar molecules is called cohesion. The force of attraction between two different molecules is called adhesion. The water molecules remain bound to each other by force of cohesion.

The water molecules remain attached to the cellulose molecules of the inner surface of the xylem vessel, by force of adhesion. The xylem vessels are tubular structures, which extend from roots to the top of plants.

” transportation of water in plants”

They are placed above one another to form a column. They are connected through their perforated end wall. This helps to create a continuous water column within the vessel.

Transpiration puli: During the daytime, water in the mesophyll cells of leaves is released into the atmosphere through stomata, by the process of transpiration. This creates a water deficit in these cells. As a result, water potential declines, and diffusion pressure increases in cells of that region.

Which establishes a negative gradient of pressure or tension along the path of the ascent of sap. mesophyll cells of leaves and cells of xylem vessels experience a suction pressure that is transmitted all the way down to the unbroken column of water through the xylem vessel up to the roots.

The molecules of water show cohesion and the vessels show adhesion to the water molecules. due to these forces water column does not break and is pulled upwards by the transpiration pull.

Evidence in support of the theory:

The facts which support this theory are—

1. Pressure in the mesophyll cells of leaves is 20 atm. At 1 atm pressure, water can rise in a column of 10m. So in tall plants, the ascent of sap can occur without any difficulty up to a height of 200 meters.
2. The ascent of sap is a physical process. For this, metabolic energy is not required. Even if energy is required, it is almost negligible. 0.5 cal energy is spent to pull 1ml of water to a height of 100m.
3. The magnitude of the force of transpiration pull in the xylem vessel is 25-300 atm; so a continuous column of water forms.
4. This is supported by the fact that, on a warm day, the diameter of the xylem vessel gets reduced. This indicates a presence of strong transpiration pull.

Demonstration of transpiration pull in plants:

The process of transpiration pull occurring in plants can be explained through a simple experiment.

Demonstration Of Transpiration Pull In Plants Ingredients: Two beakers, two glass tubes, one cork with a central hole, porous container, mercury, pure water, sealing wax, glass marker, and a freshly cut, young, leafy twig.

Demonstration Of Transpiration Pull In Plants Procedure:

1. One end of a glass tube is fixed with a holed cork. A freshly cut, young, leafy twig is fixed through the hole in the cork.
2. The gap between the glass tube and the cork and the gap at the hole of the cork is sealed with sealing wax.
3. Another glass tube is fitted with a porous container.
4. Both the tubes are now filled with water.
5. The open end is placed carefully in the beakers containing mercury, by covering the open end with a finger.
6. The initial level of mercury in the glass tube is marked with a glass marker.
7. The setups are kept in direct sunlight for a few hours.

Demonstration Of Transpiration Pull In Plants Observation: After a few hours it will be observed that the levels of mercury in both the glass tubes have risen. The levels are again marked.

Demonstration Of Transpiration Pull In Plants Inference and explanation: In the setup with the plant no evaporation has occurred as it was airtight. But the leafy shoot attached to the glass tube has transpired. Due to transpiration water is lost from the plant body. This has created a tension or transpiration pull in the plant. This has caused the rise of mercury levels through glass tubes.

Similarly, evaporation of water has occurred in the setup without a plant, through the porous container, it has generated a vacuum in the tube. To fill this vacuum mercury has risen through the tube.

Postethwalt and Rogers (1958) used radioactive phosphorus (P32) to prove that even in the presence of gas bubbles in the water column of xylem vessels, the ascent of sap occurs normally. As xylem vessels are lined laterally by other cells, water may get transported through those supporting cells. When transpiration pulls decrease at night, the air bubbles dissolve and the continuous water column is again restored.

Drawbacks of the cohesion tension and transpiration pull theory:

1. In the root pressure theory, it has been already demonstrated that water exudes from a cut stem that has no leaf. This observation contradicts the transpiration pull concept.
2. A fact against this theory is that a temperature variation in day and night can cause the formation of bubbles in the water column in the xylem vessels with larger diameters. In such a case, a continuous water column may not form.