Water absorption and ascent of sap in plants - class-XI

The water transport in the apoplastic pathway is through the cell membrane and the intercellular spaces. Water and solutes can pass freely through them.

In the Apoplast pathway water cannot enter the protoplasm.

The apoplast pathway refers to the movement of water through the cell walls and intercellular spaces without crossing any plasma membranes.

Apoplast is the system of cell walls which is continuous throughout the plant except in some parts like Casparian strips in the roots. Therefore it cannot be aided by cytoplasmic streaming and cannot occur through plasmodesmata. So, the correct answer is 'apoplast pathway is faster than the symplast pathway'

The symplast pathway involves the movement of water through the cytoplasm of cells, which requires crossing the cell membrane to enter and exit cells.

Most of the water flow in the roots occurs via Apoplast since the cortical cells are loosely packed, and hence offer no resistance to water movement.

Apoplast pathway is faster while symplast pathway is slightly slower. Metabolic state of root does not affect apoplast pathway while metabolic state of root directly affects symplast pathway.

In symplast pathway, water passes from cell to cell through their protoplasm. It does not enter cell vacuoles. The cytoplasm of the adjacent cells are connected through plasmodesmata. Symplastic movement is aided by cytoplasmic streaming of individual cells.

Apoplast refers to the non living component of plant like intercellular spaces and cell wall. Transportation through apoplast is fast. 

DPD or diffusion pressure deficit determines the flow of water. Water moves from low DPD to high DPD. 

The transportation of water and dissolved minerals from the root to the leaves is known as ascent of sap. It is transported through xylem tissues. During transportation, there is large accumulation of mineral ions in the root hair cells by active transport. This results in the higher water concentration in the soil as compared to the cell sap of the root hair. Hence, water diffuses from the soil into the root hair. The root hair cells become turgid. From root hair cells, the water moves into the cortical cells. From the cortical cells, the water reaches the endodermis by osmosis. The endodermis then pushes water into the xylem tubes through passage cells.

The ascent of sap experiment demonstrates the absorption of water by root cells if plants and their upwards movement. The sap is the water and minerals absorbed by the roots and they are pushed upwards into the xylem and then it goes to other regions of the plant.

Cohesion transpiration pull theory was put forward by Dixon and Jolly in 1894. It is also known as Dixon's theory of ascent of sap. During day time, there will be maximum water loss due to transpiration, hence, transpiration pull will also be maximum during day time. Consequently, maximum ascent of sap will occur during day time.

Transpiration causes the development of a pull or a negative pressure region which created a suction force to aid the ascent of sap. It increases the process of ascent of sap.

The root pressure theory considers that hydrostatic pressure developed in the roots is responsible for pushing the water through the column. It depends on the active accumulation of minerals in the root cells which then causes passive absorption of water creating a high pressure.

Organic material in plants is transferred by phloem while water and mineral nutrients are transferred by xylem. It is a well-known fact that xylem only has unidirectional transport of water while phloem can exhibit bidirectional transport.

Ascent of Sap : Ascent of sap in the xylem tissue of plants is the upward movement of water from the root to the crown.

Askenasy is known for experiments regarding the cohesion-tension theory and the ascent of sap, including observations on the effects of cuts.

Ascent of sap theory is based on transpiration pull of a continuous water column in the xylem elements of plants. The continuous water column is generated because of adhesion and cohesion forces, operating between water molecules and xylem elements and between water molecules respectively. The pull is generated passively due to loss of water during transpiration and does not requires input of energy. Hence, there is no requirement of ATP in ascent of sap and it is a physical process only.

The first theories proposed to explain ascent of sap were Vital force theories. These theories believed that living cells are involved in the ascent of sap, e.g., relay pump theory by Godlewski, 1883 and Pulsation theory by Bose, 1923.  

In ringing experiment the bark of a tree is removed at a height above the soil surface as a ring. This leads to accumulation of sugars just above the ring of bark removal because sugars are transported through phloem. The ascent of sap which occurs through xylem continues normally because xylem is not part of the bark and bark consists of secondary phloem along with several other tissues.The experiment clearly demonstrates the function of xylem.

In soil, the minerals can be present dissolved in soil solution or adsorbed on charged clay particles. Plants obtain minerals both from the soil solution (through absorption) as well as directly from clay particles (through ion exchange mechanisms). 

According to the most widely accepted theory, ascent of sap occurs through 'Cohesion Tension' or 'Cohesion Transpiration pull'. The cohesion and adhesion forces give rise to a continuous water column in xylem elements, which is lifted by the transpiration pull. Thus, ascent of sap is purely physical process and does not involves energy input.

Xylem tissue is a complex permanent tissue specialized to transport water in plant body. Xylem tracheids are found in a number of plant groups like angiosperms, gymnosperms and pteridophytes. Bryophytes do not possess vascular tissues. Hence, bryophytes do not attain height. Xylem tracheids are present in pteridophytes and gymnosperms. These plant groups lack vessels. Vessels are characteristics of angiosperms only. In angiosperms, xylem vessels form the main channel for ascent of sap.

Vital force theories were propagated to explain the ascent of sap. These theories believed that living cells are involved in the ascent of sap e.g., relay pump theory by Godlewski, 1883 and pulsation theory by Bose, 1923. Bose believed that the inner most cortical cells pump water into the interior, that contains xylem channels. The vital force theories have been discarded because as proved by Boucherie in 1840 and Strasburger in 1891 that water rises up the stem, even when all the living cells are killed by poison or heat.

Ascent of sap occurs through non living xylem tracheids and vessels. The force responsible for pull of water to great heights is the transpiration pull. The properties of adhesion, cohesion and capillarity helps in ascent of sap through non living tracheids and vessels.

Plants absorb water through the entire surface - roots, stems and leaves. However, mainly water is absorbed by roots. They also absorb minerals like nitrogen, phosphorus, etc., from the soil through their roots. The root hairs which are in close contact with the thin film of water
surrounding the soil particles absorb the mineral salts such as
nitrates, chlorides, sulphates, phosphates, etc., dissolved in this
water. However, the concentration of minerals in the soil is very low.

Water moves from a region of its own high concentration to a region of its own low concentration. If the cell sap is highly concentrated as compared to soil solution- it means water molecules are present in high concentration in the soil solution and in low concentration in the cell sap. In such a situation water molecules will move along their concentration gradient and readily move inside the root hairs.

"Pulsation" or "Vital force" theory was a proposed mechanism for the ascent of sap through the xylem tissue of plants according to which, the conduction of water up the xylem vessel is a result of vital action of the living cells in the xylem tissue.
Jagadish Chandra Bose suggested this mechanism for the ascent of sap in 1927. He found electrical pulsations or oscillations in electric potentials, and came to believe these were coupled with rhythmic movements in the telegraph plant Desmodium. However, this theory has now been rejected by majority of plant physiologists.

Dixon and jolly demonstrated the process of translocation of  water and minerals from base to apex of the plant and is known as the ascent of sap as proposed by Dixon and Jolly. The complex tissue xylem in plants is responsible for translocation water and minerals in plants. In blockage experiment, two small shoots of a woody plant are taken. Their cut ends are so prepared that in one shoot phloem is blocked with wax, while in the other xylem is blocked. Now the cut ends are submerged in water. After some time, the shoot with its xylem blocked shows wilting, but the shoot with its phloem blocked remains unaffected.

The first vital theory to explain ascent of sap was proposed by Godlewski who gave relay pump theory. According to him, there was a rhythmic change in the osmotic pressure of the living cells of xylem such as parenchyma and medullary rays which brought about a pumping action of water in an upward direction. Hence option B is correct.

Most widely accepted theory of ascent of sap is the 'transpiration pull -cohesion-adhesion theory'. According to this theory water rises in plants due to the tension generated by loss of water through transpiration. The transpiration loss of water pulls the continuous water column in xylary elements which is maintained by cohesion and adhesion forces.

The mechanism of the ascent of sap in plants occurs due to the activity of the living cells. The factors responsible are
root pressure-  plant roots absorb the excess of water by an active process and builds up a hydrostatic pressure within the root system, called root pressure.  This activity pushes the water upwards all along the length of the stem.
Transpiration -The pulling force that operates on xylem water columns is called Transpiration Pull or Suction Pressure.
Capillary action along with the root pressure, the ascent of sap is mainly due to passive forces that develop within the plant, due to certain environmental factors that act upon the plant.  tracheids and vessels of the xylem behave as capillary tubes. Ascent of sap is partly due to the capillarity of these tissues. Thus, the correct answer is option D.

When a Balsam plant is provided with a dilute solution of safranine, the same is absorbed by the root system and transported upwards which can be easily traced because of the red color of the safranine that binds and stains the cells responsible for ascent of sap.

Gravitational water percolates the soil layers and reaches the water table and not available by soil for plants. The amount of water retained by soil and available for plants is called field capacity. Thus, the correct option is C.

The classical vital theories attempted to explain mechanism of ascent of sap. The vital force theory was a theory presented by Berzelius. The theory stated that for the ascent of sap vital capacity of a living cell is necessary. Many scientists supported the theory by demonstrating their experiments. Some of such theories are Godlewski's relay pump theory and Bose's pulsatory movement theory. These classical theories have not been accepted much and rejected by the German scientist Wohler because of some reasons. For example, according to these theories ( Godlewski's relay pump theory), the force necessary for the ascent of sap is provided by the living cells of plants. He also assumed that a pumping action was generated due to the rythmic change in the water potential of the living cell to raise water upward against gravity. Further, it was found by Strasberger that translocation of water was continued even after the death of living cells. Many scientists like Dixon, Shull, MacDougal etc. proposed that pulsatory movement and the rate of water translocation is not related to each other. The normal rate of ascent of sap at which sap flows through pulsatory cells was observed 230 to 400 pulsatory cells per second. Whereas, earlier it was observed (by Bose) 14 seconds, for one pulsation. The remarkable difference was observed between them and the theory was rejected. Therefore, the correct option is C.

The capillary force theory was proposed by Christian Wolf. The theory was proposed in the year 1873. Water moves in the upward direction partially due to the capillary of xylem element and partially due to atmospheric pressure. This capillary acts as the tube which is responsible for the movement of water. 

Most acceptable theory for the ascent of sap is the 'Physical force theory', called as 'Cohesion-Transpiration pull theory'. This theory was put forward by Dixon and Joly. According to this theory, the ascent of sap in plants is a physical process, the lifting force for which is provided by the pulling force generated by transpiration. The cohesion forces between the water molecules help in maintaining a continuous water column and the adhesion forces between water molecules and the tracheary elements help in sticking to water to the xylary elements. Both of these forces help in maintaining a continuous water column in the xylary elements.

Girdling removes the phloem, preventing nutrient transport to the roots, which leads to the death of the tree more quickly than the other conditions listed.

The ascent of sap is the mechanism by which water moves from the roots to the top of the trees. Studies on the process of an ascent of sap show that the strong cohesive force between the water molecules (cohesion), the attraction between water molecules and the wall of the xylem elements (adhesion) and the transpiration pull creates a tension on the water column, which ensures continuity of water column in plants. The water lost from the leaves during transpiration creates a high solute potential and reduces water potential. The reduced water potential creates a suction pull and allows ascent of sap. 

The transpiration pull theory is the most accepted theory for the ascent of sap. Dixon and Jolly gave transpiration pull theory. According to this theory, the ascent of sap in plants is a passive physical process, which occurs under the influence of transpiration pull generated due to loss of water from aerial parts. The water lost from the leaves creates a high solute potential and reduces water potential. The reduced water potential creates a suction pull and allows ascent of sap. Other than the pull, a continuous column of water is also essential for the rise of water in xylem elements. The continuous column of water is maintained by cohesion forces between water molecules and adhesive forces between water molecules and vascular elements of the plant.

The movement of water in the plants is known as the ascent of the sap. This upward movement is due to the pulsatory activity of the living cell. This theory was supported by J. C. Bose. he explained that there is rhythmic form of pulse-like movement which helps the water to be transported in the upward direction. 

There are several vital theories which help in understanding the concept of the conduction of water by xylem. Westermaier explained the theory that the upward conduction of water is due to xylem parenchyma cells. The tracheids and the vessels act as water reservoirs. This allows the movement of the water in upward direction. 

Tap root is a type of roots which has a major big central root and small branching. These go very deep in the soil and are very strong.

• Down to upwards movement is known as ascent and Top to downwards movement is known as descend.
• In plants sap is water and minerals.
• Ascent of sap in the xylem tissue of plants is the upward movement of water and minerals from the root to the top. 
  

This experiment was introduced by Malpighi in 1675. It involves the removal of a ring of bark (girdle), that is, all tissues outside the vascular cambium. Thus, the correct answer is (B).

When any material movement is from down to upwards it is called as ascending movement.

Sir J.C. Bose proposed a theory of ascent of sap known as pulsation theory. The pulsation theory states that water is absorbed by the plants due to the metabolic activity of living cells which are in constant state of rythmatic pulsation. But it was not accepted as water can be passively absorbed by the roots even the cells were dead.

Transportation is the process involving the transfer of materials from one place to another place.

According to the vital force theory, ascent of sap is due to active pulsation of the innermost layer of cortex. This theory was given by J.C Bose. The pulsation theory states that water is absorbed by the plants due to the metabolic activity of living cells which are in constant state of rythmatic pulsation. But it was not accepted as water can be passively absorbed by the roots even when the cells were dead.

The rupture or fractionation do not usually occur in the water column in vessels/ tracheids during the ascent of sap because of cohesion and adhesion. This is because continuity of water column is maintained by cohesion and adhesion. The conduction of water from root to aerial parts of plants is called ascent of sap. The water molecules remain attached to each other by cohesion force. The magnitude of this force is very high. This helps in the continuous water column in the xylem. The adhesive force is present between the water molecules and the walls of xylem. This also helps in continuous water column in the xylem.

The tensile strength of water is its ability to resist the effect of external stress and maintain it's flow. The transpiration all pull exerts great stress on the water column but due to its tensile strength, it doesn't break in between.

Godlewski proposed relay pump theory to explain ascent of sap. According to him there was a rhythmic change in the osmotic pressure of the living cells of xylem parenchyma and medullary rays, which brought about a pumping action of water in an upward direction.

The tracheids and vessels acted as water reservoirs. Janse supported Godlewski and showed that if the lower portion of branch was killed, the leaves above were affected.

Capillarity Theory: It is given by Boehm (1809). According to this theory, water is translocated because water molecules adhere to the surfaces of small, or capillary, tubes. This adhesion causes water to somewhat creep upward along the sides of xylem elements. In glass tubes this upward movement is visible as the curved or crescent-shaped (concave) meniscus. However, it was shown that capillarity (or capillary rise) alone in tubes of similar diameter as that of a xylem element raises water less than 1 meter.

Hopkins (1999) explained that the relationship between the rise of water in a capillary tube and the size of the tube is inversely proportional. This means that the thinner is the tube, the higher will be the rise of water.

The passage of absorbed water from root to the aerial parts of the plant is known as the ascent of sap. It occurs through xylem (lumen of tracheary elements or vessels and tracheids) with a rate of 10 to 75 centimetres per minute. This can be demonstrated by ringing experiments (conducted by Malpighi, 1672; Stephen Hales, 1727; and Hartig in 1837) or dipping the cut end of a fresh shoot (e.g., Balsam) in 2 percent eosin solution for about 30 minutes. Then, the veins of the leaves turn red. Sections through stem and leaves indicate that the xylem vessels/tracheids turn red.

Downward translocation of the organic solutes takes place through phloem. This can be proved by the ringing experiment. Ringing experiment was carried by Hartig, Malpighi and Stephen Hales.
They took a branch with leaves immersed in beaker which is filled with water. Now remove all the tissues which are present above the xylem [cortex, cambium and phloem]. Now placed this experiment in a suitable environment for 2 - 3 days. Then they observed that leaves did not wilt. This experiment makes it clear that water moves up only through vessels and tracheids of the xylem. 
Ringing [girdling] experiment has been done only in dicotyledons because their vascular bundles lies in a ring. While vascular bundles are scattered in monocotyledons, so that this experiment can not be possible.

Atmospheric pressure theory was advanced to explain ascent of sap in plants. The protagonists of this theory have assumed that plants are closed systems. When water escapes by transpiration from the surface of the leaves, it is believed that vacuum will be created within the plant body. As the root system is submerged in soil water, with the atmospheric action on the soil water, in order to fill up the vacuum created in the xylem vessels, water just enters passively; thus, the water is translocated upwards. But it was later on concluded that plants are not closed systems but they exhibit openness, for, the gases can diffuse into and out of the plant system with ease and facility. Added to this, atmospheric pressure can support the water to be lifted only to a height of 34 feet; but there are plants which are taller than this and still there is transport of water. Hence it can be concluded that atmospheric pressure could not be the force for ascent of sap.

Imbibitions theory was proposed (1868) and supported by Sachs (1878). According to this theory the ascent of sap occurs due to imbibitonal activity of the cell walls of the xylem elements.

This theory is discarded as it has been experimentally found that water rises up through the lumen of the xylem and not through the walls.

Transpiration is the process of water movement from soil solution into the plants and then to the atmosphere. The direction and intensity of water movement are determined by the difference in the water potential gradient between the two regions. The water potential in the leaf is lower which allows the movement of water in the upward direction. When the water potential is low, the solute potential is high. 

Guttation is the loss of water in liquid form as a dilute solution from the leaf tips. The uninjured leaf tips have hydathodes or water stomata. Under conditions of low transpiration dilute solution of minerals oozes out through them under the influence of increased root pressure. Water potential depends on the number of water molecules in the system. More the number of water molecules, greater will be the water potential. Under the conditions, when there are more water molecules in the system, water potential is expected to be higher. There is more water molecules will be stuffed up inside leaf in the intercellular spaces giving rise to a positive water potential. There is low transpiration and guttation takes place.

A deficiency of 0xygen always causes a corresponding decrease in the rate of mineral absorption. It is probably due to the unavailability of ATP. The increased oxygen tension helps in increased uptake of salts. So the given statement is false.

Cohesion is the mutual attraction between water molecules. Adhesion is the attraction of water molecules to polar surfaces (such as walls of xylem vessels). These both forces together helps in the formation of a thin, unbroken column of water which leads to the mass flow of water in apoplastic pathway.