ASCENT OF SAP
to The sap (i.e., water with dissolved minerals) is absorbed mainly by roots and is moved upward to all the parts of plants via stem. The upward movement of water through stem is called ascent of sap. It occurs mainly through xylem . The following ex- periment demonstrates that upward movement of water occurs through xylem tracheids and vessels :
Expt. To demonstrate that water moves through xylem vessels or tracheids (Stain method).
Requirements.
A small potted plant of tomato or balsam, eosine solution, beaker, stand, micro- scope, slide, razor, water.
Method.
Dig out a small potted plant of tomato or balsam. Cut the stem at the base only 1 or 2 cm. above the roots under water. Immerse the cut end in eosine solution contained in a beaker and fix the shoot erect with the help of a stand. Keep the shoot in this solution for a day or so. Then cut the transverse sections of stem and leaves and examine them under the microscope.
Observations and Conclusions.
The sections show that xylem vessels and tracheids turn red in- dicating that water moves upward through the xylem elements.
Theories of Ascent of Sap
align Many theories have been put forward to ex- plain the upward movement of water. The important ones are:
- Root pressure theory 2. Vital theories
- Physical theories.
caused due to diffusion pressure gradient and is maintained by the activity of living cells. The root pressure is, therefore, referred to as an active process,
which is confirmed by the following –
(i) The pressure is not observed if the roots are placed in hypertonic or isotonic solutions;
(ii) Oxygen supply and some poisons also affect the root pressure without affecting semi permeability of protoplasm; and
(iii) Living roots are essential for it to occur.
It is believed that root pressure is largely responsible for ascent of sap in herbaceous plants. The pressure develops more in certain seasons (i.e., rainy season in tropical and spring season in temperate habitats) which favour optimum metabolic activity and reduce transpiration. The magnitude of root pressure is about 2 bars or atmospheres (sometimes, it has been observed upto 6 atm.).
Expt. To demonstrate root pressure in plants
Requirements
. Potted plant of tomato or bal- sam, knife, rubber tubing, a narrow glass tube, stand, water.
Method.
Take a potted plant of tomato or balsam and water it thoroughly. After the excess of water has drained off, cut off the top of plant only a few centimetres above the soil surface. Connect the stump to a narrow glass tube by means of a rubber tubing. Seal the joints with grafting wax. Pour a little water into the tube and mark its level. A small oil drop can be placed above the water surface in the tube to avoid surface evaporation. Keep the apparatus for some time and observe the change in the level of water in the tube Alla
Observation and Conclusions.
The water level starts rising in the glass tube. The rise of water indicates the root pressure. Explanation.
Continuous absorption of water by root hairs and its transportation to the xylem via various tissues like cortex, endodermis and pericycle causes hydrostatic pressure to develop in the xylem. This pressure of water is known as root pressure which is responsible to raise water upto a certain height.
The root-pressure can be measured by fitting a manometer instead of vertical tube.
Objections to Root Pressure Theory
(a) Strasburger observed ascent of sap plant in which roots were removed.
(b) In most of plants root pressure is about 2 in theatm. while tall plants require much more pressureto raise water to the tops.
(c) Exuded sap of detopped plant is very less as compared to the absorbed water of normal transpiring plant.
(d) Absorption of water is less in detopped plant as compared to intact plant.brm
(e) Root pressure is not observed in plants growing in cold, drought or less aerated soil, while ascent of sap is normal
.2. Vital Theories
These theories have been proposed by God- lewski (1884), Bose (1923), Molish (1929) and others which believe that vital forces of living cells are responsible for ascent of sap. Godlewski (1884) proposed ‘relay pump theory’, according to which the living cells of xylem parenchyma show rhythmic change in the osmotic pressure which causes up- ward movement of water. Sir J.C. Bose (1923) proposed the ‘pulsation theory’. According to this theory, cells outside the endodermis show simultaneous expansion and contraction. They show pumping action and pump their sap into the xylem cells. However, vital theories do not hold much support
3. Physical Theories
All those theories which consider dead cells of xylem to be responsible for ascent of sap are grouped under physical theories. These include imbibition theory, capillary theory, atmospheric pressure theory, transpiration pull and cohesion of water theory, etc. However, the cohesion-tension theory holds much support
Transpiration Pull and CohesionTot Water Theory (Ginbesion-tension theory).
Cohesion-tension theory was originally proposed by Dixon and Joly in 1894 and further improved by Dixon in 1914. The theory is based onthe following features
-(A) Cohesive and adhesive properties of water molecules to form an unbroken continuous water column in the xylem.
(B) Transpiration pull or tension exerted on water column.
Xylem vessels are tubular structures extend- ing from roots to the top of the plants. Cells are splaced one above the other, with their end walls perforated forming a continuous tube. These are csupported by xylem tracheids which are characsterized by having pores in their walls. One end of xylem tube is connected with the root hairs via pericycle, endodermis and cortex and the other end connected with the substomatal cavity in the leaves via mesophyll cells This tube is filled with water.
The water is filled inside the xylem capillaries and due to cohesion (a force holding a solid or liquid together owing to attraction between the molecules) and adhesion (sticking to a surface) properties of water, it forms a continuous water column. The water column cannot be broken or pulled away from the xylem walls because of the Leohesion and adhesion of water. The water column is subjected to various forces which try to break it.
(These forces are,
(a) weight of the column itself and
(b) the resistance put to it during translocation. Since the magnitude of the cohesive force is much high (upto 350 atm.) the column is not broken by (other forces.
An important factor which can discontinue -the water column is the introduction of air bubbles vin the xylem. Copeland (1902) believe that air bub- rbles enter into the xylem which break the tensile strength of water column, but Scholander et al., (1957) have shown that the air does not block the entire conducting system. Even if air bubbles were introduced, the individual water columns were un-broken and continuous with each other both in the Vertical and lateral directions through the pits present in cell walls
It is clear from the above discussion that water column is just like a steel rope which is extended
from substomatal cavities in leaves to the roots. If this rope is pulled from the top, the entire rope will move upward. In plants, the pull is generated by the process of transpiration which is know as transpiration-pull.
The water vapours evaporate from mesophyll cells to the intercellular space, as a result of active transpiration. The water vapours are transpired through the stomatal pores
.Loss of water from mesophyll cells causes a decrease in the water potentials (i.e., the cell sap of mesophyll cells becomes more concentrated and consequently their DPD is increased). The water moves from cell to cell along the water potential gradient (i.e., from higher water potential level to lower water potential level). Finally the movement of water within the leaf is transmitted to the water filled in the xylem elements. This exerts a pull and the water, filled in the xylem, comes in a state of tension. This pull is called transpiration pull. This tension, generated at the top of the unbroken water column, is transmitted downwards from petiole, stem and finally reaches to the roots. This tension or pull results upward movement of water
Evidence in Support of Cohesion-tension Theory
(i) Scholander provided evidences in favour of continuous freely mobile sap column and absence of metabolic pump.
(ii) All the forces combined together have been found to be 50 atm. in the tallest tree which creates obstacle, but the cohesive force of water is upto 350 atm. which prevents the breaking of the column.
iii) A leafy twig cut under water and the cut end of the twig sealed to the top of a mercury manometer has been shown to pull the mercury above barometric level (Thut, 1932).(iv) If water is under tension, the strain in the vessels should cause their diameters to decrease. A decrease in diameter of vessels in the stems have been observed when the transpiration is high.
Criticism
Cohesion theory assumes tracheids to be more efficient than vessels. If it is so, why the dominant flora adopted vessels in the place of tracheids?
Expt. To demonstrate pull or tension (water-lifting power) due to transpiration
Requirements.
A long narrow glass tube, beaker, stand, rubber tubing, oil cloth, a plant twigcut under water, mercury, water, vaseline.
Method
Take a narrow glass tube and slip a short rubber tube on one end of it. Close the lower end of tube with a finger and fill it with the air-free water. Take a plant twig, cut under water, and fix it to the upper end of glass tube through the rubber tube (It is easier if complete operation is done under water). Make the joints air tight. Dip the lower end of glass tube in the mercury contained in a beaker. Clamp the tube to a stand in an upright position. Keep the apparatus in light and observe
Observation and Conclusions.
The mercury rises in the vertical tube. This demonstrates that loss of water from the leaves (transpiration) exerts a suction which results ascent of sap. This suction is called pull or tension.
