Ascent Of Sap, Chapter Notes, Class 11, Biology PDF Download

Ascent of Sap

INTRODUCTION
Upward conduction of water from roots to leaves through stem against force of gravity is called ascent of sap.
The inorganic substances dissolved in xylem water are called sap.

PROBLEMS RELATED TO ASCENT OF SAP

(A) Path of Ascent of sap (B) Mechanism of Ascent of Sap

(A) Path of Ascent of Sap :

The path of ascent of sap is xylem though exact mechanism is not clear.

Important evidences -
(i) Ringing or Girdling Experiment (ii) Staining Experiment

(i) Ringing or Girdling Experiment :

Malpighi (1671), Stephen Hales (1731) and Hartig (1837) proved by ringing experiment that ascent of sap not occurs through phloem or food conduction occurs by phloem.
When ring or girdle upto 2-3cm of Bark (Secondry Phloem) is removed, the branches remain healthy.
Ringing experiment cannot be performed in monocots due to presence of scattered vascular bundles in stem.

Conclusion - Phloem is food conducting tissue or phloem is not water conducting tissue.

(ii) Staining Experiment–

When a cut shoot of Balsam plant is dipped in solution of Eosine, only xylem tissue is found stained.
Blockage experiment - By Dixon, xylem is blocked by wax.
From above observations following conclusions can be drawn-
Main path for ascent of sap is through xylem cavities.

MECHANISM OF ASCENT OF SAP

Two types of theories have been proposed for the mechanism of ascent of sap.

(i) Vital Force Theory (ii) Physical Force Theory

(i) Vital Force Theory :

According to this theory ascent of sap is due to vital activities of cells or living cells are responsible for ascent of sap. Main Supporters of this theory are -

Westermaier (1884) was of the opinion that xylem parenchyma is responsible for ascent of sap.
Tracheids and vessels only function as water reservoir.
Godlewski (1884) gave clambering force theory or relay pump theory :

(i) According to this theory cells of xylem parenchyma and medullary rays shows rhythmic changes
in osmotic pressure which causes upward movement of water.
(ii) When O.P. value of parenchyma rises, they absorb water from adjoining xylem vessel. Water rises up in parenchyma due to atmospheric pressure.

J.C. Bose - Pulsation theory :

(i) According to which innermost layer of cortex (=just outside the endodermis) is responsible for ascent of sap.
(ii) He concluded that these cells pulsate like heart and push water into the xylem cavity leading to ascent of sap.
(iii) He verified this with the help of electric-probe experiment. When electric probe reached in innermost layer of cortex, vibrations in galvanometer were observed.
(iv) Bose performed his experiment in Desmodium gyrans (a member of Papilionatae).
Strasburger (1891) proved experimentally that living cells are not essential for ascent of sap.
Ascent of sap can occur even after making the live cells dead by poisoning them (using Picric acid, HgCl2 etc.)
Root Pressure Theory (By Priestlye) :

Root pressure is respobsible for ascent of sap.

Objections -
Root pressure is absent in woody plants, as secondry growth occurs in wood plant roots.
When root pressure is high, during night, then ascent of sap is low.

(ii) Physical Force Theories :

Supporters of this theory are of opinion that forces responsible for upward conduction of water are physical in nature and even dead cells can conduct water.

Capillary force theory : -

Proposed by Boehm. Xylem vessels acts as capillaries and water rises up due to capillary action.
 

Jamin’s Theory : -

Also known as chain theory.
According to Jamin, there are alternate regular layers of water and air in vessels.
Ascent of sap occurs due to expansion of air.
 

Imbibition force theory :

Proposed by Unger (1868) and supported by Sachs (1878).
According to this theory imbibition by thick walls of xylem vessels is responsible for ascent of sap.
Sachs considered hydrophilic colloidal substances present in cell wall of vessels to be responsible for this process.
 

Transpiration pull-cohesion force theory :

This theory was proposed by Dixon and Jolly (1894).
Most accepted or universally accepted theory of ascent of sap. According to it three components are involved in ascent of sap.

(a) Cohesion : Mutual attraction between the water molecule is called cohesion force, which helps in forming water coloum in xylem elements.

(b) Adhesion : Attraction between xylem walls and water molecules is called adhesion force,
which helps in maintainance of water coloumn of xylem.

(c) Transpiration Pull : A tension or negative pressure develops in xylem due to rapid transpiration in leaves (because of high DPD), this creates a transpiration pull, which is responsible for the pulling up of water column in xylem. So ascent of sap is constitutive effect of cohesion, adhesion and transpiration pull.

FACTORS AFFECTING ASCENT OF SAP

Amount of water in Soil
Temperature
Atmospheric humidity
Atmospheric pressure
Movement of wind
Number of stomata in leaves
Besides them, all other factors which affects root pressure and transpiration process, also affects ascent of sap directly or indirectly.

FOOD TRANSILOCATION IN PLANTS

Food/organic material conduction in plants mainly occurs by phloem. (Proved by Girdling experiment).
Food conduction occurs in between source and sink. Source is net exporter while sink is net importer.
Generally green photosynthetic plant parts acts as source like leaves while non photosynthetic parts like root, shoot, fruits acts as sink.
Food conduction may be in any required direction or bidirectional unlike the water conduction which is a unidirectional process.
Translocation of food mainly occurs in the form of sucrose or it is non-reducing sugar and chemically inert in it’s pathway of conduction.
Pressure flow/mass flow hypothesis of food/sucrose translocation - Given by E. Munch (1930).
This is the most accepted theory of food conduction in plants.

According to it food translocation occur in between source and sink in order of turgor pressure gradient i.e., high T.P. to low T.P.
Phloem loading/sucrose loading at source - It is an active process occurs with expenditure of ATP and helped by carrier molecules. At source due to phloem loading concentration of sieve cells increase, results in increase in osmotic pressure and water will moves from nearby xylem into sieve cells results in increase in turgor pressure (T.P.) and increase in water potential (W). It establish a higher T.P
at source end in sieve tubes. Sucrose moves from source in sieve tube towards sink from high T.P./ High  to towards the low T.P./low W.
Phloem unloading/sucrose unloading at sink . It is also active process occurs with expenditure of ATP and helped by carrier molecules. At sink sucrose is unloaded results in decrease in osmotic pressure (O.P.), it results in exit of water into near by xylem leads to decrease in Turgor pressure (T.P.) and water potential (W) of phloem. In sink cells the unloaded sucrose is either changed into starch (as starch not change O.P.) or consumed, to maintain low O.P. and continuous unloading .

The translocation of food between source and sink end is passive process as occurs in order of T.P. or W gradient.
So, the process of sucrose loading at source and unloading at sink continues. This turgor pressure difference will maintained and water will continue to move in at soure and out at sink.

This mechanism was experimentally demonstrated by Bimodel exp. of Munch.

According to evidences of modern research phloem conduction is an active process (only phloem loading and unloading) and it required metabolic energy of phloem cells.