The below mentioned experiment and theories demonstrate the ascent of SAP in plants.
The process by which water and minerals translocate from base to apex of the plant is known as ascent of sap. It takes place through complex tissue xylem.
Experiment to Show that Xylem is the Path of Water Conduction:
That most of the water is trans-located in the xylem can easily be demonstrated by a simple experiment. In this 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 (see fig. 3.1). After some time the shoot with its xylem blocked shows wilting, but the shoot with its phloem blocked remains unaffected.
Theories to Account for the Mechanism of Ascent of SAP:
More recently three theories have been advanced to explain the rise of water in plants. These are:
(1) Vital force theories,
(2) Root-pressure theory and
(3) Physical Forces Theory: Cohesion of water and transpiration pull theory
(1) Vital Force Theories:
According to these theories the forces required for ascent of sap are generated in living cells of plants.
Some of the important vital force theories are as follows:
1. Westermaeir (1883):
Westermaeir considered that only cells of xylem parenchyma provide the force for ascent of sap.
2. Relay Pump theory (Clambering theory):
According to Godlewski (1884) ascent of sap takes place due to rhythmic change in the osmotic pressure of living cells of xylem parenchyma and medullary ray. These are responsible for bringing about pumping action of water in upward direction.
3. Pulsation Theory:
J.C. Bose (1923) was of the opinion that living cells of the innermost layer of cortex, just outside the endodermis, are in rhythmic pulsation. Such pulsations are responsible for pumping the water in upward direction.
All the theories of vital force concept are not accepted because of the following reasons:
1. Strasburger (1891) and Overton (1911) used poison like picric acid and excessive heat to kill the living cells of plant. When such twigs were dipped in water, ascent of sap took place indicating that mechanisms other than the vital forces were operative.
2. Pumping action of living cells should be in between two xylem elements and not on lateral sides.
(2) Root-Pressure Theory:
It was proposed by Prisetley & Supported by Stephan Hales.
(i) Root pressure is the water pressure developed in the xylem of the roots when the rate of active absorption of water is greater than the rate of transpiration.
(ii) In root cells, the xylem sap is hypertonic to the cell saps of other root cells. This is because, the salt ions absorbed by the roots from the soil solution are transported and loaded in the xylem vessels. As a result, xylem vessels develop low water potential as well as low osmotic potential.
On the other hand, soil solution is hypotonic and has higher water potential as well as higher osmotic potential. Therefore, water flows from the region of higher water potential (i.e. soil solution) to the region of lower water potential (i.e. xylem sap). This is one of the conditions which, when coupled with low transpiration helps in development of root pressure. Root pressure is a positive pressure developed in root xylem.
Objections:
(1) Root pressure is absent in woody plants, as secondary growth occurs is wood plant roots.
(2) When root pressure is high, during night, then ascent of sap is low.
Especially during the spring season root pressure does play a part in the ascent of sap in some plants. If the leafy top of a plant is cut off, the stump frequently bleeds. The exudation is sap. The pressure which causes exudation originates from osmotic forces in the roots and hence is called root pressure.
As roots absorb water from the soil, the water is forced up the stem. This can be easily demonstrated by a simple experiment (see Fig. 3.2). Attach fine glass tube to the stump of actively growing plant. As water is forced up through the stem, the water column in glass tube rises, indicating that root pressure is operating.
(3) Physical Theory (dead cell has imp. role in Ascent of Sap):
A. Sachs imbibition theory:
(i) According to this theory imbibition by thick walls of xylem vessels is responsible for ascent of sap.
(ii) Sachs considered hydrophilic colloidal substances present in cell wall of vessels to be responsible for this process.
B. Jamins chain theory:
(i) According to Jamin, there are alternate regular layers of water and air in vessels,
(ii) Ascent of sap occurs due to expansion of air.
C. Boehms capillary theory:
Xylem vessels acts as capillaries and water rises up due to capillary action.
D. Cohesion tension theory:
Cohesion-tension or Cohesion & adhesion Theory or Transpiration pull theory (Dixon’s and Jolly Theory, 1894). The water deficit, D.P.D. or the suction force developed in the leaf cells due to transpiration is described as transpiration pull. This is because, this suction force is strong enough to work against gravity and pull the water column in the xylem to the top of any group of plant.