Chapter-III

Ascent of sap

MD. RESAD MALLIK ASSISTANT PROFESSOR

DEPARTMENT OF AGRICULTURAL BOTANY PATUAKHALI SCIENCE AND TECHNOLOGY UNIVERSITY

DUMKI, PATUAKHALI-8602

Chapter-III: Ascent of sapDefinition:

The upward movement of sap from stem base to tree top is called ascent of sap.

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‘Sap’ indicates the watery solution or suspension of substances like minerals, sugar, organic acids and their salts inside the cell (vacule) ‘Ascent of sap’ refers to the movement of water from stem base to tree top against the gravitational pull.

Xylem vessel is the path of ascent of sap in angiosperm and tracheid is the path of ascent of sap in gymnosperm.

Theories of ascent of sap:

Various theories have been proposed to explain the mechanism involved in the ascent of sap. Some workers considered that living cells are actively involved in pumping water while others have explained the mechanism to be purely a physical one and independent of life activity.

1. Vital theories:

a) Godlewski’s relay pump theory (1887)5

b) Pulsation theory of J.C. Bose (1923)

2. Root pressure theory.

3. Physical force theories:

a) Capillarity theory.

b) Imbibitional theory.

c) Atmospheric pressure theory.

d) ‘Cohesion-tension’ or ‘transpiration pull’ theory.

2. Root pressure theory:

If a plant stem is cut a few inches above from its base with a sharp knife, the xylem sap is seen flowing out through the cut end, This phenomenon is called “exudation or bleeding”.

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Priestly explained firstly that the process of upward flow of water (exudation) is due to a hydrostatic pressure developed in the root system. He said that “a sort of hydrostatic pressure develops in the roots due to accumulation of absorbed water”. Its another name is root pressure.

Stephan Hales (1727) proposed this pressure as root pressure. He observed that water rises in 8 mm dia tube to a height of 6.3 meters (in several days) when a cut stump of vine system is connected (Fig: 4.5).

He proposed that the root pressure develops largely due to osmosis because it is not observed when roots are watered with a solution of isotonic or hypertonic concentration solutions.

Further studies was conducted about root pressure. It is revealed/proved that it is affected by poisons and oxygen supply. These factors affect the release of energy during respiration of living cells. But it does not affect the semipermeability of protoplasm. It is proved that there is involvement of living cells. It is also proved that root pressure is occurred by living cells of roots. It is believed that the Casparian’s strips of endodermal cells of the root help to prevent downward leakage of water and the living cells act as a semipermeable membrane to develop an osmotic pressure within the dead xylem vesssels.

Stocking (1956) considered root pressure as an active process. He defined it as “a pressure developing in the tracheary elements of the xylem as a result of metablic activities of the roots. i.e. root respiration.

Some scientists believe that root pressure is responsible to drive water to any height of the plant.

Objection to root pressure theory:

Knamer Unger, Renner, Dinon and Jolly, Steward etc. have objected very strongly the involvement of root pressure in the ascent of sap.

Knamer, Unger, Renner, Dinon and Jolly, Steward etc. have objected very strongly the involvement of root pressure in the ascent of sap.

Some points against this theory are enlisted here:

01. Root pressure has not been observed in all plants. No or little pressure is found in the members of gymnosperms and the tallest trees of the world.

02. Water continues to rise upward even in absence of roots.

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03. In actively transpiring plants, root pressure is not observed. Here, transpiration pull’ is found.

04. Root pressure is commonly observed during favourable periods of growth. When transpiration rate is high, root pressure is absent.

05.The amount of water exuded/The upward flow of water by the root pressure is very low. It constitutes about 5 % of the total water lost during excessive transpiration.

06. The rate of rise of water by root pressure is too slow to explain the actual rate of ascent of water in plant.

07. In plants, root pressure is always below 2 atmospheres. Therefore, due to root pressure water can rise up to 21 meters if all favorable conditions are available. Some plants are enormously taller (up to 120 meters or more) in which case root pressure theory cannot be applied.

However, some suggestions have been offered that root pressure plays an important role in trees by refilling xylem vessels with water during rapid transpiration and interrupted water column.

03.d) Cohesion Tension or Transpiration Pull theory:

This theory was proposed by Dixon and Jolly (1894). This theory has now been widely supported by many workers.

Transpiration pull: When transpiration takes place in the leaves at the upper parts of plant, water evaporates from the intercellular spaces of the leaves to the outer atmosphere through the

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stomata. More water is released into the intercellular spaces from the mesophyll cells. These cells draw water from the xylem of the leaf. As a result, a pull/tension is created in the xylem elements of the leaves. This tension is transmitted downward to water in xylem elements of the roots through the xylem of petiole and stem. Then Water is pulled upward in the form of continuous unbroken water column to the top of the plants.

What is cohesion?

Attraction between the similar molecules is called cohesion. The water molecules have strong mutual attraction (cohesion) due to which they cannot be easily separated from one another. The magnitude of cohesive force of water, which has been measured up to 350 atms. It is minimum required for the tallest trees to move ascent of sap /water from downward (Root) to the upward part (Shoot….).

Mechanism of ascent of sap:

Due to transpiration, the water is lost from the surface of leaf mesophyll cells. It reduces the water amount and causes an increase in the osmotic pressure (OP) of these cells.

Thus a reduced water potential is developed in mesophyll cells. i.e. DPD increase. Water from the adjacent cells and ultimately from conducting tissue (xylem) is pulled to meet this loss of water. As a result a pull is developed in mesophyll cells and xylem cells of the leaf. Now water present in the xylem cells is placed under tension which is ultimately transmitted to the root through the tracheids/ xylem vessel.

This downward transmission of tension is because of cohesive properties/ cohesion of continuous water column in the vessels and tracheids form leaves to roots through stem. The water column moves upward by mass flow due to ‘transpiration pull’. Simultaneously The process of ascent of sap is accomplished / completed simultaneously

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Features of transpiration pull theory:

01. Water forms a continuous column in the xylem from base to top.

02. Water is lost from leaf due to transpiration.

03. A pulling force/pull/suction develops in the mesophyll cell and keeps water under tension.

04. Water column is not broken due to cohesive property/cohesion.

05. The tension or transpiration pull is transmitted to root.

Evidences:

01. Osmotic pressure of leaf cell is 20 atmosphere which can lift H20 from base to upper part of the plant.

02. Physical process does not need energy even if it requires it’s negligible.

03. Cohesive force of xylem sap is measured as 25-30 atmosphere, enough to maintain the water column.

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