Ascent of Sap

 The water is absorbed by root hairs of the plant from where it reaches the xylem via cortical cells and passage cell and through the xylem, it reaches the top of the plant where it is transpired by leaves and used for other metabolic activities. Water and minerals are conducted through xylem tissue. While sieving tubes of the phloem vessels are involved in the conduct of sucrose molecules and other assimilates.

The upward movement of water from the stem base to treetop is called Ascent of Sap. It may even cover a height of more than 90 metres against the gravitational pull. Because of the fact that the pressure of one atmosphere can normally lift the water column only by about 10 metres, the rise of water up to such heights (90m) needs some special mechanism.

Theories of Ascent of Sap

Various theories have been proposed to explain the special mechanism (s) involved in the ascent of sap.

These theories can be grouped under three categories as given below:

1. Vital Theories

a. Westermeier (1883) theory: He stated that the tracheids and vessels acted as water reservoirs rather than only as conducting elements.

b. Godlewski (1884) relay pump theory: Ascent of sap takes place due to the pumping activity of the cells of xylem parenchyma which are living.

c. Janse (1887) theory: He supported Godlewski’s theory and showed that if the lower portion of a branch is killed, the leaves above are affected within a few days.

d. Sir. J.C. Bose (1923) Pulsation theory: Ascent of sap takes place due to the pulsatory activity of living cells of the innermost cortical layer just outside the endodermis.

These theories are based on the fact that water movement in the xylem is taking place only with the aid of living cells.

Objections to Vital Theories

Strasburger (1891) and Overton (1911) experimentally demonstrated that the ascent sap can take place even in the case of non-living cells as in the segments of the xylem in which living cells have been killed by heat or poisons.

2. Root Pressure Theory

Although root pressure which is developed in the xylem of the roots can raise the water to a certain height it does not seem to be an effective force in the ascent of sap due to the following points :

i. Magnitude of root pressure is very low (about 2 ATMs).

ii. Ascent of sap continues even in the absence of root pressure.

iii In gymnosperms, root pressure has rarely been observed.

3. Physical Force Theories

Contrary to the above two theories, several scientists also believed that living activities or living cells are not involved in the ascent of sap. It is purely a physical phenomenon. Many theories have been proposed to explain this aspect of ascent sap. Some of them are as follows.

a. Atmospheric Pressure Theory

According to this theory, atmospheric pressure is responsible for the ascent of sap. Water rises upward to fill up the gap in the fall of atmospheric pressure at the transpiring surface due to loss of water during transpiration.

There are two objections to this theory :

i. It can not act on water present in the xylem of roots, and

ii. In case it is working, then also it will not be able to rise water beyond 10 metres.

b. Capillary Force Theory

In plants, the xylem is placed one above the other forming a sort of continuous channel which can be compared with long capillary tubes and it was thought that water rises in the capillary tube due to capillary force; in the same manner, the ascent of sap takes place in the xylem.

This theory also faces the following objections :

i. A free surface in the xylem must be present to maintain capillarity.

ii. Soil water is not directly connected with the vessels, hence this theory cannot be functional in vessel bearing plants.

iii. Capillarity operates easily in plants having narrower vessels but tall plants have rarely such vessels.

iv. Capillary theory cannot operate due to the presence of end walls on the conducting vessels; whereas, in plants where vessels are absent, the tracheids with end walls are present for ascent sap.

v. The magnitude of the capillary force is very low

c. Imbibition Theory

As advanced by Sachs (1878, 1879), this theory assumed that ascent of sap could take place by imbibition through the walls of the xylem. This theory is also disregarded because it is well known that imbibitional force is insignificant in the ascent of sap and the walls do not carry warer; it moved through the lumen of xylem elements and not through walls.

d. Transpiration Pull Theory or Cohesion - Tension Theory

This theory was originally proposed by Dixon and Jolly (1894) and has been supported by Curtis names, Cohesion Hypothesis, Theory of Cohesive Force, Dixon and Jolly’s Theory of Cohesion, or Transpiration

Pull Theory. It is based on the following features :

i. Cohesive and adhesive properties of water molecules to form a continuous water column in the xylem.

ii. Transpiration pull exerted on the water column

It is better explained as follows :

A. What is cohesion?

The attraction between 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 the cohesive force of water has been measured up to 350 atm. and is much in excess of the minimum required for the ascent of sap in the tallest trees.

B. Cohesion-tension Theory

Water forms a continuous column from the base of the plant to its top and remains under cohesive tension due to transpiration pull. And according to the need, water is being pulled up to the top of the tree.

C. Characteristics of cohesion-tension theory

This important and widely accepted theory has the following essential features :

i. Water forms a continuous column from the base of the plant to its top.

ii. Water is lost from the mesophyll cells due to transpiration because of which a pulling force is developed. It puts these cells under tension.

iii The tension may cause a break in the water column but due to the tensile strength or cohesive property of water molecules, the continuous water column is not broken.

iv. The tension or transpiration pull is transmitted to the root region to regulate absorption.

How water is taken up in tall trees?

Because of the enormous atmospheric pressure (350 atmosphere) created due to the cohesive force of water molecules and due to the cohesive tension developed in the water column, water is taken up in tall trees.

Mechanism of Ascent of Sap

The loss of water from the surface of leaf mesophyll cells due to transpiration reduces the amount of water in the cells. It causes an increase in the OP of these cells. Thus, reduced water potential is developed in the mesophyll cells, i.e., the DPD increases. Water from the adjacent cells and ultimately from the conducting tissue is pulled to meet this loss of water and as a result, a pull is developed in the cells of mesophyll and xylem of the leaf. Now, water present in the xylem cells is placed under tension, which is ultimately transmitted to the root through the stem tracheids.

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

Phloem Transport

As described earlier, water is absorbed and translocated through the xylem vessels. Transport of sugars and other substances occurs through the phloem vessels and is called Translocation or Long-distance transport. The translocation of solutes in plants can take place either through the apoplast or symplast. Translocation of water and minerals takes place through the apoplast. The transport through phloem is symplastic.

Apoplastic vs Symplastic Transport

Each plant consists of two anatomical parts: The apoplast and the Symplast. The apoplast is in fact the dead part of the plant. This part is situated outside the plasmalemma barrier and includes a wall of cells. Symplastic parts are delineated by the plasmalemma and are living parts of the plant. They include cytoplasm but without vacuoles. The symplasts are interconnected through cytoplasmic connections called plasmodesmata.

Phloem Transport Mechanism

There are several hypotheses regarding the translocation of solutes in the phloem. Some important theories are described below :

1. Protoplasmic Streaming Hypothesis

This is proposed by Hugo de Vries (1885). Protoplasmic streaming or cyclosis might be a mechanism of solute translocation. The streaming depends upon the contraction of elongated protein molecules and is also linked to the utilization of ATP. It can account for the bi-directional flow of solutes.

2. Contractile Protein Hypothesis

This hypothesis is based on the contractile nature of P-proteins of sieve elements. The contraction and relaxation of these proteins help in translocation.

3. Mass flow Hypothesis

This was elaborated by E. Munch (1930). It is applied to the transport of solutes through phloem only. This is also called Munch’s Hypothesis. The model described in the following

Fig.

- Two vessels (A+B) made up of semi-permeable membrane.

- These are interconnected to each other by capillary tube (d)

- Both vessels have sugar solutions; but B has a higher concentrated & A is less concentrated.

- Vessels (A+B) are marked in two reservoirs of dil.Sol. connected them capillary tube C

- Now, H2O will flow into Vessel B, Therefore diff. in their water potential.

- This creates hydrostatic pressure, which forces sol.

According to the model, the flow is the result of differential pressure between vessels A&B.


Fig.4. The model describing the mass flow hypothesis

The model as applied to the plant system can be simulated as under :

* A is the sink organ (eg. root) that has low sugar concentration, because of its consumption.

* B is the source organ (e.g. a leaf) that has a high sugar concentration since it is produced there.

* D is the channel for solution transport and represents phloem tubes.

* C is the channel for water return and represents xylem vessels.

Now, water will flow into vessel B, because of differences in their water potential. This will create a hydrostatic pressure, Which will force the solution of vessel B to flow through tube D. If water returns from vessel A to B through tube C, the fluid will keep on flowing from B to A. Thus, the flow is the result of differential pressure between vessel A and B.

- from B to A through D.

- water returns from A to B through C, the fluid will keep on flowing from B to A.

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