Transport of Water

 

Transport of Water ( Ascent Of SAP) :

Sap  is water with dissolved  minerals. The upward movement of water from roots towards the stem and leaves is called ascent of sap. It occurs through  the tracheary elements of xylem. It can be demonstrated by the following  experiment. 








Stain  Test : 

Take a leafy shoot freshly cut under water. Dip the cut end of the shoot in an eosine solution contained in a  beaker. Hold the leafy shoot erect by means of a stand. After some time veins of the leaves will become red. Even stem may look reddish. Cut thin transverse  sections of the stem and leaves. Observe them under the microscope. The walls of tracheids and vessles will be found coloured. 

Ringing Experiment:

Cut two leafy shoots. Keep their  lower ends dipping in water. In one shoot remove 2__ 4 cm long ring of bank. Remove the pith of the stem by means of a needle. Remove the xylem in the second shoot. Allow the apparatus as such for 1_ 2 days. The leaves of the first shoot will remain turgid while those of the second shoot would get wilted. 


 In the first shoot the leaves remain turgid even after 24 hours showing clearly that water continues to rise upwards the leafy shoot despite removal of bark and pith. Removal of bark breaks the continuity of epidermis, cortex, and phloem. The only tissue left intact is xylem, therefore, must be the tissue taking part in the transport of water. This is confirmed  from the wilting of the second shoot in which xylem has been removed. 

Theories of Ascent of Sap: 

Water or sap is lifted from root tip to the shoot tip against the force of gravity, sometimes to height of 100 meters. The rate of translocation can be upto 75cm/minute. Several  theories  have been put forward to explain it. The main theories are vital force, root pressure, cohesion tension and capillarity.

1: Vital Force Theory:  

It was put forward by J.C. Bose. It is called pulsation theory. It postulates that the innermost cortical cells of the root absorb water from the outer side and pump the same into xylem channels. However,  living cells do not seem to be involved in the ascent  of sap as water continues to rise upward even if roots have been cut or the living cells of the stem are killed by poison and heart. 

2: Root Pressure  Theory : 

Root pressure  is a positive pressure  that develops in the xylem  sap of the root. It is a manifestation of active water absorption. Root pressure  is observed in certain seasons, which favour optimum  activity and reduce transpiration. It is maximum during rainy season in the tropical countries and during spring in temperate habitats. Effects of root pressure  are usually  observed  at night and morning when evaporation  is low.  Root pressure is retarded  under conditions of starvation, low temperature, drought and reduced availability  of oxygen.  If the stem of a plant is transversely cut above the soil surface, xylem sap will exude in the form of a drop from the cut surface. It  indicates the presence of a positive pressure  in the xylem which is due to root pressure. There are three view points about the mechanism of root pressure  development:_ 

(a) Osmotic : Tracheary elements of xylem accumulate salts and sugars. High solute concentration causes withdrawal  of water from the surrounding cells as well as from the normal pathway of water absorption.  

(b) Electro _ osmotic : A bioelectric potential  exists between  the xylem channels and surrounding cells which favour the passage of water into them. 

(c) Nonosmotic : The xylem elements produce hormones that cause movement of water towards them.  The living surrounding xylem can actively pump water into them.



Objective  to Root Pressure  Theory : 

(i) It is not found in all plants. 

(ii) It is seen only  during  the most favorable  periods like spring or rainy seasons. In summer when the water requirements are high, the root pressure  is generally absent. 

(iii) It is generally low which is unable to raise the sap to the top of trees. 

(iv) Water continues to rise upwards even in the absence of roots. 

(v) The  rapidly transpiring plants  do not show any root pressure. Rather a negative pressure  is observed  in most of the plants. 

(vi) The amount of  exudation due to root pressure is quite low as compared to the rate of passage through the xylem. 

Now it is believed  that the greatest contribution of root pressure may be to re_ establish  the continuous chains of water molecules in the xylem which often break under enormous  tensions created by transpiration. 



3: Cohesion  Tension Theory (Cohesion and Trans_  piration  Pull Theory): 

It was put forward by Dixon. It is also named after him as Dixon's theory of ascent of sap. Most of the workers believe in this theory. The main  features are: 

(a) Continuous  Water Column: There is a continuous  column of water in tracheary  elements from root through  the stem and into the leaves. The elements  operate separately  but from a continuous system through their unthickened areas. Since there are a large number of tracheary  elements running together, the blockage of a few of them does not cause any breakage  in the continuity  of water column. 


(b) Cohension or Tensile Strength: Water molecules  remain attached to one another by cohension force. It is due to hydrogen bonds formed amongst  adjacent  water molecules. One account of cohension force, the water column can bear a tension or pull of upto 100 atm. This force is also called tensile strength.  Water column does not break its connection from the tracheary  elements because of another force called adhesion force between  their walls and water molecules. Water molecules are  attached  to one another  more than the water molecules in  the  gaseous state. It produces surface tension that accounts for high capillarity through tracheids and vessels. 




(c) Development  of Tension or Transpiration Pull : Intercellular  spaces between  mesophyll cells are always  saturated  with water vapours. They come from the wet walls of mesophyll cells. These spaces are connected to the outside air through stomata. Outside air is seldom saturated with water vapours. It has a lower water potential than the most air present inside the leaf. Water vapour diffuse out of the leaves. The mesophyll cells continue to lose water to the intercellular spaces. They withdraw water from the deeper cells. The deeper cells in turn obtain water from the tracheary elements. The water in the tracheary  elements would, therefore, come under tension. As the tension develops due to transpiration, it is also called transpiration pull. On account of this tension the water column of the plant is pulled up passively  from below to the top plant like a rope. It overcomes 

(i) Gravitational  pull, 

(ii) Resistance  of narrow xylem channels and their end walls. 

(iii) Resistance  of living cells of the root and 

(iv)  Resistance  offered by water coming out of narrow capillary  pores of the soil. 

Evidences : 

(i) The rate of ascent of sap closely  follows the rate of transpiration. 

(ii) Evaporation  of water from a porous pot  can produce  a tension in the water column present in attached  tube. 

(iii) In a branch cut from a rapidly  transpiring  plant, water snaps away from the cut end showing that the water column is under tension. 

(iv) With the help of dendrograph ( an instrument  used to measure  the variations in the diameters of tree trunks without being influenced by temperature  changes) it is found that tree trunks contract during  the daytime and expand during the night. Contraction is caused by narrowing of tracheary elements  when the contained water is under tension. 


(v) The maximum tension observed in water column  is 10_ 20 atm. It is sufficient to pull the water to the top of the tallest trees. 

(vi) The cohesive strength  of xylem  sap ( 45__ 207 atm) is adequate to meet the stress of transpiration pull so that water column does not break. 



Objections 

(i) The gases dissolved  in sap shall form air bubbles under tension and high temperature. Air bubbles  would break the continuity of water column. 

(ii) A tension of upto 100 atm has been reported in the xylem sap while the cohesive force of sap can be as low as 45 atm. 

(iii) Overlapping cuts do not stop ascent of sap though they break the continuity of water column.  

(iv) Dixon believed tracheids to be more efficient  than vessels in ascent of sap. The reverse is true. 

(v) It has been found that water column breaks down frequently  even in herbaceous plants. 


4. Capillarity :

Water rises in the narrow tubes due to the force of surface tension. This phenomenon is called capillarity. It is believed  that uptake of water through  xylem vessels is also possible  in small_ sized plants through capillarity.The rise in water is due to the force of adhesion  and cohension. Force of gravity  also affects water uptake by capillarity. 

According  to this theory, water is first taken in due to the force of adhesion between  water and the walls of thin xylem vessles ( tracheids). As the water flows upward along the wall, strong cohesive forces between water molecules  come into play to pull the water upward. 

Objections to the theory :_ 

(i)  The value of capillarity is very small. Water can only rise to a height of little more than one metre in the normally occurring vessels. 

(ii) The vessels do not have uniformity of the lumen on account of presence  of different types of thickenings. 

(iii) The soil water is not connected  with the xylem vessels  directly.  

(iv) As the water rise due to capillarity will increase with the decrease  in the diameter  of the lumen, it will be imperative for: 

(a) Tall plants to have narrow vessels, while the truth is just the reverse, 

(b) Autumn wood to be more efficient than springwood while the truth is just the reverse. 



  


















































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