Tag: water absorption and ascent of sap in plants

Plants absorb water through the entire surface - roots, stems and leaves. However, mainly water is absorbed by roots. They also absorb minerals like nitrogen, phosphorus, etc., from the soil through their roots. The root hairs which are in close contact with the thin film of water surrounding the soil particles absorb the mineral salts such as nitrates, chlorides, sulphates, phosphates, etc., dissolved in this water. However, the concentration of minerals in the soil is very low.

Water moves from a region of its own high concentration to a region of its own low concentration. If the cell sap is highly concentrated as compared to soil solution- it means water molecules are present in high concentration in the soil solution and in low concentration in the cell sap. In such a situation water molecules will move along their concentration gradient and readily move inside the root hairs.

"Pulsation" or "Vital force" theory was a proposed mechanism for the ascent of sap through the xylem tissue of plants according to which, the conduction of water up the xylem vessel is a result of vital action of the living cells in the xylem tissue.
Jagadish Chandra Bose suggested this mechanism for the ascent of sap in 1927. He found electrical pulsations or oscillations in electric potentials, and came to believe these were coupled with rhythmic movements in the telegraph plant Desmodium. However, this theory has now been rejected by majority of plant physiologists.

Dixon and jolly demonstrated the process of translocation of  water and minerals from base to apex of the plant and is known as the ascent of sap as proposed by Dixon and Jolly. The complex tissue xylem in plants is responsible for translocation water and minerals in plants. In blockage 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. After some time, the shoot with its xylem blocked shows wilting, but the shoot with its phloem blocked remains unaffected.

The first vital theory to explain ascent of sap was proposed by Godlewski who gave relay pump theory. According to him, there was a rhythmic change in the osmotic pressure of the living cells of xylem such as parenchyma and medullary rays which brought about a pumping action of water in an upward direction. Hence option B is correct.

Most widely accepted theory of ascent of sap is the 'transpiration pull -cohesion-adhesion theory'. According to this theory water rises in plants due to the tension generated by loss of water through transpiration. The transpiration loss of water pulls the continuous water column in xylary elements which is maintained by cohesion and adhesion forces.

The mechanism of the ascent of sap in plants occurs due to the activity of the living cells. The factors responsible are
root pressure-  plant roots absorb the excess of water by an active process and builds up a hydrostatic pressure within the root system, called root pressure.  This activity pushes the water upwards all along the length of the stem.
Transpiration -The pulling force that operates on xylem water columns is called Transpiration Pull or Suction Pressure.
Capillary action along with the root pressure, the ascent of sap is mainly due to passive forces that develop within the plant, due to certain environmental factors that act upon the plant.  tracheids and vessels of the xylem behave as capillary tubes. Ascent of sap is partly due to the capillarity of these tissues. Thus, the correct answer is option D.

When a Balsam plant is provided with a dilute solution of safranine, the same is absorbed by the root system and transported upwards which can be easily traced because of the red color of the safranine that binds and stains the cells responsible for ascent of sap.