ASCENT OF SAP
The water is absorbed from the soil by the roots and moved upward to all the parts of the stem through xylem. Inside the plant, the water forms the part of the tissue and many other substances get dissolved in water. Hence, cell sap is a dilute solution of mineral ions absorbed from the soil. The upper movement of sap through stem is called ascent of sap or the process of translocation of sap from the roots to the top of the plant is called ascent of sap. Water is conducted upwards by tracheary elements i.e., tracheids and vessels.
PATH OF ASCENT OF SAP
At first, water is absorbed by root hair cells. This water moves through several root tissues such as cortex, endodermis and pericycle, simultaneously and finally reaches xylem tissue. Once the water enters into the xylem tissue, it continues its upward movement until reaches the mesophyll tissue of the leaves. Water then enters into each mesophyll cell and finally evaporates and transpires through the stomata. Only small amount of water is used only in metabolism and growth.
MECHANISM OF ASCENT OF SAP
Many theories have been proposed to explain the mechanism of ascent of sap. They are as follows:
- Vital theories.
- Root pressure theories.
- Physical theories.
According to vital theories, living cells are responsible for ascent of sap. Westermaier in the year 1883 for the first time suggested that living cells take part in ascent of sap. Since then, many workers have put forth different views regarding the mechanisms taking place in living cells.
- Godlewski’s clambering or relay pump theory (1984): Godlewski suggested that ascent of sap takes place through xylem parenchyma and medullar rays whereas the vessels and tracheids of xylem serve as reservoir. Upward movement of water takes place due to the periodic increase or decrease of permeability of living cells. According to him, parenchymatous cells first draw water from the vessel below it and pump it into other vessel above it. Thus, by alternative passage through living (xylem parenchyma and medullary rays) and non living cells, water raises. This theory is known as relay-pump theory.
The above theory was discarded because of the following reasons:
- Water moves upwards even in the absence of living cells.
- Metabolic inhibitors affecting the living cells do not change the rate of ascent of sap.
- Wood anatomy does not show the arrangement of living and xylem cells as proposed in Godlewski’s theory.
- Sir J.C. Bose’s pulsation theory (1923): Sir J.C. Bose invented an electrical probe by which the activity of cell inside the plant body can be detected. He inserted the needle of an electric probe up to certain distance in the stem, i.e., up to cortex and found pulsating movement in the cortical cells. He believed that when the cells of this zone expanded, they suck water from outer surface and pumped it into the inner cell on contraction. He came to the conclusion that cell associated with the xylem show pumping action and pump it sap into the xylem cells.
This theory was criticized because:
- There was no relation between the pulsatory activity and the rate of translocation.
- The pulsatory movement of sap, according to theory, involves transfer through 200-400 living cells per second. But comparison of the maximum pumping capacity of the most rapidly pulsating rate found by Bose and the actual rate of sap flow under maximal conditions of transpiration has shown that the latter is 8000-30,000 times as rapid as would be possible under the theory proposed by Bose.
- Molisch (1929) took a drug, which increases the cardiac activity of animals and injected into plants. He observed the same rhythmic pulsation as found in Bose’s electric probe experiment. Thus, Molisch supported the Bose’s theory.
ROOT PRESSURE THEORY
If a stem is cut below the first leaf or near its base, xylem sap can be seen to flow out from the cut end. This is called exudation. According to Priestley, exudation was due to hydrostatic pressure developed in the root system. Such root pressure can be shown experimentally. When a well-watered potted plant is cut a few centimeter above the soil surface, some amount of xylem sap exude at the cut surface after a lapse of time with pressure. The pressure of exudation can be demonstrated by placing a graduated glass tube to the cut end of the stem. A column of sap seems to rise in it. This pressure is actually the hydrostatic pressure developed in the root system due to active absorption of water. This pressure is called root pressure. Root pressure, though largely an osmotic phenomenon, is maintained by the activity of the living cells.
Stocking (1956) defined root pressure as the pressure developed in the tracheary element of the xylem as a result of metabolic activity of the root. It is a positive pressure developed in the roots which pump the sap in the xylem ducts.
In most of the plant, the magnitude of root pressure is about 2 atm. In some plants, it is up to 6 atm. which is sufficient to raise water level up to a few feet. Root pressure theory for ascent of sap was discarded due to the following objections:
- Strasburger observed ascent of sap in the plants in which roots were removed.
- In most of the plants, root pressure is about 2 atm. while all plants require much more pressure to raise water to the top which is about 20 atm.
- Rapidly transpiring plants do not have root pressure and guttation.
- Root pressure is not observed in plants growing in cold, draught, and less-aerated soil while ascent of sap is normal.
It may, therefore, be mentioned that when transpiration is poor, the upward movement of water is affected by root pressure. Thus it cannot be considered to be a major process for water conduction, especially in tall trees. Guttation is the best example of root pressure.