Ascent of Sap in Plants

• Ascent of Sap refers to the upward movement of water through the stem to reach the topmost parts of a plant.
• After being absorbed by the roots, water is distributed to all parts of the plant, with the excess being lost through transpiration.
• Ascent of Sap is studied under two categories: Path of Ascent of Sap and Mechanism of Ascent of Sap.

Path of Ascent of Sap

• Ascent of sap happens through the xylem.
• The ascent of sap through xylem can be shown by experiments using a leafy twig of Balsam plant with a semi-transparent stem.
• When the twig is cut under water and placed in a beaker with water and Eosin dye, colored lines move upward in the stem, with xylem elements appearing filled with colored water.
• The leaves above the ringed part of the stem stay fresh and green in the Ringing Experiment, where a ring of bark is removed from a leafy twig placed in a beaker with water.
• Water is continuously supplied to the upper part of the twig through xylem.

Mechanism of Ascent of Sap

• Ascent of sap can be explained easily in small trees and herbaceous plants; in tall trees like Australian Eucalyptus and Sequoias, water has to rise to heights of 300-400 feet therefore ascent of sap becomes a problem.
• The exact mechanism of ascent of sap is not fully understood and has a number of theories to explain it.

Vital Theories

• Vital theories state that the ascent of sap is controlled by vital activities in the stem.
• Godlewski (1884) proposed that the ascent of sap happens because of the pumping action of living Xylem parenchyma cells, where cells change their O.P. to draw and pump water upward, but this was discarded by Strasburger's experiments (1891, 1893).
• Strasburger's experiments demonstrated that ascent of sap continues with uptake of poisons to kill living cells.
• Bose (1923) proposed that upward translocation of water happens due to the pulsatory activity of living cells of the innermost cortical layer outside the endodermis, but it was rejected because many could not repeat the experiment and found no correlation between pulsatory activity and the ascent of sap.

Root Pressure Theory

• Root pressure developed in the xylem of the roots can raise water to a certain height and is not an effective force in ascent of sap because the magnitude of root pressure is very low (about 2 arms).
• Ascent of sap continues with absence of root pressure, and root pressure is rarely observed in gymnosperms.

Physical Force Theories

• Physical forces may be involved in the ascent of sap.
• Atmospheric Pressure cannot act on water present in xylem in roots as it cant raise water beyond 34'.
• Sachs (1878) stated that ascent of sap could happen by imbibition through the walls of xylem, but imbibitional force is insignificant in the ascent of sap because it takes place through the lumen of xylem elements and not through Walls.
• Water rises in xylem in the same manner as it rises in the capillary tube due to capillary force in plants where xylem vessels are placed one above the other, forming a continuous channel.
• A free surface and low magnitude of capillary force is required for capillarity.
• In Spring there is greater need for water because of development of new leaves, the wood consists of broader elements; while in autumn, when ply decreases, the wood consists of narrow elements, which goes against capillarity.
• Vessels are absent in Gymnosperms where xylem elements do not form continuous channels.

Transpiration Pull and Cohesion of Water Theory

• Dixon and Jolly (1894) proposed this theory, further elaborated by Dixon (1914, 1924).
• The widely by supported theory involves cohesive and adhesive properties, forming a continuous water column in the xylem.
• Water molecules remain joined due to the presence of H-bonds between them.
• A strong mutual force of attraction, or Cohesive force, develops between water molecules in water, with magnitude of this force being high (up to 350 atmos.).
• Continuous water column in the xylem cannot be broken due to gravity or obstructions.
• Adhesive properties of water ensure ensure continuity of water column in xylem.
• During transpiration in leaves, water evaporates from intercellular spaces to the outer atmosphere through stomata, with mesophyll cells drawing water from the xylem of the leaf.
• Water is pulled through the xylem of petiole and stem in a continuous, unbroken column, reaching the transpiring surfaces.
• Presence of air bubbles could break the continuity of the water column, but there are said to be no air bubbles, and air bubbles wont break the water column.