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Ascent of Sap in plants; Cohesion-Tension Theory; Active Absorption of Water

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The ascent of Sap in plants; Cohesion-Tension Theory; Active Absorption of Water


  • Sap is liquid that circulates in plants.
  • Sap rises from the root of the plant in the form of crude sap, a solution of material that is absorbed from the soil. Passing from cell to cell, mainly by osmosis through cell walls, the sap ascends to the leaves, where chemical changes take place under the influence of light.
  • Chief among these changes are the absorption of carbon dioxide from the atmosphere and the formation of organic compounds.
  • When the sap, now called elaborated sap, descends, the organic compounds serve as a food supply for the plant. The elaborated sap contains sugars, amino acids (the chief components of protein), and hormones.
  • The rate of sap circulation decreases during the winter; an increased flow is one of the first signs of the arrival of spring. The most familiar saps are those of the sugarcane grass and the sugar maple tree, which are used as sweetening agents or confections (sweet food).


  • Sap ascends in response to transpirational pull, and that force originates in the leaf. Transpiration is the consequence of the exchange of gas between leaf and air that is essential for photosynthesis.  As C02 diffuses into the leaf, H2O diffuses out.

Role of Matrix Potential

  • Water evaporates from mesophyll cell walls to replace that lost through the stomates by transpiration.
  • The matrix potential by which each layer of water is held on the cell wall increases on layers of water closer to the wall. If the matrix potential of the cell wall exceeds -1.5 MPa (equivalent to a radius of curvature <0.01 µm), it will exceed ψcell of most crops. In that case, water will be drawn from the vacuole into the cell wall, causing the cell to loose turgor P and perhaps wilt.
  • Species of arid climates compensate by an increase in лcell. This decreases ψcell and thus a more negative cell wall matrix potential must occur before loss of turgor.
  • The matrix potential of the cell wall causes sap to flow in the intercellular spaces, on the surface of cells, and between the fibrils of the cell wall, much like water flows across filter paper. As water evaporates from the surface of the cell into the sub-stomatal cavity, sap flows from the xylem in the leaf vein to replace it, and in response to the matrix potential gradient. Xylem sap is placed under tension as a result.  This tension is relayed (conveyed) down the xylem column to the root.

Design Of Xylem Cells

  • Xylem cells have an almost foolproof (infallible) design, but the specific pattern varies between species adapted to different environments. The water conducting cells are of two major types: vessels and tracheids.
  • Vessels are larger in diameter (80 – 200 µm), and they have end walls that may be perforated, or absent, so that a string of vessels may join in one long tube.
  • Vessels may be from a few cm to several m in length, and they conduct water rapidly and at high velocity. The longest vessels are in ring porous wood and in vines.  Vessels also have pits that permit the lateral movement of sap from one column to another.
  • Trachied are much smaller in diameter, and they do not have end walls, but their walls contain abundant pits. Water passes from one trachea to the next through pits.  The pits are small enough to permit the passage of water, but too small to permit a vapor bubble to pass through.  Surface tension at the vapor-liquid interface of the bubble is too great to permit passage through such a narrow opening.
  • The velocity of sap movement in vessels is from 5 to 40 m hr”1, whereas in tracheids velocity ranges from 0.5 – 1.2 m/ hr1.
  • Vessels are prominent in broadleaf angiosperms. These trees are found in regions with a dependable water supply during the growing season, either from precipitation or from ground water.
  • Their large leaf surfaces capture light for rapid photosynthesis (and growth), but the leaf transpires heavily as a consequence. This requires an efficient water conducting system, and vessels provide it. However, the large diameter of vessels makes them more susceptible to cavitations.
  • Vessels with end walls can confine the cavitations to a single vessel while water moves laterally around it. Nevertheless, the long vessels that transport sap so efficiently are seriously compromised by cavitations because the conduit (channel for liquid) becomes blocked for a substantial distance.
  • Gymnosperms such as conifers do not have vessels. Because their tracheas are quite narrow and without end walls, cavitations is confined to a single trachea.  In addition, the vapor is more likely to be reabsorbed than in large diameter vessels.
  • Conifers are more likely to be able to tolerate high water stress conditions that cause cavitations in angiosperms, but as importantly, they are able to survive the consequences of cold winters that freeze the sap. Gases are not soluble in ice, so any gas dissolved in the sap that is frozen will form a bubble, and cause cavitation. In the spring, bubbles in tracieds dissolve in the thawed (melted), moving sap.  The larger bubbles in vessels may not dissolve, maintaining cavitation, and compromising (adversely affecting) the water conduction capabilities of the vessel.
  • Neither vessels nor tracheids remain efficient water conducting systems indefinitely. In time, cavitation accumulates, xylem elements become plugged with gums and other chemicals, and tyloses (special cells in the wood) grow into xylem cells, plugging them.  It is by this process that the non­-conducting heartwood is formed.
  • Heartwood is not useless; it may store chemicals used in defense or nutrients, and water that may be withdrawn from heartwood in times of stress. However, nearly all ascent of sap takes place in sapwood.  There is an excellent correlation between cross-sectional sapwood area of a stem and dependent characteristics such as total leaf area or weight in the crown.
  • The pits in vessels and tracheids permit water to move laterally in the stem. This can be demonstrated by making crosscuts partially across the stem, and then attaching a cup to the stem using modeling clay below the crosscuts.  A dye, e.g. acid fuscin, can be placed in the cup, and then a whole drilled though the dye into the wood.  The dye will be drawn into the xylem stream in a transpiring plant, marking the path of transport.  Using this technique lateral paths around the crosscuts can be visualized
  • Rapid transpiration can place so much negative tension on xylem cells that they are pulled inward, and when this happens

Xylem Water Potential

  • Water tension in the xylem is measured with a pressure bomb and expressed as ψxylem. There is a relationship between the magnitude of ψXylem developed by a species and its ecological distribution.
  • There is also a relationship between the magnitude ψxylem and the height of the tree. Eg ψxylem measured at 30 m and 79 m up the stem, and throughout the day. These trees had a pre-dawn ψxylem of -0.5 and -1.0 MPa for the lower and upper branches respectively, indicating the requirement for a more negative ψ to compensate for gravity in the taller tree.
  • The ψxylem became progressively more negative during the day, reaching -1.6 and -2.25 MPa respectively by noon.  After noon ψxylem became less negative, indicating water stress caused stomatal closure.
  • The reduction of transpiration caused by stomatal closure with the same rate of water absorption should relieve moisture stress in time. However, photosynthesis is also restricted following stomatal closure. For this reason, some trees may photosynthesize more on cloudy days (reduced moisture stress and open stomata) than on sunny days.

The magnitude of the Drying Power of the Air

  • The ultimate driving force is the sun, which provides the energy required to evaporate sap in the leaf, and to warm the air. Note that at 50% relative humidity the ψair was -94 MPa.  It would require a pressure of l ton/ cm2 on the water in the atmosphere to raise its free energy equal to that of the water vapor at 100% inside the leaf!  This ψ gradient from soil-root-stem-leaf-air is far more than enough to pull water in large volume and velocity to the tops of the tallest trees, providing there is a proper conducting system.
  • The design of the xylem and the adhesive and cohesive properties of water provide that system.
  • There is a special case of the ascent of sap in mangrove. Mangrove must compensate for the salinity of the sea, which bathes its roots, and it does so by increasing лroot and лleaf.


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Naeem Javid Muhammad Hassani is working as Conservator of Forests in Balochistan Forest & Wildlife Department (BFWD). He is the CEO of Tech Urdu ( Forestrypedia (, All Pak Notifications (, Essayspedia, etc & their YouTube Channels). He is an Environmentalist, Blogger, YouTuber, Developer & Vlogger.

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