Ascent of sap [ Upward movement of water and other components from roots to aerial parts of the plant ]

Ascent of water and minerals…. Upward movement of water and minerals is called as ascent of sap or translocation of sap.

Structures involved in the process of ascent of sap … Water mainly absorbed by root hairs As xylem elements are present all over the length that’s why xylem is mainly responsible for ascent of sap

STRUCTURAL ORGANISATION OF XYLEM ELEMENTS … Xylem cells Xylem vessels Xylem tracheids Pits

Tracheids have large septal perforations that provide continuous system and do not afford much resistance for the smooth flow of water. The xylem parenchyma provides access for the lateral movement of water towards cortical cells. It contains many inorganic and organic components absorbed by the root system.  So the trachea and tracheids with their large lumen and end to end association act as excellent pipelines for the movement of water, minerals .

RATE OF MOVEMENT Using dyes and radioactive isotopes, it has been determined that the average rate of movement of water in xylem is 60-75 cms/minute.  This is under normal transpiring conditions.  However, this rate is not constant and it varies depending upon the environmental conditions, particularly Relative Humidity (RH) of the atmosphere.

DEVELOPMENT OF TRANSPIRATION PULL…… Mesophyll cells (transpiration occurs Increase in DPD Absorption of H2O from near by cells Leads to increase in DPD from cell to cell Leads to pulling of water through xylem vessels of leaf Xylem midrib xylem of root Continuous intake of water from root hairs

Theories for transport of water…. vital theories Root pressure theory Physical theories Vital theories are stated here .. Godlewski’s relay pump theory Pulsation theory Root pressure theory

Godlewski relay pump theory Due to rythmatic change in OP of xylem elements Living cells absorb water by osmosis And finally water is pumped into xylem vessel due to lowering of pressure in living cells Thus staircase type of movement occurs

Pulsation theory { sir J.C. Bose } Living cells of innermost layer of cortex had rhythmic pulsations. These pulsations are responsible for the upward movement of water. Fine needle attached to galvanometer was inserted in plant stem Galvanometer shows slow oscillations but when it touched cortex cells it turned violent indicating that the cells were pulsating.

Root pressure theory .. When watered plant is cut near base xylem sap flow out at the end with a pressure. Mainly observed in herbaceous plants Water oozes out due to bleeding This is due to hydrostatic pressure inside root c/a root pressure Root pressure appear due to osmotic pressure. If root is supplied with isotonic or hypertonic solution root pressure disappears.

Physical theories …. Capillarity :water rises in narrow tubes due to the force of surface tension It stated that it is the capillary action of narrow xylem tubes and it is further supported by atmospheric pressure 2) Imbibitional theory : ascent of sap takes place due to imbibitional forces through walls of xylem cells. Magnitude of imbibitional force may be up to 1000 atm, But discarded due to fact that water moves in lumen of vessels rather than through the walls.

COHESION TENSION THEORY……. Theory was proposed by Dixon and Jolley, but the theory has been expanded . This theory is also called as Suction force theory.

COHESIVE FORCES When fine water columns are subjected to a transpiration pull as great as 1000 bars or more, the water column is likely to snap, but it does not because inter molecular forces that hold the water molecules together are greater than the opposing forces, .

Such forces are called cohesive forces; they are nothing but hydrogen bonds between the water molecules.  The overall strength of water column in such narrow xylem elements has been estimated to be many folds higher than the transpiration pull and the gravitational pull put together

Tension : Transpirational pull develops a negative pressure or tension in xylem sap which is transferred to the roots. DPD of water in mesophyll cells increases. So water is absorbed by cell wall from protoplasm DPD of xylem cells increases. When transpiration is rapid but absorption is slow, water in xylem cells is passed into the state of tension.

Explaination : Leaves loose water by stomata by the process of transpiration Water transpired through the mesophyll cell spaces also DPD increases and water potential decreases in mesophyll cells. Due to this water moves along concentration gradient. Now the water is continuously pulled by cohesive and adhesive forces This upward force is called as transpirational pull.

Flow chart showing cohesion tension theory of Dixon Strong force of attraction in water molecules {cohesion} Tensile strength of water + adhesive force of water Water molecules remain adhere to xylem vessel walls

EVIDENCE IN SUPPORT OF COHESION ... 1.           Transpiration pull developed in the aerial regions in the air is more than 1000 bars.  2.         Cohesive force that holds the water molecules in a column as narrow as xylem vessels is very strong and they withstand the opposing transpiration and gravitational pulls.  3.         Because of the opposing forces, tension develops in the water column.  As a result, xylem elements become slightly narrow.  This results in the contraction and expansion of the stem which has been demonstrated by using dendrometer.  The diurnal behavior of rhythmic contraction and expansion is a good evidence for the water column to be in tension when the transpiration is rapid or not.

  4.     The forces that operate cohesion transpiration pull are just passive forces and no metabolic energy is involved in this phenomenon, because present the movement of water upwards.

Guttation : While transpiration loss of water in the form of liquid is c/a guttation Such plants have pore in the epidermis near xylem elements c/a hydathode. Thus rate of expiration increases.