Biological Foundations 112, Lecture 11


Water Movement in the Xylem

Chapter 32, pages 699-701


I.   Water Economy in the Plant

     A.   Uses of water in the plant

            1.   Metabolism

            2.   Retained in plant cells

            3.   Transpiration - 98% of all water is used here

     B.   Examples of transpiration

            1.   Definition - the movement of water through the plant from roots to leaves during the
                                    photosynthetic process

            2.   Single corn plant in Kansas used 196 liters of water between May 5 and
                  September 8

            3.   Red maples in a growing season - 3 inches of water

            4.   Alfalfa - 900 units of water/unit of dry matter produced

            5.   Millet - 248 units of water/unit of dry matter produced

     C.   Pathway of transpiration

            1.   Bulk (liquid not vapor or molecular water) occurs in soil and is called soil water

            2.   Root hairs absorb water and pass it through epidermis of root to cortex

            3.   The cortex passes it to the endodermis

            4.   The endodermis controls the solute absorption and passes the water into the xylem   
                  of the root

            5.   The water is carried up the xylem of the stem to the xylem of the leaf

            6.   Xylem of the leaf releases the water to the mesophyll of the leaf

            7.   Water in the cellulose fibrils of the leaf evaporates into the substomatal chambers

            8.   When stomata open, the water vapor inside of the leaf diffuses out into atmosphere

II.  Transport in Xylem (Or how does water get to the top of a 300 foot redwood tree?)

      A.   Direction of movement

             1.   Water moves toward low water potential

             2.   Water moves toward a rising solute concentration

             3.   Water moves toward where water is being removed

      B.   Sinks for water

             1.   In regions of photosynthesis

             2.   Differentiating meristems

             3.   Transpiration

      C.   Cohesion and Tension Theory

             1.   How it works:

                   a.   Matrix potential decreases as the cell walls of the spongy parenchyma begins to
                        dry when the stomata open

                   b.   The water moves from the cell to the cell wall cellulose

                   c.   Water moves from adjacent cells to replace the water in the cells

                   d.   The xylem supplies the water to these cells that are losing water to the cellulose 
                         cell walls in the spongy parenchyma

                   e.   The decrease in water potential from the loss of water in the leaves is  
                         transmitted into the xylem so that the water in the xylem is under tension all
                         the way down to the roots

                   f.    This water potential amounts to about 30 bars of tension depending upon the
                         tree height

                   g.    The taller the tree, the lower the water potential in the leaves needs to be in
                          order to pull the water up to the top of the tree from the roots

             2.   Water pulled (under tension) not pushed

                   a.    Water has enough tensile strength in columns the size of vessels to be lifted to
                          300 meters

                   b.   Gas bubbles destroy tensile strength if they enter the system.  With gas
                         bubbles a column of water can only be raised 9.75 meters at sea level

                   c.   Too much tension causes cavitation (spontaneous bubbles) in the vessels and  
                         tracheids

                   d.   If cavitation occurs in vessels or tracheids

                         (1)  Bubbles are contained within the cell and won't move because the pit pairs 
                               will not let them through

                         (2)  When the extreme tension stops (usually at night) the tension on the water  
                                is released and the bubbles are reabsorbed by the water and the complete
                                water column is restored

      D.   Root Pressure

             1.   Xylem sap is pushed up the stem as a result of osmotic pressure in the xylem cells
                   of the root

             2.   Examples of where it occurs in the plant:

                   a.   Water bleeds out of  newly cut tree stump

                   b.   Water is pushed out the tips of leaves (guttation, see below)

             3.   How it works:

                   a.   The osmotic potential in the root decreases as active transport takes up  
                         minerals from the soil

                   b.   With decreased osmotic potential in the roots the water potential drops and the  
                          plant begins to uptake water by osmosis and push it up the xylem

                   c.   It bleeds out the stump or out the leaf tips as the pressure is transmitted up the    
                         stem

      E.   Guttation

             1.   When root pressure in small plants pushes water out the tips of leaves

             2.   Hydathodes - The special structures (pressure relief valves) that allow the water    
                                           from root pressure to pass through

             3.   Usually happens in the mornings when the humidity is 100% and no transpiration is  
                   occurring

      F.   How roots come in contact with water

            1.   Roots only grow in moist soil, and will not penetrate dry soil

            2.   Important to water plants deeply and widely enough to include a good root ball

            3.   Weeds transpire water out of soil and thus deplete soil moisture

      G.   Too much fertilizer makes the soil hypertonic to the plant and dehydrates it

      H.   Saline water can do the same thing if too strong


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