Biology 1102
Dr. Neufeld's Section
T, Th 9:30 am - 10:45 am
Room 213

Lecture 9 Notes
Plant Water Relations

I. Theories of water movement in plants
    A. How does water get up tall plants?  Here are several potential theories.  Only one
            is correct.
        1. Pushed - the root pressure hypothesis
        2. Capillary Action - it wicks up the plant
        3. Vapor Movement - it vaporizes up the xylem
        4. Cohesion-Tension Theory of Sap Ascent - plants "pull" water up
    B. Root Pressure - what is it and can it push water to the tops of tall plants?
        1. Remember the structure of a root?  See Lecture 8.  The endodermis has that
            suberized layer (Casparian Strip).  Water and the dissolved nutrients contained
            therein must pass through the living cells in the endodermis to get to the stele.
            At night, when there is no transpiration, the membranes on these cells are still
            transporting nutrients, such as K+, and NO3- into the stele.  These act as
           osmotica, and can cause osmosis to begin.  This results in water entering the
            stele, even at night.  Since the plant is not transpiring, the pressure builds up in
            the root.
                The pressure that builds up is known as root pressure.  If you decapitate a
            plant, then the root pressure will force water out of the cut stem.  Stephen
            Hales did this experiment with grape back in the 1700's, and showed that
            there was enough pressure to push water to a height of 27'.  It would have
            gone even higher had the "tube not leaked".  It takes a pressure of about
            1.5 psi to push water up about 3'.  So this means that the plant generated a
            root pressure of about 13.5 psi, or just below one atmosphere of pressure.
                Some plants can be 100 m tall or more (say 300' or more).  That would
            require a pressure of 150 psi, or 10 or more bars (1 bar is 14.7 psi).  But no
            has ever measured much more than 5-6 bars (75-90 psi) root pressure.  In
            addition, root pressure is too slow to provide trees with the amount of water
            that we know they transpire.  SO - root pressure can't be the way that water
            gets to the tops of tall trees.
       2. Capillary Action - ever dip a corner of a napkin in water and then watched how
            the water wicks its way up the napkin?  This is capillary action at work.  Water
            is strongly attracted to other objects because of charges on the water.  This
            is called adhesion.  Since the walls of xylem are cellulose, it is plausible to
            suggest that perhaps water wicks its way up trees.  However, some physics
            and math quickly show that (1) it's too slow, and (2) gravity would stop the
            upward movement after only about 1 meter (3') of rise.  SO - capillary action
            can not explain how trees get water to their tops.
        3. Vapor Diffusion - what if the xylem in trees didn't contain liquid water, but
            instead, was composed of vapor?  Then, maybe the vapor could diffuse to the
            tops of tall trees.  But anatomical studies show that the xylem is indeed filled
            with liquid water, and besides, diffusion is WAY TOO SLOW.  SO - vapor
            diffusion is not the answer.
        4. Cohesion-Tension Theory - Well, that leaves just this theory.  How does this
            theory work?  We know that in an ordinary pipe, like that attached to a well
            pump, you can only "pull" water to a height of 33' (10 m) with a vacuum.  After
            that, gravity pulls on the water column and it breaks under it's own weight.  So
            if this theory is to work, we have to explain how a tree can violate this
            property.
                First off, trees don't have large diameter tubes inside.  Instead, they have
            tracheids and vessel elements.  The diameters of these cells range from 20 um
            to nearly 500 um, depending on species.  Studies with water in capillary tubes
            show that water in small diameter tubes can withstand tensions of up to 300
            bars (or 4500 psi tension!!).  Tension is simply negative pressure.  They found
            this out by putting water in small diameter tubes, bent at the ends, and then
            spun in a centrifuge.  By knowing how fast the centrifuge spun, and lengths of
            the tubes, scientists could calculate the tension on the water inside. So, water
            in small diameter tubes does not break.  When it does, that is called a
           cavitation event, and the result is an embolism.
                When water transpires from the leaf, water molecules leave the cell walls
            of the cells below the stomata.  They are replaced by other nearby water
            molecules through the process known as cohesion.  Cohesion is the tendency of
            like molecules to attract each other.  Because of charges, water is attracted
            to other water molecules.  When a water molecule replaces one that is lost via
            transpiration, another takes its place.  Then, another water replaces that one,
            and so on and son on down the line all the way to the root.  This causes the
            entire water column in the plant from root to leaf to move up.  All by
            cohesion.
                Since water can evaporate faster than it can be taken up, tension begins
            to build up in the xylem.  Eventually, the plant is under quite severe tension.
            Studies show that these tensions "pull" water from the soil into the root, up
            the stem, and then to the leaf, where it evaporates.  Henry Dixon and J. Joly
            first offered up this theory at the turn of the century, and it is still the most
            widely accepted theory for the ascent of sap in plants.
                What is needed for this theory to be viable?
                1. negative pressures in the xylem - this has indeed been found.  Trees
                    shrink during the day when transpiring due to the tensions in the xylem.
                    Further, if you cut into a tree, no water comes squirting out.  In fact,
                    if you put a drop of water over a cut, it gets sucked in!
                2. water must be able to tolerate great tensions in the xylem - verified as
                    noted above, and also recently by spinning not glass tubes in a centrifuge,
                    but stems themselves, and finding no evidence that the water columns
                    broke.
                3. Tensions that develop must be of sufficient magnitude to move water
                    fast enough to account for the transpiration rates that are measured.
                    This has indeed been found.
            So, in conclusion - the cohesion-tension theory of sap ascent is the most widely
            accepted theory.
                4. Some caveats - it is a passive process - that is, the plant need not expend
                    any energy to bring water up the stem (makes sense since the xylem cells
                    are dead at maturity).
                5. There are trade-offs.  Let's discuss these now.
    C. Cavitation and Embolisms - what happens when tensions run too high!
        1. Remember, cavitation is when the water column breaks due to too high tension.
        2. The air bubble left in the xylem after cavitation is called an embolism.
            a. when a xylem conduit cavitates, the tension is relieved, and water no longer
                moves up the plant in that tube.  Thus cavitation is bad because it takes
                active xylem out of commission.
        3. What causes cavitation?
            a. freezing
               i. when you freeze water, bubbles form (see your ice cubes).  If this happens
                    in a xylem cell, it can result in an embolism.
                    1. larger diameter cells are more prone to freezing-induced embolisms
                    2. easier for bubbles to form in large conduit xylem cells.  This is one
                        reason vines are a tropical phenomenon - they have large conduits and
                        are very prone to freezing-induced embolisms.
            b. drought
                i. drought increases tension in the xylem.
                ii. when tension increases to a certain point, air can be sucked into the xylem
                    from adjoining spaces, causing an embolism.
                iii. air sucked through pits.  Pits with large pore diameters allow air to enter
                    easier, so embolisms form more readily.  Not related so much to cell
                    diameter, like freezing-induced embolisms are.
        4. Trade-Offs
            a. plants with large diameter xylem cells can move water very easily, but are
                prone to freezing-induced embolisms.  May explain absence of vines from
                northern habitats, since vines have the largest xylem cell diameters.
            b. plants with small pit pore diameters more resistant to drought than those
                with big pores.  But small pits inhibits the flow of water.  So, plants with
                high resistance to drought may also have low capacity for water flow
                because of small pit pore diameters.
            c. remember, flow in a tube is proportional to the 4th power of the radius:
                                        Flow = r4

                  This means that cutting the diameter in half reduces flow by 16 times!
                    This means that doubling the diameter increases flow 16 times!
            d. small changes in xylem conduit diameter can have large effects on water
                flow, which in turn, can determine a plant's competitive ability.  Hence
                really drought tolerant plants often have to grow slower due to lower
                water movement in the xylem.

II. Stomatal Physiology
    A. Stomata are composed of two guard cells, and subsidiary cells that help with the
            opening and closing of the pore.
    B. How do stomata open and close?
        1. Mechanics
            a. when the guard cells become turgid, they swell and pull apart from each
                other
            b. when they lose turgor, they flab shut against each other
            c. the thickened cell walls on the inner pore side, and aspects of the cellulose
                cell walls elsewhere are responsible for this behavior.
        2. Physiology
            a. Osmosis is the process by which water enters and leaves guard cells.
            b. Early on, it was thought that the breakdown of starch in guard cells
                produced sugars, which because they dissolve in water, would act as
                osmotica and cause an inflow of water due to osmosis.
                However, some guard cells don't accumulate starch, and the breakdown
                turned out to be too slow to account for the rapidity of stomatal opening.
            c. Then, in the early 1970's, a researcher named Fisher found that opening
                was associated with in influx of K+ ions.  He calculated that enough went
                in to cause osmosis and the amount and timing were sufficient to account
                for the opening response.  So, today we know that K+ ions are the osmotica
                that cause stomatal opening.
            d. Closing is the reverse of (c).  K+ ions are sent out of the cell, osmosis now
                occurs in the other direction, and water flows out of the cell.  The loss of
                turgor causes the guard cells to close up against each other.
            e. Certain stimuli cause stomata to open or close.  Here they are in a nutshell:
                i. light - causes stomata to open; most stomata close in the dark.
                ii. CO2 - a lack of carbon dioxide inside the leaf causes stomata to open;
                        this allows more CO2 to enter when the concentration inside gets low.
                iii. temperature - more opening at higher temps.
                iv. relative humidity - low humidity often causes stomata to close somewhat.
                    This prevents excess water loss on days with dry air.
                v. drought - severe drought causes stomata to close and become
                    unresponsive to other stimuli (i through iv above).
                vi. ABA - the hormone abscisic acid (ABA), causes stomata to close.
                    Produced when plants are under drought stress - prevents excess water
                    loss.
III. Phloem Transport
    A. Again, osmosis is the driving force.  Main theory was thought up by Munch in the
        1930's - called the Munch Pressure Flow Hypothesis.
    B. Sugars, produced in photosynthesis, are transported to the sieve tube cells.
    C. Sugars are then loaded into the sieve tube cells.  This causes an inflow of water
        due to osmosis, building up turgor.
    D. Remember, sieve tube cells have sieve plates on the ends that allow movement
        of solutes and water from one cell to another.
    E. When the turgor builds up enough, water (and the dissolved sugars) are pushed
        from one cell to the next.  Since the pressure is greater at the source end (where
        the sugars are produced from photosynthesis) the flow goes out of the leaf to
        those tissues needing the sugars for metabolic purposes.
    F. At the sink end (where the sugars are used up) the sugars are moved out of the
        sieve tube cells, where they are absorbed by other cells.  This lowers the sugar
        concentration in the sieve tube cells here, and osmosis reverses - water flows
        out of the sieve tube cells, lowering the turgor pressure.
    G. The process is an active one, meaning it takes metabolic energy.  We know this
        because the process requires oxygen, ATP, and is temperature dependent.



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