Spring.wmf (18300 bytes) Plant Physiology (Biology 327)  - Dr. Stephen G. Saupe;  College of St. Benedict/ St. John's University;  Biology Department; Collegeville, MN  56321; (320) 363 - 2782; (320) 363 - 3202, fax;    ssaupe@csbsju.edu

Water Transport

I. Soil-plant-air continuum. The movement of water follows the pathway:

soil → uptake →  root →  stem →  leaf →  transpiration →  air

The driving force for water movement is the water potential gradient that exists from soil to air. Or in other words:

Ψsoil > Ψroot > Ψstem > Ψleaf > Ψair

Some typical values water potential values (in MPa) for a tree are:  trunk -0.7; twig -2.3, leaf -2.5.  In class, we may also look at some data for ivy.

II. Soil →  Plant

A.  What is soil? 
    Soil is a mixture of organic (dung, decayed organic materials, decomposers) and inorganic (weathered rock) materials, gases (oxygen, carbon dioxide, ethylene), and liquid.

B. Soil type - determined by: (1) composition; (2) texture or particle size ( sand > silt > clay. A loam is a soil with 10-25% clay and equal parts of sand and silt); and (3) structure (i.e., compaction)

C. Water and soil

  1. Saturated - soil before drained. Gravitational water - water that drains and is not tightly bound; Ψ = 0 MPa
  2. Field capacity - soil that holds all the water it can against gravity. Capillary water -water held by capillary action, water at field capacity;  Ψ = -0.015 MPa
  3. Permanent wilting percentage - soil moisture content at which plants can't get enough water. For most, Ψ = -1.5 MPa
  4. Graphic relationship of soil water potential vs. water content (%). Take-home lessons
    1. between PWP and FC is the water available for a plant to use;
    2. clay holds more water than sand at any →Ψ ; and
    3. clay holds water more tightly (i.e., @10% water Ysand > Ψclay). This is essentially a s/v problem, since smaller particles in clay they have a larger total surface and hence, has more charged surfaces that will bind water tightly.
  5. Soil water potential is a function of osmotic potential (which is usually near zero except in saline soils) and mostly pressure (used to call it matrix potential; this refers to the tension generated because of the attraction of colloidal particles; i.e., adhesion). The pressure in the soil can be calculated from the equation: Ψp = -2T/r where T = surface tension (7.28 x 10-8 MPa) and r = radius of curvature of the meniscus).  Water movement through soil - mostly due to bulk flow as a result of pressure gradients, with some diffusion.
     
  6. Spuds McSaupe plays with sponges

III. Plant → Air (= Transpiration)
   Or more simply stated, the movement of water from plant to air occurs via transpiration.  Air has a very high capacity for holding water. For example at 20 C, the water potential of water in air at 100% RH = 0 MPa; 98% RH -2.7 MPa; 50% RH = -93.5 MPa.  Conclusion - there is a very steep water potential gradient from soil to air. Essentially, the plant just inserts itself between the two and takes advantage of passive transport.


IV. Soil →  Root

  1. Root anatomy
        We will go over structure of the root including epidermis, cortex, endodermis, Casparian strip, stele, phloem, xylem, pericycle.
     
  2. Root Formation 
        Roots develop from the pericycle; film loop
     
  3. Apoplast vs. symplast 
        Recall that the apoplast refers to the "non-living" regions of the plant and the symplast is the "living" areas.
     
  4. Region of Water Absorption 
        Most of the water is absorbed near the tip of the root. The further from the tip, the less water that is taken up by the root. This roughly correlates to the region of the root that is suberized.
     
  5. Route of Water Movement 
        There are three routes water can follow: (a) Apoplastic – water follows an apoplastic route from soil through cortex. However, it must enter the stele symplast because of the Casparian strip. Once inside, it leaks back out and enters the apoplast (xylem) where it is transported to the apex of the plant. This appears to be the major route of transport; (b) Symplastic: Transmembrane – in other words, the water moves from cell-to-cell crossing membranes as it goes; and (c) Symplastic: Plasmodesmata – the water moves from cell-to-cell via the plasmodesmata.
     
  6. Root as an osmometer 
        Analogy - allows for the development of root pressures in the stem. These can be measured and are about 0.2 - 0.3 MPa.
     
  7. Guttation and hydathodes

V. Root → Leaves

A. What is the transport tissue for water? Xylem.
    Evidence comes from various tracer studies where xylem is loaded with dyes. I’ll bet you’ve done the classic "celery stalk in food coloring" experiment.

    We’ll see a film loop and maybe play with some celery

B. In which cells does water move? Vessels & Tracheids
    There are four major types of cells in the xylem: (a) tracheids - long, tapered ends, thick secondary wall; (b) vessel elements, - shorter, ends attached; (c) fibers - long and skinny with thick secondary wall, mostly for support; and (d) parenchyma - alive, thin, store starch and other materials, lateral transport. The primary water transport cells are tracheids and vessels. Note that gymnosperms only have tracheids whereas angiosperms have both and primarily rely on vessels for water transport. Both tracheids and vessels have pits, thin circular regions, in the walls.

C. How much pressure is required to move water to the top of a tall tree, that is say, 100 meters tall?
    Let's calculate. We can measure the velocity of flow in the xylem to be 4 - 13 mm s-1 in vessels with a diameter of 100 - 200 mm. For our calculations, let's use a flow rate of 4 mm s-1 (= 4 x 10-3 m s-1) and a vessel radius of 40 μm (= 0.00004 m).

    According to Poiseuille's Law - flow rate is directly proportional to the pressure gradient and the cross sectional area of the pipe but inversely proportional to the viscosity of the fluid. Thus, this is mathematically expressed as the Poiseuille equation:

Jv = ((π)(r4)( ΔP))/8 (η) where η = viscosity of water (assume it is the same as in a cell, 10-3 Pa s)

Now, divide the equation by the cross-sectional area of a vessel (π r2). Thus, the equation simplifies to:

Jv = (( r2 )( ΔP ))/8 (η)

Substituting the values for flow and vessel diameter:

4 x 10-3 m/sec = (0.00004 m )2(P)/ 8 (10-3 Pa s)

P = 20,000 Pa m-1

P = 0.02 MPa m-1

If the tree is 100 meters, then: 0.02 MPa m-1 x 100 m = 2 MPa

However, we must also take into account the effects of gravity (0.01 MPa m-1). Thus, for a 100 m tree: 0.01 MPa m-1 x 100 m = 1 MPa for gravity

Finally, the total pressure required to move water to the top of a 100 meter tree equals:

2.0 + 1.0 = 3 MPa

D. How is Water Moved to the top of trees?

1.  Is water moved to the tops of trees by a "push from the bottom" pump? - NO
    Several lines of evidence show that this type of pump doesn't exist:  (a) dissections showed there is no anatomical area in the stem or root that could serve as a pump; (b) when German physiologists cut off a tree in a vat of picric acid it continued to transport water.  This suggested that a stem pump was not involved since the picric acid should have killed living cells stopping the pump; (c) a root pump isn't involved or else when a plant is decapitated, the stump should continue to gush water; and (d) recall that root pressures only generate 0.2-0.3 MPA but that a pressure of at least 3 MPa is required to move water to the tops of tall trees.  To summarize, root pressure doesn't have nearly enough power.  

2.  Is water moved to the tops of trees by "capillary action" - NO
    Capillary action is the movement of water up a thin tube due to surface tension and the cohesive and adhesive properties of water. Essentially the meniscus "pulls" the water up the tube. Without worrying about the derivation of the equation, the height to which a column of water can move is inversely related to the radius of the pipe and is mathematically expressed as: h = 14.87/r (where r = radius in μm; and h = height in meters). Let's look at some actual data

Table 1: Capillary Heights of Water Movement
Tube Radius (μm) Column Height (m)
10 1.4877
40 (tracheid) 0.37
100 0.148
0.005 (size of pores in wall) 2975 (ca. 3 kilometers)

    Vessels are too wide to support movement very high and obviously capillarity cannot be responsible for water movement. Further, even it could, it would only move to the top of the plant once; capillary action can't continually pull the water up.

3.  Cohesion-Tension Theory - YES!
    This idea was first proposed by HH Dixon (Transpiration and the Ascent of Sap in Plants, 1914). According to this hypothesis, water is drawn up and out of the plant by the force of transpiration. Because of the cohesive/adhesive properties of water, as one water molecule evaporates at the opening it pulls the other molecules and sends this pull all the way down the column. If this is true them, water transport in plants must meet the following criteria:

  • The system must have little resistance. The vessels and tracheids are hollow at maturity. Imagine how difficult it would be to move water through a clogged pipe. Let's calculate how much pressure would be necessary if the water transport cells were "alive."  We'll use Fick's Law:

            Jv = Lp     ΔΨ   (Lp = hydraulic conductance which is the inverse of resistance)

    If we assume that water movement occurs at the rate we used earlier (Jv = 4 x 10-3 m s-1) and we use a typical value of 4 x 10-7 m s-1 MPa-1 for Lp, then:

4 x 10-3 m s-1 = (4 x 10-7 m s-1 MPa-1) (ΔΨ)

ΔΨ = 4 x 10-3 m s-1/4 x 10-7 m s-1 MPa-1

 ΔΨ = 104 MPa (this is the pressure required to move the water across just one membrane! Compare to the value we calculated above)

if, the cell length is 100 μm (10-4 m), then we can calculate the force required per meter:

104 MPa/10-4 m = 108 MPa m-1

  • The columns of water must be continuous from the leaves to the soil. 
        If not, it would be analogous to having a chain with a single broken link – it would be impossible to pull anything attached to the other end. The tracheids and vessels form a continuous water column.  If there are gaps - or air bubbles - water must be routed around these bubbles.  Cavitation, or vacuum boiling, is the fancy term for air coming out of solution when the water columns break.  

        By the way, this is one reason why you don't want to go outside and beat on the trunk of a tree on a hot sunny day...it could cause many of the columns to break so that a plant may have a difficult time transporting water.  Check out the Per Scholander stories written by Dr. V Berg.  

        If cavitation does occurs, the plant responds by: (a) transporting water around the blocked cell; or (b) redissolving the air bubble, which usually occurs at night; and/or (3) forming new xylem cells; in other words, xylem is disposable. Only the most recent cells in the latest seasons growth are actually functional. The remainder of wood in a tree is non-functional because it has cavitated and/or filled with other waste materials.   In addition, it is thought that one function of bordered pits is to stop the movement of air bubbles from a cavitated cell to another thereby isolating the impact of cavitation.
     
  • There must be sufficient pulling force. 
        Even though ca. 3 MPa are required to move water to the top of a tall tree, the water potential gradient from soil to air is considerably steeper (on the order of -100 MPa.)
     
  • The xylem should be under a tension. 
        Several lines of evidence support this prediction:  (a) Cut a stem and the water will snap up into the top and accumulate at the cut surface on the bottom; (b) Dendrometer studies - this device is essentially a band wrapped around a tree that is hooked to a pressure transducer. As the tree transpires the diameter of the tree is measured. These experiments show that the diameter of the stem is smallest during the day when transpiration is occurring and largest at night, as we would. Imagine putting your finger on the end of a straw and then sucking on the other end. The straw will get thinner (collapse) as you apply tension to the air in the straw - just like a plant stem; (c) Puncturing the xylem of an actively transpiring tree with an ice pick may result in a hissing sound as air is sucked into the stem (see Dr. Berg's water stories); and (d) Dye solutions are rapidly sucked into a tree trunk when punctured with a knife and then transported in both upward and downward directions.  Since the pressure in the stem is lower than atmospheric the dye solution is quickly sucked in.  We will see a demonstration of this in a video that a previous class made (BIOL327- Spring 2001) made.
     
  • The tensile strength of water must be able to withstand the pull. 
        In other words, the columns of water must not snap as they are being pulled.  As the water is pulled up the tree the water column puts up a resistance, much like stretching a rubber band.  Just like a rubber band will snap  if pulled too hard, so too will a water column break or cavitate.  This occurs because the reduced pressures in the water column cause gases to come out of solution and form a vapor lock in that cell.  Cavitation can be heard by placing a sensitive microphone on the plant. Tiny popping noises can be heard, a little like a bowl of rice krispies.

        The fact that water has a very high tensile strength, more than sufficient to withstand the pulling forces necessary to move water to the top of tall trees, was demonstrated by an elegant experiment in which water was centrifuged in Z-shaped tubes. 

        As an aside, the tensile strength of water may be one of the factors that limits the height of trees - the tensions in the stems of taller trees would be too great and the water columns would snap.  It's perhaps not a surprise that the tallest trees, California redwoods, grow along the fog enshrouded coast.  This helps to minimize the rate of water loss and ultimately reduce the tensions in the xylem (Zimmer, 2000).
     
  • Tracheids and vessels must be able to withstand tensions without imploding. Hence the reason that they have thick cell walls with circular thickenings. It's no surprise that wood is hard.

4.  Pressure Compensation Theory - Controversial.
    Recent work by Martin Canny and others have challenged the validity of the Cohesion-Tension Theory (see Canny, 1995).  

VII. Flow/size paradox (compromise)
    Don’t you just love a paradox?....recall that flow rate is directly related to the radius of the pipe (Poiseuille’s law). Thus, flow rates in vessels are greater than those in tracheids. But, why aren’t vessels (and tracheids for that matter) larger, especially since it means that they could transport more water? The answer - cavitation.  As the pipes get larger, the chance of cavitation increases.

VIII.  References

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Last updated:  01/07/2009     � Copyright  by SG Saupe