WATER IN THE SOIL - PLANT - ATMOSPHERE CONTINUUM 

Water constitutes 80-90% of the fresh weight of herbaceous plants, and more than 50% of woody species. It is the key medium in which the protein and lipid molecules are distributed to form protoplasm. 

It acts as a solvent for gases, mineral nutrients and organic nutrients, and the solution formed is the way that these materials enter and move around in the plant. 

Water is a key reactant in chemical processes within the plant, especially in photosynthesis and in hydrolysis reactions. 

Movement of water from cell to cell due to gradients of osmotic potential is important in the operation of stomatal guard cells and the maintenance of turgidity. The latter is important for cell expansion and activity. 

Properties of Aqueous Solutions: (Kramer & Boyer, 1995 pp 30-35) 

Water has a very large specific heat, which results in it tending to stabilize temperatures following an input of heat. The latent heat of vaporization is also very large; causing pronounced cooling when water evaporates and warming when it condenses. 

The surface tension of water is relatively large due to the cohesion between molecules. As a result it also has a large tensile strength. 

Water is only very slightly ionized but the molecules are polarized. It has a large dielectric constant so that water molecules can form clouds around a charged ion to prevent ions of opposite charge from bonding. The polarized nature of the molecule also means that it will form hydrogen bonds with organic groups. 

Water potential and water movement 

Water potential is a term expressing the ability of the water to do work. Thermodynamically this is the Gibbs free energy (G) or the chemical potential (µw) of the water (Kramer & Boyer, 1995 pp 35-37). 
µw = µ0w + R T ln Nw
    Where µ0w is the chemical potential of pure water and has a value of zero 
    R is the gas constant 
    T is the temperature in K 
    and Nw is the mole fraction of water 
Chemical potential can also be related to the vapour pressure of the water. 
Raoult's law states that the vapour pressure of solvent vapour in equilibrium with a dilute solution of a non-dissociating solute is proportional to the mole fraction of solvent in the solution. 
              e = e0 . Nw
where e0  is the vapour pressure of pure water 

The vapour pressure ratio is given by : 
 

The chemical potential therefore becomes 



Q What factors will change the water potential ?

  • Decreases in e due to:

  •  
      1. the presence of solutes 
      2. negative hydrostatic pressure 
      3. temperature but note it affects both e and e0 similarly 

      4.  
  • increase in e due to:

  •  
      1. positive hydrostatic pressure 
      2. temperature but note it affects both e and e0 similarly 
Chemical potential is expressed per mole of water, an inconvenient unit. The chemical potential can be expressed on a per unit volume basis - the volumetric water potential (Yw) by dividing by the partial molar volume of water: ie the volume occupied by the water molecules in the solution. 

The components of volumetric water potential are: 

Where 

    Ys  = potential due to solutes (-ve) 
    Yp  = pressure potential (+ve or -ve) 

    Ym  = matric potential (-ve) 

    Yg   = potential due to gravity 
The availability of soil water to plants is determined by the soil water potential and the movement of water in the soil (Kramer & Boyer, 1995 p 84-89, 89-91,93-97). 
Water moves in response to a potential gradient. The components of potential differ in effectiveness depending on the system, particularly on the presence or absence of a differentially permeable membrane. 

In the soil water is held in pores. We commonly have to do work to remove water from the soil. The equation for capillary rise is used to identify the size of pores which hold water as the soil dries. 
The change in potential across the meniscus Dp = 2t/ r 
where t = surface tension, and r is the radius of curvature of the meniscus. 

This approximates to D p = 2 t cos a /R 
where R is the radius of the pore. 

D p = hrg , where r is the density of the liquid in the pore. 
R is given by 2 t /hg if the angle of contact is very small. 
Similarly, h is given by 2 t /Rg 
Eg. for a pore of diameter 300µm the capillary rise is 10 cm.


 
 
The relationship between the volume of water in the soil and the work required to remove the water is given by the moisture release curve. Pore necks mean that the curve for a drying soil is not identical to that for a wetting soil - a phenomenon known as hysteresis.

 
 
 

In the soil the water potential is given by : 

yp is positive, so will only operate below a water table, ie when ym increases to zero. Also, ys only becomes really important for water movement to plant roots, or for vapour transfer. 

The sign for the gravitational component depends on the reference point, but it is often convenient to use the soil surface. 

ys becomes important in soils for fertilizer prills. Water can move in vapour form across an air-water interface towards a dissolving prill or fertilizer band. The soil will eventually saturate in the region of the prill. and the water will flow away from the area due to the difference in matric potential. 

In this diagram there is a water table at 80 cm. Using the soil surface as the reference level, yg will be zero at the soil surface and -80 at the water table. If the soil water in the unsaturated zone is in equilibrium with the water in the saturated zone, there will be no net vertical movement. The hydraulic head h will be constant. By definition, ym will have a value of zero at the water table. 

The value of h must have a value of -80 cm at all depths in the unsaturated layer. 

Below the water table the value of h will be influenced by the pressure potential, which will increase as the gravitational potential decreases. 
 
 
 
 
 

Q What is the available water content of a soil ?

  • This is the difference in the volume of water that is held in the soil after it has stopped draining under gravity (often referred to as field capacity) and the volume held when plants show permanent wilting. The potential often chosen to determine the upper limit is -0.033 MPa. In Ontario a value closer to -0.01 MPa is more appropriate. 
The lower limit is commonly taken to be -1.5 MPa. 

Q Does soil type have a marked effect on the available water ?

  • The tendency is for there to be less water available in sands and clays, with most being available in silt-loam soils. 

The Field Water Balance

Q How is water redistributed in the soil-plant atmosphere continuum ?
 
  • Incoming precipitation (rain, snow, hail) - P may be augmented with irrigation - I
  • Some water - R. will runoff the field into ditches or streams
  • Some water - S, will add to the soil water content and be stored
  • Some water - D, will drain through the soil and enter the water table,or be intercepted by a subsurface drainage system, but in any case it will be lost to the crop
  • And some water - E, will be evaporated from the surface of leaves, or from the soil surface, or will be taken up by plants and be used for cooling as it evaporates at the surface of the mesophyll cells surrounding the sub-stomatal cavity
          •  

            P + I = R + D + S + E

Water Transport 

The mass of water that will move through a given area (the flux) is determined by the ratio of the driving force (which results from the potential difference between the source and sink) and the resistance to flow. 

The flux of water can be represented as follows: 
 

              Jv = Lp (Dyp + ss Dys
    Where Jv = Flux 
    Lp = hydraulic conductance of the system (the reciprocal of the resistance) 
    ss = reflection coefficient for salts. 
Reflection coefficients for salts (ss) range from close to 0 in the absence of a plant membrane to close to 1 across a plant membrane. An air/water interface is a perfect example of a semi-permeable barrier where the value of ss is 1. Hence Dys has very little effect on liquid water movement in soil but has a major effect on movement into and out of plant cells or across an air/water interface. 

A temperature gradient has little effect on movement of water in the liquid phase. Water will move in vapour phase from warmer to cooler regions in response to the lowering of vapour pressure (Kramer & Boyer, 1995 P. 94-95). 

Water movement in the soil-plant-atmosphere continuum.

Q What is the driving force for water moving through soil ?

  • The total water potential gradient -dy/dx 
Q What is the appropriate resistance?
  • The resistance is expressed in reciprocal form as a conductance per unit distance or conductivity k
The flux of water (volume per unit area) is given by: 

Hydraulic conductivity is however dependent on the water content of the soil. 

 Hydraulic conductivity can decrease by several orders of magnitude between field capacity and wilting point. 
 





 The coarser the soil texture the greater the change in hydraulic conductivity with water content.

 Q What are the potentials that contribute to the driving force for water moving through plants ?

Because we are often concerned with cell to cell movement we can ignore Yg. Similarly there is a positive hydrostatic pressure inside the cell vacuole so Ym can also be ignored. Thus
Yw = Yp +Y
 
Q What is the driving force for the movement through the whole soil-plant -atmosphere continuum? 
  • The driving force for water flow in the soil-plant-atmosphere-continuum is evaporation of water from leaf surfaces - transpiration. 

Transpiration (Kramer & Boyer, 1995 pp 204-214) 

The energy balance of a leaf can be represented as follows: 
 
    Rn = H + E + hn
    where Rn = net radiation 
    H= sensible heat exchange with atmosphere (+ve or -ve) 
    E= latent heat flux and is the product of the latent heat of vaporization and the mass of water evaporated
    hn= energy used in photosynthesis (-ve) or released in other metabolic processes (+ve), usually not more than 2-3% of Rn
Normally about 80% of the net radiation is dissipated by evaporation of water from the leaf surface (transpiration). If water not available for transpiration, leaf temperature rises and heat is transferred to air. 

A well-watered crop may evaporate 5mm water per day. This is equivalent to 5 x 104 kg water per hectare 

Q What is the driving force for transpiration ?
 

  • The driving force is the difference in vapour pressure between the evaporating surface in the leaf and that in the air above the leaf : eL - eair
    ie yL - yair
Q What are the resistances to the transport of water ?
 
  • Stomatal resistance (rst
  • Cuticular resistance (rc) - large compared with the other leaf resistances 
Relative importance of components of resistance to water vapour in leaves
 
    Resistance to water vapour (sec cm-1)  
Species rst rc ra
Betula verrucosa 0.9 83 0.8
Quercus robur 6.7 380 0.7
Acer platanoides 4.7 85 0.7
Circaea lutetiana 16.1 90 0.6
Lamium galeobdolon 10.6 37 0.7
Helianthus annuus 0.4 - 0.6

 
  • resistances in the mesophyll (rm
  • the boundary layer resistance of the air (ra
    rst - varies according to the stomatal aperture. 
    ra - the boundary layer resistance is determined by wind velocity, which makes the layer thinner as velocity increases. 
    If instead of laminar flow, the velocity is such that tubulence is created, this breaks-up the boundary layer. In reality, the increased flow will cool the leaf, and reduce transpiration. The structure of the leaf surface also affects the thickness of the boundary layer. 
Transpiration rate is controlled primarily by the stomatal resistance rst
 

Q What influences stomatal aperture ?
 

  • The degree of opening depends on the turgidity of the guard cells. 
Q What affects guard cell turgidity ?
 
  • CO2 concentration in the sub-stomatal cavity 
  • light intensity (affects CO2 concentration) 
  • Water deficiency (through direct or triggered reaction) 
At a potential of about -1.7 MPa in the guard cells, stomates close. 

Q What is the vapour pressure in the sub-stomatal cavity relative to the saturated vapour pressure?

  • The vapour pressure is always very close to saturation (98-99% relative humidity) because of the wet surfaces of the mesophyll cells. 
Q What is the effect of temperature changes ?
  • In free air the vapour pressure hardly change with temperature, but the relative humidity declines because warm air can hold more water vapour than cooler air. 

  • As the leaf warms, the vapour pressure in the leaf increases, because the vapour pressure of liquid water increases markedly with temperature. 
  •  




  • Q Will transpiration occur in a lighted growth chamber at 100% relative humidity ?

    • The leaf temperature will increase so vapour pressure at the evaporating surface will increase, and water be transpired. 

    Movement in the plant 

    Q What is the driving force governing water movement between the soil and the leaf ?
    • If yL is the leaf water potential 

    • and yS is the soil water potential 
      The driving force is 




    A typical value of water potential in a transpiring leaf is -1 MPa 

    Q What constitute the resistances ?

    • In stem, petiole and leaf veins - xylem 
    • In root - root surface to xylem 
    • In soil - bulk soil to root surface 
    Q Where is the greatest resistance ?
    • In the root - the water passes through membranes. 
    There may be a resistance to flow at the peridermis (if one is present). In most cases the greatest resistance is at the endodermis 


     
     

    Water potential in the soil might be 0.05 MPa. 

    At the root-soil interface it might be 0.06 MPa 

    Resistance in the soil is unimportant until the soil water content declines such that hydraulic conductivity decreases. 
     

    In the xylem the water potential is -0.085 MPa. 

    eL, eair, rL, and rair are all very variable on a daily basis. YS, rS, rrt, and rxyl

    are all relatively constant. 

    yL is the plant parameter that has to change in response to environmental factors. 

    NB leaves will curl to reduce the evaporative surface, as well as close the stomates 

    Q At night what happens to plant water potentials ?

    • All tend to increase. 
    Q Why do leaves guttate ?
    • Ions continue to be pumped into the xylem even though transpiration has ceased. The increased concentration results in a lower osmotic potential in the xylem, and a gradient is developed across the root. Water moves into the xylem in response. 
    This is the basis of root pressure (Kramer & Boyer, 1995 pp 167=176). 
     
     

    Absorption of Water from Soil (Kramer & Boyer, 1995 182-191) 

     

     
     
     
     

    1) Transpiration of water from leaf surfaces lowers water potential in leaf, hence creating a potential gradient between leaf and soil. Water flows in response to this gradient. Resistances to water flow may occur in the soil, at the soil-root interface or in the root. 

    2) The ability of the plant to extract water from soil at a sufficient rate to prevent stress depends on: 

        Soil water availability and conductivity 
        Extent of root systems 
    Water Deficits and Plant Growth 

    1) Water stress affects the various plant processes differently. For example leaf expansion may be reduced to very low levels at water potentials of -0.2 MPa whereas stomatal closure may not begin until potential drops to -1.0 MPa. 

    The following figure (from Stypa et al., 1987: Can J. Soil Sci. 67, 293-308) gives an example of changes in leaf water potential during the day, and the corresponding values of stomatal conductance. 






    To explain these observations we can consider the driving forces and resistances: 

             Where rP = rxyl + rrt
    For July 8 at 5 am eL - ea is small. 

    Q Why ? 

    • No radiance, so temperature difference is very small - may even have conditions where leaf is cooler than the air. 

    • rL will be large too because the stomates are closed yL will also be large (ie close to zero) . 
    By midday, eL - ea is large. 

    Q Why ? 

    • Radiance is large, so temperature difference between leaf and air is large. 

    • rL will be very small because the stomates are wide open. 

      yS will not have changed much, (yS = -0.1 to -0.2 MPa on irrigated plots, and -0.2 to -0.3 MPa on non-irrigated) so rS will not have changed much either. 
      The resistances in the root and xylem will not have changed So yL gets much smaller (ie more negative). 

    Q Where will the main resistance be ? 
    • In the root 
    By midday on July 15 eL - ea is large. 

    Q Why is the value of ySt less on irrigated plots than on July 8 ?

      Q Will rSt have changed between the two dates?
    • No - conductance is the same as before. 
    • Temperature difference between leaf and air could be greater. 
    • ea could be much lower. In fact a large dry air mass entered the region, so a change in ea was the answer in this case. 
    Q Why is yL smaller on non-irrigated plots ?
    • rL large because of stomatal closure - conductance smaller. yStwas close to wilting point (-1.5 to -1.6 MPa. 
    • The resistances in the root and xylem will not have changed. So yL gets even smaller (ie more negative) 

     The importance of root distribution 

    Adding branch roots to a single axis, or new axes all reduce root resistance because the new roots are in parallel with the original root. The resistances of the two systems are given by: 



    In consequence, water needs to move less far through the soil to reach a root, and a larger volume of soil is explored for water and other nutrients. 

    Q Why do soils dry from the top down ? 

    • As xylem resistance is small, though significance, root resistance increases with distance from the stem. 

    • Root clumping also a significant factor in the exploitation of soil water reserves. 
    Effect of Root Clumping on Transpiration by Wheat in NSW Australia 
    Block Transpiration (% total 18/5-23/11)
    A B C D
    Mean root concentration factor 2.2 2.0 5.6 3.3
    Period 91 90 77 83
    18/05-28/10
    28/10-23/11 9 10 23 17
    Note: The mean root concentration factor is an index of root clumping in the soil at 30-50 cm depth. 
     
       

    Water Use Efficiency 

    Water use efficiency can be expressed in agronomic or physiological terms. (Kramer & Boyer, 1995 383-387). 


    It makes more sense when considering the processes to determine the above ground dry matter production. 


    For irrigation, the water used will include drainage and run-off as well as evaporation and transpiration. Much of the effort in irrigation schemes is to minimize losses through drainage and run-off. 

    We can also consider the physiological water use efficiency 


       
       
       
       
       


       
       
       
       
       








      If the CO2 concentration in the atmosphere increases, water-use efficiency increases. 

      If the atmosphere is is more humid, it also increases WUE. 
       

    Q Are there differences between plants in WUE ?
    • See previous notes on photosynthesis concerning differences between C3 and C4 plants. 
    • The resistances for CO2 and water are not the same in all species. 
    • Similarly, the effective concentration of CO2 in the sub-stomatal cavity is species dependent. 
    • Some plants have developed a novel method of increasing water use efficiency. They are mainly succulents and use crassulacean acid metabolism (CAM). The stomates of CAM plants are open at night. CO enters the leaf and is fixed in the cytoplasm of mesophyll cells by PEP carboxylase, which converts phosphoenolpyruvate to oxaloacetate. The latter is reduced to malic acid. The malate is stored in the vacuoles. During the day, stomates are closed. Malate is released from the vacuoles, decarboxylated, and the released CO2 is fixed by the Calvin cycle in the chloroplasts. 

    • This pathway can be switched-on as a result of water stress (or salt stress) in some halophytes, the rapidity of the response being more efficient as plants age. 
       
    Because of the strong interdependence of C02 uptake and water transpired, for a given crop there is a reasonably constant relation between yield (Y) and transpiration (T) at a given vapour pressure deficit (e). 
     
    Variation in Water Use Efficiency for Wheat 
     
    Crop management Location WUE (g kg-1H2O)
    Irrigation Utah 2.5
    No-till UK 5.2
    Plough UK 5.3
    Drained UK 5.2

    Note that vapour pressure deficits in Utah are much greater in Utah than in the UK. 

    Signalling of changes in the root zone water status

    The traditional concept was that shoot growth was affected by soil water deficit because of the changes in the water supply to the leaves and meristems. 

    Over the past 10-15 years it has been established that roots sense a drying soil and send a signal to the shoot causing reduced leaf expansion or stomatal closure before there is any detectable change in leaf water status. Davies and Zhang ( Annu. Rev. Plant Physiol. Plant Mol. Biol. 1991. 42, 55-76) report one of the most elegant demonstrations of roots signalling changes in soil water status. They measured daily increments in leaf area for apple trees which had their root systems divided between two pots of soil. Plants were either watered through additions to both pots, or to one pot only. After 24 days of these treatments, some plants, which had previously had been watered through one pot had water restored to the roots in the dry soil. Another group of plants that had also been watered through one pot only, had their roots in the dry soil excised. Growth was followed over the next 14 days. 

    After two weeks the plants that had had the roots excised grew as well as the rewatered pots, and both grew better than the half-watered plants. The evidence seems unmistakable that messages from the roots in the unwatered pot reduced the growth of the shoot. Cutting-ff the supply of the message was as effective as rewatering the pots. 

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