WATER IN THE SOIL - PLANT - ATMOSPHERE CONTINUUMWater 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 movementWater 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).
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
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.
The vapour pressure ratio is given by :
Q What factors will change the water potential ?
The components of volumetric water potential are:
Ym = matric potential (-ve)
|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.
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
Q What is the available water content of a soil ?
Q Does soil type have a marked effect on the available water ?
The Field Water BalanceQ How is water redistributed in the soil-plant atmosphere continuum ?
P + I = R + D + S + E
Water TransportThe 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:
Lp = hydraulic conductance of the system (the reciprocal of the resistance)
ss = reflection coefficient for salts.
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 ?
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 ?
|Q What is the driving force for the movement through the whole soil-plant
Transpiration (Kramer & Boyer, 1995 pp 204-214)The energy balance of a leaf can be represented as follows:
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.
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 ?
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.
Q What influences stomatal aperture ?
Q What is the vapour pressure in the sub-stomatal cavity relative to the saturated vapour pressure?
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 ?
Movement in the plantQ What is the driving force governing water movement between the soil and the leaf ?
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 ?
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 ?
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:
Extent of root systems
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:
Q Why ?
rL will be large too because the stomates are closed yL will also be large (ie close to zero) .
Q Why ?
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.
Q Why is the value of ySt less on irrigated plots than on July 8 ?
The importance of root distributionAdding 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 ?
Root clumping also a significant factor in the exploitation of soil water reserves.
Water Use EfficiencyWater use efficiency can be expressed in agronomic or physiological terms. (Kramer & Boyer, 1995 383-387).
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.
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.
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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