Summary Water availability

• Precipitation, evaporation and run off determine the water balance.

• Water loss includes evaporation, which is determined by the chemico-physical conditions in the atmosphere and transpiration from plants, which is regulated by physiological processes. Evaporation and transpiration are combined in the term évapotranspiration.

• Evaporation from the earth's surface depends primarily on available radiation.

• Global temperature distribution and hydro-logical balance determine climatic regions and thus the distribution of vegetation on earth.

creases with radius of the hydrated ion. Thus Na+ has a larger hydration shell than K+ (Liittge 1973). There are hydration shells for all polar groups in organic molecules. Five to 10% of the total water in a leaf is thereby not freely available.

• Capillary water: in cell walls, as well as in the fine pores, water is subject to capillary forces. The rise of the water column in a capillary (h, measured in m) is, as a first approximation, inversely proportional to the radius (r) of the capillary, because the water mass and the gravitational force effecting it (jihpg) must be balanced against the cohesive force produced by the surface tension (27zxi7w). Neglecting the contact angle a (cos a=l), the following applies (Nobel 1991, p. 55)

where h is the height in m to which the water column rises, p is the density of water (998 kg m"3 at 20 °C), g the acceleration of the earth (9.8 m s~2), r is the radius of the capillary (m), and aw the surface tension of water (0.0728 Nm"1 at 20 °C).

Thus the pressure in a capillary, the so-called capillary force is:

and the height of the meniscus is calculated as:

In a clean (lipid-free) glass capillary 3 (.tm in diameter (1.5 (.tm radius), water rises to 10 m. In a xylem vessel of 30 (.tm, typical for tracheids of conifers, water rises by capillarity by only 1 m and in the 300 (.tm trachea of deciduous trees the water rises only 0.1 m via capillarity. This means that the capillary force is not sufficient to lift water into the canopy of trees and thus does not provide the lifting power required for flow of water (see Chap. 2.2.2).

• Storage water: this water is osmotically bound, e.g. in the vacuole (see also Chap. 1.5). The osmotic value (77, measured as MPa) depends on the number of particles per mole (n), the concentration (cn), the gas constant R (8.3144 Pa m3 mol-1 K"1) and the temperature (T in K):

77 of a 1 molar solution of an undissociated solute at 0 °C is 2.27 MPa and at 20 °C 2.43 MPa.

The osmotic value is generally expressed as pressure, in MPa, based on an experiment which also demonstrates the phenomenon of osmosis: A closed chamber is divided into two compartments by a semipermeable membrane which allows passage of water molecules but it is impermeable for ions. One compartment of the chamber is filled with distilled water, the other with a salt solution. To balance the difference in concentration of both solutions only free water is able to flow into the chamber with the salt solution, so building up a hydrostatic pressure there (i.e. the level of water rises in comparison to the free water in the neighbouring part of the chamber). The height of the water column corresponds to the osmotic value, which is therefore called the osmotic pressure.

The flow of liquid between soil, plant and atmosphere is based on several forces. In order to describe the flow between the very heterogeneous compartments of the environment it is necessary to quantify the availability of water uniformly. This is possible by the definition of a common force for water transport, the water potential, W (see also Chap. 1.5):

where /uw is the chemical potential in the system (J mol-1) and /u0 is the chemical potential of a reference system, i.e. of pure liquid water at a given temperature and at normal pressure (atmospheric pressure). By dividing the difference in the chemical potentials (//w -fi()) by the molar volume of liquid water (Vw) the water potential is defined in units of pressure. Thus, the water potential describes the driving force for water movement in a practical way because pressures can be easily measured. Vw is slightly dependent on temperature and pressure, but this is normally neglected. V^ expresses the molar volume of pure water.

In the gaseous phase the water potential is proportional to the relative humidity:

where e/eQ expresses the vapour pressure of bound water (e.g. in solution or solid material)

relative to that of free water, and thus corresponds to the relative humidity. The right side of Eq. (2.2.5) is also called water activity, describing, e.g., the degree of swelling of colloids and thus characterises the conditions for life of microorganisms or poikilohydric plants. As the chemical potential of bound water, ¿¿TO is lower than that of free water, /u0 (energy has to be added to change, e.g. bound water in a salt solution into the state of free water), the water potential has a negative sign. Water movement occurs from sites with high potential to sites with low potential.

Water potential describes

• the state of water of plants or particles, and

• the driving force for the movement of water.

Using water potential it is possible to describe water in single-phase systems (e.g. in a plant) as well as in phase transitions (e.g. evaporation of a leaf; see Chap. 1.5, Fig. 1.5.1). In a cell with good water supply, the osmotic pressure is compensated by the counter pressure of the cell wall, the turgor pressure, Pc, the water potential is zero. With decreasing water content the turgor pressure sinks, the osmotic pressure rises because of the increasing concentration of the residual solution in the cell and the difference, 77-P, corresponds to the water potential, W, which becomes increasingly more negative. Thus, in a tissue, water flows, e.g., from the cell wall into this cell. If desiccation of the cell continues and water potential becomes equal to the osmotic pressure (77= -¥0, then plasmolysis starts.

The water balance of the cell is given by the equation:

where r represents the binding force in the membrane-free matrix of the cell wall (the so-called matrix potential, see Chap. and Chap. 1.5) and /;gh the water pressure, with p the density of water, g the gravitational force (9.807 m s"2 at 45° latitude) and h the height of the meniscus [m; see Eq. (2.2.4)].

Equation (2.2.9) shows that water potential is dependent on osmotic pressure, the chemical binding of water, and surface properties and on gravity, particularly important for tall trees. The matrix potential is an analogous value to 77, but in this case the value is dependent on surface forces and not on the number of particles in the solution. In the literature, the osmotic pressure is often also called osmotic potential and the numerical value is given as a negative number. It is basically only a different formulation of the same process. We use the term osmotic pressure because it creates a positive pressure.

If the water potential between plant, soil and air is balanced, there is no evaporation. In nature this state occurs particularly in the early morning, before dawn. Therefore, the early morning water potential, '//prct|awn, in a plant is used to characterise the water conditions of the soil in the zone from which the roots gain their water. With transpiration, a gradient in water potential develops between soil and atmosphere, which may be up to 100 MPa.

In the continuum of water transport between soil and atmosphere, water follows the potential gradient (from high to lower potential, i.e. to increasingly more negative values), whereby the flow rate is limited by resistances which are dependent on the characteristics of the soil and the types of tissue. In addition there is a phase transition in the leaf from the liquid to the vapour phase, with the rate of diffusion in the vapour phase determined by the water vapour pressure of the atmosphere (e/eQ). In the soil-plant-atmosphere continuum (SPAC), the highest water potential gradient is between the surfaces of the cell walls in the mesophyll of leaves where water evaporates and the external air, because the hydraulic resistance is highest there.

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