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Salt Stress (Osmotic Stress)

Salt lakes are almost uninhabitable for plants because of the enormous osmotic potential of the substrate, which is often also very alkaline due to the high soda (NaC03) content. Nevertheless, plant life can be found in such habitats. The white expanses in the picture of Lake Magadi in southern Kenya are not snow, but salt incrustations. The banks of sediment in the lake are overgrown with thick layers of algae. The shoreline also supports, in part, a vegetation of halotolerant bushes. Photo E. Beck

Recommended Literature

• Blumwald E (2000) Sodium transport and salt tolerance in plants. Curr Opin Cell Biol 12:431-434

• Hasegawa PM, Bressan R, Pardo JM (2000) The dawn of plant salt tolerance genetics. Trends Plant Sci 5:317-319

• Larcher W (2003) Ecophysiology of plants, 3rd edn. Springer-Telos

• Marschner H (1995) Mineral nutrition of higher plants, 2nd edn. Academic Press, chap 16.6

• Osmond CB, Bjorkman O, Anderson DJ (1980) Physiological processes in plant ecology: towards a synthesis with Atriplex. Ecological Studies 36. Springer, Berlin Heidelberg New York

• Zhu J-K (2002) Salt and drought stress signal transduction in plants. Annu Rev Plant Biol 53:247-274

That stress occurs when certain environmental factors exceed or are below the useful or tolerable range of intensities becomes particularly obvious with nutrients. The concentrations at which nutrients occur or exert their effect often differ greatly from element to element and from plant to plant (see also Chap. 2.3). This is partic ularly apparent for ions which are not obligate nutrients, such as sodium. This ion - in contrast to the case with animal metabolism - plays no essential role in the metabolism of plants, but it often occurs at high concentrations as sodium chloride where plants grow. Salts found in nature (mainly NaCl and CaS04) may occur at concentrations which are either toxic, tolerated or even required as an osmoticum, and plants are accordingly referred to as glycophytes (nonsalt plants) or halophytes (salt plants). In the following, stress caused by high NaCl concentrations will be the main topic of discussion.

The electrical conductivity (ECe) of salts in liquid solutions is used for the standardisation of salt contents and is expressed in [Siemens (S)/m] or [mS/cm]:

1 S/m corresponds to -0.36 MPa and the dimension Mho is the reciprocal of Ohm, the measure of electrical resistance.

The conductivity of seawater corresponds to 4.4 S/m; that of water used for irrigation must be lower than 0.2 S/m. Soils with a conductivity of > 0.4 S/m are termed saline soils. The salt concentration of seawater (3%: 480 mM Na+, 50 mM Mg2+ and 560 mM CI") corresponds to

Table 1.6.1. Salt tolerance of important agricultural plants grown under a range of high salt concentrations. ECe is a measure of salinity and is the electrical conductivity of the water extracted from the saturated soil solution. (After Marschner 1986)

Barley 8.0 5.0

Sugar beet 7.0 5.9

Soybean 5.0 20.0

Tomato 2.5 9.9

Maize 1.7 12.9

Green bean 1.0 19.0

Table 1.6.2. Yields of salt-tolerant barley plants from the gene and seed banks of the USA upon cultivation on Californian sand dunes and irrigation with seawater supplemented with phosphate and nitrogen fertilisers. (After Luttge et al. 1994)

Standard variety on saline soil 0.83 (average)

Var. XXI, 1 st experiment 1.08

Var. XXI, 2nd experiment 1.50

World average on non-saline soil 2.00

an osmotic potential of -2.7 MPa. Table 1.6.1 shows the salt tolerance of various crop plants.

Irrigation water in dry areas contains between 0.1 and 1 kg mineral salts per cubic metre. While this is significantly below the damage threshold (-0.15 MPa), it can still lead to soil salinity and to the loss of useful agricultural land in a few years due to the intense evaporation.

NaCI concentration in seawater

NaCI concentration in seawater

NaCI concentration (molar)

rig. 1.6.1 Salt tolerance of various barley varieties, measured by the development of young plants on salt-containing substrates. (After Luttge et al. 1994)

NaCI concentration (molar)

rig. 1.6.1 Salt tolerance of various barley varieties, measured by the development of young plants on salt-containing substrates. (After Luttge et al. 1994)

It follows from the theory that life arose in the sea that salt tolerance must have been an original characteristic of living systems; sensitivity to salt should then be seen as a loss of this original characteristic. "Regaining" salt tolerance of important crop plants is accordingly one of the most important aims of plant breeding. An example is the improvement of salt tolerance in barley (Fig. 1.6.1 and Table 1.6.2), which has per se the highest salt tolerance of all cereals (see Table 1.6.1).

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