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Effects on the Ion Metabolism of the Cell ("Primary Effects")

Sodium and chloride ions are regarded as being biologically aggressive osmolytes on account of their small ionic diameters and high surface charge densities and their consequent strong tendency to attract water. High concentrations of these ions in the apoplast accordingly lead to imbalances in water and ion relations. Stress caused by salinity is thus both dehydration stress and ionic stress. The latter rapidly leads to destruction of biomembranes, because ion imbalances result not only in altered concentration relations, but also in changes of membrane potential. Homeostatic ion concentrations, i.e. cytosolic concentrations maintained at equilibrium in glycophytes not subject to salinity stress, are in the ranges of 100-200 mM K+, 110 mM Na+ and CI" and 0.1-0.2 mM Ca2+ (Fig. 1.6.2; Niu et al. 1995).

These ion concentrations are kept constant in plant cells in the main by electrophoretic flows, i.e. passive flows which are coupled to H+-AT-Pases and H+-pyrophosphatases in the plasma membrane and the tonoplast (in contrast to animal and yeast cells, plant cells do not possess Na+/K+-ATPase pumps). In addition, import and

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)

Plant type

Maximum

Reduction of har

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