Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

Salt Stress
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Therefore, the capacity of vacuolar compartmentation, which enables effective
osmotic adjustment of plants grown in saline conditions, and liberation of cytosol
free from the presence of toxic ions, is one of the key factors in salinity tolerance. Many
halophytic species are characterized by their large vacuoles. The vacuoles occupy 77%
of the mesophyll cells of Suaeda maritima (Hajibagheri et al., 1984) and are capable of
accumulating salts to concentrations higher than 500 mM (Dracup and Greenway, 1985).
In a study with S. maritima conducted on its natural habitat in Serbia, sodium concentration
of the cell sap exceeded even 800 mM, while the total salt content contributed to
the osmotic potential in degree of up to 91 % (Dajic et al., 1997b).
In the case of salt sequestration into the vacuole, potassium ions and organic
solutes should be accumulated in the cytoplasm in order to achieve and maintain the
osmotic and ionic balance between these two compartments (Flowers et al., 1977;
Greenway and Munns, 1980; Munns, 2002).
5.2. Succulence
The feature of succulence is characteristic for many dicotyledonous halophytes
(Crawford, 1989), and represents a very efficient way of mitigation of adverse osmotic
and toxic effects of ions through dilution. In natural conditions, the content of salts in
vegetative organs of halophytes increases with ageing, and might reach toxic levels.
Therefore, halophytic species tend to lower such unfavorable salt concentrations by
increasing of water content in their tissues and/or by enhanced growth. Morpho-anatomical
alterations of succulent halophytes include increase of cell volume, especially
of spongy and water parenchyma, increase of leaf thickness and decrease in number of
stomata (Strogonov, 1973). The significance of phenotypic plasticity in adaptations to
salt stress was documented in experiments with Plantago coronopus, where salt treatment
lead to increased leaf thickness and leaf dry matter percentage (Smekens and
Vantienderen, 2001). Shoots of halophyte Salicornia bigelovii were larger and more
succulent when grown in highly saline conditions (Parks et al., 2002). Salt-induced
succulence was also reported for two subspecies of Salsola kali: subsp. kali and
subsp. tragus, where the latter, when grown in saline medium, exhibited about three
times higher leaf thickness compared with the control plants (Rilke and Reimann, 1996).
The type of salinity plays a role in the occurrence of particular alterations of
plant structure. For example, chloride salinity contributes to the succulent forms, while
sulfate salinity leads to xeromorphism (Strogonov, 1964, 1973). Succulence is important
also in regulation of temperature and water balance, especially in warm and dry
periods of the year (Fitter and Hay, 1989), and this has been established for many
halophytes of extremely dry saline habitats (Weiglin and Winter, 1991). The ability of
halophytes to dilute salts both through succulence and growth is dependent on cell
wall extensibility, enabling the increase of cell volume followed by further uptake of
water in order to balance the excess of salts (Munns et al., 1983). Hence, in order to
maintain growth under saline conditions, plants may either increase the amounts of