Mechanism of Salt Tolerance

Although plant responses to salinity are one of the most widely researched subjects in plant physiology, the mechanisms that impart salt tolerance are still unresolved (Cheeseman 1988; Munns 1993; Ashraf and Foolad 2007).

Plants which were able to obtain more water than others from a soil under low water potential would grow better in saline conditions (Cruz and Cuartero 1990). So, plants have developed various mechanisms for survival under high salinity stress. Some tolerate high concentrations of toxic ions present in their root environment by exclusion or compartmentation of ions into the vacuole, and the production of high concentrations of organic solutes in the cytoplasm that lower the osmotic potential (Greenway and Munns 1980, Parida and Das 2005). These organic solutes such as proline (Perez-Alfocea et al. 1993; Ashraf and Foolad 2007) and myo-inositol (Sacher and Staples 1985) are generally non-toxic to enzymes.

It has been reported that Na+ and Cl-ions were accumulated in the vacuolar sap of halophytes (Austin 1989; Aghaleh et al. 2009). As a result of this, plants become succulent. Succulence is usually defined as the thickening of the leaves of the plants exposed to salinity, although this condition is also applicable to the stem and the root. It is expressed as an increase of water content per unit dry weight, fresh weight or water content per unit area (Jennings 1976). It has been proposed that increases in succulence in response to salinity could be a characteristic indicative of an increased degree of salt tolerance (Tal and Shannon 1983; Dikilitas 2003).

An increase in salt uptake generally depends on transpiration loss, because the water loss will increase the flux of saline water into the root system. Consequently, most plants, especially halophytes, show morphological features that prevent water loss, such as increased succulence, a thick cuticle on leaves, a reduced number of stomata, or sunken stomata, altered stomatal distribution and rolled leaves (Begg 1980; Flowers et al. 1986; Cruz and Cuartero 1990; Ashraf and Foolad 2007), which would thereby reduce the uptake of ions and would improve salinity tolerance. Preventing water loss, by this way, might also reduce the toxic effect of excessive ion concentration (Flowers et al. 1991).

Climate and irrigation also influence salinity tolerance. As the soil dries, salts become concentrated in the soil solution, increasing salt stress. Therefore, salt problems are more severe under hot and dry conditions than under cool and humid conditions. Detailed description of adaptation to salinity is given in Fig. 16.1 following Waisel (1991).

Salt Stress Plant
Fig. 16.1 Modes of plant adaptation to salinity (Waisel 1991)
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Responses

  • Thomas
    How the plant salt resistance?
    7 months ago

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