Investigating physiological and biochemical

2.3.1.2. Intra-cellular accumulation/compartmentation 
At cell level, different salt tolerance mechanisms operate to efficiently remove 
the toxic and/or lethal concentrations of ions from the plant body. In cell, different 
types of antiporter, symporter and carrier proteins actively participate in ion 
trafficking. These are termed as ion pumps and they regulate the ion homeostasis (IH). 
SOS (salt overly sensitive) and NHX regulatory pathways are among the best IH 
pathways. SOS proteins work on plasma membrane while NHX proteins work on 
tonoplast (vacuolar membrane). SOS family (SOS1, SOS2, SOS3) shows 
hypersensitivity to NaCl concentration but not to osmotic stress as these are not 
sensitive to mannitol. In the Arabidopsis thaliana, SOS3 activates SOS2 on cell 

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40 
membrane and hence stimulates the activity of Na
+
/H
+
antiporter (Quintero et al., 
2002; Guo et al., 2004). 
After reaching the cytoplasm, Na
+
is immediately pumped into the vacuole and 
this scavenging is regulated by NHX proteins on tonoplast (Blumwald et al., 2000). 
When Na
+
is pumped into the vacuole, it is then thrown into the leaf cells before 
being toxic for different enzymes. The activity of Na
+
/H
+
antiporters is more in 
halophytes as compared to glycophytes and is induced by the presence of high salt 
concentration. Thus the over-expression of NHX proteins (vacuolar transporter) 
enhances the salt stress tolerance as is reported in tomato and rice (Zhang and 
Blumwald, 2001; Fukuda et al., 2004). These proteins enhance and facilitate the 
storage of Na
+
ions by increasing uptake of Na
+
to vacuoles and thus confer high salt 
tolerance by reducing the Na
+
concentration in the cytosol.
Glycophytes as well as halophytes are not able to tolerate high Na
+
concentration 
in their cytosol and hence both these types control Na
+ 
entry into the cell and its 
accumulation in the cytoplasm for the safety and protection of metabolic machinery. 
A salt tolerant plant “Golden Promise” exhibited significantly low concentration of 
Na
+
along with low ratios of Na
+
/K
+
and Na
+
/Ca
2+
in its younger leaf blades and 
sheath tissues as compared to a salt sensitive plant “Maythorpe” (Wenxue et al., 
2003). In durum wheat, Munns and James (2003) observed that salt sensitive 
genotypes failed to exclude salts from the transpiration stream which ultimately 
hammered the new leaves and caused plant death.
In some plant species, however, controversial observations were also recorded. 
For instance, Lupinus luteus, a salt tolerant species compartmented high Na contents 
in stem when compared with salt sensitive Lupinus angustifolius species (Van 
Steveninck et al., 1982). Such mechanisms prevail in plant cells and even in some 
special plant parts showing some sort of adaptation at cell or plant level (Carden et al., 
2003).
Keeping in view the above discussion, it can be suggested that different plant 
species specially glycophytes adopt both ion inclusion and exclusion as mechanisms 

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41 
of salt tolerance and these mechanisms depend on ion distribution pattern among 
leaves and other parts of the plant body (Munns, 2002; Ashraf, 2004; Dogan et al., 
2010; Nemati et al., 2011).